Launches a Java application.
java [options] classname [args]
java [options] -jar filename [args]
javaw [options] classname [args]
javaw [options] -jar filename [args]
The name of the class to be launched.
The name of the Java Archive (JAR) file to be called.
Used only with the -jar option.
The arguments passed to the main()
method separated by spaces.
The java command supports a wide
range of options that can be divided into the following
categories:
Standard options are guaranteed to be supported by all implementations of the Java Virtual Machine (JVM). They are used for common actions, such as checking the version of the JRE, setting the class path, enabling verbose output, and so on.
Non-standard options are general purpose options that are
specific to the Java HotSpot Virtual Machine, so they are
not guaranteed to be supported by all JVM implementations,
and are subject to change. These options start with -X.
Advanced options are not 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. They are also not
guaranteed to be supported by all JVM implementations, and
are subject to change. Advanced options start with -XX.
To keep track of the options that were deprecated or removed in the latest release, there is a section named Deprecated and Removed Options at the end of the document.
Boolean options are used to either enable a feature that is
disabled by default or disable a feature that is enabled by
default. Such options do not require a parameter. Boolean -XX options are enabled using the plus
sign (-XX:+OptionName) and
disabled using the minus sign (-XX:-OptionName).
For options that require an argument, the argument may be
separated from the option name by a space, a colon (:), or
an equal sign (=), or the argument may directly follow the
option (the exact syntax differs for each option). If you
are expected to specify the size in bytes, you can use no
suffix, or use the suffix k or K for kilobytes (KB), m
or M for megabytes (MB), g or G for
gigabytes (GB). For example, to set the size to 8 GB, you
can specify either 8g, 8192m, 8388608k,
or 8589934592 as the argument. If
you are expected to specify the percentage, use a number
from 0 to 1 (for example, specify 0.25
for 25%).
These are the most commonly used options that are supported by all implementations of the JVM.
Loads the specified native agent library. After the library name, a comma-separated list of options specific to the library can be used.
If the option -agentlib:foo
is specified, then the JVM attempts to load the
library named foo.dll in the
location specified by the PATH
system variable.
The following example shows how to load the heap profiling tool (HPROF) library and get sample CPU information every 20 ms, with a stack depth of 3:
-agentlib:hprof=cpu=samples,interval=20,depth=3
The following example shows how to load the Java Debug Wire Protocol (JDWP) library and listen for the socket connection on port 8000, suspending the JVM before the main class loads:
-agentlib:jdwp=transport=dt_socket,server=y,address=8000
For more information about the native agent libraries, refer to the following:
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").
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:
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.
These options are general purpose options that are specific to the Java HotSpot Virtual Machine.
Displays help for all available -X
options.
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.
Specifies a list of directories, JAR files, and ZIP archives separated by semicolons (;) to search for boot class files. These are used in place of the boot class files included in the JDK.
Do not deploy applications that use this option to
override a class in rt.jar,
because this violates the JRE binary code license.
Specifies a list of directories, JAR files, and ZIP archives separated by semicolons (;) to append to the end of the default bootstrap class path.
Do not deploy applications that use this option to
override a class in rt.jar,
because this violates the JRE binary code license.
Specifies a list of directories, JAR files, and ZIP archives separated by semicolons (;) to prepend to the front of the default bootstrap class path.
Do not deploy applications that use this option to
override a class in rt.jar,
because this violates the JRE binary code license.
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. Any invalid data encountered indicates a problem in the native code, and the JVM will terminate with an irrecoverable error in such cases. Expect a performance degradation when this option is used.
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.
Does nothing. Provided for backward compatibility.
Shows additional diagnostic messages.
Enables strict class-file format checks that enforce close conformance to the class-file format specification. Developers are encouraged to use this flag when developing new code because the stricter checks will become the default in future releases.
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 are not present in this mode.
Displays more detailed JVM version information than
the -version option, and
then exits.
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:
-Xloggc:garbage-collection.log
Specifies the maximum code cache size (in bytes)
for JIT-compiled code. Append the letter k or K to
indicate kilobytes, m or M to indicate megabytes, g or G to
indicate gigabytes. The default maximum code cache
size is 240 MB; if you disable tiered compilation
with the option -XX:-TieredCompilation,
then the default size is 48 MB:
-Xmaxjitcodesize=240m
This option is equivalent to -XX:ReservedCodeCacheSize.
Executes all bytecode by the interpreter except for hot methods, which are compiled to native code.
Sets the initial and maximum size (in bytes) of the
heap for the young generation (nursery). Append the
letter k or K
to indicate kilobytes, m or
M to indicate megabytes, g or G to
indicate gigabytes.
The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too small, then a lot of minor garbage collections will be performed. If the size is too large, then only full garbage collections will be performed, which can take a long time to complete. Oracle recommends that you keep the size for the young generation between a half and a quarter 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.
Sets the minimum and 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 do not set this option, then the initial
size will be set as the sum of the sizes allocated
for the old generation and the young generation. The
initial size of the heap for the young generation
can be set using the -Xmn
option or the -XX:NewSize
option.
Note that the -XX:InitalHeapSize
option can also be used to set the initial heap
size. If it appears after -Xms
on the command line, then the initial heap size gets
set to the value specified with -XX:InitalHeapSize.
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 watches for console control events to
implement shutdown hooks for unexpected termination.
Specifically, the JVM registers a console control
handler that begins shutdown-hook processing and
returns TRUE for CTRL_C_EVENT, CTRL_CLOSE_EVENT,
CTRL_LOGOFF_EVENT, and CTRL_SHUTDOWN_EVENT.
The JVM uses a similar mechanism to implement the
feature of dumping thread stacks for debugging
purposes. The JVM uses CTRL_BREAK_EVENT
to perform thread dumps.
If the JVM is run as a service (for example, as a
servlet engine for a web server), then it can receive
CTRL_LOGOFF_EVENT but should
not initiate shutdown because the operating system
will not actually terminate the process. To avoid
possible interference such as this, the -Xrs option can be used. When the -Xrs option is used, the JVM does
not install a console control handler, implying that
it does not watch for or process CTRL_C_EVENT,
CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT, or CTRL_SHUTDOWN_EVENT.
There are two consequences of specifying -Xrs:
Ctrl + Break 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:
Use CDS if possible. This is the default value for Java HotSpot 32-Bit Client VM.
Require the use of CDS. Print an error message and exit if class data sharing cannot be used.
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.
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:
Shows all categories of settings. This is the default value.
Shows settings related to locale.
Shows settings related to system properties.
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 virtual memory.
The following examples set the thread stack size to 1024 KB in different units:
-Xss1m -Xss1024k -Xss1048576
This option is equivalent to -XX:ThreadStackSize.
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:
Verifies all bytecodes not loaded by the
bootstrap class loader. This is the default
behavior if you do not specify the -Xverify option.
Enables verification of all bytecodes.
Disables verification of all bytecodes. Use of -Xverify:none is unsupported.
These options control the runtime behavior of the Java HotSpot VM.
Enables the option to prevent the java
command from running a Java application if it uses
the endorsed-standards override mechanism or the
extension mechanism. This option checks if an
application is using one of these mechanisms by
checking the following:
The java.ext.dirs or java.endorsed.dirs system
property is set.
The lib/endorsed
directory exists and is not empty.
The lib/ext directory
contains any JAR files other than those of the
JDK.
The system-wide platform-specific extension directory contains any JAR files.
Enables the option that 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 tools such as jcmd, jstack,
jmap, and jinfo.
Specifies the path and file name to which error
data is written when an irrecoverable error occurs.
By default, this file is created in the current
working directory and named hs_err_pidpid.log where pid is the
identifier of the process that 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
The following example shows how to set the error
log file to C:/log/java/java_error.log:
-XX:ErrorFile=C:/log/java/java_error.log
If the file cannot 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.
The temporary directory is specified by the value of
the TMP environment
variable; if that environment variable is not
defined, then the value of the TEMP
environment variable is used.
Enables automatic failover to the old verifier when the new type checker fails. By default, this option is disabled and it is ignored (that is, treated as disabled) for classes with a recent bytecode version. You can enable it for classes with older versions of the bytecode.
Enables the use of the Java Flight Recorder (JFR)
during the runtime of the application. This is a
commercial feature that works in conjunction with
the -XX:+UnlockCommercialFeatures
option as follows:
java -XX:+UnlockCommercialFeatures -XX:+FlightRecorder
If this option is not provided, Java Flight
Recorder can still be enabled in a running JVM by
providing the appropriate jcmd
diagnostic commands.
Disables the use of the Java Flight Recorder (JFR)
during the runtime of the application. This is a
commercial feature that works in conjunction with
the -XX:+UnlockCommercialFeatures
option as follows:
java -XX:+UnlockCommercialFeatures -XX:-FlightRecorder
If this option is provided, Java Flight Recorder cannot be enabled in a running JVM.
Sets the parameters that control the behavior of
JFR. This is a commercial feature that works in
conjunction with the -XX:+UnlockCommercialFeatures
option. This option can be used only when JFR is
enabled (that is, the -XX:+FlightRecorder
option is specified).
The following list contains all available JFR parameters:
Specifies whether a default continuous recording should be started for the Java application. By default, this parameter is set to false. To start a default recording automatically, set the parameter to true.
Specifies whether to write temporary data to the disk repository. By default, this parameter is set to false. To enable it, set the parameter to true.
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, and
also set defaultrecording=true.
The dump file is written to the location
defined by the dumponexitpath
parameter.
Specifies the path and name of the dump file
with JFR data that is created when the JVM exits
in a controlled manner if you set the dumponexit=true parameter.
Setting the path makes sense only if you also
set defaultrecording=true.
If the specified path is a directory, the JVM assigns a file name that shows the creation date and time. If the specified path includes a file name and if that file already exists, the JVM creates a new file by appending the date and time stamp to the specified file name.
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, g or G to specify the size in GB.
By default, the size is set to 462848 bytes.
Specify the amount of data written to the log
file by JFR. By default, it is set to info.
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).
This parameter is valid only if you set the disk=true parameter.
Specifies the maximum size (in bytes) of the
data chunks in a 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 data chunks is
set to 12 MB.
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.
This parameter is valid only if you set the disk=true parameter.
Specifies the repository (a directory) for temporary disk storage. By default, the system's temporary directory is used.
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.
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.
Stack depth for stack traces by JFR. By default, the depth is set to 64 method calls. The maximum is 2048, minimum is 1.
Specifies the per-thread local buffer size (in
bytes). 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. Higher values for this parameter allow
more data gathering without contention to flush
it to the global storage. It can increase
application footprint in a thread-rich
environment. By default, the local buffer size
is set to 5 KB.
You can specify values for multiple parameters by separating them with a comma. For example, to instruct JFR to write a continuous recording to disk, and set the maximum size of data chunks to 10 MB, specify the following:
-XX:FlightRecorderOptions=defaultrecording=true,disk=true,maxchunksize=10M
On Solaris, sets the maximum size (in bytes) for
large pages used for Java heap. The size
argument must be a power of 2 (2, 4, 8, 16, ...).
Append the letter k or K to indicate kilobytes, m or M to
indicate megabytes, g or G to indicate gigabytes. By
default, the size is set to 0, meaning that the JVM
chooses the size for large pages automatically.
The following example illustrates how to set the large page size to 4 megabytes (MB):
-XX:LargePageSizeInBytes=4m
Sets the maximum total size (in bytes) of the New
I/O (the java.nio package)
direct-buffer allocations. Append the letter k or K to
indicate kilobytes, m or M to indicate megabytes, g or G to
indicate gigabytes. By default, the size is set to
0, meaning that the JVM chooses the size for NIO
direct-buffer allocations automatically.
The following examples illustrate how to set the NIO size to 1024 KB in different units:
-XX:MaxDirectMemorySize=1m -XX:MaxDirectMemorySize=1024k -XX:MaxDirectMemorySize=1048576
Specifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following:
Do not track JVM native memory usage. This is
the default behavior if you do not specify the -XX:NativeMemoryTracking
option.
Only track memory usage by JVM subsystems, such as Java heap, class, code, and thread.
In addition to tracking memory usage by JVM
subsystems, track memory usage by individual CallSite, individual virtual
memory region and its committed regions.
Sets the memory alignment of Java objects (in bytes). By default, the value is set to 8 bytes. The specified value should be a power of two, and must be within the range of 8 and 256 (inclusive). This option makes it possible to use compressed pointers with large Java heap sizes.
The heap size limit in bytes is calculated as:
4GB * ObjectAlignmentInBytes
Note: As the alignment value increases, the unused space between objects will also increase. As a result, you may not realize any benefits from using compressed pointers with large Java heap sizes.
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.
The following example shows how the -XX:OnError option can be used to
run the userdump.exe utility
to obtain a crash dump in case of an irrecoverable
error (the %p designates the
current process):
-XX:OnError="userdump.exe %p"
The preceding example assumes that the path to the
userdump.exe utility is
specified in the PATH
environment variable.
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.
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>
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.
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:
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.
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.
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.
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
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.
Specifies the path and name of the JFR recording log file.
Specifies the identifier for the JFR recording.
By default, it is set to Recording
x.
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).
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.
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 thread stack size (in bytes). Append the
letter k or K
to indicate kilobytes, m or M to indicate megabytes, g or G to
indicate gigabytes. The default value depends on
virtual memory.
The following examples show how to set the thread stack size to 1024 KB in different units:
-XX:ThreadStackSize=1m -XX:ThreadStackSize=1024k -XX:ThreadStackSize=1048576
This option is equivalent 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 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 .
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.
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.
Enables installation of signal handlers by the application. By default, this option is disabled and the application is not allowed to install signal handlers.
These options provide the ability to gather system information and perform extensive debugging.
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 is not dumped when an OutOfMemoryError
exception is thrown.
Sets the path and file name for writing the heap
dump provided by the heap profiler (HPROF) when the
-XX:+HeapDumpOnOutOfMemoryError
option is set. By default, the file is created in
the current working directory, and it is named java_pidpid.hprof
where pid is the identifier of the process
that caused the error. The following example shows
how to set the default file explicitly (%p represents the current process
identificator):
-XX:HeapDumpPath=./java_pid%p.hprof
The following example shows how to set the heap
dump file to C:/log/java/java_heapdump.log:
-XX:HeapDumpPath=C:/log/java/java_heapdump.log
Sets the path and file name where log data is
written. By default, the file is created in the
current working directory, and it is named hotspot.log.
The following example shows how to set the log file
to C:/log/java/hotspot.log:
-XX:LogFile=C:/log/java/hotspot.log
Enables printing of a class instance histogram
after a Control+Break event.
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.
Enables printing of java.util.concurrent
locks after a Control+Break
event. By default, this option is disabled.
Setting this option is equivalent to running the jstack -l command or the jcmd pid Thread.print
-l command, where pid is the
current Java process identifier.
Unlocks the options intended for diagnosing the JVM. By default, this option is disabled and diagnostic options are not available.
These options control how garbage collection (GC) is performed by the Java HotSpot VM.
Overrides the number of CPUs that the VM uses to calculate the size of thread pools it uses for various operations such as Garbage Collection and ForkJoinPool.
The VM normally determines the number of available
processors from the operating system. This flag can
be useful for partitioning CPU resources when
running multiple Java processes in docker
containers. This flag is honored even if UseContainerSupport
is not enabled. See -XX:-UseContainerSupport
for a description of enabling and disabling
container support.
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 is not optimized.
Enables touching of every page on the Java heap
during JVM initialization. This gets all pages into
the memory before entering the main()
method. The option can be used in testing to
simulate a long-running system with all virtual
memory mapped to physical memory. By default, this
option is disabled and all pages are committed as
JVM heap space fills.
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.
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
Sets the percentage of the old generation occupancy
(0 to 100) at which to start a CMS collection cycle.
The default value is set to -1. Any negative value
(including the default) implies that -XX:CMSTriggerRatio is used to
define the value of the initiating occupancy
fraction.
The following example shows how to set the occupancy fraction to 20%:
-XX:CMSInitiatingOccupancyFraction=20
Enables scavenging attempts before the CMS remark step. By default, this option is disabled.
Sets the percentage (0 to 100) of the value
specified by -XX:MinHeapFreeRatio
that is allocated before a CMS collection cycle
commences. The default value is set to 80%.
The following example shows how to set the occupancy fraction to 75%:
-XX:CMSTriggerRatio=75
Sets the number of threads used for concurrent GC. 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
Enables the option that disables processing of
calls to System.gc(). This
option is disabled by default, meaning that calls to
System.gc() are processed. If
processing of calls to System.gc()
is disabled, the JVM still performs GC when
necessary.
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
option.
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.
Sets the size of the regions into which the Java heap is subdivided when using the garbage-first (G1) collector. The value can be between 1 MB and 32 MB. The default region size is determined ergonomically based on the heap size.
The following example shows how to set the size of the subdivisions to 16 MB:
-XX:G1HeapRegionSize=16m
Enables the printing of information about which regions are allocated and which are reclaimed by the G1 collector. By default, this option is disabled.
Sets the percentage of the heap (0 to 50) that is reserved as a false ceiling to reduce the possibility of promotion failure for the G1 collector. By default, this option is set to 10%.
The following example shows how to set the reserved heap to 20%:
-XX:G1ReservePercent=20
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.
-XX:InitialRAMPercentage=percentSets 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
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 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 size (in bytes) of the heap for the young generation (nursery). The default value is set ergonomically.
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.
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.
The following examples show how to use experimental tuning flags to either optimize throughput or to provide lower response time.
java -d64 -server -XX:+AggressiveOpts -XX:+UseLargePages -Xmn10g -Xms26g -Xmx26g
java -d64 -XX:+UseG1GC -Xms26g Xmx26g -XX:MaxGCPauseMillis=500 -XX:+PrintGCTimeStamp
Also known as huge pages, large pages are memory pages that are significantly larger than the standard memory page size (which varies depending on the processor and operating system). Large pages optimize processor Translation-Lookaside Buffers.
A Translation-Lookaside Buffer (TLB) is a page translation cache that holds the most-recently used virtual-to-physical address translations. TLB is a scarce system resource. A TLB miss can be costly as the processor must then read from the hierarchical page table, which may require multiple memory accesses. By using a larger memory page size, a single TLB entry can represent a larger memory range. There will be less pressure on TLB, and memory-intensive applications may have better performance.
However, large pages page memory can negatively affect system performance. For example, when a large mount of memory is pinned by an application, it may create a shortage of regular memory and cause excessive paging in other applications and slow down the entire system. Also, a system that has been up for a long time could produce excessive fragmentation, which could make it impossible to reserve enough large page memory. When this happens, either the OS or JVM reverts to using regular pages.
Only Windows Server 2003 supports large pages. To use this feature, the administrator must first assign additional privilege to the user who will be running the application:
Select Control Panel -> Administrative Tools -> Local Security Policy.
Select Local Policies -> User Rights Assignment.
Double click Lock pages in memory; add users and/or groups.
Reboot your system.
Note that these steps are required even if it is the administrator who will be running the application, as administrators by default do not have the privilege to lock pages in memory.
Application Class Data
Sharing (AppCDS) extends CDS (see https://docs.oracle.com/javase/8/docs/technotes/guides/vm/class-data-sharing.html)
to enable classes from the standard extensions directories
(specified by the system property java.ext.dirs;
see https://docs.oracle.com/javase/8/docs/technotes/guides/extensions/spec.html)
and the application class path (see "Setting the Class Path")
to be placed in the shared archive. AppCDS reduces the
footprint and decreases start-up time of your applications
provided that a substantial number of classes are loaded
from the application class path.
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.
You can share the same archive file across multiple applications processes that have the exact same class path or share a common class path prefix. This reduces memory usage as the archive is memory-mapped into the address space of the processes. The operating system automatically shares the read-only pages across these processes.
The following steps create a shared archive that both
applications Hello and Hi can use.
Create a list of all classes used by the Hello application and another
list for the Hi application:
java -XX:+UnlockCommercialFeatures
-XX:DumpLoadedClassList=hello.classlist
-XX:+UseAppCDS -cp common.jar:hello.jar Hello
java -XX:+UnlockCommercialFeatures
-XX:DumpLoadedClassList=hi.classlist
-XX:+UseAppCDS -cp common.jar:hi.jar Hi
Note that because the Hello
and Hi applications share a
common class path prefix (both of their class paths
start with common.jar),
these two applications can share a shared archive
file.
Create a single list of classes used by all the applications that will share the shared archive file.
The following commands combine the files hello.classlist and hi.classlist to one file, common.classlist:
type hello.classlist hi.classlist
> common.classlist
Create a shared archive, named common.jsa,
that contains all the classes in common.classlist:
java -XX:+UnlockCommercialFeatures
-Xshare:dump -XX:SharedArchiveFile=common.jsa
-XX:+UseAppCDS
-XX:SharedClassListFile=common.classlist -cp
common.jar
The value of the -cp
parameter is the common class path prefix shared by
the Hello and Hi applications.
Run the Hello and Hi applications with the same
shared archive:
java -XX:+UnlockCommercialFeatures
-Xshare:on -XX:SharedArchiveFile=common.jsa
-XX:+UseAppCDS -cp common.jar:hello.jar Hello
java -XX:+UnlockCommercialFeatures
-Xshare:on -XX:SharedArchiveFile=common.jsa
-XX:+UseAppCDS -cp common.jar:hi.jar Hi
The following exit values are typically returned by the
launcher when the launcher is called with the wrong
arguments, serious errors, or exceptions thrown by the
JVM. However, a Java application may choose to return any
value by using the API call System.exit(exitValue).
The values are:
0: Successful completion
>0: An error occurred
