java.1

.B \f[CB]\-Xrs\f[R]
Reduces the use of operating system signals by the JVM.
Shutdown hooks enable the orderly shutdown of a Java application by
running user cleanup code (such as closing database connections) at
shutdown, even if the JVM terminates abruptly.
.RS
.IP \[bu] 2
\f[B]Linux and macOS:\f[R]
.RS 2
.IP \[bu] 2
The JVM catches signals to implement shutdown hooks for unexpected
termination.
The JVM uses \f[CB]SIGHUP\f[R], \f[CB]SIGINT\f[R], and \f[CB]SIGTERM\f[R] to
initiate the running of shutdown hooks.
.IP \[bu] 2
Applications embedding the JVM frequently need to trap signals such as
\f[CB]SIGINT\f[R] or \f[CB]SIGTERM\f[R], which can lead to interference with
the JVM signal handlers.
The \f[CB]\-Xrs\f[R] option is available to address this issue.
When \f[CB]\-Xrs\f[R] is used, the signal masks for \f[CB]SIGINT\f[R],
\f[CB]SIGTERM\f[R], \f[CB]SIGHUP\f[R], and \f[CB]SIGQUIT\f[R] aren\[aq]t
changed by the JVM, and signal handlers for these signals aren\[aq]t
installed.
.RE
.IP \[bu] 2
\f[B]Windows:\f[R]
.RS 2
.IP \[bu] 2
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 \f[CB]TRUE\f[R] for
\f[CB]CTRL_C_EVENT\f[R], \f[CB]CTRL_CLOSE_EVENT\f[R],
\f[CB]CTRL_LOGOFF_EVENT\f[R], and \f[CB]CTRL_SHUTDOWN_EVENT\f[R].
.IP \[bu] 2
The JVM uses a similar mechanism to implement the feature of dumping
thread stacks for debugging purposes.
The JVM uses \f[CB]CTRL_BREAK_EVENT\f[R] to perform thread dumps.
.IP \[bu] 2
If the JVM is run as a service (for example, as a servlet engine for a
web server), then it can receive \f[CB]CTRL_LOGOFF_EVENT\f[R] but
shouldn\[aq]t initiate shutdown because the operating system doesn\[aq]t
actually terminate the process.
To avoid possible interference such as this, the \f[CB]\-Xrs\f[R] option
can be used.
When the \f[CB]\-Xrs\f[R] option is used, the JVM doesn\[aq]t install a
console control handler, implying that it doesn\[aq]t watch for or
process \f[CB]CTRL_C_EVENT\f[R], \f[CB]CTRL_CLOSE_EVENT\f[R],
\f[CB]CTRL_LOGOFF_EVENT\f[R], or \f[CB]CTRL_SHUTDOWN_EVENT\f[R].
.RE
.PP
There are two consequences of specifying \f[CB]\-Xrs\f[R]:
.IP \[bu] 2
\f[B]Linux and macOS:\f[R] \f[CB]SIGQUIT\f[R] thread dumps aren\[aq]t
available.
.IP \[bu] 2
\f[B]Windows:\f[R] Ctrl + Break thread dumps aren\[aq]t available.
.PP
User code is responsible for causing shutdown hooks to run, for example,
by calling the \f[CB]System.exit()\f[R] when the JVM is to be terminated.
.RE
.TP
.B \f[CB]\-Xshare:\f[R]\f[I]mode\f[R]
Sets the class data sharing (CDS) mode.
.RS
.PP
Possible \f[I]mode\f[R] arguments for this option include the following:
.TP
.B \f[CB]auto\f[R]
Use shared class data if possible (default).
.RS
.RE
.TP
.B \f[CB]on\f[R]
Require using shared class data, otherwise fail.
.RS
.RE
.RS
.PP
\f[B]Note:\f[R] The \f[CB]\-Xshare:on\f[R] option is used for testing
purposes only.
It may cause the VM to unexpectedly exit during start\-up when the CDS
archive cannot be used (for example, when certain VM parameters are
changed, or when a different JDK is used).
This option should not be used in production environments.
.RE
.TP
.B \f[CB]off\f[R]
Do not attempt to use shared class data.
.RS
.RE
.RE
.TP
.B \f[CB]\-XshowSettings\f[R]
Shows all settings and then continues.
.RS
.RE
.TP
.B \f[CB]\-XshowSettings:\f[R]\f[I]category\f[R]
Shows settings and continues.
Possible \f[I]category\f[R] arguments for this option include the
following:
.RS
.TP
.B \f[CB]all\f[R]
Shows all categories of settings.
This is the default value.
.RS
.RE
.TP
.B \f[CB]locale\f[R]
Shows settings related to locale.
.RS
.RE
.TP
.B \f[CB]properties\f[R]
Shows settings related to system properties.
.RS
.RE
.TP
.B \f[CB]vm\f[R]
Shows the settings of the JVM.
.RS
.RE
.TP
.B \f[CB]system\f[R]
\f[B]Linux:\f[R] Shows host system or container configuration and
continues.
.RS
.RE
.RE
.TP
.B \f[CB]\-Xss\f[R] \f[I]size\f[R]
Sets the thread stack size (in bytes).
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate KB, \f[CB]m\f[R] or
\f[CB]M\f[R] to indicate MB, or \f[CB]g\f[R] or \f[CB]G\f[R] to indicate GB.
The default value depends on the platform:
.RS
.IP \[bu] 2
Linux/x64 (64\-bit): 1024 KB
.IP \[bu] 2
macOS (64\-bit): 1024 KB
.IP \[bu] 2
Windows: The default value depends on virtual memory
.PP
The following examples set the thread stack size to 1024 KB in different
units:
.IP
.nf
\f[CB]
\-Xss1m
\-Xss1024k
\-Xss1048576
\f[R]
.fi
.PP
This option is similar to \f[CB]\-XX:ThreadStackSize\f[R].
.RE
.TP
.B \f[CB]\-\-add\-reads\f[R] \f[I]module\f[R]\f[CB]=\f[R]\f[I]target\-module\f[R](\f[CB],\f[R]\f[I]target\-module\f[R])*
Updates \f[I]module\f[R] to read the \f[I]target\-module\f[R], regardless
of the module declaration.
\f[I]target\-module\f[R] can be all unnamed to read all unnamed modules.
.RS
.RE
.TP
.B \f[CB]\-\-add\-exports\f[R] \f[I]module\f[R]\f[CB]/\f[R]\f[I]package\f[R]\f[CB]=\f[R]\f[I]target\-module\f[R](\f[CB],\f[R]\f[I]target\-module\f[R])*
Updates \f[I]module\f[R] to export \f[I]package\f[R] to
\f[I]target\-module\f[R], regardless of module declaration.
The \f[I]target\-module\f[R] can be all unnamed to export to all unnamed
modules.
.RS
.RE
.TP
.B \f[CB]\-\-add\-opens\f[R] \f[I]module\f[R]\f[CB]/\f[R]\f[I]package\f[R]\f[CB]=\f[R]\f[I]target\-module\f[R](\f[CB],\f[R]\f[I]target\-module\f[R])*
Updates \f[I]module\f[R] to open \f[I]package\f[R] to
\f[I]target\-module\f[R], regardless of module declaration.
.RS
.RE
.TP
.B \f[CB]\-\-limit\-modules\f[R] \f[I]module\f[R][\f[CB],\f[R]\f[I]module\f[R]...]
Specifies the limit of the universe of observable modules.
.RS
.RE
.TP
.B \f[CB]\-\-patch\-module\f[R] \f[I]module\f[R]\f[CB]=\f[R]\f[I]file\f[R](\f[CB];\f[R]\f[I]file\f[R])*
Overrides or augments a module with classes and resources in JAR files
or directories.
.RS
.RE
.TP
.B \f[CB]\-\-source\f[R] \f[I]version\f[R]
Sets the version of the source in source\-file mode.
.RS
.RE
.SH EXTRA OPTIONS FOR MACOS
.PP
The following extra options are macOS specific.
.TP
.B \f[CB]\-XstartOnFirstThread\f[R]
Runs the \f[CB]main()\f[R] method on the first (AppKit) thread.
.RS
.RE
.TP
.B \f[CB]\-Xdock:name=\f[R]\f[I]application_name\f[R]
Overrides the default application name displayed in dock.
.RS
.RE
.TP
.B \f[CB]\-Xdock:icon=\f[R]\f[I]path_to_icon_file\f[R]
Overrides the default icon displayed in dock.
.RS
.RE
.SH ADVANCED OPTIONS FOR JAVA
.PP
These \f[CB]java\f[R] options can be used to enable other advanced
options.
.TP
.B \f[CB]\-XX:+UnlockDiagnosticVMOptions\f[R]
Unlocks the options intended for diagnosing the JVM.
By default, this option is disabled and diagnostic options aren\[aq]t
available.
.RS
.PP
Command line options that are enabled with the use of this option are
not supported.
If you encounter issues while using any of these options, it is very
likely that you will be required to reproduce the problem without using
any of these unsupported options before Oracle Support can assist with
an investigation.
It is also possible that any of these options may be removed or their
behavior changed without any warning.
.RE
.TP
.B \f[CB]\-XX:+UnlockExperimentalVMOptions\f[R]
Unlocks the options that provide experimental features in the JVM.
By default, this option is disabled and experimental features aren\[aq]t
available.
.RS
.RE
.SH ADVANCED RUNTIME OPTIONS FOR JAVA
.PP
These \f[CB]java\f[R] options control the runtime behavior of the Java
HotSpot VM.
.TP
.B \f[CB]\-XX:ActiveProcessorCount=\f[R]\f[I]x\f[R]
Overrides the number of CPUs that the VM will use to calculate the size
of thread pools it will use for various operations such as Garbage
Collection and ForkJoinPool.
.RS
.PP
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 \f[CB]UseContainerSupport\f[R] is not
enabled.
See \f[CB]\-XX:\-UseContainerSupport\f[R] for a description of enabling
and disabling container support.
.RE
.TP
.B \f[CB]\-XX:AllocateHeapAt=\f[R]\f[I]path\f[R]
Takes a path to the file system and uses memory mapping to allocate the
object heap on the memory device.
Using this option enables the HotSpot VM to allocate the Java object
heap on an alternative memory device, such as an NV\-DIMM, specified by
the user.
.RS
.PP
Alternative memory devices that have the same semantics as DRAM,
including the semantics of atomic operations, can be used instead of
DRAM for the object heap without changing the existing application code.
All other memory structures (such as the code heap, metaspace, and
thread stacks) continue to reside in DRAM.
.PP
Some operating systems expose non\-DRAM memory through the file system.
Memory\-mapped files in these file systems bypass the page cache and
provide a direct mapping of virtual memory to the physical memory on the
device.
The existing heap related flags (such as \f[CB]\-Xmx\f[R] and
\f[CB]\-Xms\f[R]) and garbage\-collection related flags continue to work
as before.
.RE
.TP
.B \f[CB]\-XX:\-CompactStrings\f[R]
Disables the Compact Strings feature.
By default, this option is enabled.
When this option is enabled, Java Strings containing only single\-byte
characters are internally represented and stored as
single\-byte\-per\-character Strings using ISO\-8859\-1 / Latin\-1
encoding.
This reduces, by 50%, the amount of space required for Strings
containing only single\-byte characters.
For Java Strings containing at least one multibyte character: these are
represented and stored as 2 bytes per character using UTF\-16 encoding.
Disabling the Compact Strings feature forces the use of UTF\-16 encoding
as the internal representation for all Java Strings.
.RS
.PP
Cases where it may be beneficial to disable Compact Strings include the
following:
.IP \[bu] 2
When it\[aq]s known that an application overwhelmingly will be
allocating multibyte character Strings
.IP \[bu] 2
In the unexpected event where a performance regression is observed in
migrating from Java SE 8 to Java SE 9 and an analysis shows that Compact
Strings introduces the regression
.PP
In both of these scenarios, disabling Compact Strings makes sense.
.RE
.TP
.B \f[CB]\-XX:ErrorFile=\f[R]\f[I]filename\f[R]
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 \f[CB]hs_err_pid\f[R]\f[I]pid\f[R]\f[CB]\&.log\f[R] where \f[I]pid\f[R]
is the identifier of the process that encountered the error.
.RS
.PP
The following example shows how to set the default log file (note that
the identifier of the process is specified as \f[CB]%p\f[R]):
.RS
.PP
\f[CB]\-XX:ErrorFile=./hs_err_pid%p.log\f[R]
.RE
.IP \[bu] 2
\f[B]Linux and macOS:\f[R] The following example shows how to set the
error log to \f[CB]/var/log/java/java_error.log\f[R]:
.RS 2
.RS
.PP
\f[CB]\-XX:ErrorFile=/var/log/java/java_error.log\f[R]
.RE
.RE
.IP \[bu] 2
\f[B]Windows:\f[R] The following example shows how to set the error log
file to \f[CB]C:/log/java/java_error.log\f[R]:
.RS 2
.RS
.PP
\f[CB]\-XX:ErrorFile=C:/log/java/java_error.log\f[R]
.RE
.RE
.PP
If the file exists, and is writeable, then it will be overwritten.
Otherwise, if the file can\[aq]t be created in the specified directory
(due to insufficient space, permission problem, or another issue), then
the file is created in the temporary directory for the operating system:
.IP \[bu] 2
\f[B]Linux and macOS:\f[R] The temporary directory is \f[CB]/tmp\f[R].
.IP \[bu] 2
\f[B]Windows:\f[R] The temporary directory is specified by the value of
the \f[CB]TMP\f[R] environment variable; if that environment variable
isn\[aq]t defined, then the value of the \f[CB]TEMP\f[R] environment
variable is used.
.RE
.TP
.B \f[CB]\-XX:+ExtensiveErrorReports\f[R]
Enables the reporting of more extensive error information in the
\f[CB]ErrorFile\f[R].
This option can be turned on in environments where maximal information
is desired \- even if the resulting logs may be quite large and/or
contain information that might be considered sensitive.
The information can vary from release to release, and across different
platforms.
By default this option is disabled.
.RS
.RE
.TP
.B \f[CB]\-XX:FlightRecorderOptions=\f[R]\f[I]parameter\f[R]\f[CB]=\f[R]\f[I]value\f[R] (or)\f[CB]\-XX:FlightRecorderOptions:\f[R]\f[I]parameter\f[R]\f[CB]=\f[R]\f[I]value\f[R]
Sets the parameters that control the behavior of JFR.
.RS
.PP
The following list contains the available JFR
\f[I]parameter\f[R]\f[CB]=\f[R]\f[I]value\f[R] entries:
.TP
.B \f[CB]globalbuffersize=\f[R]\f[I]size\f[R]
Specifies the total amount of primary memory used for data retention.
The default value is based on the value specified for
\f[CB]memorysize\f[R].
Change the \f[CB]memorysize\f[R] parameter to alter the size of global
buffers.
.RS
.RE
.TP
.B \f[CB]maxchunksize=\f[R]\f[I]size\f[R]
Specifies the maximum size (in bytes) of the data chunks in a recording.
Append \f[CB]m\f[R] or \f[CB]M\f[R] to specify the size in megabytes (MB),
or \f[CB]g\f[R] or \f[CB]G\f[R] to specify the size in gigabytes (GB).
By default, the maximum size of data chunks is set to 12 MB.
The minimum allowed is 1 MB.
.RS
.RE
.TP
.B \f[CB]memorysize=\f[R]\f[I]size\f[R]
Determines how much buffer memory should be used, and sets the
\f[CB]globalbuffersize\f[R] and \f[CB]numglobalbuffers\f[R] parameters based
on the size specified.
Append \f[CB]m\f[R] or \f[CB]M\f[R] to specify the size in megabytes (MB),
or \f[CB]g\f[R] or \f[CB]G\f[R] to specify the size in gigabytes (GB).
By default, the memory size is set to 10 MB.
.RS
.RE
.TP
.B \f[CB]numglobalbuffers\f[R]
Specifies the number of global buffers used.
The default value is based on the memory size specified.
Change the \f[CB]memorysize\f[R] parameter to alter the number of global
buffers.
.RS
.RE
.TP
.B \f[CB]old\-object\-queue\-size=number\-of\-objects\f[R]
Maximum number of old objects to track.
By default, the number of objects is set to 256.
.RS
.RE
.TP
.B \f[CB]repository=\f[R]\f[I]path\f[R]
Specifies the repository (a directory) for temporary disk storage.
By default, the system\[aq]s temporary directory is used.
.RS
.RE
.TP
.B \f[CB]retransform=\f[R]{\f[CB]true\f[R]|\f[CB]false\f[R]}
Specifies whether event classes should be retransformed using JVMTI.
If false, instrumentation is added when event classes are loaded.
By default, this parameter is enabled.
.RS
.RE
.TP
.B \f[CB]samplethreads=\f[R]{\f[CB]true\f[R]|\f[CB]false\f[R]}
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.
.RS
.RE
.TP
.B \f[CB]stackdepth=\f[R]\f[I]depth\f[R]
Stack depth for stack traces.
By default, the depth is set to 64 method calls.
The maximum is 2048.
Values greater than 64 could create significant overhead and reduce
performance.
.RS
.RE
.TP
.B \f[CB]threadbuffersize=\f[R]\f[I]size\f[R]
Specifies the per\-thread local buffer size (in bytes).
By default, the local buffer size is set to 8 kilobytes, with a minimum
value of 4 kilobytes.
Overriding this parameter could reduce performance and is not
recommended.
.RS
.RE
.PP
You can specify values for multiple parameters by separating them with a
comma.
.RE
.TP
.B \f[CB]\-XX:LargePageSizeInBytes=\f[R]\f[I]size\f[R]
Sets the maximum large page size (in bytes) used by the JVM.
The \f[I]size\f[R] argument must be a valid page size supported by the
environment to have any effect.
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate kilobytes,
\f[CB]m\f[R] or \f[CB]M\f[R] to indicate megabytes, or \f[CB]g\f[R] or
\f[CB]G\f[R] to indicate gigabytes.
By default, the size is set to 0, meaning that the JVM will use the
default large page size for the environment as the maximum size for
large pages.
See \f[B]Large Pages\f[R].
.RS
.PP
The following example describes how to set the large page size to 1
gigabyte (GB):
.RS
.PP
\f[CB]\-XX:LargePageSizeInBytes=1g\f[R]
.RE
.RE
.TP
.B \f[CB]\-XX:MaxDirectMemorySize=\f[R]\f[I]size\f[R]
Sets the maximum total size (in bytes) of the \f[CB]java.nio\f[R] package,
direct\-buffer allocations.
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate kilobytes,
\f[CB]m\f[R] or \f[CB]M\f[R] to indicate megabytes, or \f[CB]g\f[R] or
\f[CB]G\f[R] to indicate gigabytes.
By default, the size is set to 0, meaning that the JVM chooses the size
for NIO direct\-buffer allocations automatically.
.RS
.PP
The following examples illustrate how to set the NIO size to 1024 KB in
different units:
.IP
.nf
\f[CB]
\-XX:MaxDirectMemorySize=1m
\-XX:MaxDirectMemorySize=1024k
\-XX:MaxDirectMemorySize=1048576
\f[R]
.fi
.RE
.TP
.B \f[CB]\-XX:\-MaxFDLimit\f[R]
Disables the attempt to set the soft limit for the number of open file
descriptors to the hard limit.
By default, this option is enabled on all platforms, but is ignored on
Windows.
The only time that you may need to disable this is on Mac OS, where its
use imposes a maximum of 10240, which is lower than the actual system
maximum.
.RS
.RE
.TP
.B \f[CB]\-XX:NativeMemoryTracking=\f[R]\f[I]mode\f[R]
Specifies the mode for tracking JVM native memory usage.
Possible \f[I]mode\f[R] arguments for this option include the following:
.RS
.TP
.B \f[CB]off\f[R]
Instructs not to track JVM native memory usage.
This is the default behavior if you don\[aq]t specify the
\f[CB]\-XX:NativeMemoryTracking\f[R] option.
.RS
.RE
.TP
.B \f[CB]summary\f[R]
Tracks memory usage only by JVM subsystems, such as Java heap, class,
code, and thread.
.RS
.RE
.TP
.B \f[CB]detail\f[R]
In addition to tracking memory usage by JVM subsystems, track memory
usage by individual \f[CB]CallSite\f[R], individual virtual memory region
and its committed regions.
.RS
.RE
.RE
.TP
.B \f[CB]\-XX:ObjectAlignmentInBytes=\f[R]\f[I]alignment\f[R]
Sets the memory alignment of Java objects (in bytes).
By default, the value is set to 8 bytes.
The specified value should be a power of 2, and must be within the range
of 8 and 256 (inclusive).
This option makes it possible to use compressed pointers with large Java
heap sizes.
.RS
.PP
The heap size limit in bytes is calculated as:
.RS
.PP
\f[CB]4GB\ *\ ObjectAlignmentInBytes\f[R]
.RE
.RS
.PP
\f[B]Note:\f[R] As the alignment value increases, the unused space
between objects also increases.
As a result, you may not realize any benefits from using compressed
pointers with large Java heap sizes.
.RE
.RE
.TP
.B \f[CB]\-XX:OnError=\f[R]\f[I]string\f[R]
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.
.RS
.IP \[bu] 2
\f[B]Linux and macOS:\f[R] The following example shows how the
\f[CB]\-XX:OnError\f[R] option can be used to run the \f[CB]gcore\f[R]
command to create a core image, and start the \f[CB]gdb\f[R] debugger to
attach to the process in case of an irrecoverable error (the \f[CB]%p\f[R]
designates the current process identifier):
.RS 2
.RS
.PP
\f[CB]\-XX:OnError="gcore\ %p;gdb\ \-p\ %p"\f[R]
.RE
.RE
.IP \[bu] 2
\f[B]Windows:\f[R] The following example shows how the
\f[CB]\-XX:OnError\f[R] option can be used to run the
\f[CB]userdump.exe\f[R] utility to obtain a crash dump in case of an
irrecoverable error (the \f[CB]%p\f[R] designates the current process
identifier).
This example assumes that the path to the \f[CB]userdump.exe\f[R] utility
is specified in the \f[CB]PATH\f[R] environment variable:
.RS 2
.RS
.PP
\f[CB]\-XX:OnError="userdump.exe\ %p"\f[R]
.RE
.RE
.RE
.TP
.B \f[CB]\-XX:OnOutOfMemoryError=\f[R]\f[I]string\f[R]
Sets a custom command or a series of semicolon\-separated commands to
run when an \f[CB]OutOfMemoryError\f[R] 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
\f[CB]\-XX:OnError\f[R] option.
.RS
.RE
.TP
.B \f[CB]\-XX:+PrintCommandLineFlags\f[R]
Enables printing of ergonomically selected JVM flags that appeared on
the command line.
It can be useful to know the ergonomic values set by the JVM, such as
the heap space size and the selected garbage collector.
By default, this option is disabled and flags aren\[aq]t printed.
.RS
.RE
.TP
.B \f[CB]\-XX:+PreserveFramePointer\f[R]
Selects between using the RBP register as a general purpose register
(\f[CB]\-XX:\-PreserveFramePointer\f[R]) and using the RBP register to
hold the frame pointer of the currently executing method
(\f[CB]\-XX:+PreserveFramePointer\f[R] .
If the frame pointer is available, then external profiling tools (for
example, Linux perf) can construct more accurate stack traces.
.RS
.RE
.TP
.B \f[CB]\-XX:+PrintNMTStatistics\f[R]
Enables printing of collected native memory tracking data at JVM exit
when native memory tracking is enabled (see
\f[CB]\-XX:NativeMemoryTracking\f[R]).
By default, this option is disabled and native memory tracking data
isn\[aq]t printed.
.RS
.RE
.TP
.B \f[CB]\-XX:SharedArchiveFile=\f[R]\f[I]path\f[R]
Specifies the path and name of the class data sharing (CDS) archive file
.RS
.PP
See \f[B]Application Class Data Sharing\f[R].
.RE
.TP
.B \f[CB]\-XX:SharedArchiveConfigFile\f[R]=\f[I]shared_config_file\f[R]
Specifies additional shared data added to the archive file.
.RS
.RE
.TP
.B \f[CB]\-XX:SharedClassListFile=\f[R]\f[I]file_name\f[R]
Specifies the text file that contains the names of the classes to store
in the class data sharing (CDS) archive.
This file contains the full name of one class per line, except slashes
(\f[CB]/\f[R]) replace dots (\f[CB]\&.\f[R]).
For example, to specify the classes \f[CB]java.lang.Object\f[R] and
\f[CB]hello.Main\f[R], create a text file that contains the following two
lines:
.RS
.IP
.nf
\f[CB]
java/lang/Object
hello/Main
\f[R]
.fi
.PP
The classes that you specify in this text file should include the
classes that are commonly used by the application.
They may include any classes from the application, extension, or
bootstrap class paths.
.PP
See \f[B]Application Class Data Sharing\f[R].
.RE
.TP
.B \f[CB]\-XX:+ShowCodeDetailsInExceptionMessages\f[R]
Enables printing of improved \f[CB]NullPointerException\f[R] messages.
When an application throws a \f[CB]NullPointerException\f[R], the option
enables the JVM to analyze the program\[aq]s bytecode instructions to
determine precisely which reference is \f[CB]null\f[R], and describes the
source with a null\-detail message.
The null\-detail message is calculated and returned by
\f[CB]NullPointerException.getMessage()\f[R], and will be printed as the
exception message along with the method, filename, and line number.
By default, this option is enabled.
.RS
.RE
.TP
.B \f[CB]\-XX:+ShowMessageBoxOnError\f[R]
Enables the display of a dialog box when the JVM experiences an
irrecoverable error.
This prevents the JVM from exiting and keeps the process active so that
you can attach a debugger to it to investigate the cause of the error.
By default, this option is disabled.
.RS
.RE
.TP
.B \f[CB]\-XX:StartFlightRecording=\f[R]\f[I]parameter\f[R]\f[CB]=\f[R]\f[I]value\f[R]
Starts a JFR recording for the Java application.
This option is equivalent to the \f[CB]JFR.start\f[R] diagnostic command
that starts a recording during runtime.
You can set the following \f[I]parameter\f[R]\f[CB]=\f[R]\f[I]value\f[R]
entries when starting a JFR recording:
.RS
.TP
.B \f[CB]delay=\f[R]\f[I]time\f[R]
Specifies the delay between the Java application launch time and the
start of the recording.
Append \f[CB]s\f[R] to specify the time in seconds, \f[CB]m\f[R] for
minutes, \f[CB]h\f[R] for hours, or \f[CB]d\f[R] for days (for example,
specifying \f[CB]10m\f[R] means 10 minutes).
By default, there\[aq]s no delay, and this parameter is set to 0.
.RS
.RE
.TP
.B \f[CB]disk=\f[R]{\f[CB]true\f[R]|\f[CB]false\f[R]}
Specifies whether to write data to disk while recording.
By default, this parameter is enabled.
.RS
.RE
.TP
.B \f[CB]dumponexit=\f[R]{\f[CB]true\f[R]|\f[CB]false\f[R]}
Specifies if the running recording is dumped when the JVM shuts down.
If enabled and a \f[CB]filename\f[R] is not entered, the recording is
written to a file in the directory where the process was started.
The file name is a system\-generated name that contains the process ID,
recording ID, and current timestamp, similar to
\f[CB]hotspot\-pid\-47496\-id\-1\-2018_01_25_19_10_41.jfr\f[R].
By default, this parameter is disabled.
.RS
.RE
.TP
.B \f[CB]duration=\f[R]\f[I]time\f[R]
Specifies the duration of the recording.
Append \f[CB]s\f[R] to specify the time in seconds, \f[CB]m\f[R] for
minutes, \f[CB]h\f[R] for hours, or \f[CB]d\f[R] for days (for example,
specifying \f[CB]5h\f[R] means 5 hours).
By default, the duration isn\[aq]t limited, and this parameter is set to
0.
.RS
.RE
.TP
.B \f[CB]filename=\f[R]\f[I]path\f[R]
Specifies the path and name of the file to which the recording is
written when the recording is stopped, for example:
.RS
.IP \[bu] 2
\f[CB]recording.jfr\f[R]
.IP \[bu] 2
\f[CB]/home/user/recordings/recording.jfr\f[R]
.IP \[bu] 2
\f[CB]c:\\recordings\\recording.jfr\f[R]
.RE
.TP
.B \f[CB]name=\f[R]\f[I]identifier\f[R]
Takes both the name and the identifier of a recording.
.RS
.RE
.TP
.B \f[CB]maxage=\f[R]\f[I]time\f[R]
Specifies the maximum age of disk data to keep for the recording.
This parameter is valid only when the \f[CB]disk\f[R] parameter is set to
\f[CB]true\f[R].
Append \f[CB]s\f[R] to specify the time in seconds, \f[CB]m\f[R] for
minutes, \f[CB]h\f[R] for hours, or \f[CB]d\f[R] for days (for example,
specifying \f[CB]30s\f[R] means 30 seconds).
By default, the maximum age isn\[aq]t limited, and this parameter is set
to \f[CB]0s\f[R].
.RS
.RE
.TP
.B \f[CB]maxsize=\f[R]\f[I]size\f[R]
Specifies the maximum size (in bytes) of disk data to keep for the
recording.
This parameter is valid only when the \f[CB]disk\f[R] parameter is set to
\f[CB]true\f[R].
The value must not be less than the value for the \f[CB]maxchunksize\f[R]
parameter set with \f[CB]\-XX:FlightRecorderOptions\f[R].
Append \f[CB]m\f[R] or \f[CB]M\f[R] to specify the size in megabytes, or
\f[CB]g\f[R] or \f[CB]G\f[R] to specify the size in gigabytes.
By default, the maximum size of disk data isn\[aq]t limited, and this
parameter is set to \f[CB]0\f[R].
.RS
.RE
.TP
.B \f[CB]path\-to\-gc\-roots=\f[R]{\f[CB]true\f[R]|\f[CB]false\f[R]}
Specifies whether to collect the path to garbage collection (GC) roots
at the end of a recording.
By default, this parameter is disabled.
.RS
.PP
The path to GC roots is useful for finding memory leaks, but collecting
it is time\-consuming.
Enable this option only when you start a recording for an application
that you suspect has a memory leak.
If the \f[CB]settings\f[R] parameter is set to \f[CB]profile\f[R], the stack
trace from where the potential leaking object was allocated is included
in the information collected.
.RE
.TP
.B \f[CB]settings=\f[R]\f[I]path\f[R]
Specifies the path and name of the event settings file (of type JFC).
By default, the \f[CB]default.jfc\f[R] file is used, which is located in
\f[CB]JAVA_HOME/lib/jfr\f[R].
This default settings file collects a predefined set of information with
low overhead, so it has minimal impact on performance and can be used
with recordings that run continuously.
.RS
.PP
A second settings file is also provided, profile.jfc, which provides
more data than the default configuration, but can have more overhead and
impact performance.
Use this configuration for short periods of time when more information
is needed.
.RE
.PP
You can specify values for multiple parameters by separating them with a
comma.
Event settings and .jfc options can be specified using the following
syntax:
.TP
.B \f[CB]option=\f[R]\f[I]value\f[R]
Specifies the option value to modify.
To list available options, use the \f[CB]JAVA_HOME\f[R]/bin/jfr tool.
.RS
.RE
.TP
.B \f[CB]event\-setting\f[R]=\f[I]value\f[R]
Specifies the event setting value to modify.
Use the form: #= To add a new event setting, prefix the event name with
\[aq]+\[aq].
.RS
.RE
.PP
You can specify values for multiple event settings and .jfc options by
separating them with a comma.
In case of a conflict between a parameter and a .jfc option, the
parameter will take precedence.
The whitespace delimiter can be omitted for timespan values, i.e.
20ms.
For more information about the settings syntax, see Javadoc of the
jdk.jfr package.
.RE
.TP
.B \f[CB]\-XX:ThreadStackSize=\f[R]\f[I]size\f[R]
Sets the Java thread stack size (in kilobytes).
Use of a scaling suffix, such as \f[CB]k\f[R], results in the scaling of
the kilobytes value so that \f[CB]\-XX:ThreadStackSize=1k\f[R] sets the
Java thread stack size to 1024*1024 bytes or 1 megabyte.
The default value depends on the platform:
.RS
.IP \[bu] 2
Linux/x64 (64\-bit): 1024 KB
.IP \[bu] 2
macOS (64\-bit): 1024 KB
.IP \[bu] 2
Windows: The default value depends on virtual memory
.PP
The following examples show how to set the thread stack size to 1
megabyte in different units:
.IP
.nf
\f[CB]
\-XX:ThreadStackSize=1k
\-XX:ThreadStackSize=1024
\f[R]
.fi
.PP
This option is similar to \f[CB]\-Xss\f[R].
.RE
.TP
.B \f[CB]\-XX:\-UseCompressedOops\f[R]
Disables the use of compressed pointers.
By default, this option is enabled, and compressed pointers are used.
This will automatically limit the maximum ergonomically determined Java
heap size to the maximum amount of memory that can be covered by
compressed pointers.
By default this range is 32 GB.
.RS
.PP
With compressed oops enabled, object references are represented as
32\-bit offsets instead of 64\-bit pointers, which typically increases
performance when running the application with Java heap sizes smaller
than the compressed oops pointer range.
This option works only for 64\-bit JVMs.
.PP
It\[aq]s possible to use compressed pointers with Java heap sizes
greater than 32 GB.
See the \f[CB]\-XX:ObjectAlignmentInBytes\f[R] option.
.RE
.TP
.B \f[CB]\-XX:\-UseContainerSupport\f[R]
The VM now provides automatic container detection support, which allows
the VM to determine the amount of memory and number of processors that
are available to a Java process running in docker containers.
It uses this information to allocate system resources.
This support is only available on Linux x64 platforms.
If supported, the default for this flag is \f[CB]true\f[R], and container
support is enabled by default.
It can be disabled with \f[CB]\-XX:\-UseContainerSupport\f[R].
.RS
.PP
Unified Logging is available to help to diagnose issues related to this
support.
.PP
Use \f[CB]\-Xlog:os+container=trace\f[R] for maximum logging of container
information.
See \f[B]Enable Logging with the JVM Unified Logging Framework\f[R] for a
description of using Unified Logging.
.RE
.TP
.B \f[CB]\-XX:+UseHugeTLBFS\f[R]
\f[B]Linux only:\f[R] This option is the equivalent of specifying
\f[CB]\-XX:+UseLargePages\f[R].
This option is disabled by default.
This option pre\-allocates all large pages up\-front, when memory is
reserved; consequently the JVM can\[aq]t dynamically grow or shrink
large pages memory areas; see \f[CB]\-XX:UseTransparentHugePages\f[R] if
you want this behavior.
.RS
.PP
See \f[B]Large Pages\f[R].
.RE
.TP
.B \f[CB]\-XX:+UseLargePages\f[R]
Enables the use of large page memory.
By default, this option is disabled and large page memory isn\[aq]t
used.
.RS
.PP
See \f[B]Large Pages\f[R].
.RE
.TP
.B \f[CB]\-XX:+UseTransparentHugePages\f[R]
\f[B]Linux only:\f[R] 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.
.RS
.RE
.TP
.B \f[CB]\-XX:+AllowUserSignalHandlers\f[R]
Enables installation of signal handlers by the application.
By default, this option is disabled and the application isn\[aq]t
allowed to install signal handlers.
.RS
.RE
.TP
.B \f[CB]\-XX:VMOptionsFile=\f[R]\f[I]filename\f[R]
Allows user to specify VM options in a file, for example,
\f[CB]java\ \-XX:VMOptionsFile=/var/my_vm_options\ HelloWorld\f[R].
.RS
.RE
.SH ADVANCED JIT COMPILER OPTIONS FOR JAVA
.PP
These \f[CB]java\f[R] options control the dynamic just\-in\-time (JIT)
compilation performed by the Java HotSpot VM.
.TP
.B \f[CB]\-XX:AllocateInstancePrefetchLines=\f[R]\f[I]lines\f[R]
Sets the number of lines to prefetch ahead of the instance allocation
pointer.
By default, the number of lines to prefetch is set to 1:
.RS
.RS
.PP
\f[CB]\-XX:AllocateInstancePrefetchLines=1\f[R]
.RE
.RE
.TP
.B \f[CB]\-XX:AllocatePrefetchDistance=\f[R]\f[I]size\f[R]
Sets the size (in bytes) of the prefetch distance for object allocation.
Memory about to be written with the value of new objects is prefetched
up to this distance starting from the address of the last allocated
object.
Each Java thread has its own allocation point.
.RS
.PP
Negative values denote that prefetch distance is chosen based on the
platform.
Positive values are bytes to prefetch.
Append the letter \f[CB]k\f[R] or \f[CB]K\f[R] to indicate kilobytes,
\f[CB]m\f[R] or \f[CB]M\f[R] to indicate megabytes, or \f[CB]g\f[R] or
\f[CB]G\f[R] to indicate gigabytes.
The default value is set to \-1.
.PP
The following example shows how to set the prefetch distance to 1024
bytes:
.RS
.PP
\f[CB]\-XX:AllocatePrefetchDistance=1024\f[R]
.RE
.RE
.TP
.B \f[CB]\-XX:AllocatePrefetchInstr=\f[R]\f[I]instruction\f[R]
Sets the prefetch instruction to prefetch ahead of the allocation
pointer.
Possible values are from 0 to 3.
The actual instructions behind the values depend on the platform.
By default, the prefetch instruction is set to 0: