Pre-General Availability: 2017-05-24

java

You can use the java command to launch a Java application.

Synopsis

To run a class:

java [options] classname [args...] 

To execute a JAR file:

java [options] -jar filename [args...]

To execute the main class in a module:

java [options] -p modulepath -m modulename[/mainclass] [args...]
options

Command-line options separated by spaces. See Overview of java Options for a description of available options.

classname

The name of the class to be launched. Command-line entries following classname are the arguments for the main method.

filename

The name of the Java Archive (JAR) file to be called. Used only with the -jar option.

modulepath

A semicolon (;) separated list of directories in which each directory is a directory of modules. Used only with the -p option. See Standard Options for java.

modulename[/mainclass]

The name of the initial module to resolve and, if it isn’t specified by the module, the name of the main class to execute. Used only with the -m option. See Standard Options for java.

args

The arguments passed to the main method separated by spaces.

Description

The java command starts a Java application. It does this by starting the Java Runtime Environment (JRE), 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)

The java command can be used to launch a JavaFX application by loading a class that either has a main() method or that extends javafx.application.Application. In the latter case, the launcher constructs an instance of the Application class, calls its init() method, and then calls the start(javafx.stage.Stage) method.

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 the -jar option is specified, 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.

Windows: The javaw command is identical to java, except that with javaw there is 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.

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 are 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.

Boolean options are used to either enable a feature that is disabled by default or disable a feature that is 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 are expected to specify the size in bytes, you can use no suffix, or use the suffix k or K for kilobytes (KB), m or M for megabytes (MB), 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, use a number from 0 to 1. For example, specify 0.25 for 25%.

The following sections describe the options are obsolete, deprecated, and removed in JDK 9:

Standard Options for java

These are the most commonly used options supported by all implementations of the JVM:

-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.

Oracle Solaris, Linux, and OS X: 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 OS X 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 * 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 * expands to a list of all the jar files in the current directory. The CLASSPATH environment variable, where defined, will be similarly expanded. Any class path wildcard expansion that occurs before the Java VM is started. Java programs never see wild cards that aren’t expanded except by querying the environment, such as by calling System.getenv("CLASSPATH").

--module-path module path... or -p module path

A semicolon (;) separated list of directories. Each directory is a directory of modules.

--upgrade-module-path module path...

A semicolon (;) separated list of directories. Each directory is a directory of modules that replace upgradeable modules in the runtime image.

--module modulename[/mainclass] or -m module[/mainclass]

Specifies the initial module to resolve and the name of the main class to execute if not specified by the module.

--add-modules modulename[,modulename...]

Specifies the root modules to resolve in addition to the initial module. modulename can also be ALL-DEFAULT, ALL-SYSTEM, and ALL-MODULE-PATH.

--limit-modules modulename[,modulename...]

Specifies the limit of the universe of observable modules.

--list-modules [modulename[,modulename...]]

Lists the observable modules and exits.

--dry-run

Creates the VM but doesn’t execute the main method. This --dry-run option may be useful for validating the command-line options such as the module system configuration.

-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 is 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 package com.wombat.fruitbat (and any subpackages) but disabled in 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 is 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 are 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 or -?

Prints the help message to the error stream.

--help

Prints the help message to the output stream.

-jar filename

Executes a program encapsulated in a JAR file. The filename 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 the -jar option, the specified JAR file is the source of all user classes, and other class path settings are ignored. If you are using JAR files, then see: jar

-javaagent:jarpath[=options]

Loads the specified Java programming language agent.

--permit-illegal-access

This option, if present at run time, permits illegal access operations by code in any unnamed module (such as, code on the class path) to members of types in any named module by using the standard reflective APIs (java.lang.reflect and java.lang.invoke), including in particular those that would otherwise result in an IllegalAccessException or an InaccessibleObjectException.

This option doesn’t permit illegal access operations by code in named modules to members of types in other named modules. For that, you must use the --add-opens or --add-exports options. Those options can be combined with --permit-illegal-access.

Note:

This option is only supported in JDK 9.

--show-version or -showversion

Displays version information and continues execution of the application. This option is equivalent to the -version option except that the latter instructs the JVM to exit after displaying version information.

-splash:imgname

Shows the splash screen with the image specified by imgname. HiDPI scaled images are automatically supported and used if available. The unscaled image filename, such as image.ext, should always be passed as the argument to the -splash option. The most appropriate scaled image provided will be 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
-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.

--version or -version

Displays version information and then exits. This option is equivalent to the -showversion option except that the latter doesn’t instruct the JVM to exit after displaying version information.

-X

Prints the help on extra options to the error stream.

--help-extra

Prints the help on extra options to the output stream.

@argument files

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 @argument files 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 as they would be specified on the command line until the -Xdisable-@files option is 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, the command line could simply be

java @argument files

See java Command-Line Argument Files for a description and examples of using @argument files .

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:path

Specifies a list of directories, JAR files, and ZIP archives to append to the end of the default bootstrap class path.

Oracle Solaris, Linux, and OS X: 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 will terminate with an irrecoverable error in such cases. Expect a performance degradation when this option is used.

-Xcomp

Forces compilation of methods on first invocation. By default, the Client VM (-client) performs 1,000 interpreted method invocations and the Server VM (-server) performs 10,000 interpreted method invocations to gather information for efficient compilation. Specifying the -Xcomp option disables interpreted method invocations to increase compilation performance at the expense of efficiency. You can also change the number of interpreted method invocations before compilation using the -XX:CompileThreshold option.

-Xdebug

Does nothing. Provided for backward compatibility.

-Xdiag

Shows additional diagnostic messages.

-Xdiag:resolver

Shows the resolver diagnostic messages.

-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.

-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.

-Xloggc:option

Enables the JVM unified logging framework. Logs GC status to a file with time stamps.

-Xlog:help

Prints a short option reference with examples, and then exits the VM. 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.

-Xmn size

Sets the initial and maximum 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, 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 will be performed. If the size is too large, then only full garbage collections will be performed, which can take a long time to complete. Oracle recommends 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 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 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

If you don’t set this option, then the initial size will be 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 memory allocation pool in bytes. 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, 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 will be left untouched during GC and will always be considered live. This can result in more memory being permanently occupied which, if not used carefully, will throw an out-of-memory exception.

-Xprof

Profiles the running program and sends profiling data to standard output. This option is provided as a utility that is useful in program development and isn’t intended to be used in production systems.

-Xrs

Reduces the use of operating system signals by the JVM. Shutdown hooks enable orderly shutdown of a Java application by running user cleanup code (such as closing database connections) at shutdown, even if the JVM terminates abruptly.

Oracle Solaris, Linux, and OS X:

  • 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.

  • 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 will not 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:

  • Oracle Solaris, Linux, and OS X: 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 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 CDS if possible. This is the default value for Java HotSpot 32-Bit Client VM.

on

Require the use of CDS. Print an error message and exit if class data sharing can’t be used.

off

Don’t use CDS.

-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.

-Xss size

Sets the thread stack size (in bytes). Append the letter k or K to indicate KB, m or M to indicate MB, g or G to indicate GB. The default value depends on the platform:

  • Linux/ARM (32-bit): 320 KB

  • Linux/ARM (64-bit): 1024 KB

  • Linux/x64 (64-bit): 1024 KB

  • OS X (64-bit): 1024 KB

  • Oracle Solaris/i386 (32-bit): 320 KB

  • Oracle Solaris/x64 (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.

-Xverify:mode

Sets the mode of the bytecode verifier. Bytecode verification ensures that class files are properly formed and satisfy the constraints listed in Verification of class Files in the The Java Virtual Machine Specification.

Don’t turn off verification as this reduces the protection provided by Java and could cause problems due to ill-formed class files.

Possible mode arguments for this option include the following:

remote

Verify those classes that aren’t loaded by the bootstrap class loader. This is the default behavior if you don’t specify the -Xverify option.

all

Enables verification of all bytecodes.

none

Disables verification of all bytecodes. Use of -Xverify:none is unsupported.

--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.

--disable-@files

Can be used anywhere on the command line, including in an argument file, to prevent further @filename expansion. Will stop expanding @argfiles after the option.

--patch-module module=file(;file)*

Override or augment a module with classes and resources in JAR files or directories.

Advanced Runtime Options for java

These java options control the run-time behavior of the Java HotSpot VM.

-XX:+CheckEndorsedAndExtDirs

Enables the option to prevent the java command from running a Java application if any of these directories exist and aren’t empty:

  • lib/endorsed

  • lib/ext

  • The system-wide platform-specific extension directory

The endorsed standards override mechanism and the extension mechanism are no longer supported.

-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 will be 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 multi-byte character, these are represented and stored as two 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 is known that an application will overwhelmingly be allocating multi-byte 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 Compact Strings introduces the regression.

In both of these scenarios, disabling Compact Strings makes sense.

-XX:CompilerDirectivesFile=file

Adds directives from a file to the directives stack when a program starts. See Compiler Directives and the Command Line.

-XX:CompilerDirectivesPrint

Prints the directives stack when the program starts or when a new directive is added..

-XX:ConcGCThreads=n

Sets the number of parallel marking threads. Sets n to approximately 1/4 of the number of parallel garbage collection threads (ParallelGCThreads).

-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: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_pid pid.log where pid is the identifier of the process that caused 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
  • Oracle Solaris, Linux, and OS X: 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 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:

  • Oracle Solaris, Linux, and OS X: 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:+FailOverToOldVerifier

Enables automatic failover to the old verifier when the new type checker fails. By default, this option is disabled and it is ignored (that is, treated as disabled) for classes with a recent byte code version. You can enable it for classes with older versions of the byte code.

-XX:InitiatingHeapOccupancyPercent=45

Sets the Java heap occupancy threshold that triggers a marking cycle. The default occupancy is 45 percent of the entire Java heap.

-XX:LargePageSizeInBytes=size

Oracle Solaris: Sets the maximum size (in bytes) for large pages used for Java heap. The size argument must be a power of 2 (2, 4, 8, 16, ...). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, 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, 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:MaxGCPauseMillis=200

Sets a target value for desired maximum pause time. The default value is 200 milliseconds. The specified value doesn’t adapt to your heap size.

-XX:NativeMemoryTracking=mode

Specifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following:

off

Don’t track JVM native memory usage. This is the default behavior if you don’t specify the -XX:NativeMemoryTracking option.

summary

Only track memory usage 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 two, 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:

Note: As the alignment value increases, the unused space between objects will also increase. 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.

  • Oracle Solaris, Linux, and OS X: The following example shows how the -XX:OnError option can be used to run the gcore command to create the core image, and the debugger is started to attach to the process in case of an irrecoverable error (the %p designates the current process):

    -XX:OnError="gcore %p;dbx - %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). 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:ParallelGCThreads=n

Sets the value of the STW worker threads. Sets the value of n to the number of logical processors. The value of n is the same as the number of logical processors up to a value of 8. If there are more than eight logical processors, sets the value of n to approximately 5/8 of the logical processors. This works in most cases except for larger SPARC systems where the value of n can be approximately 5/16 of the logical processors.

-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 you ran. Use jstat to display the performance data contained in this file as follows:

jstat -class file:///path/hsperfdata_pid
jstat -gc file:///path/hsperfdata_pid
-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 

The PreserveFramePointer flag 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, 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:+RelaxAccessControlCheck

Decreases the amount of access control checks in the verifier. By default, this option is disabled, and it is ignored (that is, treated as disabled) for classes with a recent byte code version. You can enable it for classes with older versions of the byte code.

-XX:+ShowMessageBoxOnError

Enables displaying 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:ThreadStackSize=size

Sets the Java thread stack size (in kilobytes). Please note that use of a scaling suffix, such as k, results in 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/ARM (32-bit): 320 KB

  • Linux/ARM (64-bit): 1024 KB

  • Linux/i386 (32-bit): 320 KB

  • Linux/x64 (64-bit): 1024 KB

  • OS X (64-bit): 1024 KB

  • Oracle Solaris/x64 (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:-UseBiasedLocking

Disables the use of biased locking. Some applications with significant amounts of uncontended synchronization may attain significant speedups with this flag enabled, whereas applications with certain patterns of locking may see slowdowns. .

By default, this option is enabled.

-XX:-UseCompressedOops

Disables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used when Java heap sizes are less than 32 GB. When this option is 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 less than 32 GB. This option works only for 64-bit JVMs.

It is also possible to use compressed pointers when Java heap sizes are greater than 32 GB. See the -XX:ObjectAlignmentInBytes option.

XX:+UseGCLogRotation

Used to handle large log files. Must be used with -Xloggc:filename

-XX:NumberOfGClogFiles=number of files

Used to handle large log files. The number of files must be greater than or equal to 1. The default is 1.

-XX:GCLogFileSize=number

Used to handle large log files. The number can be in the form of numberM or numberK. The default is set to 512K.

-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:+UseMembar

Enables issuing of membars on thread state transitions. This option is disabled by default on all platforms except ARM servers, where it is enabled. (It is recommended that you don’t disable this option on ARM servers.)

-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.

-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:+AggressiveOpts

Enables the use of aggressive performance optimization features. By default, this option is disabled and experimental performance features aren’t used.

-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

Only the Java HotSpot Server VM supports this option.

-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, 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

Only the Java HotSpot Server VM supports this option.

-XX:AllocatePrefetchInstr=instruction

Sets the prefetch instruction to prefetch ahead of the allocation pointer. Only the Java HotSpot Server VM supports this option. 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

Only the Java HotSpot Server VM supports this option.

-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

Only the Java HotSpot Server VM supports this option.

-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

Only the Java HotSpot Server VM supports this option.

-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 parameter.

2

Use the thread-local allocation block (TLAB) watermark pointer to determine when prefetch instructions are executed.

3

Use BIS instruction on SPARC for allocation prefetch.

Only the Java HotSpot Server VM supports this option.

-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 threads is set to 2 for the server JVM, to 1 for the client JVM, and it scales to the number of cores if tiered compilation is used. The following example shows how to set the number of threads to 2:

-XX:CICompilerCount=2
-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 is 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, the command will be 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 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 newline separator (\n). The following commands are available:

break

Set a breakpoint when debugging the JVM to stop at the beginning of compilation of the specified method.

compileonly

Exclude all methods from compilation except for the specified method. As an alternative, you can use the -XX:CompileOnly option, which allows to specify several methods.

dontinline

Prevent inlining of the specified method.

exclude

Exclude the specified method from compilation.

help

Print a help message for the -XX:CompileCommand option.

inline

Attempt to inline the specified method.

log

Exclude 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

This command can be used to pass 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

Print generated assembler code after compilation of the specified method.

quiet

Don’t 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 will be 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 are using commands for the JIT compiler to perform on methods, then see the -XX:CompileCommand option.

-XX:CompileOnly=methods

Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods will be 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 is 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:CompileThreshold=invocations

Sets the number of interpreted method invocations before compilation. By default, in the server JVM, the JIT compiler performs 10,000 interpreted method invocations to gather information for efficient compilation. For the client JVM, the default setting is 1,500 invocations. This option is ignored when tiered compilation is enabled; see the option -XX:-TieredCompilation. The following example shows how to set the number of interpreted method invocations to 5,000:

-XX:CompileThreshold=5000

You can completely disable interpretation of Java methods before compilation by specifying the -Xcomp option.

-XX:CompileThresholdScaling=scale

Provides unified control of first compilation. Controls when methods are first compiled for both the tiered and the non-tiered modes of operation. CompileThresholdScaling has an integer value between 0 and +Inf and scales the thresholds corresponding to the current mode of operation (both tiered and non-tiered). 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. Only the Java HotSpot Server VM supports this option.

-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, g or G to indicate gigabytes. The default value is set to 500 KB. 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 compiled methods that should be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. Only compiled methods with the size smaller than the specified size will be inlined. By default, the maximum code size 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:MaxInlineSize=size

Sets the maximum bytecode size (in bytes) of a method to be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. By default, the maximum bytecode size is set to 35 bytes:

-XX:MaxInlineSize=35
-XX:MaxNodeLimit=nodes

Sets the maximum number of nodes to be used during single method compilation. By default, the maximum number of nodes is set to 65,000:

-XX:MaxNodeLimit=65000
-XX:NonNMethodCodeHeapSize=size

Sets the size in bytes of the code segment containing non-method code.

A non-method code segment containing non-method code, such as compiler buffers and the bytecode interpreter. This code type stays in the code cache forever. This flag is only used if —XX:SegmentedCodeCache is enabled.

—XX:NonProfiledCodeHeapSize=size

Sets the size in bytes of the code segment containing non-profiled methods. This flag is only used if —XX:SegmentedCodeCache is enabled.

-XX:MaxTrivialSize=size

Sets the maximum bytecode size (in bytes) of a trivial method to be inlined. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, 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:+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. Only the Java HotSpot Server VM supports this option.

-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 will be hsdis-amd64.dll. This enables 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 only used 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 enables 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 enables you to 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, 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. This option is equivalent to -Xmaxjitcodesize.

-XX:RTMAbortRatio=abort_ratio

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 will be 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 will be deoptimized if 50% of all transactions are aborted.

-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 non-method, profiled method and non-profiled 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:RTMRetryCount=number_of_retries

RTM locking code will be retried, when it is aborted or busy, the number of times specified by this option before falling back to the normal locking mechanism. The default value for this option is 5. The -XX:UseRTMLocking option must be enabled.

-XX:-TieredCompilation

Disables the use of tiered compilation. By default, this option is enabled. Only the Java HotSpot Server VM supports this option.

-XX:+UseAES

Enables hardware-based AES intrinsics for Intel, AMD, and SPARC hardware. Intel Westmere (2010 and newer), AMD Bulldozer (2011 and newer), and SPARC (T4 and newer) are the supported hardware. -XX:+UseAES is used in conjunction with UseAESIntrinsics. Flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.

-XX:+UseAESIntrinsics

Use -XX:+UseAES and -XX:+UseAESIntrinsics flags are enabled by default and are supported only for Java HotSpot Server VM 32-bit and 64-bit. 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 -XX:+UnlockDiagnosticVMOptions. To support UseAES and UseAESIntrinsics flags for 32-bit and 64-bit, use the -server option to choose Java HotSpot Server VM. These flags aren’t supported on Client VM.

-XX:+UseCMoveUnconditionally

Generate 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 only be used on machines with multiple sockets, where it will increase the performance of Java applications that rely on concurrent operations. Only the Java HotSpot Server VM supports this option.

-XX:+UseCountedLoopSafepoints

Keeps safepoints in counted loops. Its default value will be false. 

-XX:+UseFMA

Enables hardware-based FMA intrinsics for hardware where FMA instructions are available (such as, Intel, SPARC, and ARM64). FMA intrinsics are generated for the java.lang.Math.fma(a, b, c) methods which calculate the value of (a * b + c) expressions.

-XX:+UseRTMDeopt

Auto-tunes RTM locking depending on the abort ratio. This ratio is specified by -XX:RTMAbortRatio option. If the number of aborted transactions exceeds the abort ratio, then the method containing the lock will be deoptimized and recompiled with all locks as normal locks. This option is disabled by default. The -XX:+UseRTMLocking option must be enabled.

-XX:+UseRTMLocking

Generate 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 only available for the Java HotSpot Server VM 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, 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 XEND instruction to check if a set of instructions are 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:+UseSHA

Enables hardware-based intrinsics for SHA crypto hash functions for SPARC hardware. UseSHA is used in conjunction with the UseSHA1Intrinsics, UseSHA256Intrinsics, and UseSHA512Intrinsics options.

The UseSHA and UseSHA*Intrinsics flags are enabled by default, and are supported only for Java HotSpot Server VM 64-bit on SPARC T4 and newer.

This feature is only applicable when using the sun.security.provider.Sun provider for SHA operations. Flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.

To disable all hardware-based SHA intrinsics, specify -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 -XX:+UnlockDiagnosticVMOptions.

-XX:+UseSHA256Intrinsics

Enables intrinsics for SHA-224 and SHA-256 crypto hash functions. Flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.

-XX:+UseSHA512Intrinsics

Enables intrinsics for SHA-384 and SHA-512 crypto hash functions. Flags that control intrinsics now require -XX:+UnlockDiagnosticVMOptions.

-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. Only the Java HotSpot Server VM supports this option.

Advanced Serviceability Options for java

These java options provide the ability to gather system information and perform extensive debugging.

-XX:+ExtendedDTraceProbes

Oracle Solaris, Linux, and OS X: Enables additional dtrace tool probes that impact 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 is named java_pidpid.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 identificator):

-XX:HeapDumpPath=./java_pid%p.hprof
  • Oracle Solaris, Linux, and OS X: 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 where log data is written. By default, the file is created in the current working directory, and it is named hotspot.log.

  • Oracle Solaris, Linux, and OS X: 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:

  • Oracle Solaris, Linux, and OS X: 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:

  • Oracle Solaris, Linux, and OS X: Control+Break

  • Windows: Control+C (SIGTERM)

By default, this option is disabled.

Setting this option is equivalent to running the jstack -l command or thejcmd 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:+UnlockDiagnosticVMOptions

Unlocks the options intended for diagnosing the JVM. By default, this option is disabled and diagnostic options aren’t available.

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 isn’t optimized.

-XX:+AlwaysPreTouch

Enables touching of every page on the Java heap during JVM initialization. This gets all pages into the memory before entering the main() method. The option can be used in testing to simulate a long-running system with all virtual memory mapped to physical memory. By default, this option is disabled and all pages are committed as JVM heap space fills.

-XX:+CMSClassUnloadingEnabled

Enables class unloading when using the concurrent mark-sweep (CMS) garbage collector. This option is enabled by default. To disable class unloading for the CMS garbage collector, specify -XX:-CMSClassUnloadingEnabled.

-XX:CMSExpAvgFactor=percent

Sets the percentage of time (0 to 100) used to weight the current sample when computing exponential averages for the concurrent collection statistics. By default, the exponential averages factor is set to 25%. The following example shows how to set the factor to 15%:

-XX:CMSExpAvgFactor=15
-XX:CMSIncrementalDutyCycle=percent

Sets the percentage (0 to 100) of time between minor collections that the CMS collector is allowed to run. If CMSIncrementalPacing is enabled, then this is just the initial value. The default value is 10.

-XX:CMSIncrementalDutyCycleMin=percent

Sets the percentage (0 to 100) that is the lower bound on the duty cycle when CMSIncrementalPacing is enabled. The default value is 0.

-XX:CMSIncrementalDutySafetyFactor=percent

The percentage (0 to 100) used to add conservatism when computing the duty cycle. The default value is 10.

-XX:CMSIncrementalOffset=percent

Sets the percentage (0 to 100) by which the incremental mode duty cycle is shifted to the right within the period between minor collections. The default value is 0

-XX:+CMSIncrementalPacing

Enables automatic pacing. The incremental mode duty cycle is automatically adjusted based on statistics collected while the JVM is running. By default, this option is disabled.

-XX:+CMSScavengeBeforeRemark

Enables scavenging attempts before the CMS remark step. By default, this option is disabled.

-XX:CMSTriggerRatio=percent

Sets the percentage (0 to 100) of the value specified by -XX:MinHeapFreeRatio that is allocated before a CMS collection cycle commences. The default value is set to 80%.

The following example shows how to set the occupancy fraction to 75%:

-XX:CMSTriggerRatio=75
-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 System.gc(). This option is disabled by default, meaning that calls to System.gc() are processed. If processing of calls to System.gc() is disabled, 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 together with the -XX:+UseConcMarkSweepGC and -XX:+UseG1GC options.

-XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses

Enables invoking of concurrent GC by using the System.gc() request and unloading of classes during the concurrent GC cycle. This option is disabled by default and can be enabled only together with the -XX:+UseConcMarkSweepGC option.

-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 two and can range from 1 MB to 32 MB. The goal is to have around 2048 regions based on the minimum Java heap size. The default region size is determined ergonomically based on the heap size.

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 are 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 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.

This setting isn’t available in Java HotSpot VM build 23 or earlier.

-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.

This setting isn’t available in Java HotSpot VM build 23 or earlier.

-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.

This setting isn’t available in Java HotSpot VM build 23 or earlier.

-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.

This setting isn’t available in Java HotSpot VM build 23 or earlier.

-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.

This setting isn’t available in Java HotSpot VM build 23 or earlier.

-XX:G1ReservePercent=percent

Sets the percentage of the heap (0 to 50) that is reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector. When you increase or decrease the percentage, make sure to 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:InitialHeapOccupancyPercent=percent

Sets the Java heap occupancy threshold that triggers a marking cycle. The default occupancy is 45 percent of the entire Java heap.

-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, g or G to indicate gigabytes. The default value is chosen at runtime based on 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 will be 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:InitialSurvivorRatio=ratio

Sets the initial survivor space ratio used by the throughput garbage collector (which is enabled by the -XX:+UseParallelGC and/or -XX:+UseParallelOldGC options). Adaptive sizing is enabled by default with the throughput garbage collector by using the -XX:+UseParallelGC and -XX:+UseParallelOldGC options, and 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), the initial size of the survivor space will be 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 heap occupancy (0 to 100) at which to start a concurrent GC cycle. It is used by garbage collectors that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (for example, the G1 garbage collector).

By default, the initiating value is set to 45%. A value of 0 implies nonstop GC cycles. 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, there is no maximum pause time value.

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, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -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

On Oracle Solaris 7 and Oracle Solaris 8 SPARC platforms, the upper limit for this value is approximately 4,000 MB minus overhead amounts. On Oracle Solaris 2.6 and x86 platforms, the upper limit is approximately 2,000 MB minus overhead amounts. On Linux platforms, the upper limit is approximately 2,000 MB minus overhead amounts.

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 will be shrunk. By default, this value is set to 70%.

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 are 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: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, and 6 for the CMS 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 will trigger a garbage collection the first time it is 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 will be 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 are 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: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, 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 will be performed. If the size is too high, then only full GCs will be performed, which can take a long time to complete. Oracle recommends 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 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 value of the stop-the-world (STW) worker threads. Sets the value of threads to the number of logical processors. The value of threads is the same as the number of logical processors up to a value of 8.

If there are more than eight logical processors, sets the value of threads to approximately 5/8 of the logical processors. This works in most cases except for larger SPARC systems where the value of threads can be approximately 5/16 of the logical processors.

The default value depends on the number of CPUs available to the JVM.

For example, to set the number of threads for parallel 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. Oracle recommends that you do not 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 -XX:-ScavengeBeforeFullGC.

-XX:-ShrinkHeapInSteps

Incrementally reduces the Java heap to the target size, specified by —XX:MaxHeapFreeRatio. This option is enabled by default. If disabled, 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 MaxHeapFreeRatio to keep the Java heap small by reducing the dynamic footprint for embedded applications.

–XX:StringDeduplicationAgeThreshold=threshold

String objects reaching the specified age 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. See the -XX:+PrintTenuringDistribution option.

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, g or G to indicate gigabytes. If this option is set to 0, then the JVM chooses 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:+UseCMSInitiatingOccupancyOnly

Enables the use of the occupancy value as the only criterion for initiating the CMS collector. By default, this option is disabled and other criteria may be used.

-XX:+UseG1GC

Enables the use of the garbage-first (G1) garbage collector. It is a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. It 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 (stable and predictable pause time below 0.5 seconds). By default, this option is enabled and 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 -XX:-UseGCOverheadLimit.

-XX:+UseNUMA

Enables performance optimization of an application on a machine with non-uniform 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 only available 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 collector is chosen automatically based on the configuration of the machine and type of the JVM. If it is enabled, then the -XX:+UseParallelOldGC option is automatically enabled, unless you explicitly disable it.

-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.

-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 collector is chosen automatically based on the configuration of the machine and type of the JVM.

-XX:+UseSHM

Linux only: Enables the JVM to use shared memory to setup 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 -XX:-UseTLAB.

Obsolete java Options

These java options are still accepted but ignored, and a warning is issued when they are used.

-Xusealtsigs / -XX:+UseAltSigs

Oracle Solaris only: Use alternative signals instead of SIGUSR1 and SIGUSR2 for JVM internal signals. Since Solaris 10, two dedicated signals have been made available to the VM and so, since JDK 6, these flags have been documented as having no affect. The flags have now been made obsolete, and their use will generate a warning. In a future release these flags will be removed completely.

Deprecated java Options

These java options are deprecated and might be removed in a future JDK release. They are still accepted and acted upon, but a warning is issued when they are used.

-d32

Oracle Solaris, Linux, and OS X: Runs the application in a 32-bit environment. If a 32-bit environment isn’t installed or isn’t supported, then an error will be reported. By default, the application is run in a 32-bit environment unless a 64-bit system is used.

Note:

The -d32 and -d64 options were added to allow multiple architectures (data model) JDK and JREs to co-exist on the same system. The user could invoke the other data model by using these launcher options. Oracle Solaris was the only platform supporting these options, and the 32-bit JDK/JREs are no longer supported. Therefore these options are obsolete and will be removed in a future release.

-d64

Oracle Solaris, Linux, and OS X: Runs the application in a 64-bit environment. If a 64-bit environment isn’t installed or isn’t supported, then an error will be reported. By default, the application is run in a 32-bit environment unless a 64-bit system is used.

Only the Java HotSpot Server VM supports 64-bit operation and the -server option is implicit with the use of -d64. The -client option is ignored with the use of -d64.

-Xloggc:garbage-collection.log

Sets the file to which verbose GC events information should be redirected for logging. The information written to this file is similar to the output of -verbose:gc with the time elapsed since the first GC event preceding each logged event. The -Xloggc option overrides -verbose:gc if both are given with the same java command.

Example:

-Xlog:gc:garbage-collection.log
-XX:CMSInitiatingOccupancyFraction=percent

Sets the percentage of the old generation occupancy (0 to 100) at which to start a CMS collection cycle. The default value is set to -1. Any negative value (including the default) implies that -XX:CMSTriggerRatio is used to define the value of the initiating occupancy fraction.

The following example shows how to set the occupancy fraction to 20%:

-XX:CMSInitiatingOccupancyFraction=20
-XX:CMSInitiatingPermOccupancyFraction=percent

Sets the percentage of the permanent generation occupancy (0 to 100) at which to start a GC. This option was deprecated in JDK 8 with no replacement.

-XX:+G1PrintHeapRegions

Enables the printing of information about which regions are allocated and which are reclaimed by the G1 collector. By default, this option is disabled. See Enable Logging with the JVM Unified Logging Framework.

-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 is exceeded. This option was deprecated in JDK 8 and superseded by the -XX:MetaspaceSize option.

-XX:+PrintGC

Enables printing of messages at every GC. By default, this option is disabled. If you are using this flag, then see Enable Logging with the JVM Unified Logging Framework. In JDK 9, this option is deprecated.

-XX:+PrintGCApplicationConcurrentTime

Enables printing of how much time elapsed since the last pause (for example, a GC pause). By default, this option is deprecated.

-XX:+PrintGCApplicationStoppedTime

Enables printing of how much time the pause (for example, a GC pause) lasted. By default, this option is deprecated

-XX:+PrintGCDateStamps

Enables printing of a date stamp at every GC. By default, this option is deprecated.

-XX:+PrintGCDetails

Enables printing of detailed messages at every GC. By default, this option is disabled. See Enable Logging with the JVM Unified Logging Framework.

-XX:+PrintGCTaskTimeStamps

Enables printing of time stamps for every individual GC worker thread task. By default, this option is disabled. See Enable Logging with the JVM Unified Logging Framework.

-XX:+PrintGCTimeStamps

Enables printing of time stamps at every GC. By default, this option is disabled. See Enable Logging with the JVM Unified Logging Framework.

-XX:+PrintStringDeduplicationStatistics

Prints detailed deduplication statistics. By default, this option is disabled. See the -XX:+UseStringDeduplication option.

-XX:+PrintTenuringDistribution

Enables printing of tenuring age information. The following is an example of the output:

Desired survivor size 48286924 bytes, new threshold 10 (max 10)
- age 1: 28992024 bytes, 28992024 total
- age 2: 1366864 bytes, 30358888 total
- age 3: 1425912 bytes, 31784800 total
...

Age 1 objects are the youngest survivors (they were created after the previous scavenge, survived the latest scavenge, and moved from eden to survivor space). Age 2 objects have survived two scavenges (during the second scavenge they were copied from one survivor space to the next). This pattern is repeated for all objects in the output.

In the preceding example, 28,992,24 bytes survived one scavenge and were copied from eden to survivor space, 1,366,864 bytes are occupied by age 2 objects, etc. The third value in each row is the cumulative size of objects of age nor less.

By default, this option is disabled.

-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:+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 are 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:+TraceClassUnloading

Enables tracing of classes as they are unloaded. By default, this option is disabled and classes aren’t traced.

The replacement Unified Logging syntax is -Xlog:class+unload=level. See Enable Logging with the JVM Unified Logging Framework.

Use level=info for regular information, and level=trace for additional information. In Unified Logging syntax, -verbose:class equals -Xlog:class+unload=info,class+unload=info .

-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.

-XX:+UseConcMarkSweepGC

Enables the use of the CMS garbage collector for the old generation. CMS is an alternative to the default garbage collector (G1) which also focuses on meeting application latency requirements. By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. In JDK 9, the CMS garbage collector is deprecated.

-XX:+UseParNewGC

Enables the use of parallel threads for collection in the young generation. By default, this option is disabled. It is automatically enabled when you set the -XX:+UseConcMarkSweepGC option. Using the -XX:+UseParNewGC option without the -XX:+UseConcMarkSweepGC option was deprecated in JDK 8. Starting with JDK 9, all uses of -XX:+UseParNewGC option are deprecated. Using the option without -XX:+UseConcMarkSweepGC isn’t possible.

-XX:+UseSplitVerifier

Enables splitting the verification process. By default, this option was enabled in the previous releases, and verification was split into two phases: type referencing (performed by the compiler) and type checking (performed by the JVM runtime). Verification is now split by default without a way to disable it.

Removed java Options

These java options were removed in JDK 9 and using them results in an error of
Unrecognized VM option XXX
-Xincgc

Enabled incremental garbage collection. This option and the GC mode are removed in JDK 9.

-Xmaxjitcodesize=size

Specified 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, 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:

-Xmaxjitcodesize=240m

This option is equivalent to -XX:ReservedCodeCacheSize.

-Xrunlibname

Loaded the specified debugging/profiling library. This option was superseded by the -agentlib option.

-XX:CMSIncrementalDutyCycle=percent

Set the percentage of time (0 to 100) between minor collections that the concurrent collector is allowed to run.

-XX:CMSIncrementalDutyCycleMin=percent

Set the percentage of time (0 to 100) between minor collections that is the lower bound for the duty cycle when -XX:+CMSIncrementalPacing is enabled. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option. The option was removed in JDK 9, because the entire incremental mode is removed.

-XX:+CMSIncrementalMode

Enabled incremental mode. Note that the CMS collector must also be enabled (with -XX:+UseConcMarkSweepGC) for this option to work. The option was removed in JDK 9, because the entire incremental mode is removed.

-XX:CMSIncrementalOffset=percent

Set the percentage of time (0 to 100) by which the incremental mode duty cycle is shifted to the right within the period between minor collections.

-XX:+CMSIncrementalPacing

Enabled automatic adjustment of the incremental mode duty cycle based on statistics collected while the JVM was running. This option was deprecated with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option. The option was removed, because the entire incremental mode is removed.

-XX:CMSIncrementalSafetyFactor=percent

Set the percentage of time (0 to 100) used to add conservatism when computing the duty cycle. This option was deprecated in JDK 8 with no replacement, following the deprecation of the -XX:+CMSIncrementalMode option. The option was removed, because the entire incremental mode is removed.

-XX:CodeCacheMinimumFreeSpace=size

Set the minimum free space (in bytes) required for compilation. Appended the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. When less than the minimum free space remained, compiling stopped. By default, this option was set to 500 KB.

java Command-Line Argument Files

You can shorten or simplify the java command by using @argument files to specify a text file that contains arguments, such as options and class names, passed to the java command. This enables 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 -Xdisable-@files option. You can use the -Xdisable-@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 only contain ASCII characters or characters in system default encoding that is 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 present within an argument file.

  • Use white space or new line characters to separate arguments included in the file.

  • White space includes white space character and \t, \n, \r, and \f.

  • Any option that contains spaces, such as a path component, must be quoted using quotation ('"') characters in its entirety.

  • A quoted string may contain the characters \n, \r, \t, and \f. They will be 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 the location of the argument file.

  • Use # in the argument file to identify comments. All characters following the# are ignored until the end of line.

  • Additional @ 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 will be 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 will be 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.

  • Wild cards (*) 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

will be interpreted as:

-cp lib/cool/app/jars  

Example of a Backslash Character Escaped with Another Back-Slash Character in an Argument File

To output

-cp c:\Program Files (x86)\Java\jre\lib\ext;c:\Program Files\Java\jre9\lib\ext

it 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"

will 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” 

will be 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 are 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

Enable Logging with the JVM Unified Logging Framework

You use -Xlog option to configure or enable logging with the Java Virtual Machine (JVM) unified logging framework.

Synopsis

-Xlog[:[what][:[output][:[decorators][:output-options [,...]]]]]
what

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 will be matched. See -Xlog Tags and Levels.

output

Sets the type of output. Omitting the output type defaults to stdout. See -Xlog Outputs.

decorators

Configures each 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 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, 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]

Multiple arguments are applied in the order 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 * in a tag set definition denotes a wildcard tag match. Matching with 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 -Xlog 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 the warning or error regardless of what tags the message is associated with. The default configuration is equivalent to entering the following command line:

-Xlog:all=warning:stdout:uptime,level,tags

Controlling Logging at Runtime

Logging can also be controlled at runtime 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 runtime.

-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, compiler, or threads). 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:

The following are the available log tags. Specifying all instead of a tag combination matches all tag combinations.

  • add

  • age

  • alloc

  • annotation

  • aot

  • arguments

  • attach

  • barrier

  • biasedlocking

  • blocks

  • bot

  • breakpoint

  • bytecode

  • census

  • class

  • classhisto

  • cleanup

  • compaction

  • comparator

  • constraints

  • constantpool

  • coops

  • cpu

  • cset

  • data

  • defaultmethods

  • dump

  • ergo

  • event

  • exceptions

  • exit

  • fingerprint

  • freelist

  • gc

  • hashtables

  • heap

  • humongous

  • ihop

  • iklass

  • init

  • itables

  • jfr

  • jni

  • jvmti

  • liveness

  • load

  • loader

  • logging

  • mark

  • marking

  • metadata

  • metaspace

  • method

  • mmu

  • modules

  • monitorinflation

  • monitormismatch

  • nmethod

  • normalize

  • objecttagging

  • obsolete

  • oopmap

  • os

  • pagesize

  • parser

  • patch

  • path

  • phases

  • plab

  • preorder

  • promotion

  • protectiondomain

  • purge

  • redefine

  • ref

  • refine

  • region

  • remset

  • resolve

  • safepoint

  • scavenge

  • scrub

  • setting

  • stackmap

  • stacktrace

  • stackwalk

  • start

  • startuptime

  • state

  • stats

  • stringdedup

  • stringtable

  • subclass

  • survivor

  • sweep

  • system

  • task

  • thread

  • time

  • timer

  • tlab

  • unload

  • update

  • verification

  • verify

  • vmoperation

  • vtables

  • workgang

-Xlog Outputs

The -Xlog option supports the following types of outputs:

  • stdout — Outputs to stdout

  • stderr — Outputs to stderr

  • file=filename — Outputs to text file(s).

When using file=filename, specifying %p and/or %t in the filename will expand 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 is 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 is 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 runtime. 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

Table 2-1 Mapping Legacy Garbage Collection Logging Flags to the Xlog Configuration

Legacy Garbage Collection (GC) Flag Xlog Configuration Comment
G1PrintHeapRegions -Xlog:gc+region=trace NA
GCLogFileSize

No configuration available

Log rotation is handled by the framework.
NumberOfGCLogFiles NA Log rotation is handled by the framework.
PrintAdaptiveSizePolicy -Xlog: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 NA
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 NA GC cause is now always logged.
PrintGCDateStamps NA

Date stamps are logged by the framework.

PrintGCDetails -Xlog:gc* NA
PrintGCID NA

GC ID Is now always logged.

PrintGCTaskTimeStamps -Xlog:task*=debug NA
PrintGCTimeStamps NA

Time stamps are logged by the framework.

PrintHeapAtGC -Xlog:gc+heap=trace NA
PrintReferenceGC -Xlog:ref*=debug

Note that in the old logging, PrintReferenceGC only had any effect if PrintGCDetails was also enabled.

PrintStringDeduplicationStatistics -Xlog:stringdedup*=debug NA
PrintTenuringDistribution -Xlog: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 NA

What was logged for PrintTenuringDistribution.

Convert Runtime Logging Flags to Xlog

Table 2-2 Mapping Runtime Logging Flags to the Xlog Configuration

Legacy Runtime Flag Xlog Configuration Comment
TraceExceptions -Xlog:exceptions=info

NA

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

NA

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

NA

TraceClassPaths -Xlog:class+path=info

NA

TraceClassResolution -Xlog:class+resolve=debug

NA

TraceClassInitialization -Xlog:class+init=info

NA

TraceLoaderConstraints -Xlog:class+loader+constraints=info

NA

TraceClassLoaderData -Xlog:class+loader+data=level

Use level=debug for regular information or level=trace for additional information.

TraceSafepointCleanupTime -Xlog:safepoint+cleanup=info

NA

TraceSafepoint -Xlog:safepoint=debug

NA

TraceMonitorInflation -Xlog:monitorinflation=debug

NA

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 infolevel 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

Log messages tagged either with gc or safepoint tags, both using info level, to stdout, with default decorations. Messages tagged with both gc and safepointwill not be logged.

-Xlog:gc+ref=debug

Log messages tagged with both gc and ref tags, using debug level, to stdout, with default decorations. Messages tagged only with one of the two tags will not be logged.

-Xlog:gc=debug:file=gc.txt:none

Log messages tagged with gc tag using 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

Log messages tagged with gc tag using trace level to a rotating fileset with 5 files with size 1MB with base name gctrace.txt and use decorations uptimemillis and pid.

The default configuration for all other messages at levelwarning is still in effect.

-Xlog:gc::uptime,tid

Log messages tagged with gc tag using default 'info' level to default outputstdout and use decorations uptime and tid. The default configuration for all other messages at levelwarning is still in effect.

-Xlog:gc*=info,safepoint*=off

Log messages tagged with at least gc using info level, but turn off logging of messages tagged with safepoint. Messages tagged with both gc and safepoint will not be logged.

-Xlog:disable -Xlog:safepoint=trace:safepointtrace.txt

Turn off all logging, including warnings and errors, and then enable messages tagged with safepointusing tracelevel to file safepointtrace.txt. The default configuration doesn’t apply, since 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 tag using debug level to stdout. The default configuration for all other messages at level warning is still in effect

-Xlog:gc+meta*=trace,class*=off:file=gcmetatrace.txt

Logs messages tagged with at least gc and meta tag usingtrace level to file metatrace.txt but turn off all messages tagged with class. Messages tagged with gc, meta andclass aren’t be logged asclass* 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 gc and meta tag using 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 and class and heap tag usingtrace level to stdout but only log messages tagged with meta with level. The default configuration for all other messages at level warning are 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 are 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 the 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 via jcmd tool). The JVM will be 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, the JVM will exit 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. TLB is a scarce system resource. A TLB miss can be costly as 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. There will be less pressure on 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.

Large Pages Support

Oracle Solaris, Linux, and Windows Server 2003 support large pages.

Large Pages Support for Oracle Solaris

Oracle Solaris 9 and later include Multiple Page Size Support (MPSS). No additional configuration is necessary. See  Features and Benefits - Scalability.

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 are 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 will make all the memory sharable:

    # echo 4294967295 > /proc/sys/kernel/shmmax
    
  2. If you are 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:

  • Note that the values contained in /proc will reset 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 Server 2003

Only Windows Server 2003 supports large pages. To use this feature, the administrator must first assign additional privilege to the user who will be 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 is the administrator who will be running the application, as administrators by default don’t have the privilege to lock pages in memory.

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 runtime options to achieve higher throughput performance for your application:

java -d64 -server -XX:+UseParallelGC -XX:+AggressiveOpts -XX:+UseLargePages -Xmn10g  -Xms26g -Xmx26g

Tuning for Lower Response Time

Use the following commands and advanced runtime options to achieve lower response times for your application:

java -d64 -XX:+UseG1GC -Xms26g Xmx26g -XX:MaxGCPauseMillis=500 -XX:+PrintGCTimeStamp 

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