java

-agentlib:hprof=cpu=samples,interval=20,depth=3

-agentlib:jdwp=transport=dt_socket,server=y,address=8000

• The java.lang.instrument package description at http://docs.oracle.com/javase/8/docs/api/java/lang/instrument/package-summary.html

• Agent Command Line Options in the JVM Tools Interface guide at http://docs.oracle.com/javase/8/docs/platform/jvmti/jvmti.html#starting

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.

Selects the Java HotSpot Client VM. The 64-bit version of the Java SE Development Kit (JDK) currently ignores this option and instead uses the Server JVM.

For default JVM selection, see Server-Class Machine Detection at
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/server-class.html

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").

Runs the application in a 32-bit environment. If a 32-bit environment is not installed or is not supported, then an error will be reported. By default, the application is run in a 32-bit environment unless a 64-bit system is used.

Runs the application in a 64-bit environment. If a 64-bit environment is not installed or is not supported, then an error will be reported. By default, the application is run in a 32-bit environment unless a 64-bit system is used.

Currently 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. This is subject to change in a future release.

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 do not have a class loader). There is one exception to this rule: if the option is provided with no arguments, then it does not apply to system classes. This makes it easy to disable assertions in all classes except for system classes. The -disablesystemassertions option enables you to disable assertions in all system classes.

To explicitly enable assertions in specific packages or classes, use the -enableassertions (-ea) option. Both options can be used at the same time. For example, to run the MyClass application with assertions enabled in package com.wombat.fruitbat (and any subpackages) but disabled in class com.wombat.fruitbat.Brickbat, use the following command:

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

Disables assertions in all system classes.

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 do not have a class loader). There is one exception to this rule: if the option is provided with no arguments, then it does not apply to system classes. This makes it easy to enable assertions in all classes except for system classes. The -enablesystemassertions option provides a separate switch to enable assertions in all system classes.

To explicitly disable assertions in specific packages or classes, use the -disableassertions (-da) option. If a single command contains multiple instances of these switches, then they are processed in order before loading any classes. For example, to run the MyClass application with assertions enabled only in package com.wombat.fruitbat (and any subpackages) but disabled in class com.wombat.fruitbat.Brickbat, use the following command:

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

Enables assertions in all system classes.

Displays usage information for the java command without actually running the JVM.

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.

For more information about JAR files, see the following resources:

• jar(1)

• The Java Archive (JAR) Files guide at http://docs.oracle.com/javase/8/docs/technotes/guides/jar/index.html

• Lesson: Packaging Programs in JAR Files at
  http://docs.oracle.com/javase/tutorial/deployment/jar/index.html

Loads the specified Java programming language agent. For more information about instrumenting Java applications, see the java.lang.instrument package description in the Java API documentation at http://docs.oracle.com/javase/8/docs/api/java/lang/instrument/package-summary.html

Includes user-private JREs in the version search.

Excludes user-private JREs from the version search.

Selects the Java HotSpot Server VM. The 64-bit version of the JDK supports only the Server VM, so in that case the option is implicit.

For default JVM selection, see Server-Class Machine Detection at
http://docs.oracle.com/javase/8/docs/technotes/guides/vm/server-class.html

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.

Shows the splash screen with the image specified by imgname. For example, to show the splash.gif file from the images directory when starting your application, use the following option:

-splash:images/splash.gif

Displays information about each loaded class.

Displays information about each garbage collection (GC) event.

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

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

Specifies the release version to be used for running the application. If the version of the java command called does not meet this specification and an appropriate implementation is found on the system, then the appropriate implementation will be used.

The release argument specifies either the exact version string, or a list of version strings and ranges separated by spaces. A version string is the developer designation of the version number in the following form: 1.x.0_u (where x is the major version number, and u is the update version number). A version range is made up of a version string followed by a plus sign (+) to designate this version or later, or a part of a version string followed by an asterisk (*) to designate any version string with a matching prefix. Version strings and ranges can be combined using a space for a logical OR combination, or an ampersand (&) for a logical AND combination of two version strings/ranges. For example, if running the class or JAR file requires either JRE 6u13 (1.6.0_13), or any JRE 6 starting from 6u10 (1.6.0_10), specify the following:

-version:"1.6.0_13 1.6* & 1.6.0_10+"

Quotation marks are necessary only if there are spaces in the release parameter.

For JAR files, the preference is to specify version requirements in the JAR file manifest rather than on the command line.

Specifies the maximum size (in bytes) of the memory allocation pool in bytes. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -Xms and -Xmx are often set to the same value. See the section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.

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.

Disables garbage collection (GC) of classes. This can save some GC time, which shortens interruptions during the application run.

When you specify -Xnoclassgc at startup, the class objects in the application will be left untouched during GC and will always be considered live. This can result in more memory being permanently occupied which, if not used carefully, will throw an out of memory exception.

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

Reduces the use of operating system signals by the JVM.

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

The JVM catches signals to implement shutdown hooks for unexpected termination. The JVM uses SIGHUP, SIGINT, and SIGTERM to initiate the running of shutdown hooks.

The JVM uses a similar mechanism to implement the feature of dumping thread stacks for debugging purposes. The JVM uses SIGQUIT to perform thread dumps.

Applications embedding the JVM frequently need to trap signals such as SIGINT or SIGTERM, which can lead to interference with the JVM signal handlers. The -Xrs option is available to address this issue. When -Xrs is used, the signal masks for SIGINT, SIGTERM, SIGHUP, and SIGQUIT are not changed by the JVM, and signal handlers for these signals are not installed.

There are two consequences of specifying -Xrs:

• SIGQUIT thread dumps are not available.

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

Sets the class data sharing (CDS) mode. Possible mode arguments for this option include the following:

auto

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

on

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

off

Do not use CDS. This is the default value for Java HotSpot 32-Bit Server VM, Java HotSpot 64-Bit Client VM, and Java HotSpot 64-Bit Server VM.

dump

Manually generate the CDS archive. Specify the application class path as described in "Setting the Class Path".

You should regenerate the CDS archive with each new JDK release.

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.

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

• Linux/ARM (32-bit): 320 KB
• Linux/i386 (32-bit): 320 KB
• Linux/x64 (64-bit): 1024 KB
• macOS (64-bit): 1024 KB
• Oracle Solaris/i386 (32-bit): 320 KB
• Oracle Solaris/x64 (64-bit): 1024 KB
• Windows: The default value depends on virtual memory

The following examples set the thread stack size to 1024 KB in different units:

-Xss1m
-Xss1024k
-Xss1048576

This option is similar to -XX:ThreadStackSize.

Use alternative signals instead of SIGUSR1 and SIGUSR2 for JVM internal signals. This option is equivalent to -XX:+UseAltSigs.

Sets the mode of the bytecode verifier. Bytecode verification ensures that class files are properly formed and satisfy the constraints listed in section 4.10, Verification of class Files in the The Java Virtual Machine Specification.

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

Possible mode arguments for this option include the following:

remote

Verifies all bytecodes not loaded by the bootstrap class loader. This is the default behavior if you do not specify the -Xverify option.

all

Enables verification of all bytecodes.

none

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

If enabled, saves jstat(1) binary data when the Java application exits. This binary data is saved in a file named hsperfdata_<pid>, where <pid> is the process identifier of the Java application you ran. Use jstat to display the performance data contained in this file as follows:

jstat -class file:///<path>/hsperfdata_<pid>
jstat -gc file:///<path>/hsperfdata_<pid>

If true, then calculate the container CPU availability based on the value of its CPU CFS (Completely Fair Scheduler) quota (if set). If false, then use the CPU shares value instead, provided it is less than the CPU quota value.

The VM uses the container CPU availability value to calculate the number of available processors if you haven't specified it with the -XX:ActiveProcessorCount option.

Note: The CPU count will never exceed the number of active processors available to the process. Active processors may have been limited by having them disabled or by selecting a cpuset (a list of CPUs; see the PrintContainerInfo option) for a container.

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 are not printed.

Prints the following information about the container:

• cpuset.cpus: A comma-separated list or hyphen-separated range of CPUs or cores a container can use provided that you have more than one CPU or core. For example, the value 0-3 means that the container can use the first, second, third, and fourth CPU.
• cpuset.mems: A comma-separated list or hyphen-separated range of memory nodes (MEMs) in which to allow execution; running of applications; only effective on non-uniform memory access (NUMA) systems.
• CPU Shares: The amount of CPU shares available to the process.
• CPU Quota: The number of milliseconds per period process is guaranteed to run.
• CPU Period: The CPU Completely Fair Scheduler (CFS) period, in ms.
• OSContainer::active_processor_count: Number of active processors for the VM to use.
• Memory Limit: The maximum amount of memory the container can use, in bytes.
• Memory Soft Limit: The maximum amount of memory a container should use, in bytes; this limit is smaller than Memory Limit, but because this is a soft limit, the container may exceed it.
• Memory Usage: The amount of used memory for this process, in bytes.
• Maximum Memory Usage: The maximum amount of used memory for this process, in bytes.

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 is not printed.

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

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

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).

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

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.

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

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:

compress={true|false}

Specifies whether to compress the JFR recording log file (of type JFR) on the disk using the gzip file compression utility. This parameter is valid only if the filename parameter is specified. By default it is set to false (recording is not compressed). To enable compression, set the parameter to true.

defaultrecording={true|false}

Specifies whether the recording is a continuous background recording or if it runs for a limited time. By default, this parameter is set to false (recording runs for a limited time). To make the recording run continuously, set the parameter to true.

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 is no delay, and this parameter is set to 0.

dumponexit={true|false}

Specifies whether a dump file of JFR data should be generated when the JVM terminates in a controlled manner. By default, this parameter is set to false (dump file on exit is not generated). To enable it, set the parameter to true.

The dump file is written to the location defined by the filename parameter.

Example:

-XX:StartFlightRecording=name=test,filename=D:\test.jfr,dumponexit=true

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 is not limited, and this parameter is set to 0.

filename=path

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

name=identifier

Specifies the identifier for the JFR recording. By default, it is set to Recording x.

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, g or G to specify the size in GB. By default, the maximum size of disk data is not 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. For example, to save the recording to test.jfr in the current working directory, and instruct JFR to compress the log file, specify the following:

-XX:StartFlightRecording=filename=test.jfr,compress=true

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

• Linux/ARM (32-bit): 320 KB
• Linux/i386 (32-bit): 320 KB
• Linux/x64 (64-bit): 1024 KB
• macOS (64-bit): 1024 KB
• Oracle Solaris/i386 (32-bit): 320 KB
• Oracle Solaris/x64 (64-bit): 1024 KB

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.

Enables tracing of classes as they are loaded. By default, this option is disabled and classes are not traced.

Enables tracing of all loaded classes in the order in which they are referenced. By default, this option is disabled and classes are not traced.

Enables tracing of constant pool resolutions. By default, this option is disabled and constant pool resolutions are not traced.

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

Enables tracing of the loader constraints recording. By default, this option is disabled and loader constraints recording is not traced.

Enables the use of commercial features. Commercial features are included with Oracle Java SE Advanced or Oracle Java SE Suite packages, as defined on the Java SE Products page at http://www.oracle.com/technetwork/java/javase/terms/products/index.html

By default, this option is disabled and the JVM runs without the commercial features. Once they were enabled for a JVM process, it is not possible to disable their use for that process.

If this option is not provided, commercial features can still be unlocked in a running JVM by using the appropriate jcmd diagnostic commands.

Enables the use of alternative signals instead of SIGUSR1 and SIGUSR2 for JVM internal signals. By default, this option is disabled and alternative signals are not used. This option is equivalent to -Xusealtsigs.

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".

Disables the use of biased locking. Some applications with significant amounts of uncontended synchronization may attain significant speedups with this flag enabled, whereas applications with certain patterns of locking may see slowdowns. For more information about the biased locking technique, see the example in Java Tuning White Paper at http://www.oracle.com/technetwork/java/tuning-139912.html#section4.2.5

By default, this option is enabled.

Disables the use of compressed pointers. By default, this option is enabled, and compressed pointers are used when Java heap sizes are less than 32 GB. When this option is enabled, object references are represented as 32-bit offsets instead of 64-bit pointers, which typically increases performance when running the application with Java heap sizes less than 32 GB. This option works only for 64-bit JVMs.

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

The VM provides automatic container detection support, which enables the VM to determine the amount of memory and number of processors that are available to a Java process running in docker containers. It uses this information to allocate system resources. This support is only available on Linux x64 platforms. If supported, then the default value for this flag is true and container support is enabled by default. You can disable it with -XX:-UseContainerSupport.

This option for Linux 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 cannot dynamically grow or shrink large pages memory areas; see -XX:UseTransparentHugePages if you want this behavior.

For more information, see "Large Pages".

Enables the use of large page memory. By default, this option is disabled and large page memory is not used.

For more information, see "Large Pages".

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

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.

On Linux, 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.

For more information, see "Large Pages".

Enables installation of signal handlers by the application. By default, this option is disabled and the application is not allowed to install signal handlers.

Sets the initial size (in bytes) of the memory allocation pool. This value must be either 0, or a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. See the section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.

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

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

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

Note that the -Xms option sets both the minimum and the initial heap size of the heap. If -Xms appears after -XX:InitialHeapSize on the command line, then the initial heap size gets set to the value specified with -Xms.

Sets the initial amount of memory that the JVM will use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 1.5625 percent.

The following example shows how to set the percentage of the initial amount of memory used for the Java heap:

-XX:InitialRAMPercentage=5

Sets the initial survivor space ratio used by the throughput garbage collector (which is enabled by the -XX:+UseParallelGC and/or -XX:+UseParallelOldGC options). Adaptive sizing is enabled by default with the throughput garbage collector by using the -XX:+UseParallelGC and -XX:+UseParallelOldGC options, and survivor space is resized according to the application behavior, starting with the initial value. If adaptive sizing is disabled (using the -XX:-UseAdaptiveSizePolicy option), then the -XX:SurvivorRatio option should be used to set the size of the survivor space for the entire execution of the application.

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

S=Y/(R+2)

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

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

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

-XX:InitialSurvivorRatio=4

Sets the percentage of the heap occupancy (0 to 100) at which to start a concurrent GC cycle. It is used by garbage collectors that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (for example, the G1 garbage collector).

By default, the initiating value is set to 45%. A value of 0 implies nonstop GC cycles. The following example shows how to set the initiating heap occupancy to 75%:

-XX:InitiatingHeapOccupancyPercent=75

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. By default, there is no maximum pause time value.

The following example shows how to set the maximum target pause time to 500 ms:

-XX:MaxGCPauseMillis=500

Sets the maximum size (in byes) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value. See the section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage Collection Tuning Guide at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.

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.

Sets the maximum allowed percentage of free heap space (0 to 100) after a GC event. If free heap space expands above this value, then the heap will be shrunk. By default, this value is set to 70%.

The following example shows how to set the maximum free heap ratio to 75%:

-XX:MaxHeapFreeRatio=75

Sets the maximum amount of native memory that can be allocated for class metadata. By default, the size is not 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

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

Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option. The default value is 25 percent.

Specifying this option disables automatic use of compressed oops if the combined result of this and other options influencing the maximum amount of memory is larger than the range of memory addressable by compressed oops. See -XX:UseCompressedOops for further information about compressed oops.

The following example shows how to set the percentage of the maximum amount of memory used for the Java heap:

-XX:MaxRAMPercentage=75

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

Sets the size of the allocated class metadata space that will trigger a garbage collection the first time it is exceeded. This threshold for a garbage collection is increased or decreased depending on the amount of metadata used. The default size depends on the platform.

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

The following example shows how to set the minimum free heap ratio to 25%:

-XX:MinHeapFreeRatio=25

Sets the maximum amount of memory that the JVM may use for the Java heap before applying ergonomics heuristics as a percentage of the maximum amount determined as described in the -XX:MaxRAM option for small heaps. A small heap is a heap of approximately 125 MB. The default value is 50 percent.

The following example shows how to set the percentage of the maximum amount of memory used for the Java heap for small heaps:

-XX:MinRAMPercentage=75

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/old ratio to 1:

-XX:NewRatio=1

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

The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too low, then a large number of minor GCs will be performed. If the size is too high, then only full GCs will be performed, which can take a long time to complete. Oracle recommends that you keep the size for the young generation between a half and a quarter of the overall heap size.

The following examples show how to set the initial size of young generation to 256 MB using various units:

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

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

Sets the number of threads used for parallel garbage collection in the young and old generations. 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

Enables parallel reference processing. By default, this option is disabled.

Enables printing of information about adaptive generation sizing. By default, this option is disabled.

Enables printing of messages at every GC. By default, this option is disabled.

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

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

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

Enables printing of detailed messages at every GC. By default, this option is disabled.

Enables printing of time stamps for every individual GC worker thread task. By default, this option is disabled.

Enables printing of time stamps at every GC. By default, this option is disabled.

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

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

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

Age 1 objects are the youngest survivors (they were created after the previous scavenge, survived the latest scavenge, and moved from eden to survivor space). Age 2 objects have survived two scavenges (during the second scavenge they were copied from one survivor space to the next). And so on.

In the preceding example, 28 992 024 bytes survived one scavenge and were copied from eden to survivor space, 1 366 864 bytes are occupied by age 2 objects, etc. The third value in each row is the cumulative size of objects of age n or less.

By default, this option is disabled.

Enables GC of the young generation before each full GC. This option is enabled by default. Oracle recommends that you do not disable it, because scavenging the young generation before a full GC can reduce the number of objects reachable from the old generation space into the young generation space. To disable GC of the young generation before each full GC, specify -XX:-ScavengeBeforeFullGC.

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

String objects reaching the specified age are considered candidates for deduplication. An object's age is a measure of how many times it has survived garbage collection. This is sometimes referred to as tenuring; see the -XX:+PrintTenuringDistribution option. Note that 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.

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

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

Sets the initial size (in bytes) of a thread-local allocation buffer (TLAB). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. If this option is set to 0, then the JVM chooses the initial size automatically.

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

-XX:TLABSize=512k

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).

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.

Enables the use of the CMS garbage collector for the old generation. Oracle recommends that you use the CMS garbage collector when application latency requirements cannot be met by the throughput (-XX:+UseParallelGC) garbage collector. The G1 garbage collector (-XX:+UseG1GC) is another alternative.

By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. When this option is enabled, the -XX:+UseParNewGC option is automatically set and you should not disable it, because the following combination of options has been deprecated in JDK 8: -XX:+UseConcMarkSweepGC -XX:-UseParNewGC.

Enables the use of the garbage-first (G1) garbage collector. It is a server-style garbage collector, targeted for multiprocessor machines with a large amount of RAM. It meets GC pause time goals with high probability, while maintaining good throughput. The G1 collector is recommended for applications requiring large heaps (sizes of around 6 GB or larger) with limited GC latency requirements (stable and predictable pause time below 0.5 seconds).

By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM.

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

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 only available when the parallel garbage collector is used (-XX:+UseParallelGC).

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

By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM. If it is enabled, then the -XX:+UseParallelOldGC option is automatically enabled, unless you explicitly disable it.

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.

Enables the use of parallel threads for collection in the young generation. By default, this option is disabled. It is automatically enabled when you set the -XX:+UseConcMarkSweepGC option. Using the -XX:+UseParNewGC option without the -XX:+UseConcMarkSweepGC option was deprecated in JDK 8.

Enables the use of the serial garbage collector. This is generally the best choice for small and simple applications that do not require any special functionality from garbage collection. By default, this option is disabled and the collector is chosen automatically based on the configuration of the machine and type of the JVM.

On Linux, enables the JVM to use shared memory to setup large pages.

For more information, see "Large Pages".

Enables string deduplication. By default, this option is disabled. To use this option, you must enable the garbage-first (G1) garbage collector. See the -XX:+UseG1GC option.

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.

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