JAVA
Section: JDK Commands (1)
Updated: 2021
Page Index
NAME
java - launch a Java application
SYNOPSIS
To launch a class file:
java [options] mainclass [args ...]
To launch the main class in a JAR file:
java [options] -jar jarfile
[args ...]
To launch the main class in a module:
java [options] -m
module[/mainclass] [args ...]
or
java [options] --module
module[/mainclass] [args ...]
To launch a single source-file program:
java [options] source-file [args
...]
- options
-
Optional: Specifies command-line options separated by spaces.
See Overview of Java Options for a description of available
options.
-
- mainclass
-
Specifies the name of the class to be launched.
Command-line entries following classname are the arguments for
the main method.
-
- -jar jarfile
-
Executes a program encapsulated in a JAR file.
The jarfile argument is the name of a JAR file with a manifest
that contains a line in the form Main-Class:classname
that defines the class with the
public static void main(String[] args) method that serves
as your application's starting point.
When you use -jar, the specified JAR file is the source of all
user classes, and other class path settings are ignored.
If you're using JAR files, then see jar.
-
- -m or --module module[/mainclass]
-
Executes the main class in a module specified by mainclass if
it is given, or, if it is not given, the value in the module.
In other words, mainclass can be used when it is not specified
by the module, or to override the value when it is specified.
-
See Standard Options for Java.
- source-file
-
Only used to launch a single source-file program.
Specifies the source file that contains the main class when using
source-file mode.
See Using Source-File Mode to Launch Single-File Source-Code
Programs
-
- args ...
-
Optional: Arguments following mainclass, source-file,
-jar jarfile, and -m or --module
module/mainclass are passed as arguments to
the main class.
-
DESCRIPTION
The java command starts a Java application.
It does this by starting the Java Virtual Machine (JVM), loading the
specified class, and calling that class's main() method.
The method must be declared public and static, it must
not return any value, and it must accept a String array as a
parameter.
The method declaration has the following form:
-
public static void main(String[] args)
In source-file mode, the java command can launch a class
declared in a source file.
See Using Source-File Mode to Launch Single-File Source-Code
Programs for a description of using the source-file mode.
-
Note: You can use the JDK_JAVA_OPTIONS launcher
environment variable to prepend its content to the actual command line
of the java launcher.
See Using the JDK_JAVA_OPTIONS Launcher Environment Variable.
By default, the first argument that isn't an option of the
java command is the fully qualified name of the class to be
called.
If -jar is specified, then its argument is the name of the JAR
file containing class and resource files for the application.
The startup class must be indicated by the Main-Class manifest
header in its manifest file.
Arguments after the class file name or the JAR file name are passed to
the main() method.
javaw
Windows: The javaw command is identical to
java, except that with javaw there's no associated
console window.
Use javaw when you don't want a command prompt window to
appear.
The javaw launcher will, however, display a dialog box with
error information if a launch fails.
USING SOURCE-FILE MODE TO LAUNCH SINGLE-FILE SOURCE-CODE PROGRAMS
To launch a class declared in a source file, run the java
launcher in source-file mode.
Entering source-file mode is determined by two items on the
java command line:
- •
-
The first item on the command line that is not an option or part of an
option.
In other words, the item in the command line that would otherwise be the
main class name.
- •
-
The --source version option, if present.
If the class identifies an existing file that has a .java
extension, or if the --source option is specified, then
source-file mode is selected.
The source file is then compiled and run.
The --source option can be used to specify the source
version or N of the source code.
This determines the API that can be used.
When you set --source N, you can only use the public
API that was defined in JDK N.
-
Note: The valid values of N change for each release,
with new values added and old values removed.
You'll get an error message if you use a value of N that is
no longer supported.
The supported values of N are the current Java SE release
(16) and a limited number of previous releases, detailed in the
command-line help for javac, under the --source and
--release options.
If the file does not have the .java extension, the
--source option must be used to tell the java
command to use the source-file mode.
The --source option is used for cases when the source file is
a "script" to be executed and the name of the source file does not
follow the normal naming conventions for Java source files.
In source-file mode, the effect is as though the source file is
compiled into memory, and the first class found in the source file is
executed.
Any arguments placed after the name of the source file in the original
command line are passed to the compiled class when it is executed.
For example, if a file were named HelloWorld.java and contained
a class named hello.World, then the source-file mode command
to launch the class would be:
-
java HelloWorld.java
The example illustrates that the class can be in a named package, and
does not need to be in the unnamed package.
This use of source-file mode is informally equivalent to using the
following two commands where hello.World is the name of the
class in the package:
-
javac -d <memory> HelloWorld.java
java -cp <memory> hello.World
In source-file mode, any additional command-line options are
processed as follows:
- •
-
The launcher scans the options specified before the source file for any
that are relevant in order to compile the source file.
-
This includes: --class-path, --module-path,
--add-exports, --add-modules,
--limit-modules, --patch-module,
--upgrade-module-path, and any variant forms of those
options.
It also includes the new --enable-preview option, described
in JEP 12.
- •
-
No provision is made to pass any additional options to the compiler,
such as -processor or -Werror.
- •
-
Command-line argument files (@-files) may be used in the
standard way.
Long lists of arguments for either the VM or the program being invoked
may be placed in files specified on the command-line by prefixing the
filename with an @ character.
In source-file mode, compilation proceeds as follows:
- •
-
Any command-line options that are relevant to the compilation
environment are taken into account.
- •
-
No other source files are found and compiled, as if the source path is
set to an empty value.
- •
-
Annotation processing is disabled, as if -proc:none is in
effect.
- •
-
If a version is specified, via the --source option, the value
is used as the argument for an implicit --release option for
the compilation.
This sets both the source version accepted by compiler and the system
API that may be used by the code in the source file.
- •
-
The source file is compiled in the context of an unnamed module.
- •
-
The source file should contain one or more top-level classes, the first
of which is taken as the class to be executed.
- •
-
The compiler does not enforce the optional restriction defined at the
end of JLS ??7.6, that a type in a named package should exist in a file
whose name is composed from the type name followed by the
.java extension.
- •
-
If the source file contains errors, appropriate error messages are
written to the standard error stream, and the launcher exits with a
non-zero exit code.
In source-file mode, execution proceeds as follows:
- •
-
The class to be executed is the first top-level class found in the
source file.
It must contain a declaration of the standard
public static void main(String[]) method.
- •
-
The compiled classes are loaded by a custom class loader, that delegates
to the application class loader.
This implies that classes appearing on the application class path cannot
refer to any classes declared in the source file.
- •
-
The compiled classes are executed in the context of an unnamed module,
as though --add-modules=ALL-DEFAULT is in effect.
This is in addition to any other --add-module options that
may be have been specified on the command line.
- •
-
Any arguments appearing after the name of the file on the command line
are passed to the standard main method in the obvious way.
- •
-
It is an error if there is a class on the application class path whose
name is the same as that of the class to be executed.
See JEP 330: Launch Single-File Source-Code Programs
[http://openjdk.java.net/jeps/330] for complete details.
USING THE JDK_JAVA_OPTIONS LAUNCHER ENVIRONMENT VARIABLE
JDK_JAVA_OPTIONS prepends its content to the options parsed
from the command line.
The content of the JDK_JAVA_OPTIONS environment variable is a
list of arguments separated by white-space characters (as determined by
isspace()).
These are prepended to the command line arguments passed to
java launcher.
The encoding requirement for the environment variable is the same as the
java command line on the system.
JDK_JAVA_OPTIONS environment variable content is treated in the
same manner as that specified in the command line.
Single (') or double (") quotes can be used to
enclose arguments that contain whitespace characters.
All content between the open quote and the first matching close quote
are preserved by simply removing the pair of quotes.
In case a matching quote is not found, the launcher will abort with an
error message.
@-files are supported as they are specified in the command
line.
However, as in @-files, use of a wildcard is not supported.
In order to mitigate potential misuse of JDK_JAVA_OPTIONS
behavior, options that specify the main class (such as -jar)
or cause the java launcher to exit without executing the main
class (such as -h) are disallowed in the environment variable.
If any of these options appear in the environment variable, the launcher
will abort with an error message.
When JDK_JAVA_OPTIONS is set, the launcher prints a message to
stderr as a reminder.
Example:
-
$ export JDK_JAVA_OPTIONS='-g @file1 -Dprop=value @file2 -Dws.prop="white spaces"'
$ java -Xint @file3
is equivalent to the command line:
-
java -g @file1 -Dprop=value @file2 -Dws.prop="white spaces" -Xint @file3
OVERVIEW OF JAVA OPTIONS
The java command supports a wide range of options in the
following categories:
- •
-
Standard Options for Java: Options guaranteed to be supported
by all implementations of the Java Virtual Machine (JVM).
They're used for common actions, such as checking the version of the
JRE, setting the class path, enabling verbose output, and so on.
- •
-
Extra Options for Java: General purpose options that are
specific to the Java HotSpot Virtual Machine.
They aren't guaranteed to be supported by all JVM implementations,
and are subject to change.
These options start with -X.
The advanced options aren't recommended for casual use.
These are developer options used for tuning specific areas of the Java
HotSpot Virtual Machine operation that often have specific system
requirements and may require privileged access to system configuration
parameters.
Several examples of performance tuning are provided in Performance
Tuning Examples.
These options aren't guaranteed to be supported by all JVM
implementations and are subject to change.
Advanced options start with -XX.
- •
-
Advanced Runtime Options for Java: Control the runtime behavior
of the Java HotSpot VM.
- •
-
Advanced JIT Compiler Options for java: Control the dynamic
just-in-time (JIT) compilation performed by the Java HotSpot VM.
- •
-
Advanced Serviceability Options for Java: Enable gathering
system information and performing extensive debugging.
- •
-
Advanced Garbage Collection Options for Java: Control how
garbage collection (GC) is performed by the Java HotSpot
Boolean options are used to either enable a feature that's disabled
by default or disable a feature that's enabled by default.
Such options don't require a parameter.
Boolean -XX options are enabled using the plus sign
(-XX:+OptionName) and disabled using the minus sign
(-XX:-OptionName).
For options that require an argument, the argument may be separated from
the option name by a space, a colon (:), or an equal sign (=), or the
argument may directly follow the option (the exact syntax differs for
each option).
If you're expected to specify the size in bytes, then you can use no
suffix, or use the suffix k or K for kilobytes (KB),
m or M for megabytes (MB), or g or G
for gigabytes (GB).
For example, to set the size to 8 GB, you can specify either
8g, 8192m, 8388608k, or 8589934592
as the argument.
If you are expected to specify the percentage, then use a number from 0
to 1.
For example, specify 0.25 for 25%.
The following sections describe the options that are obsolete,
deprecated, and removed:
- •
-
Deprecated Java Options: Accepted and acted upon --- a
warning is issued when they're used.
- •
-
Obsolete Java Options: Accepted but ignored --- a warning is
issued when they're used.
- •
-
Removed Java Options: Removed --- using them results in an
error.
STANDARD OPTIONS FOR JAVA
These are the most commonly used options supported by all
implementations of the JVM.
-
Note: To specify an argument for a long option, you can use
either --name=value or
--name value.
- -agentlib:libname[=options]
-
Loads the specified native agent library.
After the library name, a comma-separated list of options specific to
the library can be used.
-
- •
-
Linux and macOS: If the option -agentlib:foo is
specified, then the JVM attempts to load the library named
libfoo.so in the location specified by the
LD_LIBRARY_PATH system variable (on macOS this variable is
DYLD_LIBRARY_PATH).
- •
-
Windows: If the option -agentlib:foo is specified,
then the JVM attempts to load the library named foo.dll in the
location specified by the PATH system variable.
-
The following example shows how to load the Java Debug Wire Protocol
(JDWP) library and listen for the socket connection on port 8000,
suspending the JVM before the main class loads:
-
-agentlib:jdwp=transport=dt_socket,server=y,address=8000
- -agentpath:pathname[=options]
-
Loads the native agent library specified by the absolute path name.
This option is equivalent to -agentlib but uses the full path
and file name of the library.
-
- --class-path classpath, -classpath classpath, or -cp classpath
-
A semicolon (;) separated list of directories, JAR archives,
and ZIP archives to search for class files.
-
Specifying classpath overrides any setting of the
CLASSPATH environment variable.
If the class path option isn't used and classpath isn't
set, then the user class path consists of the current directory (.).
As a special convenience, a class path element that contains a base name
of an asterisk (*) is considered equivalent to specifying a list of all
the files in the directory with the extension .jar or
.JAR .
A Java program can't tell the difference between the two
invocations.
For example, if the directory mydir contains a.jar and
b.JAR, then the class path element mydir/* is expanded to
A.jar:b.JAR, except that the order of JAR files is unspecified.
All .jar files in the specified directory, even hidden ones,
are included in the list.
A class path entry consisting of an asterisk (*) expands to a list of
all the jar files in the current directory.
The CLASSPATH environment variable, where defined, is similarly
expanded.
Any class path wildcard expansion that occurs before the Java VM is
started.
Java programs never see wildcards that aren't expanded except by
querying the environment, such as by calling
System.getenv("CLASSPATH").
- --disable-@files
-
Can be used anywhere on the command line, including in an argument file,
to prevent further @filename expansion.
This option stops expanding @-argfiles after the option.
-
- --enable-preview
-
Allows classes to depend on preview features
[https://docs.oracle.com/en/java/javase/12/language/index.html#JSLAN-GUID-5A82FE0E-0CA4-4F1F-B075-564874FE2823]
of the release.
-
- --module-path modulepath... or -p modulepath
-
A semicolon (;) separated list of directories in which each
directory is a directory of modules.
-
- --upgrade-module-path modulepath...
-
A semicolon (;) separated list of directories in which each
directory is a directory of modules that replace upgradeable modules in
the runtime image.
-
- --add-modules module[,module...]
-
Specifies the root modules to resolve in addition to the initial module.
module also can be ALL-DEFAULT, ALL-SYSTEM,
and ALL-MODULE-PATH.
-
- --list-modules
-
Lists the observable modules and then exits.
-
- -d module_name or --describe-module module_name
-
Describes a specified module and then exits.
-
- --dry-run
-
Creates the VM but doesn't execute the main method.
This --dry-run option might be useful for validating the
command-line options such as the module system configuration.
-
- --validate-modules
-
Validates all modules and exit.
This option is helpful for finding conflicts and other errors with
modules on the module path.
-
- -Dproperty=value
-
Sets a system property value.
The property variable is a string with no spaces that
represents the name of the property.
The value variable is a string that represents the value of the
property.
If value is a string with spaces, then enclose it in quotation
marks (for example -Dfoo="foo bar").
-
- -disableassertions[:[packagename]...|:classname] or -da[:[packagename]...|:classname]
-
Disables assertions.
By default, assertions are disabled in all packages and classes.
With no arguments, -disableassertions (-da) disables
assertions in all packages and classes.
With the packagename argument ending in ..., the
switch disables assertions in the specified package and any subpackages.
If the argument is simply ..., then the switch disables
assertions in the unnamed package in the current working directory.
With the classname argument, the switch disables assertions in
the specified class.
-
The -disableassertions (-da) option applies to all
class loaders and to system classes (which don't have a class
loader).
There's one exception to this rule: If the option is provided with
no arguments, then it doesn't apply to system classes.
This makes it easy to disable assertions in all classes except for
system classes.
The -disablesystemassertions option enables you to disable
assertions in all system classes.
To explicitly enable assertions in specific packages or classes, use the
-enableassertions (-ea) option.
Both options can be used at the same time.
For example, to run the MyClass application with assertions
enabled in the package com.wombat.fruitbat (and any
subpackages) but disabled in the class
com.wombat.fruitbat.Brickbat, use the following command:
-
java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass
- -disablesystemassertions or -dsa
-
Disables assertions in all system classes.
-
- -enableassertions[:[packagename]...|:classname] or -ea[:[packagename]...|:classname]
-
Enables assertions.
By default, assertions are disabled in all packages and classes.
With no arguments, -enableassertions (-ea) enables
assertions in all packages and classes.
With the packagename argument ending in ..., the
switch enables assertions in the specified package and any subpackages.
If the argument is simply ..., then the switch enables
assertions in the unnamed package in the current working directory.
With the classname argument, the switch enables assertions in
the specified class.
-
The -enableassertions (-ea) option applies to all
class loaders and to system classes (which don't have a class
loader).
There's one exception to this rule: If the option is provided with
no arguments, then it doesn't apply to system classes.
This makes it easy to enable assertions in all classes except for system
classes.
The -enablesystemassertions option provides a separate switch
to enable assertions in all system classes.
To explicitly disable assertions in specific packages or classes, use
the -disableassertions (-da) option.
If a single command contains multiple instances of these switches, then
they're processed in order, before loading any classes.
For example, to run the MyClass application with assertions
enabled only in the package com.wombat.fruitbat (and any
subpackages) but disabled in the class
com.wombat.fruitbat.Brickbat, use the following command:
-
java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass
- -enablesystemassertions or -esa
-
Enables assertions in all system classes.
-
- -help, -h, or -?
-
Prints the help message to the error stream.
-
- --help
-
Prints the help message to the output stream.
-
- -javaagent:jarpath[=options]
-
Loads the specified Java programming language agent.
See java.lang.instrument.
-
- --show-version
-
Prints the product version to the output stream and continues.
-
- -showversion
-
Prints the product version to the error stream and continues.
-
- --show-module-resolution
-
Shows module resolution output during startup.
-
- -splash:imagepath
-
Shows the splash screen with the image specified by imagepath.
HiDPI scaled images are automatically supported and used if available.
The unscaled image file name, such as image.ext, should always
be passed as the argument to the -splash option.
The most appropriate scaled image provided is picked up automatically.
-
For example, to show the splash.gif file from the
images directory when starting your application, use the
following option:
-
-splash:images/splash.gif
See the SplashScreen API documentation for more information.
- -verbose:class
-
Displays information about each loaded class.
-
- -verbose:gc
-
Displays information about each garbage collection (GC) event.
-
- -verbose:jni
-
Displays information about the use of native methods and other Java
Native Interface (JNI) activity.
-
- -verbose:module
-
Displays information about the modules in use.
-
- --version
-
Prints product version to the output stream and exits.
-
- -version
-
Prints product version to the error stream and exits.
-
- -X
-
Prints the help on extra options to the error stream.
-
- --help-extra
-
Prints the help on extra options to the output stream.
-
- @argfile
-
Specifies one or more argument files prefixed by @ used by the
java command.
It isn't uncommon for the java command line to be very long
because of the .jar files needed in the classpath.
The @argfile option overcomes command-line length
limitations by enabling the launcher to expand the contents of argument
files after shell expansion, but before argument processing.
Contents in the argument files are expanded because otherwise, they
would be specified on the command line until the
--disable-@files option was encountered.
-
The argument files can also contain the main class name and all options.
If an argument file contains all of the options required by the
java command, then the command line could simply be:
-
java @argfile
See java Command-Line Argument Files for a description and
examples of using @-argfiles.
EXTRA OPTIONS FOR JAVA
The following java options are general purpose options that are
specific to the Java HotSpot Virtual Machine.
- -Xbatch
-
Disables background compilation.
By default, the JVM compiles the method as a background task, running
the method in interpreter mode until the background compilation is
finished.
The -Xbatch flag disables background compilation so that
compilation of all methods proceeds as a foreground task until
completed.
This option is equivalent to -XX:-BackgroundCompilation.
-
- -Xbootclasspath/a:directories|zip|JAR-files
-
Specifies a list of directories, JAR files, and ZIP archives to append
to the end of the default bootstrap class path.
-
Linux and macOS: Colons (:) separate entities in this
list.
Windows: Semicolons (;) separate entities in this
list.
- -Xcheck:jni
-
Performs additional checks for Java Native Interface (JNI) functions.
Specifically, it validates the parameters passed to the JNI function and
the runtime environment data before processing the JNI request.
It also checks for pending exceptions between JNI calls.
Any invalid data encountered indicates a problem in the native code, and
the JVM terminates with an irrecoverable error in such cases.
Expect a performance degradation when this option is used.
-
- -Xdebug
-
Does nothing.
Provided for backward compatibility.
-
- -Xdiag
-
Shows additional diagnostic messages.
-
- -Xint
-
Runs the application in interpreted-only mode.
Compilation to native code is disabled, and all bytecode is executed by
the interpreter.
The performance benefits offered by the just-in-time (JIT) compiler
aren't present in this mode.
-
- -Xinternalversion
-
Displays more detailed JVM version information than the
-version option, and then exits.
-
- -Xlog:option
-
Configure or enable logging with the Java Virtual Machine (JVM) unified
logging framework.
See Enable Logging with the JVM Unified Logging Framework.
-
- -Xmixed
-
Executes all bytecode by the interpreter except for hot methods, which
are compiled to native code.
On by default.
Use -Xint to switch off.
-
- -Xmn size
-
Sets the initial and maximum size (in bytes) of the heap for the young
generation (nursery) in the generational collectors.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The young generation region of the heap is used for new objects.
GC is performed in this region more often than in other regions.
If the size for the young generation is too small, then a lot of minor
garbage collections are performed.
If the size is too large, then only full garbage collections are
performed, which can take a long time to complete.
It is recommended that you do not set the size for the young generation
for the G1 collector, and keep the size for the young generation greater
than 25% and less than 50% of the overall heap size for other
collectors.
The following examples show how to set the initial and maximum size of
young generation to 256 MB using various units:
-
-
-Xmn256m
-Xmn262144k
-Xmn268435456
Instead of the -Xmn option to set both the initial and maximum
size of the heap for the young generation, you can use
-XX:NewSize to set the initial size and
-XX:MaxNewSize to set the maximum size.
- -Xms size
-
Sets the minimum and initial size (in bytes) of the heap.
This value must be a multiple of 1024 and greater than 1 MB.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, g or G
to indicate gigabytes.
The following examples show how to set the size of allocated memory to 6
MB using various units:
-
-
-Xms6291456
-Xms6144k
-Xms6m
Instead of the -Xms option to set both the minimum and initial
size of the heap, you can use -XX:MinHeapSize to set the
minimum size and -XX:InitialHeapSize to set the initial size.
If you don't set this option, the initial size is set as the sum of
the sizes allocated for the old generation and the young generation.
The initial size of the heap for the young generation can be set using
the -Xmn option or the -XX:NewSize option.
- -Xmx size
-
Specifies the maximum size (in bytes) of the heap.
This value must be a multiple of 1024 and greater than 2 MB.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default value is chosen at runtime based on system configuration.
For server deployments, -Xms and -Xmx are often set
to the same value.
The following examples show how to set the maximum allowed size of
allocated memory to 80 MB using various units:
-
-
-Xmx83886080
-Xmx81920k
-Xmx80m
The -Xmx option is equivalent to -XX:MaxHeapSize.
- -Xnoclassgc
-
Disables garbage collection (GC) of classes.
This can save some GC time, which shortens interruptions during the
application run.
When you specify -Xnoclassgc at startup, the class objects in
the application are left untouched during GC and are always be
considered live.
This can result in more memory being permanently occupied which, if not
used carefully, throws an out-of-memory exception.
-
- -Xrs
-
Reduces the use of operating system signals by the JVM.
Shutdown hooks enable the orderly shutdown of a Java application by
running user cleanup code (such as closing database connections) at
shutdown, even if the JVM terminates abruptly.
-
- •
-
Linux and macOS:
-
- •
-
The JVM catches signals to implement shutdown hooks for unexpected
termination.
The JVM uses SIGHUP, SIGINT, and SIGTERM to
initiate the running of shutdown hooks.
- •
-
Applications embedding the JVM frequently need to trap signals such as
SIGINT or SIGTERM, which can lead to interference with
the JVM signal handlers.
The -Xrs option is available to address this issue.
When -Xrs is used, the signal masks for SIGINT,
SIGTERM, SIGHUP, and SIGQUIT aren't
changed by the JVM, and signal handlers for these signals aren't
installed.
- •
-
Windows:
-
- •
-
The JVM watches for console control events to implement shutdown hooks
for unexpected termination.
Specifically, the JVM registers a console control handler that begins
shutdown-hook processing and returns TRUE for
CTRL_C_EVENT, CTRL_CLOSE_EVENT,
CTRL_LOGOFF_EVENT, and CTRL_SHUTDOWN_EVENT.
- •
-
The JVM uses a similar mechanism to implement the feature of dumping
thread stacks for debugging purposes.
The JVM uses CTRL_BREAK_EVENT to perform thread dumps.
- •
-
If the JVM is run as a service (for example, as a servlet engine for a
web server), then it can receive CTRL_LOGOFF_EVENT but
shouldn't initiate shutdown because the operating system doesn't
actually terminate the process.
To avoid possible interference such as this, the -Xrs option
can be used.
When the -Xrs option is used, the JVM doesn't install a
console control handler, implying that it doesn't watch for or
process CTRL_C_EVENT, CTRL_CLOSE_EVENT,
CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.
There are two consequences of specifying -Xrs:
- •
-
Linux and macOS: SIGQUIT thread dumps aren't
available.
- •
-
Windows: Ctrl + Break thread dumps aren't available.
User code is responsible for causing shutdown hooks to run, for example,
by calling the System.exit() when the JVM is to be terminated.
- -Xshare:mode
-
Sets the class data sharing (CDS) mode.
-
Possible mode arguments for this option include the following:
- auto
-
Use shared class data if possible (default).
-
- on
-
Require using shared class data, otherwise fail.
-
-
Note: The -Xshare:on option is used for testing
purposes only and may cause intermittent failures due to the use of
address space layout randomization by the operation system.
This option should not be used in production environments.
- off
-
Do not attempt to use shared class data.
-
- -XshowSettings
-
Shows all settings and then continues.
-
- -XshowSettings:category
-
Shows settings and continues.
Possible category arguments for this option include the
following:
-
- all
-
Shows all categories of settings.
This is the default value.
-
- locale
-
Shows settings related to locale.
-
- properties
-
Shows settings related to system properties.
-
- vm
-
Shows the settings of the JVM.
-
- system
-
Linux: Shows host system or container configuration and
continues.
-
- -Xss size
-
Sets the thread stack size (in bytes).
Append the letter k or K to indicate KB, m or
M to indicate MB, or g or G to indicate GB.
The default value depends on the platform:
-
- •
-
Linux/x64 (64-bit): 1024 KB
- •
-
macOS (64-bit): 1024 KB
- •
-
Windows: The default value depends on virtual memory
The following examples set the thread stack size to 1024 KB in different
units:
-
-Xss1m
-Xss1024k
-Xss1048576
This option is similar to -XX:ThreadStackSize.
- --add-reads module=target-module(,target-module)*
-
Updates module to read the target-module, regardless
of the module declaration.
target-module can be all unnamed to read all unnamed modules.
-
- --add-exports module/package=target-module(,target-module)*
-
Updates module to export package to
target-module, regardless of module declaration.
The target-module can be all unnamed to export to all unnamed
modules.
-
- --add-opens module/package=target-module(,target-module)*
-
Updates module to open package to
target-module, regardless of module declaration.
-
- --limit-modules module[,module...]
-
Specifies the limit of the universe of observable modules.
-
- --patch-module module=file(;file)*
-
Overrides or augments a module with classes and resources in JAR files
or directories.
-
- --source version
-
Sets the version of the source in source-file mode.
-
EXTRA OPTIONS FOR MACOS
The following extra options are macOS specific.
- -XstartOnFirstThread
-
Runs the main() method on the first (AppKit) thread.
-
- -Xdock:name=application_name
-
Overrides the default application name displayed in dock.
-
- -Xdock:icon=path_to_icon_file
-
Overrides the default icon displayed in dock.
-
ADVANCED OPTIONS FOR JAVA
These java options can be used to enable other advanced
options.
- -XX:+UnlockDiagnosticVMOptions
-
Unlocks the options intended for diagnosing the JVM.
By default, this option is disabled and diagnostic options aren't
available.
-
Command line options that are enabled with the use of this option are
not supported.
If you encounter issues while using any of these options, it is very
likely that you will be required to reproduce the problem without using
any of these unsupported options before Oracle Support can assist with
an investigation.
It is also possible that any of these options may be removed or their
behavior changed without any warning.
- -XX:+UnlockExperimentalVMOptions
-
Unlocks the options that provide experimental features in the JVM.
By default, this option is disabled and experimental features aren't
available.
-
ADVANCED RUNTIME OPTIONS FOR JAVA
These java options control the runtime behavior of the Java
HotSpot VM.
- -XX:ActiveProcessorCount=x
-
Overrides the number of CPUs that the VM will use to calculate the size
of thread pools it will use for various operations such as Garbage
Collection and ForkJoinPool.
-
The VM normally determines the number of available processors from the
operating system.
This flag can be useful for partitioning CPU resources when running
multiple Java processes in docker containers.
This flag is honored even if UseContainerSupport is not
enabled.
See -XX:-UseContainerSupport for a description of enabling
and disabling container support.
- -XX:AllocateHeapAt=path
-
Takes a path to the file system and uses memory mapping to allocate the
object heap on the memory device.
Using this option enables the HotSpot VM to allocate the Java object
heap on an alternative memory device, such as an NV-DIMM, specified by
the user.
-
Alternative memory devices that have the same semantics as DRAM,
including the semantics of atomic operations, can be used instead of
DRAM for the object heap without changing the existing application code.
All other memory structures (such as the code heap, metaspace, and
thread stacks) continue to reside in DRAM.
Some operating systems expose non-DRAM memory through the file system.
Memory-mapped files in these file systems bypass the page cache and
provide a direct mapping of virtual memory to the physical memory on the
device.
The existing heap related flags (such as -Xmx and
-Xms) and garbage-collection related flags continue to work
as before.
- -XX:-CompactStrings
-
Disables the Compact Strings feature.
By default, this option is enabled.
When this option is enabled, Java Strings containing only single-byte
characters are internally represented and stored as
single-byte-per-character Strings using ISO-8859-1 / Latin-1
encoding.
This reduces, by 50%, the amount of space required for Strings
containing only single-byte characters.
For Java Strings containing at least one multibyte character: these are
represented and stored as 2 bytes per character using UTF-16 encoding.
Disabling the Compact Strings feature forces the use of UTF-16 encoding
as the internal representation for all Java Strings.
-
Cases where it may be beneficial to disable Compact Strings include the
following:
- •
-
When it's known that an application overwhelmingly will be
allocating multibyte character Strings
- •
-
In the unexpected event where a performance regression is observed in
migrating from Java SE 8 to Java SE 9 and an analysis shows that Compact
Strings introduces the regression
In both of these scenarios, disabling Compact Strings makes sense.
- -XX:ErrorFile=filename
-
Specifies the path and file name to which error data is written when an
irrecoverable error occurs.
By default, this file is created in the current working directory and
named hs_err_pidpid.log where pid
is the identifier of the process that encountered the error.
-
The following example shows how to set the default log file (note that
the identifier of the process is specified as %p):
-
-XX:ErrorFile=./hs_err_pid%p.log
- •
-
Linux and macOS: The following example shows how to set the
error log to /var/log/java/java_error.log:
-
-
-XX:ErrorFile=/var/log/java/java_error.log
- •
-
Windows: The following example shows how to set the error log
file to C:/log/java/java_error.log:
-
-
-XX:ErrorFile=C:/log/java/java_error.log
If the file exists, and is writeable, then it will be overwritten.
Otherwise, if the file can't be created in the specified directory
(due to insufficient space, permission problem, or another issue), then
the file is created in the temporary directory for the operating system:
- •
-
Linux and macOS: The temporary directory is /tmp.
- •
-
Windows: The temporary directory is specified by the value of
the TMP environment variable; if that environment variable
isn't defined, then the value of the TEMP environment
variable is used.
- -XX:+ExtensiveErrorReports
-
Enables the reporting of more extensive error information in the
ErrorFile.
This option can be turned on in environments where maximal information
is desired - even if the resulting logs may be quite large and/or
contain information that might be considered sensitive.
The information can vary from release to release, and across different
platforms.
By default this option is disabled.
-
- -XX:FlightRecorderOptions=parameter=value (or)-XX:FlightRecorderOptions:parameter=value
-
Sets the parameters that control the behavior of JFR.
-
The following list contains the available JFR
parameter=value entries:
- globalbuffersize=size
-
Specifies the total amount of primary memory used for data retention.
The default value is based on the value specified for
memorysize.
Change the memorysize parameter to alter the size of global
buffers.
-
- maxchunksize=size
-
Specifies the maximum size (in bytes) of the data chunks in a recording.
Append m or M to specify the size in megabytes (MB),
or g or G to specify the size in gigabytes (GB).
By default, the maximum size of data chunks is set to 12 MB.
The minimum allowed is 1 MB.
-
- memorysize=size
-
Determines how much buffer memory should be used, and sets the
globalbuffersize and numglobalbuffers parameters based
on the size specified.
Append m or M to specify the size in megabytes (MB),
or g or G to specify the size in gigabytes (GB).
By default, the memory size is set to 10 MB.
-
- numglobalbuffers
-
Specifies the number of global buffers used.
The default value is based on the memory size specified.
Change the memorysize parameter to alter the number of global
buffers.
-
- old-object-queue-size=number-of-objects
-
Maximum number of old objects to track.
By default, the number of objects is set to 256.
-
- repository=path
-
Specifies the repository (a directory) for temporary disk storage.
By default, the system's temporary directory is used.
-
- retransform={true|false}
-
Specifies whether event classes should be retransformed using JVMTI.
If false, instrumentation is added when event classes are loaded.
By default, this parameter is enabled.
-
- samplethreads={true|false}
-
Specifies whether thread sampling is enabled.
Thread sampling occurs only if the sampling event is enabled along with
this parameter.
By default, this parameter is enabled.
-
- stackdepth=depth
-
Stack depth for stack traces.
By default, the depth is set to 64 method calls.
The maximum is 2048.
Values greater than 64 could create significant overhead and reduce
performance.
-
- threadbuffersize=size
-
Specifies the per-thread local buffer size (in bytes).
By default, the local buffer size is set to 8 kilobytes, with a minimum
value of 4 kilobytes.
Overriding this parameter could reduce performance and is not
recommended.
-
You can specify values for multiple parameters by separating them with a
comma.
- -XX:LargePageSizeInBytes=size
-
Sets the maximum size (in bytes) for large pages used for the Java heap.
The size argument must be a power of 2 (2, 4, 8, 16, and so
on).
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
By default, the size is set to 0, meaning that the JVM chooses the size
for large pages automatically.
See Large Pages.
-
The following example describes how to set the large page size to 4
megabytes (MB):
-
-XX:LargePageSizeInBytes=4m
- -XX:MaxDirectMemorySize=size
-
Sets the maximum total size (in bytes) of the java.nio package,
direct-buffer allocations.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
By default, the size is set to 0, meaning that the JVM chooses the size
for NIO direct-buffer allocations automatically.
-
The following examples illustrate how to set the NIO size to 1024 KB in
different units:
-
-XX:MaxDirectMemorySize=1m
-XX:MaxDirectMemorySize=1024k
-XX:MaxDirectMemorySize=1048576
- -XX:-MaxFDLimit
-
Disables the attempt to set the soft limit for the number of open file
descriptors to the hard limit.
By default, this option is enabled on all platforms, but is ignored on
Windows.
The only time that you may need to disable this is on Mac OS, where its
use imposes a maximum of 10240, which is lower than the actual system
maximum.
-
- -XX:NativeMemoryTracking=mode
-
Specifies the mode for tracking JVM native memory usage.
Possible mode arguments for this option include the following:
-
- off
-
Instructs not to track JVM native memory usage.
This is the default behavior if you don't specify the
-XX:NativeMemoryTracking option.
-
- summary
-
Tracks memory usage only by JVM subsystems, such as Java heap, class,
code, and thread.
-
- detail
-
In addition to tracking memory usage by JVM subsystems, track memory
usage by individual CallSite, individual virtual memory region
and its committed regions.
-
- -XX:ObjectAlignmentInBytes=alignment
-
Sets the memory alignment of Java objects (in bytes).
By default, the value is set to 8 bytes.
The specified value should be a power of 2, and must be within the range
of 8 and 256 (inclusive).
This option makes it possible to use compressed pointers with large Java
heap sizes.
-
The heap size limit in bytes is calculated as:
-
4GB * ObjectAlignmentInBytes
-
Note: As the alignment value increases, the unused space
between objects also increases.
As a result, you may not realize any benefits from using compressed
pointers with large Java heap sizes.
- -XX:OnError=string
-
Sets a custom command or a series of semicolon-separated commands to
run when an irrecoverable error occurs.
If the string contains spaces, then it must be enclosed in quotation
marks.
-
- •
-
Linux and macOS: The following example shows how the
-XX:OnError option can be used to run the gcore
command to create a core image, and start the gdb debugger to
attach to the process in case of an irrecoverable error (the %p
designates the current process identifier):
-
-
-XX:OnError="gcore %p;gdb -p %p"
- •
-
Windows: The following example shows how the
-XX:OnError option can be used to run the
userdump.exe utility to obtain a crash dump in case of an
irrecoverable error (the %p designates the current process
identifier).
This example assumes that the path to the userdump.exe utility
is specified in the PATH environment variable:
-
-
-XX:OnError="userdump.exe %p"
- -XX:OnOutOfMemoryError=string
-
Sets a custom command or a series of semicolon-separated commands to
run when an OutOfMemoryError exception is first thrown.
If the string contains spaces, then it must be enclosed in quotation
marks.
For an example of a command string, see the description of the
-XX:OnError option.
-
- -XX:+PrintCommandLineFlags
-
Enables printing of ergonomically selected JVM flags that appeared on
the command line.
It can be useful to know the ergonomic values set by the JVM, such as
the heap space size and the selected garbage collector.
By default, this option is disabled and flags aren't printed.
-
- -XX:+PreserveFramePointer
-
Selects between using the RBP register as a general purpose register
(-XX:-PreserveFramePointer) and using the RBP register to
hold the frame pointer of the currently executing method
(-XX:+PreserveFramePointer .
If the frame pointer is available, then external profiling tools (for
example, Linux perf) can construct more accurate stack traces.
-
- -XX:+PrintNMTStatistics
-
Enables printing of collected native memory tracking data at JVM exit
when native memory tracking is enabled (see
-XX:NativeMemoryTracking).
By default, this option is disabled and native memory tracking data
isn't printed.
-
- -XX:SharedArchiveFile=path
-
Specifies the path and name of the class data sharing (CDS) archive file
-
See Application Class Data Sharing.
- -XX:SharedArchiveConfigFile=shared_config_file
-
Specifies additional shared data added to the archive file.
-
- -XX:SharedClassListFile=file_name
-
Specifies the text file that contains the names of the classes to store
in the class data sharing (CDS) archive.
This file contains the full name of one class per line, except slashes
(/) replace dots (.).
For example, to specify the classes java.lang.Object and
hello.Main, create a text file that contains the following two
lines:
-
-
java/lang/Object
hello/Main
The classes that you specify in this text file should include the
classes that are commonly used by the application.
They may include any classes from the application, extension, or
bootstrap class paths.
See Application Class Data Sharing.
- -XX:+ShowCodeDetailsInExceptionMessages
-
Enables printing of improved NullPointerException messages.
When an application throws a NullPointerException, the option
enables the JVM to analyze the program's bytecode instructions to
determine precisely which reference is null, and describes the
source with a null-detail message.
The null-detail message is calculated and returned by
NullPointerException.getMessage(), and will be printed as the
exception message along with the method, filename, and line number.
By default, this option is enabled.
-
- -XX:+ShowMessageBoxOnError
-
Enables the display of a dialog box when the JVM experiences an
irrecoverable error.
This prevents the JVM from exiting and keeps the process active so that
you can attach a debugger to it to investigate the cause of the error.
By default, this option is disabled.
-
- -XX:StartFlightRecording=parameter=value
-
Starts a JFR recording for the Java application.
This option is equivalent to the JFR.start diagnostic command
that starts a recording during runtime.
You can set the following parameter=value
entries when starting a JFR recording:
-
- delay=time
-
Specifies the delay between the Java application launch time and the
start of the recording.
Append s to specify the time in seconds, m for
minutes, h for hours, or d for days (for example,
specifying 10m means 10 minutes).
By default, there's no delay, and this parameter is set to 0.
-
- disk={true|false}
-
Specifies whether to write data to disk while recording.
By default, this parameter is enabled.
-
- dumponexit={true|false}
-
Specifies if the running recording is dumped when the JVM shuts down.
If enabled and a filename is not entered, the recording is
written to a file in the directory where the process was started.
The file name is a system-generated name that contains the process ID,
recording ID, and current timestamp, similar to
hotspot-pid-47496-id-1-2018_01_25_19_10_41.jfr.
By default, this parameter is disabled.
-
- duration=time
-
Specifies the duration of the recording.
Append s to specify the time in seconds, m for
minutes, h for hours, or d for days (for example,
specifying 5h means 5 hours).
By default, the duration isn't limited, and this parameter is set to
0.
-
- filename=path
-
Specifies the path and name of the file to which the recording is
written when the recording is stopped, for example:
-
- •
-
recording.jfr
- •
-
/home/user/recordings/recording.jfr
- •
-
c:\recordings\recording.jfr
- name=identifier
-
Takes both the name and the identifier of a recording.
-
- maxage=time
-
Specifies the maximum age of disk data to keep for the recording.
This parameter is valid only when the disk parameter is set to
true.
Append s to specify the time in seconds, m for
minutes, h for hours, or d for days (for example,
specifying 30s means 30 seconds).
By default, the maximum age isn't limited, and this parameter is set
to 0s.
-
- maxsize=size
-
Specifies the maximum size (in bytes) of disk data to keep for the
recording.
This parameter is valid only when the disk parameter is set to
true.
The value must not be less than the value for the maxchunksize
parameter set with -XX:FlightRecorderOptions.
Append m or M to specify the size in megabytes, or
g or G to specify the size in gigabytes.
By default, the maximum size of disk data isn't limited, and this
parameter is set to 0.
-
- path-to-gc-roots={true|false}
-
Specifies whether to collect the path to garbage collection (GC) roots
at the end of a recording.
By default, this parameter is disabled.
-
The path to GC roots is useful for finding memory leaks, but collecting
it is time-consuming.
Enable this option only when you start a recording for an application
that you suspect has a memory leak.
If the settings parameter is set to profile, the stack
trace from where the potential leaking object was allocated is included
in the information collected.
- settings=path
-
Specifies the path and name of the event settings file (of type JFC).
By default, the default.jfc file is used, which is located in
JAVA_HOME/lib/jfr.
This default settings file collects a predefined set of information with
low overhead, so it has minimal impact on performance and can be used
with recordings that run continuously.
-
A second settings file is also provided, profile.jfc, which provides
more data than the default configuration, but can have more overhead and
impact performance.
Use this configuration for short periods of time when more information
is needed.
You can specify values for multiple parameters by separating them with a
comma.
- -XX:ThreadStackSize=size
-
Sets the Java thread stack size (in kilobytes).
Use of a scaling suffix, such as k, results in the scaling of
the kilobytes value so that -XX:ThreadStackSize=1k sets the
Java thread stack size to 1024*1024 bytes or 1 megabyte.
The default value depends on the platform:
-
- •
-
Linux/x64 (64-bit): 1024 KB
- •
-
macOS (64-bit): 1024 KB
- •
-
Windows: The default value depends on virtual memory
The following examples show how to set the thread stack size to 1
megabyte in different units:
-
-XX:ThreadStackSize=1k
-XX:ThreadStackSize=1024
This option is similar to -Xss.
- -XX:-UseCompressedOops
-
Disables the use of compressed pointers.
By default, this option is enabled, and compressed pointers are used.
This will automatically limit the maximum ergonomically determined Java
heap size to the maximum amount of memory that can be covered by
compressed pointers.
By default this range is 32 GB.
-
With compressed oops enabled, object references are represented as
32-bit offsets instead of 64-bit pointers, which typically increases
performance when running the application with Java heap sizes smaller
than the compressed oops pointer range.
This option works only for 64-bit JVMs.
It's possible to use compressed pointers with Java heap sizes
greater than 32 GB.
See the -XX:ObjectAlignmentInBytes option.
- -XX:-UseContainerSupport
-
The VM now provides automatic container detection support, which allows
the VM to determine the amount of memory and number of processors that
are available to a Java process running in docker containers.
It uses this information to allocate system resources.
This support is only available on Linux x64 platforms.
If supported, the default for this flag is true, and container
support is enabled by default.
It can be disabled with -XX:-UseContainerSupport.
-
Unified Logging is available to help to diagnose issues related to this
support.
Use -Xlog:os+container=trace for maximum logging of container
information.
See Enable Logging with the JVM Unified Logging Framework for a
description of using Unified Logging.
- -XX:+UseHugeTLBFS
-
Linux only: This option is the equivalent of specifying
-XX:+UseLargePages.
This option is disabled by default.
This option pre-allocates all large pages up-front, when memory is
reserved; consequently the JVM can't dynamically grow or shrink
large pages memory areas; see -XX:UseTransparentHugePages if
you want this behavior.
-
See Large Pages.
- -XX:+UseLargePages
-
Enables the use of large page memory.
By default, this option is disabled and large page memory isn't
used.
-
See Large Pages.
- -XX:+UseTransparentHugePages
-
Linux only: Enables the use of large pages that can dynamically
grow or shrink.
This option is disabled by default.
You may encounter performance problems with transparent huge pages as
the OS moves other pages around to create huge pages; this option is
made available for experimentation.
-
- -XX:+AllowUserSignalHandlers
-
Enables installation of signal handlers by the application.
By default, this option is disabled and the application isn't
allowed to install signal handlers.
-
- -XX:VMOptionsFile=filename
-
Allows user to specify VM options in a file, for example,
java -XX:VMOptionsFile=/var/my_vm_options HelloWorld.
-
ADVANCED JIT COMPILER OPTIONS FOR JAVA
These java options control the dynamic just-in-time (JIT)
compilation performed by the Java HotSpot VM.
- -XX:AllocateInstancePrefetchLines=lines
-
Sets the number of lines to prefetch ahead of the instance allocation
pointer.
By default, the number of lines to prefetch is set to 1:
-
-
-XX:AllocateInstancePrefetchLines=1
- -XX:AllocatePrefetchDistance=size
-
Sets the size (in bytes) of the prefetch distance for object allocation.
Memory about to be written with the value of new objects is prefetched
up to this distance starting from the address of the last allocated
object.
Each Java thread has its own allocation point.
-
Negative values denote that prefetch distance is chosen based on the
platform.
Positive values are bytes to prefetch.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default value is set to -1.
The following example shows how to set the prefetch distance to 1024
bytes:
-
-XX:AllocatePrefetchDistance=1024
- -XX:AllocatePrefetchInstr=instruction
-
Sets the prefetch instruction to prefetch ahead of the allocation
pointer.
Possible values are from 0 to 3.
The actual instructions behind the values depend on the platform.
By default, the prefetch instruction is set to 0:
-
-
-XX:AllocatePrefetchInstr=0
- -XX:AllocatePrefetchLines=lines
-
Sets the number of cache lines to load after the last object allocation
by using the prefetch instructions generated in compiled code.
The default value is 1 if the last allocated object was an instance, and
3 if it was an array.
-
The following example shows how to set the number of loaded cache lines
to 5:
-
-XX:AllocatePrefetchLines=5
- -XX:AllocatePrefetchStepSize=size
-
Sets the step size (in bytes) for sequential prefetch instructions.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, g or G
to indicate gigabytes.
By default, the step size is set to 16 bytes:
-
-
-XX:AllocatePrefetchStepSize=16
- -XX:AllocatePrefetchStyle=style
-
Sets the generated code style for prefetch instructions.
The style argument is an integer from 0 to 3:
-
- 0
-
Don't generate prefetch instructions.
-
- 1
-
Execute prefetch instructions after each allocation.
This is the default setting.
-
- 2
-
Use the thread-local allocation block (TLAB) watermark pointer to
determine when prefetch instructions are executed.
-
- 3
-
Generate one prefetch instruction per cache line.
-
- -XX:+BackgroundCompilation
-
Enables background compilation.
This option is enabled by default.
To disable background compilation, specify
-XX:-BackgroundCompilation (this is equivalent to specifying
-Xbatch).
-
- -XX:CICompilerCount=threads
-
Sets the number of compiler threads to use for compilation.
By default, the number of compiler threads is selected automatically
depending on the number of CPUs and memory available for compiled code.
The following example shows how to set the number of threads to 2:
-
-
-XX:CICompilerCount=2
- -XX:+UseDynamicNumberOfCompilerThreads
-
Dynamically create compiler thread up to the limit specified by
-XX:CICompilerCount.
This option is enabled by default.
-
- -XX:CompileCommand=command,method[,option]
-
Specifies a command to perform on a method.
For example, to exclude the indexOf() method of the
String class from being compiled, use the following:
-
-
-XX:CompileCommand=exclude,java/lang/String.indexOf
Note that the full class name is specified, including all packages and
subpackages separated by a slash (/).
For easier cut-and-paste operations, it's also possible to use the
method name format produced by the -XX:+PrintCompilation and
-XX:+LogCompilation options:
-
-XX:CompileCommand=exclude,java.lang.String::indexOf
If the method is specified without the signature, then the command is
applied to all methods with the specified name.
However, you can also specify the signature of the method in the class
file format.
In this case, you should enclose the arguments in quotation marks,
because otherwise the shell treats the semicolon as a command end.
For example, if you want to exclude only the indexOf(String)
method of the String class from being compiled, use the
following:
-
-XX:CompileCommand="exclude,java/lang/String.indexOf,(Ljava/lang/String;)I"
You can also use the asterisk (*) as a wildcard for class and method
names.
For example, to exclude all indexOf() methods in all classes
from being compiled, use the following:
-
-XX:CompileCommand=exclude,*.indexOf
The commas and periods are aliases for spaces, making it easier to pass
compiler commands through a shell.
You can pass arguments to -XX:CompileCommand using spaces as
separators by enclosing the argument in quotation marks:
-
-XX:CompileCommand="exclude java/lang/String indexOf"
Note that after parsing the commands passed on the command line using
the -XX:CompileCommand options, the JIT compiler then reads
commands from the .hotspot_compiler file.
You can add commands to this file or specify a different file using the
-XX:CompileCommandFile option.
To add several commands, either specify the -XX:CompileCommand
option multiple times, or separate each argument with the new line
separator (\n).
The following commands are available:
- break
-
Sets a breakpoint when debugging the JVM to stop at the beginning of
compilation of the specified method.
-
- compileonly
-
Excludes all methods from compilation except for the specified method.
As an alternative, you can use the -XX:CompileOnly option,
which lets you specify several methods.
-
- dontinline
-
Prevents inlining of the specified method.
-
- exclude
-
Excludes the specified method from compilation.
-
- help
-
Prints a help message for the -XX:CompileCommand option.
-
- inline
-
Attempts to inline the specified method.
-
- log
-
Excludes compilation logging (with the -XX:+LogCompilation
option) for all methods except for the specified method.
By default, logging is performed for all compiled methods.
-
- option
-
Passes a JIT compilation option to the specified method in place of the
last argument (option).
The compilation option is set at the end, after the method name.
For example, to enable the BlockLayoutByFrequency option for
the append() method of the StringBuffer class, use the
following:
-
-
-XX:CompileCommand=option,java/lang/StringBuffer.append,BlockLayoutByFrequency
You can specify multiple compilation options, separated by commas or
spaces.
- print
-
Prints generated assembler code after compilation of the specified
method.
-
- quiet
-
Instructs not to print the compile commands.
By default, the commands that you specify with the
-XX:CompileCommand option are printed; for example, if you
exclude from compilation the indexOf() method of the
String class, then the following is printed to standard output:
-
-
CompilerOracle: exclude java/lang/String.indexOf
You can suppress this by specifying the
-XX:CompileCommand=quiet option before other
-XX:CompileCommand options.
- -XX:CompileCommandFile=filename
-
Sets the file from which JIT compiler commands are read.
By default, the .hotspot_compiler file is used to store
commands performed by the JIT compiler.
-
Each line in the command file represents a command, a class name, and a
method name for which the command is used.
For example, this line prints assembly code for the toString()
method of the String class:
-
print java/lang/String toString
If you're using commands for the JIT compiler to perform on methods,
then see the -XX:CompileCommand option.
- -XX:CompilerDirectivesFile=file
-
Adds directives from a file to the directives stack when a program
starts.
See Compiler Control
[https://docs.oracle.com/en/java/javase/12/vm/compiler-control1.html#GUID-94AD8194-786A-4F19-BFFF-278F8E237F3A].
-
The -XX:CompilerDirectivesFile option has to be used together
with the -XX:UnlockDiagnosticVMOptions option that unlocks
diagnostic JVM options.
- -XX:+CompilerDirectivesPrint
-
Prints the directives stack when the program starts or when a new
directive is added.
-
The -XX:+CompilerDirectivesPrint option has to be used
together with the -XX:UnlockDiagnosticVMOptions option that
unlocks diagnostic JVM options.
- -XX:CompileOnly=methods
-
Sets the list of methods (separated by commas) to which compilation
should be restricted.
Only the specified methods are compiled.
Specify each method with the full class name (including the packages and
subpackages).
For example, to compile only the length() method of the
String class and the size() method of the
List class, use the following:
-
-
-XX:CompileOnly=java/lang/String.length,java/util/List.size
Note that the full class name is specified, including all packages and
subpackages separated by a slash (/).
For easier cut and paste operations, it's also possible to use the
method name format produced by the -XX:+PrintCompilation and
-XX:+LogCompilation options:
-
-XX:CompileOnly=java.lang.String::length,java.util.List::size
Although wildcards aren't supported, you can specify only the class
or package name to compile all methods in that class or package, as well
as specify just the method to compile methods with this name in any
class:
-
-XX:CompileOnly=java/lang/String
-XX:CompileOnly=java/lang
-XX:CompileOnly=.length
- -XX:CompileThresholdScaling=scale
-
Provides unified control of first compilation.
This option controls when methods are first compiled for both the tiered
and the nontiered modes of operation.
The CompileThresholdScaling option has a floating point value
between 0 and +Inf and scales the thresholds corresponding to the
current mode of operation (both tiered and nontiered).
Setting CompileThresholdScaling to a value less than 1.0
results in earlier compilation while values greater than 1.0 delay
compilation.
Setting CompileThresholdScaling to 0 is equivalent to disabling
compilation.
-
- -XX:+DoEscapeAnalysis
-
Enables the use of escape analysis.
This option is enabled by default.
To disable the use of escape analysis, specify
-XX:-DoEscapeAnalysis.
-
- -XX:InitialCodeCacheSize=size
-
Sets the initial code cache size (in bytes).
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default value depends on the platform.
The initial code cache size shouldn't be less than the system's
minimal memory page size.
The following example shows how to set the initial code cache size to 32
KB:
-
-
-XX:InitialCodeCacheSize=32k
- -XX:+Inline
-
Enables method inlining.
This option is enabled by default to increase performance.
To disable method inlining, specify -XX:-Inline.
-
- -XX:InlineSmallCode=size
-
Sets the maximum code size (in bytes) for already compiled methods that
may be inlined.
This flag only applies to the C2 compiler.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default value depends on the platform and on whether tiered
compilation is enabled.
In the following example it is set to 1000 bytes:
-
-
-XX:InlineSmallCode=1000
- -XX:+LogCompilation
-
Enables logging of compilation activity to a file named
hotspot.log in the current working directory.
You can specify a different log file path and name using the
-XX:LogFile option.
-
By default, this option is disabled and compilation activity isn't
logged.
The -XX:+LogCompilation option has to be used together with
the -XX:UnlockDiagnosticVMOptions option that unlocks
diagnostic JVM options.
You can enable verbose diagnostic output with a message printed to the
console every time a method is compiled by using the
-XX:+PrintCompilation option.
- -XX:FreqInlineSize=size
-
Sets the maximum bytecode size (in bytes) of a hot method to be inlined.
This flag only applies to the C2 compiler.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default value depends on the platform.
In the following example it is set to 325 bytes:
-
-
-XX:FreqInlineSize=325
- -XX:MaxInlineSize=size
-
Sets the maximum bytecode size (in bytes) of a cold method to be
inlined.
This flag only applies to the C2 compiler.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
By default, the maximum bytecode size is set to 35 bytes:
-
-
-XX:MaxInlineSize=35
- -XX:C1MaxInlineSize=size
-
Sets the maximum bytecode size (in bytes) of a cold method to be
inlined.
This flag only applies to the C1 compiler.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
By default, the maximum bytecode size is set to 35 bytes:
-
-
-XX:MaxInlineSize=35
- -XX:MaxTrivialSize=size
-
Sets the maximum bytecode size (in bytes) of a trivial method to be
inlined.
This flag only applies to the C2 compiler.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
By default, the maximum bytecode size of a trivial method is set to 6
bytes:
-
-
-XX:MaxTrivialSize=6
- -XX:C1MaxTrivialSize=size
-
Sets the maximum bytecode size (in bytes) of a trivial method to be
inlined.
This flag only applies to the C1 compiler.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
By default, the maximum bytecode size of a trivial method is set to 6
bytes:
-
-
-XX:MaxTrivialSize=6
- -XX:MaxNodeLimit=nodes
-
Sets the maximum number of nodes to be used during single method
compilation.
By default the value depends on the features enabled.
In the following example the maximum number of nodes is set to 100,000:
-
-
-XX:MaxNodeLimit=100000
- -XX:NonNMethodCodeHeapSize=size
-
Sets the size in bytes of the code segment containing nonmethod code.
-
A nonmethod code segment containing nonmethod code, such as compiler
buffers and the bytecode interpreter.
This code type stays in the code cache forever.
This flag is used only if -XX:SegmentedCodeCache is enabled.
- -XX:NonProfiledCodeHeapSize=size
-
Sets the size in bytes of the code segment containing nonprofiled
methods.
This flag is used only if -XX:SegmentedCodeCache is enabled.
-
- -XX:+OptimizeStringConcat
-
Enables the optimization of String concatenation operations.
This option is enabled by default.
To disable the optimization of String concatenation operations,
specify -XX:-OptimizeStringConcat.
-
- -XX:+PrintAssembly
-
Enables printing of assembly code for bytecoded and native methods by
using the external hsdis-<arch>.so or .dll library.
For 64-bit VM on Windows, it's hsdis-amd64.dll.
This lets you to see the generated code, which may help you to diagnose
performance issues.
-
By default, this option is disabled and assembly code isn't printed.
The -XX:+PrintAssembly option has to be used together with the
-XX:UnlockDiagnosticVMOptions option that unlocks diagnostic
JVM options.
- -XX:ProfiledCodeHeapSize=size
-
Sets the size in bytes of the code segment containing profiled methods.
This flag is used only if -XX:SegmentedCodeCache is enabled.
-
- -XX:+PrintCompilation
-
Enables verbose diagnostic output from the JVM by printing a message to
the console every time a method is compiled.
This lets you to see which methods actually get compiled.
By default, this option is disabled and diagnostic output isn't
printed.
-
You can also log compilation activity to a file by using the
-XX:+LogCompilation option.
- -XX:+PrintInlining
-
Enables printing of inlining decisions.
This let's you see which methods are getting inlined.
-
By default, this option is disabled and inlining information isn't
printed.
The -XX:+PrintInlining option has to be used together with the
-XX:+UnlockDiagnosticVMOptions option that unlocks diagnostic
JVM options.
- -XX:ReservedCodeCacheSize=size
-
Sets the maximum code cache size (in bytes) for JIT-compiled code.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default maximum code cache size is 240 MB; if you disable tiered
compilation with the option -XX:-TieredCompilation, then the
default size is 48 MB.
This option has a limit of 2 GB; otherwise, an error is generated.
The maximum code cache size shouldn't be less than the initial code
cache size; see the option -XX:InitialCodeCacheSize.
-
- -XX:RTMAbortRatio=abort_ratio
-
Specifies the RTM abort ratio is specified as a percentage (%) of all
executed RTM transactions.
If a number of aborted transactions becomes greater than this ratio,
then the compiled code is deoptimized.
This ratio is used when the -XX:+UseRTMDeopt option is
enabled.
The default value of this option is 50.
This means that the compiled code is deoptimized if 50% of all
transactions are aborted.
-
- -XX:RTMRetryCount=number_of_retries
-
Specifies the number of times that the RTM locking code is retried, when
it is aborted or busy, before falling back to the normal locking
mechanism.
The default value for this option is 5.
The -XX:UseRTMLocking option must be enabled.
-
- -XX:+SegmentedCodeCache
-
Enables segmentation of the code cache.
Without the -XX:+SegmentedCodeCache, the code cache consists
of one large segment.
With -XX:+SegmentedCodeCache, we have separate segments for
nonmethod, profiled method, and nonprofiled method code.
These segments aren't resized at runtime.
The feature is enabled by default if tiered compilation is enabled
(-XX:+TieredCompilation ) and
-XX:ReservedCodeCacheSize >= 240 MB.
The advantages are better control of the memory footprint, reduced code
fragmentation, and better iTLB/iCache behavior due to improved locality.
iTLB/iCache is a CPU-specific term meaning Instruction Translation
Lookaside Buffer (ITLB).
ICache is an instruction cache in theCPU.
The implementation of the code cache can be found in the file:
/share/vm/code/codeCache.cpp.
-
- -XX:StartAggressiveSweepingAt=percent
-
Forces stack scanning of active methods to aggressively remove unused
code when only the given percentage of the code cache is free.
The default value is 10%.
-
- -XX:-TieredCompilation
-
Disables the use of tiered compilation.
By default, this option is enabled.
-
- -XX:UseSSE=version
-
Enables the use of SSE instruction set of a specified version.
Is set by default to the highest supported version available (x86 only).
-
- -XX:UseAVX=version
-
Enables the use of AVX instruction set of a specified version.
Is set by default to the highest supported version available (x86 only).
-
- -XX:+UseAES
-
Enables hardware-based AES intrinsics for hardware that supports it.
This option is on by default on hardware that has the necessary
instructions.
The -XX:+UseAES is used in conjunction with UseAESIntrinsics.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseAESIntrinsics
-
Enables AES intrinsics.
Specifying-XX:+UseAESIntrinsics is equivalent to also enabling
-XX:+UseAES.
To disable hardware-based AES intrinsics, specify
-XX:-UseAES -XX:-UseAESIntrinsics.
For example, to enable hardware AES, use the following flags:
-
-
-XX:+UseAES -XX:+UseAESIntrinsics
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
- -XX:+UseAESCTRIntrinsics
-
Analogous to -XX:+UseAESIntrinsics enables AES/CTR intrinsics.
-
- -XX:+UseGHASHIntrinsics
-
Controls the use of GHASH intrinsics.
Enabled by default on platforms that support the corresponding
instructions.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseBASE64Intrinsics
-
Controls the use of accelerated BASE64 encoding routines for
java.util.Base64.
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseAdler32Intrinsics
-
Controls the use of Adler32 checksum algorithm intrinsic for
java.util.zip.Adler32.
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseCRC32Intrinsics
-
Controls the use of CRC32 intrinsics for java.util.zip.CRC32.
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseCRC32CIntrinsics
-
Controls the use of CRC32C intrinsics for java.util.zip.CRC32C.
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseSHA
-
Enables hardware-based intrinsics for SHA crypto hash functions for
some hardware.
The UseSHA option is used in conjunction with the
UseSHA1Intrinsics, UseSHA256Intrinsics, and
UseSHA512Intrinsics options.
-
The UseSHA and UseSHA*Intrinsics flags are enabled by
default on machines that support the corresponding instructions.
This feature is applicable only when using the
sun.security.provider.Sun provider for SHA operations.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
To disable all hardware-based SHA intrinsics, specify the
-XX:-UseSHA.
To disable only a particular SHA intrinsic, use the appropriate
corresponding option.
For example: -XX:-UseSHA256Intrinsics.
- -XX:+UseSHA1Intrinsics
-
Enables intrinsics for SHA-1 crypto hash function.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseSHA256Intrinsics
-
Enables intrinsics for SHA-224 and SHA-256 crypto hash functions.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseSHA512Intrinsics
-
Enables intrinsics for SHA-384 and SHA-512 crypto hash functions.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseMathExactIntrinsics
-
Enables intrinsification of various java.lang.Math.*Exact()
functions.
Enabled by default.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseMultiplyToLenIntrinsic
-
Enables intrinsification of BigInteger.multiplyToLen().
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseSquareToLenIntrinsic
-
Enables intrinsification of BigInteger.squareToLen().
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseMulAddIntrinsic
-
Enables intrinsification of BigInteger.mulAdd().
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseMontgomeryMultiplyIntrinsic
-
Enables intrinsification of BigInteger.montgomeryMultiply().
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseMontgomerySquareIntrinsic
-
Enables intrinsification of BigInteger.montgomerySquare().
Enabled by default on platforms that support it.
Flags that control intrinsics now require the option
-XX:+UnlockDiagnosticVMOptions.
-
- -XX:+UseCMoveUnconditionally
-
Generates CMove (scalar and vector) instructions regardless of
profitability analysis.
-
- -XX:+UseCodeCacheFlushing
-
Enables flushing of the code cache before shutting down the compiler.
This option is enabled by default.
To disable flushing of the code cache before shutting down the compiler,
specify -XX:-UseCodeCacheFlushing.
-
- -XX:+UseCondCardMark
-
Enables checking if the card is already marked before updating the card
table.
This option is disabled by default.
It should be used only on machines with multiple sockets, where it
increases the performance of Java applications that rely on concurrent
operations.
-
- -XX:+UseCountedLoopSafepoints
-
Keeps safepoints in counted loops.
Its default value depends on whether the selected garbage collector
requires low latency safepoints.
-
- -XX:LoopStripMiningIter=number_of_iterations
-
Controls the number of iterations in the inner strip mined loop.
Strip mining transforms counted loops into two level nested loops.
Safepoints are kept in the outer loop while the inner loop can execute
at full speed.
This option controls the maximum number of iterations in the inner loop.
The default value is 1,000.
-
- -XX:LoopStripMiningIterShortLoop=number_of_iterations
-
Controls loop strip mining optimization.
Loops with the number of iterations less than specified will not have
safepoints in them.
Default value is 1/10th of -XX:LoopStripMiningIter.
-
- -XX:+UseFMA
-
Enables hardware-based FMA intrinsics for hardware where FMA
instructions are available (such as, Intel and ARM64).
FMA intrinsics are generated for the
java.lang.Math.fma(a, b,
c) methods that calculate the value of (
a * b + c )
expressions.
-
- -XX:+UseRTMDeopt
-
Autotunes RTM locking depending on the abort ratio.
This ratio is specified by the -XX:RTMAbortRatio option.
If the number of aborted transactions exceeds the abort ratio, then the
method containing the lock is deoptimized and recompiled with all locks
as normal locks.
This option is disabled by default.
The -XX:+UseRTMLocking option must be enabled.
-
- -XX:+UseRTMLocking
-
Generates Restricted Transactional Memory (RTM) locking code for all
inflated locks, with the normal locking mechanism as the fallback
handler.
This option is disabled by default.
Options related to RTM are available only on x86 CPUs that support
Transactional Synchronization Extensions (TSX).
-
RTM is part of Intel's TSX, which is an x86 instruction set
extension and facilitates the creation of multithreaded applications.
RTM introduces the new instructions XBEGIN, XABORT,
XEND, and XTEST.
The XBEGIN and XEND instructions enclose a set of
instructions to run as a transaction.
If no conflict is found when running the transaction, then the memory
and register modifications are committed together at the XEND
instruction.
The XABORT instruction can be used to explicitly abort a
transaction and the XTEST instruction checks if a set of
instructions is being run in a transaction.
A lock on a transaction is inflated when another thread tries to access
the same transaction, thereby blocking the thread that didn't
originally request access to the transaction.
RTM requires that a fallback set of operations be specified in case a
transaction aborts or fails.
An RTM lock is a lock that has been delegated to the TSX's system.
RTM improves performance for highly contended locks with low conflict in
a critical region (which is code that must not be accessed by more than
one thread concurrently).
RTM also improves the performance of coarse-grain locking, which
typically doesn't perform well in multithreaded applications.
(Coarse-grain locking is the strategy of holding locks for long periods
to minimize the overhead of taking and releasing locks, while
fine-grained locking is the strategy of trying to achieve maximum
parallelism by locking only when necessary and unlocking as soon as
possible.) Also, for lightly contended locks that are used by different
threads, RTM can reduce false cache line sharing, also known as cache
line ping-pong.
This occurs when multiple threads from different processors are
accessing different resources, but the resources share the same cache
line.
As a result, the processors repeatedly invalidate the cache lines of
other processors, which forces them to read from main memory instead of
their cache.
- -XX:+UseSuperWord
-
Enables the transformation of scalar operations into superword
operations.
Superword is a vectorization optimization.
This option is enabled by default.
To disable the transformation of scalar operations into superword
operations, specify -XX:-UseSuperWord.
-
ADVANCED SERVICEABILITY OPTIONS FOR JAVA
These java options provide the ability to gather system
information and perform extensive debugging.
- -XX:+DisableAttachMechanism
-
Disables the mechanism that lets tools attach to the JVM.
By default, this option is disabled, meaning that the attach mechanism
is enabled and you can use diagnostics and troubleshooting tools such as
jcmd, jstack, jmap, and jinfo.
-
-
Note: The tools such as jcmd, jinfo,
jmap, and jstack shipped with the JDK aren't
supported when using the tools from one JDK version to troubleshoot a
different JDK version.
- -XX:+ExtendedDTraceProbes
-
Linux and macOS: Enables additional dtrace tool probes
that affect the performance.
By default, this option is disabled and dtrace performs only
standard probes.
-
- -XX:+HeapDumpOnOutOfMemoryError
-
Enables the dumping of the Java heap to a file in the current directory
by using the heap profiler (HPROF) when a
java.lang.OutOfMemoryError exception is thrown.
You can explicitly set the heap dump file path and name using the
-XX:HeapDumpPath option.
By default, this option is disabled and the heap isn't dumped when
an OutOfMemoryError exception is thrown.
-
- -XX:HeapDumpPath=path
-
Sets the path and file name for writing the heap dump provided by the
heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemoryError
option is set.
By default, the file is created in the current working directory, and
it's named java_pid<pid>.hprof where <pid> is the
identifier of the process that caused the error.
The following example shows how to set the default file explicitly
(%p represents the current process identifier):
-
-
-XX:HeapDumpPath=./java_pid%p.hprof
- •
-
Linux and macOS: The following example shows how to set the
heap dump file to /var/log/java/java_heapdump.hprof:
-
-
-XX:HeapDumpPath=/var/log/java/java_heapdump.hprof
- •
-
Windows: The following example shows how to set the heap dump
file to C:/log/java/java_heapdump.log:
-
-
-XX:HeapDumpPath=C:/log/java/java_heapdump.log
- -XX:LogFile=path
-
Sets the path and file name to where log data is written.
By default, the file is created in the current working directory, and
it's named hotspot.log.
-
- •
-
Linux and macOS: The following example shows how to set the log
file to /var/log/java/hotspot.log:
-
-
-XX:LogFile=/var/log/java/hotspot.log
- •
-
Windows: The following example shows how to set the log file to
C:/log/java/hotspot.log:
-
-
-XX:LogFile=C:/log/java/hotspot.log
- -XX:+PrintClassHistogram
-
Enables printing of a class instance histogram after one of the
following events:
-
- •
-
Linux and macOS: Control+Break
- •
-
Windows: Control+C (SIGTERM)
By default, this option is disabled.
Setting this option is equivalent to running the jmap -histo
command, or the jcmd pid GC.class_histogram
command, where pid is the current Java process identifier.
- -XX:+PrintConcurrentLocks
-
Enables printing of java.util.concurrent locks after one of the
following events:
-
- •
-
Linux and macOS: Control+Break
- •
-
Windows: Control+C (SIGTERM)
By default, this option is disabled.
Setting this option is equivalent to running the jstack -l
command or the jcmd pid Thread.print -l
command, where pid is the current Java process identifier.
- -XX:+PrintFlagsRanges
-
Prints the range specified and allows automatic testing of the values.
See Validate Java Virtual Machine Flag Arguments.
-
- -XX:+PerfDataSaveToFile
-
If enabled, saves jstat binary data when the Java application
exits.
This binary data is saved in a file named
hsperfdata_pid, where pid is the process
identifier of the Java application that you ran.
Use the jstat command to display the performance data contained
in this file as follows:
-
-
jstat -class file:///path/hsperfdata_pid
-
jstat -gc file:///path/hsperfdata_pid
- -XX:+UsePerfData
-
Enables the perfdata feature.
This option is enabled by default to allow JVM monitoring and
performance testing.
Disabling it suppresses the creation of the hsperfdata_userid
directories.
To disable the perfdata feature, specify
-XX:-UsePerfData.
-
ADVANCED GARBAGE COLLECTION OPTIONS FOR JAVA
These java options control how garbage collection (GC) is
performed by the Java HotSpot VM.
- -XX:+AggressiveHeap
-
Enables Java heap optimization.
This sets various parameters to be optimal for long-running jobs with
intensive memory allocation, based on the configuration of the computer
(RAM and CPU).
By default, the option is disabled and the heap sizes are configured
less aggressively.
-
- -XX:+AlwaysPreTouch
-
Requests the VM to touch every page on the Java heap after requesting it
from the operating system and before handing memory out to the
application.
By default, this option is disabled and all pages are committed as the
application uses the heap space.
-
- -XX:ConcGCThreads=threads
-
Sets the number of threads used for concurrent GC.
Sets threads to approximately 1/4 of the number of
parallel garbage collection threads.
The default value depends on the number of CPUs available to the JVM.
-
For example, to set the number of threads for concurrent GC to 2,
specify the following option:
-
-XX:ConcGCThreads=2
- -XX:+DisableExplicitGC
-
Enables the option that disables processing of calls to the
System.gc() method.
This option is disabled by default, meaning that calls to
System.gc() are processed.
If processing of calls to System.gc() is disabled, then the JVM
still performs GC when necessary.
-
- -XX:+ExplicitGCInvokesConcurrent
-
Enables invoking of concurrent GC by using the System.gc()
request.
This option is disabled by default and can be enabled only with the
-XX:+UseG1GC option.
-
- -XX:G1AdaptiveIHOPNumInitialSamples=number
-
When -XX:UseAdaptiveIHOP is enabled, this option sets the
number of completed marking cycles used to gather samples until G1
adaptively determines the optimum value of
-XX:InitiatingHeapOccupancyPercent.
Before, G1 uses the value of
-XX:InitiatingHeapOccupancyPercent directly for this purpose.
The default value is 3.
-
- -XX:G1HeapRegionSize=size
-
Sets the size of the regions into which the Java heap is subdivided when
using the garbage-first (G1) collector.
The value is a power of 2 and can range from 1 MB to 32 MB.
The default region size is determined ergonomically based on the heap
size with a goal of approximately 2048 regions.
-
The following example sets the size of the subdivisions to 16 MB:
-
-XX:G1HeapRegionSize=16m
- -XX:G1HeapWastePercent=percent
-
Sets the percentage of heap that you're willing to waste.
The Java HotSpot VM doesn't initiate the mixed garbage collection
cycle when the reclaimable percentage is less than the heap waste
percentage.
The default is 5 percent.
-
- -XX:G1MaxNewSizePercent=percent
-
Sets the percentage of the heap size to use as the maximum for the young
generation size.
The default value is 60 percent of your Java heap.
-
This is an experimental flag.
This setting replaces the -XX:DefaultMaxNewGenPercent setting.
- -XX:G1MixedGCCountTarget=number
-
Sets the target number of mixed garbage collections after a marking
cycle to collect old regions with at most
G1MixedGCLIveThresholdPercent live data.
The default is 8 mixed garbage collections.
The goal for mixed collections is to be within this target number.
-
- -XX:G1MixedGCLiveThresholdPercent=percent
-
Sets the occupancy threshold for an old region to be included in a mixed
garbage collection cycle.
The default occupancy is 85 percent.
-
This is an experimental flag.
This setting replaces the
-XX:G1OldCSetRegionLiveThresholdPercent setting.
- -XX:G1NewSizePercent=percent
-
Sets the percentage of the heap to use as the minimum for the young
generation size.
The default value is 5 percent of your Java heap.
-
This is an experimental flag.
This setting replaces the -XX:DefaultMinNewGenPercent setting.
- -XX:G1OldCSetRegionThresholdPercent=percent
-
Sets an upper limit on the number of old regions to be collected during
a mixed garbage collection cycle.
The default is 10 percent of the Java heap.
-
- -XX:G1ReservePercent=percent
-
Sets the percentage of the heap (0 to 50) that's reserved as a false
ceiling to reduce the possibility of promotion failure for the G1
collector.
When you increase or decrease the percentage, ensure that you adjust the
total Java heap by the same amount.
By default, this option is set to 10%.
-
The following example sets the reserved heap to 20%:
-
-XX:G1ReservePercent=20
- -XX:+G1UseAdaptiveIHOP
-
Controls adaptive calculation of the old generation occupancy to start
background work preparing for an old generation collection.
If enabled, G1 uses -XX:InitiatingHeapOccupancyPercent for the
first few times as specified by the value of
-XX:G1AdaptiveIHOPNumInitialSamples, and after that adaptively
calculates a new optimum value for the initiating occupancy
automatically.
Otherwise, the old generation collection process always starts at the
old generation occupancy determined by
-XX:InitiatingHeapOccupancyPercent.
-
The default is enabled.
- -XX:InitialHeapSize=size
-
Sets the initial size (in bytes) of the memory allocation pool.
This value must be either 0, or a multiple of 1024 and greater than 1
MB.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default value is selected at run time based on the system
configuration.
-
The following examples show how to set the size of allocated memory to 6
MB using various units:
-
-XX:InitialHeapSize=6291456
-XX:InitialHeapSize=6144k
-XX:InitialHeapSize=6m
If you set this option to 0, then the initial size is set as the sum of
the sizes allocated for the old generation and the young generation.
The size of the heap for the young generation can be set using the
-XX:NewSize option.
- -XX:InitialRAMPercentage=percent
-
Sets the initial amount of memory that the JVM will use for the Java
heap before applying ergonomics heuristics as a percentage of the
maximum amount determined as described in the -XX:MaxRAM
option.
The default value is 1.5625 percent.
-
The following example shows how to set the percentage of the initial
amount of memory used for the Java heap:
-
-XX:InitialRAMPercentage=5
- -XX:InitialSurvivorRatio=ratio
-
Sets the initial survivor space ratio used by the throughput garbage
collector (which is enabled by the -XX:+UseParallelGC option).
Adaptive sizing is enabled by default with the throughput garbage
collector by using the -XX:+UseParallelGC option, and the
survivor space is resized according to the application behavior,
starting with the initial value.
If adaptive sizing is disabled (using the
-XX:-UseAdaptiveSizePolicy option), then the
-XX:SurvivorRatio option should be used to set the size of the
survivor space for the entire execution of the application.
-
The following formula can be used to calculate the initial size of
survivor space (S) based on the size of the young generation (Y), and
the initial survivor space ratio (R):
-
S=Y/(R+2)
The 2 in the equation denotes two survivor spaces.
The larger the value specified as the initial survivor space ratio, the
smaller the initial survivor space size.
By default, the initial survivor space ratio is set to 8.
If the default value for the young generation space size is used (2 MB),
then the initial size of the survivor space is 0.2 MB.
The following example shows how to set the initial survivor space ratio
to 4:
-
-XX:InitialSurvivorRatio=4
- -XX:InitiatingHeapOccupancyPercent=percent
-
Sets the percentage of the old generation occupancy (0 to 100) at which
to start the first few concurrent marking cycles for the G1 garbage
collector.
-
By default, the initiating value is set to 45%.
A value of 0 implies nonstop concurrent GC cycles from the beginning
until G1 adaptively sets this value.
See also the -XX:G1UseAdaptiveIHOP and
-XX:G1AdaptiveIHOPNumInitialSamples options.
The following example shows how to set the initiating heap occupancy to
75%:
-
-XX:InitiatingHeapOccupancyPercent=75
- -XX:MaxGCPauseMillis=time
-
Sets a target for the maximum GC pause time (in milliseconds).
This is a soft goal, and the JVM will make its best effort to achieve
it.
The specified value doesn't adapt to your heap size.
By default, for G1 the maximum pause time target is 200 milliseconds.
The other generational collectors do not use a pause time goal by
default.
-
The following example shows how to set the maximum target pause time to
500 ms:
-
-XX:MaxGCPauseMillis=500
- -XX:MaxHeapSize=size
-
Sets the maximum size (in byes) of the memory allocation pool.
This value must be a multiple of 1024 and greater than 2 MB.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default value is selected at run time based on the system
configuration.
For server deployments, the options -XX:InitialHeapSize and
-XX:MaxHeapSize are often set to the same value.
-
The following examples show how to set the maximum allowed size of
allocated memory to 80 MB using various units:
-
-XX:MaxHeapSize=83886080
-XX:MaxHeapSize=81920k
-XX:MaxHeapSize=80m
The -XX:MaxHeapSize option is equivalent to -Xmx.
- -XX:MaxHeapFreeRatio=percent
-
Sets the maximum allowed percentage of free heap space (0 to 100) after
a GC event.
If free heap space expands above this value, then the heap is shrunk.
By default, this value is set to 70%.
-
Minimize the Java heap size by lowering the values of the parameters
MaxHeapFreeRatio (default value is 70%) and
MinHeapFreeRatio (default value is 40%) with the command-line
options -XX:MaxHeapFreeRatio and
-XX:MinHeapFreeRatio.
Lowering MaxHeapFreeRatio to as low as 10% and
MinHeapFreeRatio to 5% has successfully reduced the heap size
without too much performance regression; however, results may vary
greatly depending on your application.
Try different values for these parameters until they're as low as
possible yet still retain acceptable performance.
-
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
Customers trying to keep the heap small should also add the option
-XX:-ShrinkHeapInSteps.
See Performance Tuning Examples for a description of using this
option to keep the Java heap small by reducing the dynamic footprint for
embedded applications.
- -XX:MaxMetaspaceSize=size
-
Sets the maximum amount of native memory that can be allocated for class
metadata.
By default, the size isn't limited.
The amount of metadata for an application depends on the application
itself, other running applications, and the amount of memory available
on the system.
-
The following example shows how to set the maximum class metadata size
to 256 MB:
-
-XX:MaxMetaspaceSize=256m
- -XX:MaxNewSize=size
-
Sets the maximum size (in bytes) of the heap for the young generation
(nursery).
The default value is set ergonomically.
-
- -XX:MaxRAM=size
-
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics.
The default value is the maximum amount of available memory to the JVM
process or 128 GB, whichever is lower.
-
The maximum amount of available memory to the JVM process is the minimum
of the machine's physical memory and any constraints set by the
environment (e.g.
container).
Specifying this option disables automatic use of compressed oops if the
combined result of this and other options influencing the maximum amount
of memory is larger than the range of memory addressable by compressed
oops.
See -XX:UseCompressedOops for further information about
compressed oops.
The following example shows how to set the maximum amount of available
memory for sizing the Java heap to 2 GB:
-
-XX:MaxRAM=2G
- -XX:MaxRAMPercentage=percent
-
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a percentage of the maximum
amount determined as described in the -XX:MaxRAM option.
The default value is 25 percent.
-
Specifying this option disables automatic use of compressed oops if the
combined result of this and other options influencing the maximum amount
of memory is larger than the range of memory addressable by compressed
oops.
See -XX:UseCompressedOops for further information about
compressed oops.
The following example shows how to set the percentage of the maximum
amount of memory used for the Java heap:
-
-XX:MaxRAMPercentage=75
- -XX:MinRAMPercentage=percent
-
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a percentage of the maximum
amount determined as described in the -XX:MaxRAM option for
small heaps.
A small heap is a heap of approximately 125 MB.
The default value is 50 percent.
-
The following example shows how to set the percentage of the maximum
amount of memory used for the Java heap for small heaps:
-
-XX:MinRAMPercentage=75
- -XX:MaxTenuringThreshold=threshold
-
Sets the maximum tenuring threshold for use in adaptive GC sizing.
The largest value is 15.
The default value is 15 for the parallel (throughput) collector.
-
The following example shows how to set the maximum tenuring threshold to
10:
-
-XX:MaxTenuringThreshold=10
- -XX:MetaspaceSize=size
-
Sets the size of the allocated class metadata space that triggers a
garbage collection the first time it's exceeded.
This threshold for a garbage collection is increased or decreased
depending on the amount of metadata used.
The default size depends on the platform.
-
- -XX:MinHeapFreeRatio=percent
-
Sets the minimum allowed percentage of free heap space (0 to 100) after
a GC event.
If free heap space falls below this value, then the heap is expanded.
By default, this value is set to 40%.
-
Minimize Java heap size by lowering the values of the parameters
MaxHeapFreeRatio (default value is 70%) and
MinHeapFreeRatio (default value is 40%) with the command-line
options -XX:MaxHeapFreeRatio and
-XX:MinHeapFreeRatio.
Lowering MaxHeapFreeRatio to as low as 10% and
MinHeapFreeRatio to 5% has successfully reduced the heap size
without too much performance regression; however, results may vary
greatly depending on your application.
Try different values for these parameters until they're as low as
possible, yet still retain acceptable performance.
-
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
Customers trying to keep the heap small should also add the option
-XX:-ShrinkHeapInSteps.
See Performance Tuning Examples for a description of using this
option to keep the Java heap small by reducing the dynamic footprint for
embedded applications.
- -XX:MinHeapSize=size
-
Sets the minimum size (in bytes) of the memory allocation pool.
This value must be either 0, or a multiple of 1024 and greater than 1
MB.
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
The default value is selected at run time based on the system
configuration.
-
The following examples show how to set the mimimum size of allocated
memory to 6 MB using various units:
-
-XX:MinHeapSize=6291456
-XX:MinHeapSize=6144k
-XX:MinHeapSize=6m
If you set this option to 0, then the minimum size is set to the same
value as the initial size.
- -XX:NewRatio=ratio
-
Sets the ratio between young and old generation sizes.
By default, this option is set to 2.
The following example shows how to set the young-to-old ratio to 1:
-
-
-XX:NewRatio=1
- -XX:NewSize=size
-
Sets the initial size (in bytes) of the heap for the young generation
(nursery).
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
-
The young generation region of the heap is used for new objects.
GC is performed in this region more often than in other regions.
If the size for the young generation is too low, then a large number of
minor GCs are performed.
If the size is too high, then only full GCs are performed, which can
take a long time to complete.
It is recommended that you keep the size for the young generation
greater than 25% and less than 50% of the overall heap size.
The following examples show how to set the initial size of the young
generation to 256 MB using various units:
-
-XX:NewSize=256m
-XX:NewSize=262144k
-XX:NewSize=268435456
The -XX:NewSize option is equivalent to -Xmn.
- -XX:ParallelGCThreads=threads
-
Sets the number of the stop-the-world (STW) worker threads.
The default value depends on the number of CPUs available to the JVM and
the garbage collector selected.
-
For example, to set the number of threads for G1 GC to 2, specify the
following option:
-
-XX:ParallelGCThreads=2
- -XX:+ParallelRefProcEnabled
-
Enables parallel reference processing.
By default, this option is disabled.
-
- -XX:+PrintAdaptiveSizePolicy
-
Enables printing of information about adaptive-generation sizing.
By default, this option is disabled.
-
- -XX:+ScavengeBeforeFullGC
-
Enables GC of the young generation before each full GC.
This option is enabled by default.
It is recommended that you don't disable it, because
scavenging the young generation before a full GC can reduce the number
of objects reachable from the old generation space into the young
generation space.
To disable GC of the young generation before each full GC, specify the
option -XX:-ScavengeBeforeFullGC.
-
- -XX:SoftRefLRUPolicyMSPerMB=time
-
Sets the amount of time (in milliseconds) a softly reachable object is
kept active on the heap after the last time it was referenced.
The default value is one second of lifetime per free megabyte in the
heap.
The -XX:SoftRefLRUPolicyMSPerMB option accepts integer values
representing milliseconds per one megabyte of the current heap size (for
Java HotSpot Client VM) or the maximum possible heap size (for Java
HotSpot Server VM).
This difference means that the Client VM tends to flush soft references
rather than grow the heap, whereas the Server VM tends to grow the heap
rather than flush soft references.
In the latter case, the value of the -Xmx option has a
significant effect on how quickly soft references are garbage collected.
-
The following example shows how to set the value to 2.5 seconds:
-XX:SoftRefLRUPolicyMSPerMB=2500
- -XX:-ShrinkHeapInSteps
-
Incrementally reduces the Java heap to the target size, specified by the
option -XX:MaxHeapFreeRatio.
This option is enabled by default.
If disabled, then it immediately reduces the Java heap to the target
size instead of requiring multiple garbage collection cycles.
Disable this option if you want to minimize the Java heap size.
You will likely encounter performance degradation when this option is
disabled.
-
See Performance Tuning Examples for a description of using the
MaxHeapFreeRatio option to keep the Java heap small by reducing
the dynamic footprint for embedded applications.
- -XX:StringDeduplicationAgeThreshold=threshold
-
Identifies String objects reaching the specified age that are
considered candidates for deduplication.
An object's age is a measure of how many times it has survived
garbage collection.
This is sometimes referred to as tenuring.
-
-
Note: String objects that are promoted to an old heap
region before this age has been reached are always considered candidates
for deduplication.
The default value for this option is 3.
See the -XX:+UseStringDeduplication option.
- -XX:SurvivorRatio=ratio
-
Sets the ratio between eden space size and survivor space size.
By default, this option is set to 8.
The following example shows how to set the eden/survivor space ratio to
4:
-
-
-XX:SurvivorRatio=4
- -XX:TargetSurvivorRatio=percent
-
Sets the desired percentage of survivor space (0 to 100) used after
young garbage collection.
By default, this option is set to 50%.
-
The following example shows how to set the target survivor space ratio
to 30%:
-
-XX:TargetSurvivorRatio=30
- -XX:TLABSize=size
-
Sets the initial size (in bytes) of a thread-local allocation buffer
(TLAB).
Append the letter k or K to indicate kilobytes,
m or M to indicate megabytes, or g or
G to indicate gigabytes.
If this option is set to 0, then the JVM selects the initial size
automatically.
-
The following example shows how to set the initial TLAB size to 512 KB:
-
-XX:TLABSize=512k
- -XX:+UseAdaptiveSizePolicy
-
Enables the use of adaptive generation sizing.
This option is enabled by default.
To disable adaptive generation sizing, specify
-XX:-UseAdaptiveSizePolicy and set the size of the memory
allocation pool explicitly.
See the -XX:SurvivorRatio option.
-
- -XX:+UseG1GC
-
Enables the use of the garbage-first (G1) garbage collector.
It's a server-style garbage collector, targeted for multiprocessor
machines with a large amount of RAM.
This option meets GC pause time goals with high probability, while
maintaining good throughput.
The G1 collector is recommended for applications requiring large heaps
(sizes of around 6 GB or larger) with limited GC latency requirements (a
stable and predictable pause time below 0.5 seconds).
By default, this option is enabled and G1 is used as the default garbage
collector.
-
- -XX:+UseGCOverheadLimit
-
Enables the use of a policy that limits the proportion of time spent by
the JVM on GC before an OutOfMemoryError exception is thrown.
This option is enabled, by default, and the parallel GC will throw an
OutOfMemoryError if more than 98% of the total time is spent on
garbage collection and less than 2% of the heap is recovered.
When the heap is small, this feature can be used to prevent applications
from running for long periods of time with little or no progress.
To disable this option, specify the option
-XX:-UseGCOverheadLimit.
-
- -XX:+UseNUMA
-
Enables performance optimization of an application on a machine with
nonuniform memory architecture (NUMA) by increasing the
application's use of lower latency memory.
By default, this option is disabled and no optimization for NUMA is
made.
The option is available only when the parallel garbage collector is used
(-XX:+UseParallelGC).
-
- -XX:+UseParallelGC
-
Enables the use of the parallel scavenge garbage collector (also known
as the throughput collector) to improve the performance of your
application by leveraging multiple processors.
-
By default, this option is disabled and the default collector is used.
- -XX:+UseSerialGC
-
Enables the use of the serial garbage collector.
This is generally the best choice for small and simple applications that
don't require any special functionality from garbage collection.
By default, this option is disabled and the default collector is used.
-
- -XX:+UseSHM
-
Linux only: Enables the JVM to use shared memory to set up
large pages.
-
See Large Pages for setting up large pages.
- -XX:+UseStringDeduplication
-
Enables string deduplication.
By default, this option is disabled.
To use this option, you must enable the garbage-first (G1) garbage
collector.
-
String deduplication reduces the memory footprint of String
objects on the Java heap by taking advantage of the fact that many
String objects are identical.
Instead of each String object pointing to its own character
array, identical String objects can point to and share the same
character array.
- -XX:+UseTLAB
-
Enables the use of thread-local allocation blocks (TLABs) in the young
generation space.
This option is enabled by default.
To disable the use of TLABs, specify the option -XX:-UseTLAB.
-
- -XX:+UseZGC
-
Enables the use of the Z garbage collector (ZGC).
This is a low latency garbage collector, providing max pause times of a
few milliseconds, at some throughput cost.
Pause times are independent of what heap size is used.
Supports heap sizes from 8MB to 16TB.
-
- -XX:ZAllocationSpikeTolerance=factor
-
Sets the allocation spike tolerance for ZGC.
By default, this option is set to 2.0.
This factor describes the level of allocation spikes to expect.
For example, using a factor of 3.0 means the current allocation rate can
be expected to triple at any time.
-
- -XX:ZCollectionInterval=seconds
-
Sets the maximum interval (in seconds) between two GC cycles when using
ZGC.
By default, this option is set to 0 (disabled).
-
- -XX:ZFragmentationLimit=percent
-
Sets the maximum acceptable heap fragmentation (in percent) for ZGC.
By default, this option is set to 25.
Using a lower value will cause the heap to be compacted more
aggressively, to reclaim more memory at the cost of using more CPU time.
-
- -XX:+ZProactive
-
Enables proactive GC cycles when using ZGC.
By default, this option is enabled.
ZGC will start a proactive GC cycle if doing so is expected to have
minimal impact on the running application.
This is useful if the application is mostly idle or allocates very few
objects, but you still want to keep the heap size down and allow
reference processing to happen even when there are a lot of free space
on the heap.
-
- -XX:+ZUncommit
-
Enables uncommitting of unused heap memory when using ZGC.
By default, this option is enabled.
Uncommitting unused heap memory will lower the memory footprint of the
JVM, and make that memory available for other processes to use.
-
- -XX:ZUncommitDelay=seconds
-
Sets the amount of time (in seconds) that heap memory must have been
unused before being uncommitted.
By default, this option is set to 300 (5 minutes).
Committing and uncommitting memory are relatively expensive operations.
Using a lower value will cause heap memory to be uncommitted earlier, at
the risk of soon having to commit it again.
-
DEPRECATED JAVA OPTIONS
These java options are deprecated and might be removed in a
future JDK release.
They're still accepted and acted upon, but a warning is issued when
they're used.
- --illegal-access=parameter
-
When present at run time, --illegal-access= takes a keyword
parameter to specify a mode of operation:
-
-
Note: This option will be removed in a future release.
- •
-
permit: This mode opens each package in each module in the
run-time image to code in all unnamed modules ( such as code on the
class path), if that package existed in JDK 8.
This enables both static access, (for example, by compiled bytecode, and
deep reflective access) through the platform's various reflection
APIs.
The first reflective-access operation to any such package causes a
warning to be issued.
However, no warnings are issued after the first occurrence.
This single warning describes how to enable further warnings.
- •
-
warn: This mode is identical to permit except that a
warning message is issued for each illegal reflective-access operation.
- •
-
debug: This mode is identical to warn except that both
a warning message and a stack trace are issued for each illegal
reflective-access operation.
- •
-
deny: This mode disables all illegal-access operations except
for those enabled by other command-line options, such as
--add-opens.
This mode is the default.
If your application does not work with the default mode of
--illegal-access=deny then you can learn more about what is
going on with the warn and debug modes.
For each library or framework on the class path that requires illegal
access, you have two options:
- •
-
If the component's maintainers have already released a fixed version
that no longer uses JDK-internal APIs then you can consider upgrading
to that version.
- •
-
If the component still needs to be fixed, then you can contact its
maintainers and ask them to replace their use of JDK-internal APIs with
the proper exported APIs.
If you must continue to use a component that requires illegal access,
then you can eliminate the warning messages by using one or more
--add-opens options to open only those internal packages to
which access is required.
To verify that your application is ready for a future version of the
JDK, run it with --illegal-access=deny along with any
necessary --add-opens options.
Any remaining illegal-access errors will most likely be due to static
references from compiled code to JDK-internal APIs.
You can identify those by running the jdeps tool with the
--jdk-internals option.
For performance reasons, the current JDK does not issue warnings for
illegal static-access operations.
- -Xfuture
-
Enables strict class-file format checks that enforce close conformance
to the class-file format specification.
Developers should use this flag when developing new code.
Stricter checks may become the default in future releases.
-
- -Xloggc:filename
-
Sets the file to which verbose GC events information should be
redirected for logging.
The -Xloggc option overrides -verbose:gc if both are
given with the same java command.
-Xloggc:filename is replaced by
-Xlog:gc:filename.
See Enable Logging with the JVM Unified Logging Framework.
-
Example:
-Xlog:gc:garbage-collection.log
- -XX:+FlightRecorder
-
Enables the use of Java Flight Recorder (JFR) during the runtime of the
application.
Since JDK 8u40 this option has not been required to use JFR.
-
- -XX:InitialRAMFraction=ratio
-
Sets the initial amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a ratio of the maximum amount
determined as described in the -XX:MaxRAM option.
The default value is 64.
-
Use the option -XX:InitialRAMPercentage instead.
- -XX:MaxRAMFraction=ratio
-
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a fraction of the maximum
amount determined as described in the -XX:MaxRAM option.
The default value is 4.
-
Specifying this option disables automatic use of compressed oops if the
combined result of this and other options influencing the maximum amount
of memory is larger than the range of memory addressable by compressed
oops.
See -XX:UseCompressedOops for further information about
compressed oops.
Use the option -XX:MaxRAMPercentage instead.
- -XX:MinRAMFraction=ratio
-
Sets the maximum amount of memory that the JVM may use for the Java heap
before applying ergonomics heuristics as a fraction of the maximum
amount determined as described in the -XX:MaxRAM option for
small heaps.
A small heap is a heap of approximately 125 MB.
The default value is 2.
-
Use the option -XX:MinRAMPercentage instead.
- -XX:+UseBiasedLocking
-
Enables the use of biased locking.
Some applications with significant amounts of uncontended
synchronization may attain significant speedups with this flag enabled,
but applications with certain patterns of locking may see slowdowns.
-
By default, this option is disabled.
OBSOLETE JAVA OPTIONS
These java options are still accepted but ignored, and a
warning is issued when they're used.
- -XX:+UseMembar
-
Enabled issuing membars on thread-state transitions.
This option was disabled by default on all platforms except ARM servers,
where it was enabled.
-
- -XX:MaxPermSize=size
-
Sets the maximum permanent generation space size (in bytes).
This option was deprecated in JDK 8 and superseded by the
-XX:MaxMetaspaceSize option.
-
- -XX:PermSize=size
-
Sets the space (in bytes) allocated to the permanent generation that
triggers a garbage collection if it's exceeded.
This option was deprecated in JDK 8 and superseded by the
-XX:MetaspaceSize option.
-
- -XX:+TraceClassLoading
-
Enables tracing of classes as they are loaded.
By default, this option is disabled and classes aren't traced.
-
The replacement Unified Logging syntax is
-Xlog:class+load=level.
See Enable Logging with the JVM Unified Logging Framework
Use level=info for regular information, or
level=debug for additional information.
In Unified Logging syntax, -verbose:class equals
-Xlog:class+load=info,class+unload=info.
- -XX:+TraceClassLoadingPreorder
-
Enables tracing of all loaded classes in the order in which they're
referenced.
By default, this option is disabled and classes aren't traced.
-
The replacement Unified Logging syntax is
-Xlog:class+preorder=debug.
See Enable Logging with the JVM Unified Logging Framework.
- -XX:+TraceClassResolution
-
Enables tracing of constant pool resolutions.
By default, this option is disabled and constant pool resolutions
aren't traced.
-
The replacement Unified Logging syntax is
-Xlog:class+resolve=debug.
See Enable Logging with the JVM Unified Logging Framework.
- -XX:+TraceLoaderConstraints
-
Enables tracing of the loader constraints recording.
By default, this option is disabled and loader constraints recording
isn't traced.
-
The replacement Unified Logging syntax is
-Xlog:class+loader+constraints=info.
See Enable Logging with the JVM Unified Logging Framework.
REMOVED JAVA OPTIONS
These java options have been removed in JDK 16 and using them
results in an error of:
-
Unrecognized VM option option-name
- -XX:+UseParallelOldGC
-
Enables the use of the parallel garbage collector for full GCs.
By default, this option is disabled.
Enabling it automatically enables the -XX:+UseParallelGC
option.
-
For the lists and descriptions of options removed in previous releases
see the Removed Java Options section in:
- •
-
Java Platform, Standard Edition Tools Reference, Release 15
[https://docs.oracle.com/en/java/javase/15/docs/specs/man/java.html]
- •
-
Java Platform, Standard Edition Tools Reference, Release 14
[https://docs.oracle.com/en/java/javase/14/docs/specs/man/java.html]
- •
-
Java Platform, Standard Edition Tools Reference, Release 13
[https://docs.oracle.com/en/java/javase/13/docs/specs/man/java.html]
- •
-
Java Platform, Standard Edition Tools Reference, Release 12
[https://docs.oracle.com/en/java/javase/12/tools/java.html#GUID-3B1CE181-CD30-4178-9602-230B800D4FAE]
- •
-
Java Platform, Standard Edition Tools Reference, Release 11
[https://docs.oracle.com/en/java/javase/11/tools/java.html#GUID-741FC470-AA3E-494A-8D2B-1B1FE4A990D1]
- •
-
Java Platform, Standard Edition Tools Reference, Release 10
[https://docs.oracle.com/javase/10/tools/java.htm#JSWOR624]
- •
-
Java Platform, Standard Edition Tools Reference, Release 9
[https://docs.oracle.com/javase/9/tools/java.htm#JSWOR624]
- •
-
Java Platform, Standard Edition Tools Reference, Release 8 for
Oracle JDK on Windows
[https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html#BGBCIEFC]
- •
-
Java Platform, Standard Edition Tools Reference, Release 8 for
Oracle JDK on Solaris, Linux, and macOS
[https://docs.oracle.com/javase/8/docs/technotes/tools/unix/java.html#BGBCIEFC]
JAVA COMMAND-LINE ARGUMENT FILES
You can shorten or simplify the java command by using
@ argument files to specify one or more text files that
contain arguments, such as options and class names, which are passed to
the java command.
This let's you to create java commands of any length on any
operating system.
In the command line, use the at sign (@) prefix to identify an
argument file that contains java options and class names.
When the java command encounters a file beginning with the at
sign (@), it expands the contents of that file into an
argument list just as they would be specified on the command line.
The java launcher expands the argument file contents until it
encounters the --disable-@files option.
You can use the --disable-@files option anywhere on the
command line, including in an argument file, to stop @
argument files expansion.
The following items describe the syntax of java argument files:
- •
-
The argument file must contain only ASCII characters or characters in
system default encoding that's ASCII friendly, such as UTF-8.
- •
-
The argument file size must not exceed MAXINT (2,147,483,647) bytes.
- •
-
The launcher doesn't expand wildcards that are present within an
argument file.
- •
-
Use white space or new line characters to separate arguments included in
the file.
- •
-
White space includes a white space character, \t,
\n, \r, and \f.
-
For example, it is possible to have a path with a space, such as
c:\Program Files that can be specified as either
"c:\\Program Files" or, to avoid an escape,
c:\Program" "Files.
- •
-
Any option that contains spaces, such as a path component, must be
within quotation marks using quotation ('"') characters in its
entirety.
- •
-
A string within quotation marks may contain the characters \n,
\r, \t, and \f.
They are converted to their respective ASCII codes.
- •
-
If a file name contains embedded spaces, then put the whole file name in
double quotation marks.
- •
-
File names in an argument file are relative to the current directory,
not to the location of the argument file.
- •
-
Use the number sign # in the argument file to identify
comments.
All characters following the # are ignored until the end of
line.
- •
-
Additional at sign @ prefixes to @ prefixed options
act as an escape, (the first @ is removed and the rest of the
arguments are presented to the launcher literally).
- •
-
Lines may be continued using the continuation character (\) at
the end-of-line.
The two lines are concatenated with the leading white spaces trimmed.
To prevent trimming the leading white spaces, a continuation character
(\) may be placed at the first column.
- •
-
Because backslash (\) is an escape character, a backslash character
must be escaped with another backslash character.
- •
-
Partial quote is allowed and is closed by an end-of-file.
- •
-
An open quote stops at end-of-line unless \ is the last
character, which then joins the next line by removing all leading white
space characters.
- •
-
Wildcards (*) aren't allowed in these lists (such as specifying
*.java).
- •
-
Use of the at sign (@) to recursively interpret files
isn't supported.
Example of Open or Partial Quotes in an Argument File
In the argument file,
-
-cp "lib/
cool/
app/
jars
this is interpreted as:
-
-cp lib/cool/app/jars
Example of a Backslash Character Escaped with Another Backslash
Character in an Argument File
To output the following:
-
-cp c:\Program Files (x86)\Java\jre\lib\ext;c:\Program Files\Java\jre9\lib\ext
The backslash character must be specified in the argument file as:
-
-cp "c:\\Program Files (x86)\\Java\\jre\\lib\\ext;c:\\Program Files\\Java\\jre9\\lib\\ext"
Example of an EOL Escape Used to Force Concatenation of Lines in an
Argument File
In the argument file,
-
-cp "/lib/cool app/jars:\
/lib/another app/jars"
This is interpreted as:
-
-cp /lib/cool app/jars:/lib/another app/jars
Example of Line Continuation with Leading Spaces in an Argument File
In the argument file,
-
-cp "/lib/cool\
\app/jars???
This is interpreted as:
-cp /lib/cool app/jars
Examples of Using Single Argument File
You can use a single argument file, such as myargumentfile in
the following example, to hold all required java arguments:
-
java @myargumentfile
Examples of Using Argument Files with Paths
You can include relative paths in argument files; however, they're
relative to the current working directory and not to the paths of the
argument files themselves.
In the following example, path1/options and
path2/options represent argument files with different paths.
Any relative paths that they contain are relative to the current working
directory and not to the argument files:
-
java @path1/options @path2/classes
CODE HEAP STATE ANALYTICS
Overview
There are occasions when having insight into the current state of the
JVM code heap would be helpful to answer questions such as:
- •
-
Why was the JIT turned off and then on again and again?
- •
-
Where has all the code heap space gone?
- •
-
Why is the method sweeper not working effectively?
To provide this insight, a code heap state analytics feature has been
implemented that enables on-the-fly analysis of the code heap.
The analytics process is divided into two parts.
The first part examines the entire code heap and aggregates all
information that is believed to be useful or important.
The second part consists of several independent steps that print the
collected information with an emphasis on different aspects of the data.
Data collection and printing are done on an "on request" basis.
Syntax
Requests for real-time, on-the-fly analysis can be issued with the
following command:
-
jcmd pid Compiler.CodeHeap_Analytics
[function] [granularity]
If you are only interested in how the code heap looks like after running
a sample workload, you can use the command line option:
-
-Xlog:codecache=Trace
To see the code heap state when a "CodeCache full" condition exists,
start the VM with the command line option:
-
-Xlog:codecache=Debug
See CodeHeap State Analytics (OpenJDK)
[https://bugs.openjdk.java.net/secure/attachment/75649/JVM_CodeHeap_StateAnalytics_V2.pdf]
for a detailed description of the code heap state analytics feature, the
supported functions, and the granularity options.
ENABLE LOGGING WITH THE JVM UNIFIED LOGGING FRAMEWORK
You use the -Xlog option to configure or enable logging with
the Java Virtual Machine (JVM) unified logging framework.
Synopsis
-
-Xlog[:[what][:[output][:[decorators][:output-options[,...]]]]]
- what
-
Specifies a combination of tags and levels of the form
tag1[+tag2...][*][=level][,...].
Unless the wildcard (*) is specified, only log messages tagged
with exactly the tags specified are matched.
See -Xlog Tags and Levels.
-
- output
-
Sets the type of output.
Omitting the output type defaults to stdout.
See -Xlog Output.
-
- decorators
-
Configures the output to use a custom set of decorators.
Omitting decorators defaults to uptime,
level, and tags.
See Decorations.
-
- output-options
-
Sets the -Xlog logging output options.
-
Description
The Java Virtual Machine (JVM) unified logging framework provides a
common logging system for all components of the JVM.
GC logging for the JVM has been changed to use the new logging
framework.
The mapping of old GC flags to the corresponding new Xlog configuration
is described in Convert GC Logging Flags to Xlog.
In addition, runtime logging has also been changed to use the JVM
unified logging framework.
The mapping of legacy runtime logging flags to the corresponding new
Xlog configuration is described in Convert Runtime Logging Flags to
Xlog.
The following provides quick reference to the -Xlog command
and syntax for options:
- -Xlog
-
Enables JVM logging on an info level.
-
- -Xlog:help
-
Prints -Xlog usage syntax and available tags, levels, and
decorators along with example command lines with explanations.
-
- -Xlog:disable
-
Turns off all logging and clears all configuration of the logging
framework including the default configuration for warnings and errors.
-
- -Xlog[:option]
-
Applies multiple arguments in the order that they appear on the command
line.
Multiple -Xlog arguments for the same output override each
other in their given order.
-
The option is set as:
-
[tag-selection][:[output][:[decorators][:output-options]]]
Omitting the tag-selection defaults to a tag-set of
all and a level of info.
-
tag[+...] all
The all tag is a meta tag consisting of all tag-sets
available.
The asterisk * in a tag set definition denotes a wildcard tag
match.
Matching with a wildcard selects all tag sets that contain at
least the specified tags.
Without the wildcard, only exact matches of the specified tag sets are
selected.
output-options is
-
filecount=file-count filesize=file size
with optional K, M or G suffix
Default Configuration
When the -Xlog option and nothing else is specified on the
command line, the default configuration is used.
The default configuration logs all messages with a level that matches
either warning or error regardless of what tags the message is
associated with.
The default configuration is equivalent to entering the following on the
command line:
-
-Xlog:all=warning:stdout:uptime,level,tags
Controlling Logging at Runtime
Logging can also be controlled at run time through Diagnostic Commands
(with the jcmd utility).
Everything that can be specified on the command line can also be
specified dynamically with the VM.log command.
As the diagnostic commands are automatically exposed as MBeans, you can
use JMX to change logging configuration at run time.
-Xlog Tags and Levels
Each log message has a level and a tag set associated with it.
The level of the message corresponds to its details, and the tag set
corresponds to what the message contains or which JVM component it
involves (such as, gc, jit, or os).
Mapping GC flags to the Xlog configuration is described in Convert
GC Logging Flags to Xlog.
Mapping legacy runtime logging flags to the corresponding Xlog
configuration is described in Convert Runtime Logging Flags to
Xlog.
Available log levels:
- •
-
off
- •
-
trace
- •
-
debug
- •
-
info
- •
-
warning
- •
-
error
Available log tags:
There are literally dozens of log tags, which in the right combinations,
will enable a range of logging output.
The full set of available log tags can be seen using
-Xlog:help.
Specifying all instead of a tag combination matches all tag
combinations.
-Xlog Output
The -Xlog option supports the following types of outputs:
- •
-
stdout --- Sends output to stdout
- •
-
stderr --- Sends output to stderr
- •
-
file=filename --- Sends output to text file(s).
When using file=filename, specifying %p
and/or %t in the file name expands to the JVM's PID and
startup timestamp, respectively.
You can also configure text files to handle file rotation based on file
size and a number of files to rotate.
For example, to rotate the log file every 10 MB and keep 5 files in
rotation, specify the options filesize=10M, filecount=5.
The target size of the files isn't guaranteed to be exact, it's
just an approximate value.
Files are rotated by default with up to 5 rotated files of target size
20 MB, unless configured otherwise.
Specifying filecount=0 means that the log file shouldn't be
rotated.
There's a possibility of the pre-existing log file getting
overwritten.
Decorations
Logging messages are decorated with information about the message.
You can configure each output to use a custom set of decorators.
The order of the output is always the same as listed in the table.
You can configure the decorations to be used at run time.
Decorations are prepended to the log message.
For example:
-
[6.567s][info][gc,old] Old collection complete
Omitting decorators defaults to uptime,
level, and tags.
The none decorator is special and is used to turn off all
decorations.
time (t), utctime (utc),
uptime (u), timemillis (tm),
uptimemillis (um), timenanos (tn),
uptimenanos (un), hostname (hn),
pid (p), tid (ti), level
(l), tags (tg) decorators can also be
specified as none for no decoration.
|
Decorations
|
Description
|
|
|
time or t
|
Current time and date in ISO-8601 format.
|
|
utctime or utc
|
Universal Time Coordinated or Coordinated Universal Time.
|
|
uptime or u
|
Time since the start of the JVM in seconds and milliseconds.
For example, 6.567s.
|
|
timemillis or tm
|
The same value as generated by System.currentTimeMillis()
|
|
uptimemillis or um
|
Milliseconds since the JVM started.
|
|
timenanos or tn
|
The same value generated by System.nanoTime().
|
|
uptimenanos or un
|
Nanoseconds since the JVM started.
|
|
hostname or hn
|
The host name.
|
|
pid or p
|
The process identifier.
|
|
tid or ti
|
The thread identifier.
|
|
level or l
|
The level associated with the log message.
|
|
tags or tg
|
The tag-set associated with the log message.
|
Convert GC Logging Flags to Xlog
|
Legacy Garbage Collection (GC) Flag
|
Xlog Configuration
|
Comment
|
|
|
G1PrintHeapRegions
|
-Xlog:gc+region=trace
|
Not Applicable
|
|
GCLogFileSize
|
No configuration available
|
Log rotation is handled by the framework.
|
|
NumberOfGCLogFiles
|
Not Applicable
|
Log rotation is handled by the framework.
|
|
PrintAdaptiveSizePolicy
|
-Xlog:gc+ergo*=level
|
Use a level of debug for most of the information, or a
level of trace for all of what was logged for
PrintAdaptiveSizePolicy.
|
|
PrintGC
|
-Xlog:gc
|
Not Applicable
|
|
PrintGCApplicationConcurrentTime
|
-Xlog:safepoint
|
Note that PrintGCApplicationConcurrentTime and
PrintGCApplicationStoppedTime are logged on the same tag and
aren't separated in the new logging.
|
|
PrintGCApplicationStoppedTime
|
-Xlog:safepoint
|
Note that PrintGCApplicationConcurrentTime and
PrintGCApplicationStoppedTime are logged on the same tag and
not separated in the new logging.
|
|
PrintGCCause
|
Not Applicable
|
GC cause is now always logged.
|
|
PrintGCDateStamps
|
Not Applicable
|
Date stamps are logged by the framework.
|
|
PrintGCDetails
|
-Xlog:gc*
|
Not Applicable
|
|
PrintGCID
|
Not Applicable
|
GC ID is now always logged.
|
|
PrintGCTaskTimeStamps
|
-Xlog:gc+task*=debug
|
Not Applicable
|
|
PrintGCTimeStamps
|
Not Applicable
|
Time stamps are logged by the framework.
|
|
PrintHeapAtGC
|
-Xlog:gc+heap=trace
|
Not Applicable
|
|
PrintReferenceGC
|
-Xlog:gc+ref*=debug
|
Note that in the old logging, PrintReferenceGC had an effect
only if PrintGCDetails was also enabled.
|
|
PrintStringDeduplicationStatistics
|
`-Xlog:gc+stringdedup*=debug
|
` Not Applicable
|
|
PrintTenuringDistribution
|
-Xlog:gc+age*=level
|
Use a level of debug for the most relevant
information, or a level of trace for all of what was
logged for PrintTenuringDistribution.
|
|
UseGCLogFileRotation
|
Not Applicable
|
What was logged for PrintTenuringDistribution.
|
Convert Runtime Logging Flags to Xlog
|
Legacy Runtime Flag
|
Xlog Configuration
|
Comment
|
|
|
TraceExceptions
|
-Xlog:exceptions=info
|
Not Applicable
|
|
TraceClassLoading
|
-Xlog:class+load=level
|
Use level=info for regular information, or
level=debug for additional information.
In Unified Logging syntax, -verbose:class equals
-Xlog:class+load=info,class+unload=info.
|
|
TraceClassLoadingPreorder
|
-Xlog:class+preorder=debug
|
Not Applicable
|
|
TraceClassUnloading
|
-Xlog:class+unload=level
|
Use level=info for regular information, or
level=trace for additional information.
In Unified Logging syntax, -verbose:class equals
-Xlog:class+load=info,class+unload=info.
|
|
VerboseVerification
|
-Xlog:verification=info
|
Not Applicable
|
|
TraceClassPaths
|
-Xlog:class+path=info
|
Not Applicable
|
|
TraceClassResolution
|
-Xlog:class+resolve=debug
|
Not Applicable
|
|
TraceClassInitialization
|
-Xlog:class+init=info
|
Not Applicable
|
|
TraceLoaderConstraints
|
-Xlog:class+loader+constraints=info
|
Not Applicable
|
|
TraceClassLoaderData
|
-Xlog:class+loader+data=level
|
Use level=debug for regular information or
level=trace for additional information.
|
|
TraceSafepointCleanupTime
|
-Xlog:safepoint+cleanup=info
|
Not Applicable
|
|
TraceSafepoint
|
-Xlog:safepoint=debug
|
Not Applicable
|
|
TraceMonitorInflation
|
-Xlog:monitorinflation=debug
|
Not Applicable
|
|
TraceBiasedLocking
|
-Xlog:biasedlocking=level
|
Use level=info for regular information, or
level=trace for additional information.
|
|
TraceRedefineClasses
|
-Xlog:redefine+class*=level
|
level=info, debug, and trace provide
increasing amounts of information.
|
-Xlog Usage Examples
The following are -Xlog examples.
- -Xlog
-
Logs all messages by using the info level to stdout
with uptime, levels, and tags decorations.
This is equivalent to using:
-
-
-Xlog:all=info:stdout:uptime,levels,tags
- -Xlog:gc
-
Logs messages tagged with the gc tag using info level
to stdout.
The default configuration for all other messages at level
warning is in effect.
-
- -Xlog:gc,safepoint
-
Logs messages tagged either with the gc or safepoint
tags, both using the info level, to stdout, with
default decorations.
Messages tagged with both gc and safepoint won't
be logged.
-
- -Xlog:gc+ref=debug
-
Logs messages tagged with both gc and ref tags, using
the debug level to stdout, with default decorations.
Messages tagged only with one of the two tags won't be logged.
-
- -Xlog:gc=debug:file=gc.txt:none
-
Logs messages tagged with the gc tag using the debug
level to a file called gc.txt with no decorations.
The default configuration for all other messages at level
warning is still in effect.
-
- -Xlog:gc=trace:file=gctrace.txt:uptimemillis,pids:filecount=5,filesize=1024
-
Logs messages tagged with the gc tag using the trace
level to a rotating file set with 5 files with size 1 MB with the base
name gctrace.txt and uses decorations uptimemillis and
pid.
-
The default configuration for all other messages at level
warning is still in effect.
- -Xlog:gc::uptime,tid
-
Logs messages tagged with the gc tag using the default
'info' level to default the output stdout and uses
decorations uptime and tid.
The default configuration for all other messages at level
warning is still in effect.
-
- -Xlog:gc*=info,safepoint*=off
-
Logs messages tagged with at least gc using the info
level, but turns off logging of messages tagged with safepoint.
Messages tagged with both gc and safepoint won't
be logged.
-
- -Xlog:disable -Xlog:safepoint=trace:safepointtrace.txt
-
Turns off all logging, including warnings and errors, and then enables
messages tagged with safepointusing tracelevel to the
file safepointtrace.txt.
The default configuration doesn't apply, because the command line
started with -Xlog:disable.
-
Complex -Xlog Usage Examples
The following describes a few complex examples of using the
-Xlog option.
- -Xlog:gc+class*=debug
-
Logs messages tagged with at least gc and class tags
using the debug level to stdout.
The default configuration for all other messages at the level
warning is still in effect
-
- -Xlog:gc+meta*=trace,class*=off:file=gcmetatrace.txt
-
Logs messages tagged with at least the gc and meta
tags using the trace level to the file metatrace.txt
but turns off all messages tagged with class.
Messages tagged with gc, meta, and class
aren't be logged as class* is set to off.
The default configuration for all other messages at level
warning is in effect except for those that include
class.
-
- -Xlog:gc+meta=trace
-
Logs messages tagged with exactly the gc and meta tags
using the trace level to stdout.
The default configuration for all other messages at level
warning is still be in effect.
-
- -Xlog:gc+class+heap*=debug,meta*=warning,threads*=off
-
Logs messages tagged with at least gc, class, and
heap tags using the trace level to stdout but
only log messages tagged with meta with level.
The default configuration for all other messages at the level
warning is in effect except for those that include
threads.
-
VALIDATE JAVA VIRTUAL MACHINE FLAG ARGUMENTS
You use values provided to all Java Virtual Machine (JVM) command-line
flags for validation and, if the input value is invalid or
out-of-range, then an appropriate error message is displayed.
Whether they're set ergonomically, in a command line, by an input
tool, or through the APIs (for example, classes contained in the package
java.lang.management) the values provided to all Java Virtual
Machine (JVM) command-line flags are validated.
Ergonomics are described in Java Platform, Standard Edition HotSpot
Virtual Machine Garbage Collection Tuning Guide.
Range and constraints are validated either when all flags have their
values set during JVM initialization or a flag's value is changed
during runtime (for example using the jcmd tool).
The JVM is terminated if a value violates either the range or constraint
check and an appropriate error message is printed on the error stream.
For example, if a flag violates a range or a constraint check, then the
JVM exits with an error:
-
java -XX:AllocatePrefetchStyle=5 -version
intx AllocatePrefetchStyle=5 is outside the allowed range [ 0 ... 3 ]
Improperly specified VM option 'AllocatePrefetchStyle=5'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.
The flag -XX:+PrintFlagsRanges prints the range of all the
flags.
This flag allows automatic testing of the flags by the values provided
by the ranges.
For the flags that have the ranges specified, the type, name, and the
actual range is printed in the output.
For example,
-
intx ThreadStackSize [ 0 ... 9007199254740987 ] {pd product}
For the flags that don't have the range specified, the values
aren't displayed in the print out.
For example:
-
size_t NewSize [ ... ] {product}
This helps to identify the flags that need to be implemented.
The automatic testing framework can skip those flags that don't have
values and aren't implemented.
LARGE PAGES
You use large pages, also known as huge pages, as memory pages that are
significantly larger than the standard memory page size (which varies
depending on the processor and operating system).
Large pages optimize processor Translation-Lookaside Buffers.
A Translation-Lookaside Buffer (TLB) is a page translation cache that
holds the most-recently used virtual-to-physical address
translations.
A TLB is a scarce system resource.
A TLB miss can be costly because the processor must then read from the
hierarchical page table, which may require multiple memory accesses.
By using a larger memory page size, a single TLB entry can represent a
larger memory range.
This results in less pressure on a TLB, and memory-intensive
applications may have better performance.
However, large pages page memory can negatively affect system
performance.
For example, when a large mount of memory is pinned by an application,
it may create a shortage of regular memory and cause excessive paging in
other applications and slow down the entire system.
Also, a system that has been up for a long time could produce excessive
fragmentation, which could make it impossible to reserve enough large
page memory.
When this happens, either the OS or JVM reverts to using regular pages.
Linux and Windows support large pages.
Large Pages Support for Linux
The 2.6 kernel supports large pages.
Some vendors have backported the code to their 2.4-based releases.
To check if your system can support large page memory, try the
following:
-
# cat /proc/meminfo | grep Huge
HugePages_Total: 0
HugePages_Free: 0
Hugepagesize: 2048 kB
If the output shows the three "Huge" variables, then your system can
support large page memory but it needs to be configured.
If the command prints nothing, then your system doesn't support
large pages.
To configure the system to use large page memory, login as
root, and then follow these steps:
- 1.
-
If you're using the option -XX:+UseSHM (instead of
-XX:+UseHugeTLBFS), then increase the SHMMAX value.
It must be larger than the Java heap size.
On a system with 4 GB of physical RAM (or less), the following makes all
the memory sharable:
-
-
# echo 4294967295 > /proc/sys/kernel/shmmax
- 2.
-
If you're using the option -XX:+UseSHM or
-XX:+UseHugeTLBFS, then specify the number of large pages.
In the following example, 3 GB of a 4 GB system are reserved for large
pages (assuming a large page size of 2048kB, then 3 GB = 3 * 1024 MB =
3072 MB = 3072 * 1024 kB = 3145728 kB and 3145728 kB / 2048 kB = 1536):
-
-
# echo 1536 > /proc/sys/vm/nr_hugepages
-
Note: The values contained in /proc resets after you
reboot your system, so may want to set them in an initialization script
(for example, rc.local or sysctl.conf).
- •
-
If you configure (or resize) the OS kernel parameters
/proc/sys/kernel/shmmax or /proc/sys/vm/nr_hugepages,
Java processes may allocate large pages for areas in addition to the
Java heap.
These steps can allocate large pages for the following areas:
-
- •
-
Java heap
- •
-
Code cache
- •
-
The marking bitmap data structure for the parallel GC
Consequently, if you configure the nr_hugepages parameter to
the size of the Java heap, then the JVM can fail in allocating the code
cache areas on large pages because these areas are quite large in size.
Large Pages Support for Windows
To use large pages support on Windows, the administrator must first
assign additional privileges to the user who is running the application:
- 1.
-
Select Control Panel, Administrative Tools, and then
Local Security Policy.
- 2.
-
Select Local Policies and then User Rights Assignment.
- 3.
-
Double-click Lock pages in memory, then add users and/or
groups.
- 4.
-
Reboot your system.
Note that these steps are required even if it's the administrator
who's running the application, because administrators by default
don't have the privilege to lock pages in memory.
APPLICATION CLASS DATA SHARING
Application Class Data Sharing (AppCDS) extends class data sharing (CDS)
to enable application classes to be placed in a shared archive.
In addition to the core library classes, AppCDS supports Class Data
Sharing
[https://docs.oracle.com/en/java/javase/12/vm/class-data-sharing.html#GUID-7EAA3411-8CF0-4D19-BD05-DF5E1780AA91]
from the following locations:
- •
-
Platform classes from the runtime image
- •
-
Application classes from the runtime image
- •
-
Application classes from the class path
- •
-
Application classes from the module path
Archiving application classes provides better start up time at runtime.
When running multiple JVM processes, AppCDS also reduces the runtime
footprint with memory sharing for read-only metadata.
CDS/AppCDS supports archiving classes from JAR files only.
Prior to JDK 11, a non-empty directory was reported as a fatal error in
the following conditions:
- •
-
For base CDS, a non-empty directory cannot exist in the
-Xbootclasspath/a path
- •
-
With -XX:+UseAppCDS, a non-empty directory could not exist in
the -Xbootclasspath/a path, class path, and module path.
In JDK 11 and later, -XX:+UseAppCDS is obsolete and the
behavior for a non-empty directory is based on the class types in the
classlist.
A non-empty directory is reported as a fatal error in the following
conditions:
- •
-
If application classes or platform classes are not loaded, dump time
only reports an error if a non-empty directory exists in
-Xbootclasspath/a path
- •
-
If application classes or platform classes are loaded, dump time reports
an error for a non-empty directory that exists in
-Xbootclasspath/a path, class path, or module path
In JDK 11 and later, using
-XX:DumpLoadedClassList=class_list_file results a
generated classlist with all classes (both system library classes and
application classes) included.
You no longer have to specify -XX:+UseAppCDS with
-XX:DumpLoadedClassList to produce a complete class list.
In JDK 11 and later, because UseAppCDS is obsolete,
SharedArchiveFile becomes a product flag by default.
Specifying +UnlockDiagnosticVMOptions for
SharedArchiveFile is no longer needed in any configuration.
Class Data Sharing (CDS)/AppCDS does not support archiving array classes
in a class list.
When an array in the class list is encountered, CDS dump time gives the
explicit error message:
-
Preload Warning: Cannot find array_name
Although an array in the class list is not allowed, some array classes
can still be created at CDS/AppCDS dump time.
Those arrays are created during the execution of the Java code used by
the Java class loaders (PlatformClassLoader and the system
class loader) to load classes at dump time.
The created arrays are archived with the rest of the loaded classes.
Extending Class Data Sharing to Support the Module Path
In JDK 11, Class Data Sharing (CDS) has been improved to support
archiving classes from the module path.
- •
-
To create a CDS archive using the --module-path VM option,
use the following command line syntax:
-
-
java -Xshare:dump -XX:SharedClassListFile=class_list_file
-XX:SharedArchiveFile=shared_archive_file
--module-path=path_to_modular_jar -m
module_name
- •
-
To run with a CDS archive using the --module-path VM option,
use the following the command line syntax:
-
-
java -XX:SharedArchiveFile=shared_archive_file
--module-path=path_to_modular_jar -m
module_name
The following table describes how the VM options related to module paths
can be used along with the -Xshare option.
|
Option
|
-Xshare:dump
|
-Xshare:{on,auto}
|
|
|
--module-path[1] mp
|
Allowed
|
Allowed[2]
|
|
--module
|
Allowed
|
Allowed
|
|
--add-module
|
Allowed
|
Allowed
|
|
--upgrade-module-path[3]
|
Disallowed (exits if specified)
|
Allowed (disables CDS)
|
|
--patch-module[4]
|
Disallowed (exits if specified)
|
Allowed (disables CDS)
|
|
--limit-modules[5]
|
Disallowed (exits if specified)
|
Allowed (disables CDS)
|
[1] Although there are two ways of specifying a module in a
--module-path, that is, modular JAR or exploded module, only
modular JARs are supported.
[2] Different mp can be specified during dump time versus run
time.
If an archived class K was loaded from mp1.jar at dump time,
but changes in mp cause it to be available from a different
mp2.jar at run time, then the archived version of K will be
disregarded at run time; K will be loaded dynamically.
[3] Currently, only two system modules are upgradeable
(java.compiler and jdk.internal.vm.compiler).
However, these modules are seldom upgraded in production software.
[4] As documented in JEP 261, using --patch-module is
strongly discouraged for production use.
[5] --limit-modules is intended for testing purposes.
It is seldom used in production software.
If --upgrade-module-path, --patch-module, or
--limit-modules is specified at dump time, an error will be
printed and the JVM will exit.
For example, if the --limit-modules option is specified at
dump time, the user will see the following error:
-
Error occurred during initialization of VM
Cannot use the following option when dumping the shared archive: --limit-modules
If --upgrade-module-path, --patch-module, or
--limit-modules is specified at run time, a warning message
will be printed indicating that CDS is disabled.
For example, if the --limit-modules options is specified at
run time, the user will see the following warning:
-
Java HotSpot(TM) 64-Bit Server VM warning: CDS is disabled when the --limit-modules option is specified.
Several other noteworthy things include:
- •
-
Any valid combinations of -cp and --module-path
are supported.
- •
-
A non-empty directory in the module path causes a fatal error.
The user will see the following error messages:
-
-
Error: non-empty directory <directory> Hint: enable -Xlog:class+path=info to diagnose the failure Error occurred during initialization of VM Cannot have non-empty directory in paths
- •
-
Unlike the class path, there's no restriction that the module path
at dump time must be equal to or be a prefix of the module path at run
time.
- •
-
The archive is invalidated if an existing JAR in the module path is
updated after archive generation.
- •
-
Removing a JAR from the module path does not invalidate the shared
archive.
Archived classes from the removed JAR are not used at runtime.
Dynamic CDS archive
Dynamic CDS archive extends AppCDS to allow archiving of classes when a
Java application exits.
It improves the usability of AppCDS by eliminating the trial run step
for creating a class list for each application.
The archived classes include all loaded application classes and library
classes that are not present in the default CDS archive which is
included in the JDK.
A base archive is required when creating a dynamic archive.
If the base archive is not specified, the default CDS archive is used as
the base archive.
To create a dynamic CDS archive with the default CDS archive as the base
archive, just add the
-XX:ArchiveClassesAtExit=<dynamic archive> option to the
command line for running the Java application.
If the default CDS archive does not exist, the VM will exit with the
following error:
-
ArchiveClassesAtExit not supported when base CDS archive is not loaded
To run the Java application using a dynamic CDS archive, just add the
-XX:SharedArchiveFile=<dynamic archive> option to the command
line for running the Java application.
The base archive is not required to be specified in the command line.
The base archive information, including its name and full path, will be
retrieved from the dynamic archive header.
Note that the user could also use the -XX:SharedArchiveFile
option for specifying a regular AppCDS archive.
Therefore, the specified archive in the -XX:SharedArchiveFile
option could be either a regular or dynamic archive.
During VM start up the specified archive header will be read.
If -XX:SharedArchiveFile refers to a regular archive, then the
behavior will be unchanged.
If -XX:SharedArchiveFile refers to a dynamic archive, the VM
will retrieve the base archive location from the dynamic archive.
If the dynamic archive was created with the default CDS archive, then
the current default CDS archive will be used, and will be found relative
to the current run time environment.
Please refer to JDK-8221706
[https://bugs.openjdk.java.net/browse/JDK-8221706] for details on error
checking during dynamic CDS archive dump time and run time.
Creating a Shared Archive File and Using It to Run an Application
AppCDS archive
The following steps create a shared archive file that contains all the
classes used by the test.Hello application.
The last step runs the application with the shared archive file.
- 1.
-
Create a list of all classes used by the test.Hello
application.
The following command creates a file named hello.classlist that
contains a list of all classes used by this application:
-
-
java -Xshare:off -XX:DumpLoadedClassList=hello.classlist -cp hello.jar test.Hello
Note that the classpath specified by the -cp parameter must
contain only JAR files.
- 2.
-
Create a shared archive, named hello.jsa, that contains all the
classes in hello.classlist:
-
-
java -Xshare:dump -XX:SharedArchiveFile=hello.jsa -XX:SharedClassListFile=hello.classlist -cp hello.jar
Note that the classpath used at archive creation time must be the same
as (or a prefix of) the classpath used at run time.
- 3.
-
Run the application test.Hello with the shared archive
hello.jsa:
-
-
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello
- 4.
-
Optional Verify that the test.Hello application is
using the class contained in the hello.jsa shared archive:
-
-
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar -verbose:class test.Hello
The output of this command should contain the following text:
-
Loaded test.Hello from shared objects file by sun/misc/Launcher$AppClassLoader
Dynamic CDS archive
The following steps create a dynamic CDS archive file that contains the
classes used by the test.Hello application and are not included
in the default CDS archive.
The second step runs the application with the dynamic CDS archive.
- 1.
-
Create a dynamic CDS archive, named hello.jsa, that contains
all the classes in hello.jar loaded by the application
test.Hello:
-
-
java -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello
Note that the classpath used at archive creation time must be the same
as (or a prefix of) the classpath used at run time.
- 2.
-
Run the application test.Hello with the shared archive
hello.jsa:
-
-
java -XX:SharedArchiveFile=hello.jsa -cp hello.jar test.Hello
- 3.
-
Optional Repeat step 4 of the previous section to verify that
the test.Hello application is using the class contained in the
hello.jsa shared archive.
To automate the above steps 1 and 2, one can write a script such as the
following:
-
ARCHIVE=hello.jsa
if test -f $ARCHIVE; then
FLAG="-XX:SharedArchiveFile=$ARCHIVE"
else
FLAG="-XX:ArchiveClassesAtExit=$ARCHIVE"
fi
$JAVA_HOME/bin/java -cp hello.jar $FLAG test.Hello
Like an AppCDS archive, the archive needs to be re-generated if the
Java version has changed.
The above script could be adjusted to account for the Java version as
follows:
-
ARCHIVE=hello.jsa
VERSION=foo.version
if test -f $ARCHIVE -a -f $VERSION && cmp -s $VERSION $JAVA_HOME/release; then
FLAG="-XX:SharedArchiveFile=$ARCHIVE"
else
FLAG="-XX:ArchiveClassesAtExit=$ARCHIVE"
cp -f $JAVA_HOME/release $VERSION
fi
$JAVA_HOME/bin/java -cp hello.jar $FLAG test.Hello
Currently, we don't support concurrent dumping operations to the
same CDS archive.
Care should be taken to avoid multiple writers to the same CDS archive.
The user could also create a dynamic CDS archive with a specific base
archive, e.g.
named as base.jsa as follows:
-
java -XX:SharedArchiveFile=base.jsa -XX:ArchiveClassesAtExit=hello.jsa -cp hello.jar Hello
To run the application using the dynamic CDS archive hello.jsa
and a specific base CDS archive base.jsa:
-
java -XX:SharedArchiveFile=base.jsa:hello.jsa -cp hello.jar Hello
Note that on Windows, the above path delimiter : should be
replaced with ;.
The above command for specifying a base archive is useful if the base
archive used for creating the dynamic archive has been moved.
Normally, just specifying the dynamic archive should be sufficient since
the base archive info can be retrieved from the dynamic archive header.
Sharing a Shared Archive Across Multiple Application Processes
You can share the same archive file across multiple applications
processes.
This reduces memory usage because the archive is memory-mapped into the
address space of the processes.
The operating system automatically shares the read-only pages across
these processes.
The following steps demonstrate how to create a common archive that can
be shared by different applications.
Classes from common.jar, hello.jar and hi.jar
are archived in the common.jsa because they are all in the
classpath during the archiving step (step 3).
To include classes from hello.jar and hi.jar, the
.jar files must be added to the classpath specified by the
-cp parameter.
- 1.
-
Create a list of all classes used by the Hello application and
another list for the Hi application:
-
-
java -XX:DumpLoadedClassList=hello.classlist -cp common.jar:hello.jar Hello
-
java -XX:DumpLoadedClassList=hi.classlist -cp common.jar:hi.jar Hi
- 2.
-
Create a single list of classes used by all the applications that will
share the shared archive file.
-
Linux and macOS The following commands combine the files
hello.classlist and hi.classlist into one file,
common.classlist:
-
cat hello.classlist hi.classlist > common.classlist
Windows The following commands combine the files
hello.classlist and hi.classlist into one file,
common.classlist:
-
type hello.classlist hi.classlist > common.classlist
- 3.
-
Create a shared archive named common.jsa that contains all the
classes in common.classlist:
-
-
java -Xshare:dump -XX:SharedArchiveFile=common.jsa -XX:SharedClassListFile=common.classlist -cp common.jar:hello.jar:hi.jar
The classpath parameter used is the common class path prefix shared by
the Hello and Hi applications.
- 4.
-
Run the Hello and Hi applications with the same shared
archive:
-
-
java -XX:SharedArchiveFile=common.jsa -cp common.jar:hello.jar:hi.jar Hello
-
java -XX:SharedArchiveFile=common.jsa -cp common.jar:hello.jar:hi.jar Hi
Specifying Additional Shared Data Added to an Archive File
The SharedArchiveConfigFile option is used to specify
additional shared data to add to the archive file.
-
-XX:SharedArchiveConfigFile=shared_config_file
JDK 9 and later supports adding both symbols and string objects to an
archive for memory sharing when you have multiple JVM processes running
on the same host.
An example of this is having multiple JVM processes that use the same
set of Java EE classes.
When these common classes are loaded and used, new symbols and strings
may be created and added to the JVM's internal "symbol" and "string"
tables.
At runtime, the symbols or string objects mapped from the archive file
can be shared across multiple JVM processes, resulting in a reduction of
overall memory usage.
In addition, archiving strings also provides added performance benefits
in both startup time and runtime execution.
In JDK 10 and later, CONSTANT_String entries in archived classes are
resolved to interned String objects at dump time, and all interned
String objects are archived.
However, even though all CONSTANT_String literals in all archived
classes are resolved, it might still beneficial to add additional
strings that are not string literals in class files, but are likely to
be used by your application at run time.
Symbol data should be generated by the jcmd tool attaching to a
running JVM process.
See jcmd.
The following is an example of the symbol dumping command in
jcmd:
-
jcmd pid VM.symboltable -verbose
-
Note: The first line (process ID) and the second line
(@VERSION ...) of this jcmd output should be
excluded from the configuration file.
Example of a Configuration File
The following is an example of a configuration file:
-
VERSION: 1.0
@SECTION: Symbol
10 -1: linkMethod
In the configuration file example, the @SECTION: Symbol entry
uses the following format:
-
length refcount: symbol
The refcount for a shared symbol is always -1.
@SECTION specifies the type of the section that follows it.
All data within the section must be the same type that's specified
by @SECTION.
Different types of data can't be mixed.
Multiple separated data sections for the same type specified by
different @SECTION are allowed within one
shared_config_file .
PERFORMANCE TUNING EXAMPLES
You can use the Java advanced runtime options to optimize the
performance of your applications.
Tuning for Higher Throughput
Use the following commands and advanced options to achieve higher
throughput performance for your application:
-
java -server -XX:+UseParallelGC -XX:+UseLargePages -Xmn10g -Xms26g -Xmx26g
Tuning for Lower Response Time
Use the following commands and advanced options to achieve lower
response times for your application:
-
java -XX:+UseG1GC -XX:MaxGCPauseMillis=100
Keeping the Java Heap Small and Reducing the Dynamic Footprint of
Embedded Applications
Use the following advanced runtime options to keep the Java heap small
and reduce the dynamic footprint of embedded applications:
-
-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5
-
Note: The defaults for these two options are 70% and 40%
respectively.
Because performance sacrifices can occur when using these small
settings, you should optimize for a small footprint by reducing these
settings as much as possible without introducing unacceptable
performance degradation.
EXIT STATUS
The following exit values are typically returned by the launcher when
the launcher is called with the wrong arguments, serious errors, or
exceptions thrown by the JVM.
However, a Java application may choose to return any value by using the
API call System.exit(exitValue).
The values are:
- •
-
0: Successful completion
- •
-
>0: An error occurred