java (original) (raw)

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

Synopsis

To execute a class:

java [options] mainclass [args...]

To execute a JAR file:

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

To execute the main class in a module:

java [options] [--module-path modulepath] --module module[/mainclass] [args...]

options

Specifies command-line options separated by spaces. See Overview of Java Options for a description of available options.

mainclass

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

jarfile

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

modulepath

Specifies the path to a semicolon-separated (;) list of directories in which each directory is a directory of modules Used only with the --module-path option. See Standard Options for Java.

module[/mainclass]

Specifies the name of the initial module to resolve and, if it isn’t specified by the module, then specifies the name of the mainclass to execute. Used only with the -m or --module option. See Standard Options for Java.

args

Specifies the arguments passed to the main method separated by spaces.

Note:

Arguments following the main class, -jar jarfile , -m or --module module/mainclass are passed as the arguments to the main class.

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)

In JDK 9, a new launcher environment variable, JDK_JAVA_OPTIONS, has been introduced that prepends its content to the actual command line of the java launcher. See Using the JDK_JAVA_OPTIONS Launcher Environment Variable.

The java command can be used to launch a JavaFX application by loading a class that either has a main() method or that extends the 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, then its argument is the name of the JAR file containing class and resource files for the application. The startup class must be indicated by the Main-Class manifest header in its manifest file.

Arguments after the class file name or the JAR file name are passed to the main() method.

Windows: The javaw command is identical to java, except that with javaw there’s no associated console window. Use javaw when you don’t want a command prompt window to appear. The javaw launcher will, however, display a dialog box with error information if a launch fails.

Using the JDK_JAVA_OPTIONS Launcher Environment Variable

JDK_JAVA_OPTIONS prepends its content to the options parsed from the command line. The content of the JDK_JAVA_OPTIONS environment variable is a list of arguments separated by white-space characters (as determined by isspace()). These are prepended to the command line arguments passed to java launcher. The encoding requirement for the environment variable is the same as the java command line on the system. JDK_JAVA_OPTIONS environment variable content is treated in the same manner as that specified in the command line.

Single (') or double (") quotes can be used to enclose arguments that contain whitespace characters. All content between the open quote and the first matching close quote are preserved by simply removing the pair of quotes. In case a matching quote is not found, the launcher will abort with an error message. @files are supported as they are specified in the command line. However, as in @files, use of a wildcard is not supported. In order to mitigate potential misuse of JDK_JAVA_OPTIONS behavior, options that specify the main class (such as -jar) or cause the java launcher to exit without executing the main class (such as -h) are disallowed in the environment variable. If any of these options appear in the environment variable, the launcher will abort with an error message. When JDK_JAVA_OPTIONS is set, the launcher prints a message to stderr as a reminder.

Example:

export JDK_JAVA_OPTIONS='-g @file1 -Dprop=value @file2 -Dws.prop="white spaces”' $ java -Xint @file3

is equivalent to the command line:

java -g @file1 -Dprop=value @file2 -Dws.prop="white spaces" -Xint @file3

Overview of Java Options

The java command supports a wide range of options in the following categories:

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’s disabled by default or disable a feature that’s enabled by default. Such options don’t require a parameter. Boolean -XX options are enabled using the plus sign (-XX:+OptionName) and disabled using the minus sign (-XX:-OptionName).

For options that require an argument, the argument may be separated from the option name by a space, a colon (:), or an equal sign (=), or the argument may directly follow the option (the exact syntax differs for each option). If you’re expected to specify the size in bytes, then you can use no suffix, or use the suffix k or K for kilobytes (KB), m or M for megabytes (MB), or g or G for gigabytes (GB). For example, to set the size to 8 GB, you can specify either 8g, 8192m, 8388608k, or 8589934592 as the argument. If you are expected to specify the percentage, then use a number from 0 to 1. For example, specify 0.25 for 25%.

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

Standard Options for Java

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

Note:

To specify an argument for a long option, you can use either --name=value or --name value.

-agentlib:libname[=options]

Loads the specified native agent library. After the library name, a comma-separated list of options specific to the library can be used.

-agentpath:pathname[=options]

Loads the native agent library specified by the absolute path name. This option is equivalent to -agentlib but uses the full path and file name of the library.

--class-path classpath, -classpath classpath , or -cp classpath

A semicolon (;) separated list of directories, JAR archives, and ZIP archives to search for class files.

Specifying classpath overrides any setting of the CLASSPATH environment variable. If the class path option isn’t used and classpath isn’t set, then the user class path consists of the current directory (.).

As a special convenience, a class path element that contains a base name of an asterisk (*) is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR . A Java program can’t tell the difference between the two invocations. For example, if the directory mydir contains a.jar and b.JAR, then the class path element mydir/* is expanded to A.jar:b.JAR, except that the order of JAR files is unspecified. All .jar files in the specified directory, even hidden ones, are included in the list. A class path entry consisting of an asterisk (*) expands to a list of all the jar files in the current directory. The CLASSPATH environment variable, where defined, is similarly expanded. Any class path wildcard expansion that occurs before the Java VM is started. Java programs never see wildcards that aren’t expanded except by querying the environment, such as by calling System.getenv("CLASSPATH").

--disable-@files

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

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

Searches for directories from a semicolon-separated (;) list of directories. Each directory is a directory of modules.

--upgrade-module-path modulepath...

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

--add-modules module[,module...]

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

--list-modules

Lists the observable modules and then exits.

-d module or --describe-module module

Describes a specified module and then exits.

--dry-run

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

--validate-modules

Validates all modules and exit. This option is helpful for finding conflicts and other errors with modules on the module path.

-Dproperty=value

Sets a system property value. The property variable is a string with no spaces that represents the name of the property. The value variable is a string that represents the value of the property. If value is a string with spaces, then enclose it in quotation marks (for example -Dfoo="foo bar").

-disableassertions[:[packagename]...|:classname] or -da[:[packagename]...|:classname]

Disables assertions. By default, assertions are disabled in all packages and classes. With no arguments, -disableassertions (-da) disables assertions in all packages and classes. With the packagename argument ending in ..., the switch disables assertions in the specified package and any subpackages. If the argument is simply ..., then the switch disables assertions in the unnamed package in the current working directory. With the classname argument, the switch disables assertions in the specified class.

The -disableassertions (-da) option applies to all class loaders and to system classes (which don’t have a class loader). There’s one exception to this rule: If the option is provided with no arguments, then it doesn’t apply to system classes. This makes it easy to disable assertions in all classes except for system classes. The -disablesystemassertions option enables you to disable assertions in all system classes. To explicitly enable assertions in specific packages or classes, use the -enableassertions (-ea) option. Both options can be used at the same time. For example, to run the MyClass application with assertions enabled in the package com.wombat.fruitbat (and any subpackages) but disabled in the class com.wombat.fruitbat.Brickbat, use the following command:

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass

-disablesystemassertions or -dsa

Disables assertions in all system classes.

-enableassertions[:[packagename]...|:classname] or -ea[:[packagename]...|:classname]

Enables assertions. By default, assertions are disabled in all packages and classes. With no arguments, -enableassertions (-ea) enables assertions in all packages and classes. With the packagename argument ending in ..., the switch enables assertions in the specified package and any subpackages. If the argument is simply ..., then the switch enables assertions in the unnamed package in the current working directory. With the classname argument, the switch enables assertions in the specified class.

The -enableassertions (-ea) option applies to all class loaders and to system classes (which don’t have a class loader). There’s one exception to this rule: If the option is provided with no arguments, then it doesn’t apply to system classes. This makes it easy to enable assertions in all classes except for system classes. The -enablesystemassertions option provides a separate switch to enable assertions in all system classes. To explicitly disable assertions in specific packages or classes, use the -disableassertions (-da) option. If a single command contains multiple instances of these switches, then they’re processed in order, before loading any classes. For example, to run the MyClass application with assertions enabled only in the package com.wombat.fruitbat (and any subpackages) but disabled in the class com.wombat.fruitbat.Brickbat, use the following command:

java -ea:com.wombat.fruitbat... -da:com.wombat.fruitbat.Brickbat MyClass

-enablesystemassertions or -esa

Enables assertions in all system classes.

-help 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’re using JAR files, then see: jar

-javaagent:jarpath[=options]

Loads the specified Java programming language agent.

--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.

--show-module-resolution

Shows module resolution output during startup.

-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 file name, such as image.ext, should always be passed as the argument to the -splash option. The most appropriate scaled image provided is picked up automatically.

For example, to show the splash.gif file from the images directory when starting your application, use the following option:

-splash:images/splash.gif

-verbose:class

Displays information about each loaded class.

-verbose:gc

Displays information about each garbage collection (GC) event.

-verbose:jni

Displays information about the use of native methods and other Java Native Interface (JNI) activity.

-verbose:module

Displays information about the modules in use.

--version 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

Specifies one or more argument files prefixed by @ used by the java command. It isn’t uncommon for the java command line to be very long because of the .jar files needed in the classpath. The @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 because otherwise, they would be specified on the command line until the -Xdisable-@files option was encountered.

The argument files can also contain the main class name and all options. If an argument file contains all of the options required by the java command, then the command line could simply be:

java @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:directories| zip|JAR files

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

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 terminates 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.

-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:option

Configure or enable logging with the Java Virtual Machine (JVM) unified logging framework. See Enable Logging with the JVM Unified Logging Framework.

-Xmixed

Executes all bytecode by the interpreter except for hot methods, which are compiled to native code.

-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, or g or G to indicate gigabytes. The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too small, then a lot of minor garbage collections are performed. If the size is too large, then only full garbage collections are performed, which can take a long time to complete. 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 is set as the sum of the sizes allocated for the old generation and the young generation. The initial size of the heap for the young generation can be set using the -Xmn option or the -XX:NewSize option.

-Xmx size

Specifies the maximum size (in bytes) of the 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, or g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -Xms and -Xmx are often set to the same value. The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

-Xmx83886080 -Xmx81920k -Xmx80m

The -Xmx option is equivalent to -XX:MaxHeapSize.

-Xnoclassgc

Disables garbage collection (GC) of classes. This can save some GC time, which shortens interruptions during the application run. When you specify -Xnoclassgc at startup, the class objects in the application are left untouched during GC and are always be considered live. This can result in more memory being permanently occupied which, if not used carefully, throws an out-of-memory exception.

-Xprof

Profiles the running program and sends profiling data to standard output. This option is provided as a utility that’s 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 the orderly shutdown of a Java application by running user cleanup code (such as closing database connections) at shutdown, even if the JVM terminates abruptly.

There are two consequences of specifying -Xrs:

User code is responsible for causing shutdown hooks to run, for example, by calling the System.exit() when the JVM is to be terminated.

-Xshare:mode

Sets the class data sharing (CDS) mode.

Possible mode arguments for this option include the following:

auto

Uses CDS if possible. This is the default value for Java HotSpot 32-Bit Client VM.

on

Requires the use of CDS. This option prints an error message and exits if class data sharing can’t be used.

off

Instructs not to use CDS.

-XshowSettings

Shows all settings and then continues.

-XshowSettings:category

Shows settings and continues. Possible category arguments for this option include the following:

all

Shows all categories of settings. This is the default value.

locale

Shows settings related to locale.

properties

Shows settings related to system properties.

vm

Shows the settings of the JVM.

-Xss size

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

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 because 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

Verifies 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.

--illegal-access=parameter

Note:

This option is a new option in JDK 9 and may not be available in future JDK versions.

When present at run time, --illegal-access= takes a keyword parameter to specify a mode of operation:

Note:

Illegal-access operations to internal APIs from code on the class path are allowed by default in JDK 9.

The default mode, --illegal-access=permit, is intended to make you aware of code on the class path that reflectively accesses any JDK-internal APIs at least once. To learn about all such accesses, you can use the warn or the debug modes. For each library or framework on the class path that requires illegal access, you have two options:

If you must continue to use a component that requires illegal access, then you can eliminate the warning messages by using one or more --add-opens options to open only those internal packages to which access is required.

To verify that your application is ready for a future version of the JDK, run it with --illegal-access=deny along with any necessary --add-opens options. Any remaining illegal-access errors will most likely be due to static references from compiled code to JDK-internal APIs. You can identify those by running the jdeps tool with the --jdk-internals option. For performance reasons, JDK 9 does not issue warnings for illegal static-access operations.

--limit-modules module[,module...]

Specifies the limit of the universe of observable modules.

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

Overrides or augments a module with classes and resources in JAR files or directories.

--disable-@files

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

Extra Options for Mac OS X

The following extra options are Mac OS X specific.

-XstartOnFirstThread

Runs the main() method on the first (AppKit) thread.

-Xdock:name=application name

Overrides the default application name displayed in dock.

-Xdock:icon=path to icon file

Overrides the default icon displayed in dock.

Advanced Runtime Options for Java

These java options control the runtime 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 exists and isn't empty:

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 are internally represented and stored as single-byte-per-character Strings using ISO-8859-1 / Latin-1 encoding. This reduces, by 50%, the amount of space required for Strings containing only single-byte characters. For Java Strings containing at least one multibyte character: these are represented and stored as 2 bytes per character using UTF-16 encoding. Disabling the Compact Strings feature forces the use of UTF-16 encoding as the internal representation for all Java Strings.

Cases where it may be beneficial to disable Compact Strings include the following:

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

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:

-XX:+FailOverToOldVerifier

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

-XX:+FlightRecorder

.Enables the use of the Java Flight Recorder (JFR) during the runtime of the application. This is a commercial feature that requires that you also specify the -XX:+UnlockCommercialFeatures option as follows:

java -XX:+UnlockCommercialFeatures -XX:+FlightRecorder

Note:

The -XX:+FlightRecorder option is no longer required to use JFR. This was a change made in JDK 8u40.

-XX:FlightRecorderOptions=parameter=value

Sets the parameters that control the behavior of JFR. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option. The -XX:+FlightRecorder option is no longer required to use JFR. This was a change made in JDK 8u40.

The following list contains all available JFR parameters:

globalbuffersize=size

Specifies the total amount of primary memory (in bytes) used for data retention. Append k or K, to specify the size in KB, m or M to specify the size in MB, or g or G to specify the size in GB. By default, the size is set to 462848 bytes.

loglevel={quiet|error|warning|info|debug|trace}

Specify the amount of data written to the log file by JFR. By default, it’s set to info.

maxchunksize=size

Specifies the maximum size (in bytes) of the data chunks in a recording. Append k or K, to specify the size in KB, or m or M to specify the size in MB, or g or G to specify the size in GB. By default, the maximum size of data chunks is set to 12 MB.

memorysize=size

Determines how much buffer memory should be used.

repository=path

Specifies the repository (a directory) for temporary disk storage. By default, the system's temporary directory is used.

samplethreads={true|false}

Specifies whether thread sampling is enabled. Thread sampling occurs only if the sampling event is enabled along with this parameter. By default, this parameter is enabled.

stackdepth=depth

Stack depth for stack traces by JFR. By default, the depth is set to 64 method calls. The maximum is 2048, and the minimum is 1.

threadbuffersize=size

Specifies the per-thread local buffer size (in bytes). Append k or K, to specify the size in KB, or m or M to specify the size in MB, g or G to specify the size in GB. Higher values for this parameter allow more data gathering without contention to flush it to the global storage. It can increase an application footprint in a thread-rich environment. By default, the local buffer size is set to 5 KB.

transform={true|false}

Specifies if event classes should be retransformed using JVMTI. If false, instrumentation will be added when event classes are loaded. By default it is true.

You can specify values for multiple parameters by separating them with a comma.

-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 the Java heap. The size argument must be a power of 2 (2, 4, 8, 16, and so on). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for large pages automatically. See Large Pages.

The following example describes how to set the large page size to 4 megabytes (MB):

-XX:LargePageSizeInBytes=4m

-XX:MaxDirectMemorySize=size

Sets the maximum total size (in bytes) of the java.nio package, direct-buffer allocations. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. By default, the size is set to 0, meaning that the JVM chooses the size for NIO direct-buffer allocations automatically.

The following examples illustrate how to set the NIO size to 1024 KB in different units:

-XX:MaxDirectMemorySize=1m -XX:MaxDirectMemorySize=1024k -XX:MaxDirectMemorySize=1048576

-XX:-MaxFDLimit

Disables the attempt to set the soft limit for the number of open file descriptors to the hard limit. By default, this option is enabled on all platforms, but is ignored on Windows. The only time that you may need to disable this is on Mac OS, where its use imposes a maximum of 10240, which is lower than the actual system maximum.

-XX:MaxGCPauseMillis=200

Sets a target value for the 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

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

summary

Tracks memory usage only by JVM subsystems, such as Java heap, class, code, and thread.

detail

In addition to tracking memory usage by JVM subsystems, track memory usage by individual CallSite, individual virtual memory region and its committed regions.

-XX:ObjectAlignmentInBytes=alignment

Sets the memory alignment of Java objects (in bytes). By default, the value is set to 8 bytes. The specified value should be a power of 2, and must be within the range of 8 and 256 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes.

The heap size limit in bytes is calculated as:

4GB * ObjectAlignmentInBytes

Note:

As the alignment value increases, the unused space between objects also increases. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes.

-XX:OnError=string

Sets a custom command or a series of semicolon-separated commands to run when an irrecoverable error occurs. If the string contains spaces, then it must be enclosed in quotation marks.

-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 8 logical processors, then this option 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 that you ran. Use thejstat command to display the performance data contained in this file as follows:

jstat -class file:///path/hsperfdata_pid

jstat -gc file:///path/hsperfdata_pid

-XX:+PrintCommandLineFlags

Enables printing of ergonomically selected JVM flags that appeared on the command line. It can be useful to know the ergonomic values set by the JVM, such as the heap space size and the selected garbage collector. By default, this option is disabled and flags aren’t printed.

-XX:+PreserveFramePointer

Selects between using the RBP register as a general purpose register (-XX:-PreserveFramePointer) and using the RBP register to hold the frame pointer of the currently executing method (-XX:+PreserveFramePointer). If the frame pointer is available, then external profiling tools (for example, Linux perf) can construct more accurate stack traces.

-XX:+PrintNMTStatistics

Enables printing of collected native memory tracking data at JVM exit when native memory tracking is enabled (see -XX:NativeMemoryTracking). By default, this option is disabled and native memory tracking data isn’t printed.

-XX:+RelaxAccessControlCheck

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

-XX:+ResourceManagement

Enables the use of Resource Management during the runtime of the application.

This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option as follows:

java -XX:+UnlockCommercialFeatures -XX:+ResourceManagement

-XX:ResourceManagementSampleInterval=value in milliseconds

Sets the parameter that controls the sampling interval for Resource Management measurements, in milliseconds.

This option can be used only when Resource Management is enabled (that is, the -XX:+ResourceManagement option is specified).

-XX:SharedArchiveFile=path

Specifies the path and name of the class data sharing (CDS) archive file

See Application Class Data Sharing.

-XX:SharedArchiveConfigFile=shared_config_file

Specifies additional shared data added to the archive file.

-XX:SharedClassListFile=file_name

Specifies the text file that contains the names of the class files to store in the class data sharing (CDS) archive. This file contains the full name of one class file per line, except slashes (/) replace dots (.). For example, to specify the classes java.lang.Object and hello.Main, create a text file that contains the following two lines:

java/lang/Object hello/Main

The class files that you specify in this text file should include the classes that are commonly used by the application. They may include any classes from the application, extension, or bootstrap class paths.

See Application Class Data Sharing.

-XX:+ShowMessageBoxOnError

Enables the display of a dialog box when the JVM experiences an irrecoverable error. This prevents the JVM from exiting and keeps the process active so that you can attach a debugger to it to investigate the cause of the error. By default, this option is disabled.

-XX:StartFlightRecording=parameter=value

Starts a JFR recording for the Java application. This is a commercial feature that works in conjunction with the -XX:+UnlockCommercialFeatures option. This option is equivalent to the JFR.start diagnostic command that starts a recording during runtime. You can set the following parameters when starting a JFR recording:

delay=time

Specifies the delay between the Java application launch time and the start of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 10m means 10 minutes). By default, there’s no delay, and this parameter is set to 0.

duration=time

Specifies the duration of the recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 5h means 5 hours). By default, the duration isn’t limited, and this parameter is set to 0.

filename=path

Specifies the path and name of the JFR recording log file.

name=identifier

Takes both the name and the identifier of a recording.

maxage=time

Specifies the maximum age of disk data to keep for the default recording. Append s to specify the time in seconds, m for minutes, h for hours, or d for days (for example, specifying 30s means 30 seconds). By default, the maximum age is set to 15 minutes (15m).

maxsize=size

Specifies the maximum size (in bytes) of disk data to keep for the default recording. Append k or K, to specify the size in KB, m or M to specify the size in MB, or g or G to specify the size in GB. By default, the maximum size of disk data isn’t limited, and this parameter is set to 0.

settings=path

Specifies the path and name of the event settings file (of type JFC). By default, the default.jfc file is used, which is located in JAVA_HOME/jre/lib/jfr.

You can specify values for multiple parameters by separating them with a comma.

-XX:ThreadStackSize=size

Sets the Java thread stack size (in kilobytes). Use of a scaling suffix, such as k, results in the scaling of the kilobytes value so that -XX:ThreadStackSize=1k sets the Java thread stack size to 1024*1024 bytes or 1 megabyte. The default value depends on the platform:

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:+UnlockCommercialFeatures

Enables the use of commercial features. Commercial features are included with Oracle Java SE Advanced or Oracle Java SE Suite packages, as defined in theOracle Java SE and Oracle Java Embedded Products page.

By default, this option is disabled and the JVM runs without the commercial features. After they're enabled for a JVM process, it isn’t possible to disable their use for that process.

-XX:+UseAppCDS

Enables application class data sharing (AppCDS). To use AppCDS, you must also specify values for the options -XX:SharedClassListFile and -XX:SharedArchiveFile during both CDS dump time (see the option -Xshare:dump) and application run time.

This is a commercial feature that requires you to also specify the -XX:+UnlockCommercialFeatures option. This is also an experimental feature; it may change in future releases.

See Application Class Data Sharing.

-XX:-UseBiasedLocking

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

By default, this option is 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 of less than 32 GB. This option works only for 64-bit JVMs.

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

XX:+UseGCLogRotation

Handles large log files. This option must be used with -Xloggc:filename.

-XX:NumberOfGClogFiles=number of files

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

-XX:GCLogFileSize=number

Handles 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’s enabled. (It’s 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, or g or G to indicate gigabytes. The default value is set to -1.

The following example shows how to set the prefetch distance to 1024 bytes:

-XX:AllocatePrefetchDistance=1024

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’s also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:

-XX:CompileCommand=exclude,java.lang.String::indexOf

If the method is specified without the signature, then the command isapplied to all methods with the specified name. However, you can also specify the signature of the method in the class file format. In this case, you should enclose the arguments in quotation marks, because otherwise the shell treats the semicolon as a command end. For example, if you want to exclude only the indexOf(String) method of the String class from being compiled, use the following:

-XX:CompileCommand="exclude,java/lang/String.indexOf,(Ljava/lang/String;)I"

You can also use the asterisk (*) as a wildcard for class and method names. For example, to exclude all indexOf() methods in all classes from being compiled, use the following:

-XX:CompileCommand=exclude,*.indexOf

The commas and periods are aliases for spaces, making it easier to pass compiler commands through a shell. You can pass arguments to -XX:CompileCommand using spaces as separators by enclosing the argument in quotation marks:

-XX:CompileCommand="exclude java/lang/String indexOf"

Note that after parsing the commands passed on the command line using the -XX:CompileCommand options, the JIT compiler then reads commands from the .hotspot_compiler file. You can add commands to this file or specify a different file using the -XX:CompileCommandFile option.

To add several commands, either specify the -XX:CompileCommand option multiple times, or separate each argument with the new line separator (\n). The following commands are available:

break

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

compileonly

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

dontinline

Prevents inlining of the specified method.

exclude

Excludes the specified method from compilation.

help

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

inline

Attempts to inline the specified method.

log

Excludes compilation logging (with the -XX:+LogCompilation option) for all methods except for the specified method. By default, logging is performed for all compiled methods.

option

Passes a JIT compilation option to the specified method in place of the last argument (option). The compilation option is set at the end, after the method name. For example, to enable the BlockLayoutByFrequency option for the append() method of the StringBuffer class, use the following:

-XX:CompileCommand=option,java/lang/StringBuffer.append,BlockLayoutByFrequency

You can specify multiple compilation options, separated by commas or spaces.

print

Prints generated assembler code after compilation of the specified method.

quiet

Instructs not to print the compile commands. By default, the commands that you specify with the -XX:CompileCommand option are printed; for example, if you exclude from compilation the indexOf() method of the String class, then the following is printed to standard output:

CompilerOracle: exclude java/lang/String.indexOf

You can suppress this by specifying the -XX:CompileCommand=quiet option before other -XX:CompileCommand options.

-XX:CompileCommandFile=filename

Sets the file from which JIT compiler commands are read. By default, the .hotspot_compiler file is used to store commands performed by the JIT compiler.

Each line in the command file represents a command, a class name, and a method name for which the command is used. For example, this line prints assembly code for the toString() method of the String class:

print java/lang/String toString

If you’re using commands for the JIT compiler to perform on methods, then see the -XX:CompileCommand option.

-XX:CompileOnly=methods

Sets the list of methods (separated by commas) to which compilation should be restricted. Only the specified methods are compiled. Specify each method with the full class name (including the packages and subpackages). For example, to compile only the length() method of the String class and the size() method of the List class, use the following:

-XX:CompileOnly=java/lang/String.length,java/util/List.size

Note that the full class name is specified, including all packages and subpackages separated by a slash (/). For easier cut and paste operations, it’s also possible to use the method name format produced by the -XX:+PrintCompilation and -XX:+LogCompilation options:

-XX:CompileOnly=java.lang.String::length,java.util.List::size

Although wildcards aren’t supported, you can specify only the class or package name to compile all methods in that class or package, as well as specify just the method to compile methods with this name in any class:

-XX:CompileOnly=java/lang/String -XX:CompileOnly=java/lang -XX:CompileOnly=.length

-XX: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. This option controls when methods are first compiled for both the tiered and the nontiered modes of operation. The CompileThresholdScaling option has an integer value between 0 and +Inf and scales the thresholds corresponding to the current mode of operation (both tiered and nontiered). Setting CompileThresholdScaling to a value less than 1.0 results in earlier compilation while values greater than 1.0 delay compilation. Setting CompileThresholdScaling to 0 is equivalent to disabling compilation.

-XX:+DoEscapeAnalysis

Enables the use of escape analysis. This option is enabled by default. To disable the use of escape analysis, specify -XX:-DoEscapeAnalysis. 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, or 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, or g or G to indicate gigabytes. Only compiled methods with the size smaller than the specified size is 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, or 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 nonmethod code.

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

—XX:NonProfiledCodeHeapSize=size

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

-XX: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, or g or G to indicate gigabytes. By default, the maximum bytecode size of a trivial method is set to 6 bytes:

-XX:MaxTrivialSize=6

-XX:+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’s hsdis-amd64.dll. This let’s you to see the generated code, which may help you to diagnose performance issues.

By default, this option is disabled and assembly code isn’t printed. The -XX:+PrintAssembly option has to be used together with the -XX:UnlockDiagnosticVMOptions option that unlocks diagnostic JVM options.

-XX:ProfiledCodeHeapSize=size

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

-XX:+PrintCompilation

Enables verbose diagnostic output from the JVM by printing a message to the console every time a method is compiled. This let’s you to see which methods actually get compiled. By default, this option is disabled and diagnostic output isn’t printed.

You can also log compilation activity to a file by using the -XX:+LogCompilation option.

-XX:+PrintInlining

Enables printing of inlining decisions. This let’s you 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, or g or G to indicate gigabytes. The default maximum code cache size is 240 MB; if you disable tiered compilation with the option -XX:-TieredCompilation, then the default size is 48 MB. This option has a limit of 2 GB; otherwise, an error is generated. The maximum code cache size shouldn’t be less than the initial code cache size; see the option -XX:InitialCodeCacheSize. This option is equivalent to -Xmaxjitcodesize.

-XX:RTMAbortRatio=abort_ratio

Specifies the RTM abort ratio is specified as a percentage (%) of all executed RTM transactions. If a number of aborted transactions becomes greater than this ratio, then the compiled code is deoptimized. This ratio is used when the -XX:+UseRTMDeopt option is enabled. The default value of this option is 50. This means that the compiled code is deoptimized if 50% of all transactions are aborted.

-XX:+SegmentedCodeCache

Enables segmentation of the code cache. Without the —XX:+SegmentedCodeCache, the code cache consists of one large segment. With —XX:+SegmentedCodeCache, we have separate segments for nonmethod, profiled method, and nonprofiled method code. These segments aren’t resized at runtime. The feature is enabled by default if tiered compilation is enabled (-XX:+TieredCompilation ) and -XX:ReservedCodeCacheSize >= 240 MB. The advantages are better control of the memory footprint, reduced code fragmentation, and better iTLB/iCache behavior due to improved locality. iTLB/iCache is a CPU-specific term meaning Instruction Translation Lookaside Buffer (ITLB). ICache is an instruction cache in theCPU. The implementation of the code cache can be found in the file: /share/vm/code/codeCache.cpp.

-XX:StartAggressiveSweepingAt=percent

Forces stack scanning of active methods to aggressively remove unused code when only the given percentage of the code cache is free. The default value is 10%.

-XX:RTMRetryCount=number_of_retries

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

-XX:-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. The -XX:+UseAES is used in conjunction with UseAESIntrinsics. Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions.

-XX:+UseAESIntrinsics

Enables -XX:+UseAES and -XX:+UseAESIntrinsics flags by default and are supported only for the Java HotSpot Server VM. To disable hardware-based AES intrinsics, specify -XX:-UseAES -XX:-UseAESIntrinsics. For example, to enable hardware AES, use the following flags:

-XX:+UseAES -XX:+UseAESIntrinsics

Flags that control intrinsics now require the option -XX:+UnlockDiagnosticVMOptions. To support UseAES and UseAESIntrinsics flags, use the -server option to select the Java HotSpot Server VM. These flags aren’t supported on Client VM.

-XX:+UseCMoveUnconditionally

Generates CMove (scalar and vector) instructions regardless of profitability analysis.

-XX:+UseCodeCacheFlushing

Enables flushing of the code cache before shutting down the compiler. This option is enabled by default. To disable flushing of the code cache before shutting down the compiler, specify -XX:-UseCodeCacheFlushing.

-XX:+UseCondCardMark

Enables checking if the card is already marked before updating the card table. This option is disabled by default. It should be used only on machines with multiple sockets, where it increases the performance of Java applications that rely on concurrent operations. Only the Java HotSpot Server VM supports this option.

-XX:+UseCountedLoopSafepoints

Keeps safepoints in counted loops. Its default value is 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 that calculate the value of (a * b + c) expressions.

-XX:+UseRTMDeopt

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

-XX:+UseRTMLocking

Generates Restricted Transactional Memory (RTM) locking code for all inflated locks, with the normal locking mechanism as the fallback handler. This option is disabled by default. Options related to RTM are available only 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, then the memory and register modifications are committed together at the XEND instruction. The XABORT instruction can be used to explicitly abort a transaction and the XEND instruction checks if a set of instructions is being run in a transaction.

A lock on a transaction is inflated when another thread tries to access the same transaction, thereby blocking the thread that didn’t originally request access to the transaction. RTM requires that a fallback set of operations be specified in case a transaction aborts or fails. An RTM lock is a lock that has been delegated to the TSX's system.

RTM improves performance for highly contended locks with low conflict in a critical region (which is code that must not be accessed by more than one thread concurrently). RTM also improves the performance of coarse-grain locking, which typically doesn’t perform well in multithreaded applications. (Coarse-grain locking is the strategy of holding locks for long periods to minimize the overhead of taking and releasing locks, while fine-grained locking is the strategy of trying to achieve maximum parallelism by locking only when necessary and unlocking as soon as possible.) Also, for lightly contended locks that are used by different threads, RTM can reduce false cache line sharing, also known as cache line ping-pong. This occurs when multiple threads from different processors are accessing different resources, but the resources share the same cache line. As a result, the processors repeatedly invalidate the cache lines of other processors, which forces them to read from main memory instead of their cache.

-XX:+UseSHA

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

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

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

To disable all hardware-based SHA intrinsics, specify the -XX:-UseSHA. To disable only a particular SHA intrinsic, use the appropriate corresponding option. For example: -XX:-UseSHA256Intrinsics.

-XX:+UseSHA1Intrinsics

Enables intrinsics for SHA-1 crypto hash function. Flags that control intrinsics now require the option-XX:+UnlockDiagnosticVMOptions.

-XX:+UseSHA256Intrinsics

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

-XX:+UseSHA512Intrinsics

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

-XX:+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 affect the performance. By default, this option is disabled and dtrace performs only standard probes.

-XX:+HeapDumpOnOutOfMemoryError

Enables the dumping of the Java heap to a file in the current directory by using the heap profiler (HPROF) when a java.lang.OutOfMemoryError exception is thrown. You can explicitly set the heap dump file path and name using the -XX:HeapDumpPath option. By default, this option is disabled and the heap isn’t dumped when an OutOfMemoryError exception is thrown.

-XX:HeapDumpPath=path

Sets the path and file name for writing the heap dump provided by the heap profiler (HPROF) when the -XX:+HeapDumpOnOutOfMemoryError option is set. By default, the file is created in the current working directory, and it’s named java_pid pid .hprof where pid is the identifier of the process that caused the error. The following example shows how to set the default file explicitly (%p represents the current process identifier):

-XX:HeapDumpPath=./java_pid%p.hprof

-XX:LogFile=path

Sets the path and file name to where log data is written. By default, the file is created in the current working directory, and it’s named hotspot.log.

-XX:+PrintClassHistogram

Enables printing of a class instance histogram after one of the following events:

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:

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 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’s the lower bound on the duty cycle when CMSIncrementalPacing is enabled. The default value is 0.

-XX:CMSIncrementalDutySafetyFactor=percent

Sets 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 the option -XX:MinHeapFreeRatio that’s 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 the System.gc() method. This option is disabled by default, meaning that calls to System.gc() are processed. If processing of calls to System.gc() is disabled, then the JVM still performs GC when necessary.

-XX:+ExplicitGCInvokesConcurrent

Enables invoking of concurrent GC by using the System.gc() request. This option is disabled by default and can be enabled only 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 2 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’re willing to waste. The Java HotSpot VM doesn’t initiate the mixed garbage collection cycle when the reclaimable percentage is less than the heap waste percentage. The default is 5 percent.

-XX:G1MaxNewSizePercent=percent

Sets the percentage of the heap size to use as the maximum for the young generation size. The default value is 60 percent of your Java heap.

This is an experimental flag. This setting replaces the -XX:DefaultMaxNewGenPercent setting.

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’s reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector. When you increase or decrease the percentage, ensure that you adjust the total Java heap by the same amount. By default, this option is set to 10%.

The following example sets the reserved heap to 20%:

-XX:G1ReservePercent=20

-XX: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, or g or G to indicate gigabytes. The default value is selected at run time based on the system configuration.

The following examples show how to set the size of allocated memory to 6 MB using various units:

-XX:InitialHeapSize=6291456 -XX:InitialHeapSize=6144k -XX:InitialHeapSize=6m

If you set this option to 0, then the initial size is set as the sum of the sizes allocated for the old generation and the young generation. The size of the heap for the young generation can be set using the -XX:NewSize option.

-XX: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 the survivor space is resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (using the -XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio option should be used to set the size of the survivor space for the entire execution of the application.

The following formula can be used to calculate the initial size of survivor space (S) based on the size of the young generation (Y), and the initial survivor space ratio (R):

S=Y/(R+2)

The 2 in the equation denotes two survivor spaces. The larger the value specified as the initial survivor space ratio, the smaller the initial survivor space size.

By default, the initial survivor space ratio is set to 8. If the default value for the young generation space size is used (2 MB), then the initial size of the survivor space is 0.2 MB.

The following example shows how to set the initial survivor space ratio to 4:

-XX:InitialSurvivorRatio=4

-XX:InitiatingHeapOccupancyPercent=percent

Sets the percentage of the heap occupancy (0 to 100) at which to start a concurrent GC cycle. It’s 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’s 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, or g or G to indicate gigabytes. The default value is selected at run time based on the system configuration. For server deployments, the options -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value.

The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

-XX:MaxHeapSize=83886080 -XX:MaxHeapSize=81920k -XX:MaxHeapSize=80m

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

Minimize the Java heap size by lowering the values of the parameters MaxHeapFreeRatio (default value is 70%) and MinHeapFreeRatio (default value is 40%) with the command-line options -XX:MaxHeapFreeRatio and -XX:MinHeapFreeRatio. Lowering MaxHeapFreeRatio to as low as 10% and MinHeapFreeRatio to 5% has successfully reduced the heap size without too much performance regression; however, results may vary greatly depending on your application. Try different values for these parameters until they’re as low as possible yet still retain acceptable performance.

-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5

Customers trying to keep the heap small should also add the option -XX:-ShrinkHeapInSteps. See Performance Tuning Examples for a description of using this option to keep the Java heap small by reducing the dynamic footprint for embedded applications.

-XX:MaxMetaspaceSize=size

Sets the maximum amount of native memory that can be allocated for class metadata. By default, the size isn’t limited. The amount of metadata for an application depends on the application itself, other running applications, and the amount of memory available on the system.

The following example shows how to set the maximum class metadata size to 256 MB:

-XX:MaxMetaspaceSize=256m

-XX:MaxNewSize=size

Sets the maximum size (in bytes) of the heap for the young generation (nursery). The default value is set ergonomically.

-XX: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 triggers a garbage collection the first time it’s exceeded. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends on the platform.

-XX:MinHeapFreeRatio=percent

Sets the minimum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space falls below this value, then the heap is expanded. By default, this value is set to 40%.

Minimize Java heap size by lowering the values of the parameters MaxHeapFreeRatio (default value is 70%) and MinHeapFreeRatio (default value is 40%) with the command-line options -XX:MaxHeapFreeRatio and -XX:MinHeapFreeRatio. Lowering MaxHeapFreeRatio to as low as 10% and MinHeapFreeRatio to 5% has successfully reduced the heap size without too much performance regression; however, results may vary greatly depending on your application. Try different values for these parameters until they’re as low as possible, yet still retain acceptable performance.

-XX:MaxHeapFreeRatio=10 -XX:MinHeapFreeRatio=5

Customers trying to keep the heap small should also add the option -XX:-ShrinkHeapInSteps. See Performance Tuning Examples for a description of using this option to keep the Java heap small by reducing the dynamic footprint for embedded applications.

-XX:NewRatio=ratio

Sets the ratio between young and old generation sizes. By default, this option is set to 2. The following example shows how to set the young-to-old ratio to 1:

-XX:NewRatio=1

-XX:NewSize=size

Sets the initial size (in bytes) of the heap for the young generation (nursery). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes.

The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too low, then a large number of minor GCs are performed. If the size is too high, then only full GCs are performed, which can take a long time to complete. 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 the young generation to 256 MB using various units:

-XX:NewSize=256m -XX:NewSize=262144k -XX:NewSize=268435456

The -XX:NewSize option is equivalent to -Xmn.

-XX:ParallelGCThreads=threads

Sets the value of the stop-the-world (STW) worker threads. This option 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 8 logical processors, then this option 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 don’t disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space. To disable GC of the young generation before each full GC, specify the option -XX:-ScavengeBeforeFullGC.

-XX:-ShrinkHeapInSteps

Incrementally reduces the Java heap to the target size, specified by the option —XX:MaxHeapFreeRatio. This option is enabled by default. If disabled, then it immediately reduces the Java heap to the target size instead of requiring multiple garbage collection cycles. Disable this option if you want to minimize the Java heap size. You will likely encounter performance degradation when this option is disabled.

See Performance Tuning Examples for a description of using the MaxHeapFreeRatio option to keep the Java heap small by reducing the dynamic footprint for embedded applications.

–XX:StringDeduplicationAgeThreshold=threshold

Identifies String objects reaching the specified age that are considered candidates for deduplication. An object's age is a measure of how many times it has survived garbage collection. This is sometimes referred to as tenuring. 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, or g or G to indicate gigabytes. If this option is set to 0, then the JVM selects the initial size automatically.

The following example shows how to set the initial TLAB size to 512 KB:

-XX:TLABSize=512k

-XX:+UseAdaptiveSizePolicy

Enables the use of adaptive generation sizing. This option is enabled by default. To disable adaptive generation sizing, specify -XX:-UseAdaptiveSizePolicy and set the size of the memory allocation pool explicitly. See the -XX:SurvivorRatio option.

-XX:+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’s a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. This option meets GC pause time goals with high probability, while maintaining good throughput. The G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (a stable and predictable pause time below 0.5 seconds). By default, this option is enabled and G1 is used as the default garbage collector.

-XX:+UseGCOverheadLimit

Enables the use of a policy that limits the proportion of time spent by the JVM on GC before an OutOfMemoryError exception is thrown. This option is enabled, by default, and the parallel GC will throw an OutOfMemoryError if more than 98% of the total time is spent on garbage collection and less than 2% of the heap is recovered. When the heap is small, this feature can be used to prevent applications from running for long periods of time with little or no progress. To disable this option, specify the option -XX:-UseGCOverheadLimit.

-XX:+UseNUMA

Enables performance optimization of an application on a machine with nonuniform memory architecture (NUMA) by increasing the application's use of lower latency memory. By default, this option is disabled and no optimization for NUMA is made. The option is available only when the parallel garbage collector is used (-XX:+UseParallelGC).

-XX:+UseParallelGC

Enables the use of the parallel scavenge garbage collector (also known as the throughput collector) to improve the performance of your application by leveraging multiple processors.

By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. If it’s 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 selected 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 set up large pages.

See Large Pages for setting up large pages.

-XX:+UseStringDeduplication

Enables string deduplication. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector.

String deduplication reduces the memory footprint of String objects on the Java heap by taking advantage of the fact that many String objects are identical. Instead of each String object pointing to its own character array, identical String objects can point to and share the same character array.

-XX:+UseTLAB

Enables the use of thread-local allocation blocks (TLABs) in the young generation space. This option is enabled by default. To disable the use of TLABs, specify the option -XX:-UseTLAB.

Obsolete Java Options

These java options are still accepted but ignored, and a warning is issued when they’re 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 effect. The flags have now been made obsolete, and their use generates 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’re still accepted and acted upon, but a warning is issued when they’re used.

-d32

This option is deprecated and will be removed in a future release.

-d64

This option is deprecated and will be removed in a future release.

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 is reported.

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 the option -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’s 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’re 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 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, 28,992,024bytes survived one scavenge and were copied from eden to survivor space, 1,366,864 bytes are occupied by age 2 objects, and so on. The third value in each row is the cumulative size of objects of age n or 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’re referenced. By default, this option is disabled and classes aren’t traced.

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

-XX:+TraceClassResolution

Enables tracing of constant pool resolutions. By default, this option is disabled and constant pool resolutions aren’t traced.

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

-XX:+TraceClassUnloading

Enables tracing of classes as they’re 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 selected 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’s 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 the -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 option-name

-d32

Oracle Solaris, Linux, and OS X: Ran the application in a 32-bit environment. 32-bit JDKs/JREs are no longer supported.

Note:

The -d32 and -d64 options were added to allow multiple architectures (data model) JDKs and JREs to coexist 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 JDKs/JREs are no longer supported.

-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. Appended the letter k or K to indicate kilobytes, m or M to indicate megabytes, or g or G to indicate gigabytes. The default maximum code cache size is 240 MB; if you disable tiered compilation with the option -XX:-TieredCompilation, then the default size is 48 MB:

-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 was allowed to run.

-XX:CMSIncrementalDutyCycleMin=percent

Sets the percentage of time (0 to 100) between minor collections that was the lower bound for the duty cycle when -XX:+CMSIncrementalPacing option was 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 was 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 was removed.

-XX:CMSIncrementalOffset=percent

Set the percentage of time (0 to 100) by which the incremental mode duty cycle was 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 was 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 was 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, or 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.