Java Platform, Standard Edition Troubleshooting Guide
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2.6 The jcmd Utility

The jcmd utility is used to send diagnostic command requests to the JVM, where these requests are useful for controlling Java Flight Recordings, troubleshoot, and diagnose JVM and Java Applications. It must be used on the same machine where the JVM is running, and have the same effective user and group identifiers that were used to launch the JVM.

For more details on jcmd syntax and other usage details, see the jcmd command man page.

A special command jcmd <process id/main class> PerfCounter.print prints all performance counters in the process.

The command jcmd <process id/main class> <command> [options] sends the actual command to the JVM.

Example 2-1 shows diagnostic command requests to JVM using jcmd utility.

The following sections describe some useful commands and troubleshooting techniques with jcmd utility.

2.6.1 Useful Commands for jcmd Utility

The available diagnostic command may be different in different versions of HotSpot VM; therefore, using jcmd <process id/main class> help is the best way to see all available options. The following are some of the most useful commands that were in the tool since JDK 8. Remember you can always use jcmd <process id/main class> help <command> to get any additional options to these commands.

  • Print full HotSpot and JDK version ID

    jcmd <process id/main class> VM.version

  • Print all the system properties set for a VM

    There can be several hundred lines of information displayed.

    jcmd <process id/main class> VM.system_properties

  • Print all the flags used for a VM

    Even if you have provided no flags, some of the default values will be printed, for example initial and maximum heap size.

    jcmd <process id/main class> VM.flags

  • Print the uptime in seconds

    jcmd <process id/main class> VM.uptime

  • Create a class histogram

    The results can be rather verbose, so you can redirect the output to a file. Both internal and application specific classes are included in the list. Classes taking the most memory are listed at the top, and classes are listed in a descending order.

    jcmd <process id/main class> GC.class_histogram

  • Create a heap dump (hprof dump)

    jcmd GC.heap_dump filename=Myheapdump

    This is the same as using jmap -dump:file=<file> <pid>, but jcmd is the recommended tool to use.

  • Create a heap histogram

    jcmd <process id/main class> GC.class_histogram filename=Myheaphistogram

    This is the same as using jmap -histo <pid>, but jcmd is the recommended tool to use.

  • Print all threads with stack traces

    jcmd <process id/main class> Thread.print

2.6.2 Troubleshoot with jcmd Utility

The jcmd utility provides the following troubleshooting options:

  • Start a recording

    For example, to start a 2-minute recording on the running Java process with the identifier 7060 and save it to myrecording.jfr in the current directory, use the following:

    jcmd 7060 JFR.start name=MyRecording settings=profile delay=20s duration=2m filename=C:\TEMP\myrecording.jfr

  • Check a recording

    The JFR.check diagnostic command checks a running recording. For example:

    jcmd 7060 JFR.check

  • Stop a recording

    The JFR.stop diagnostic command stops a running recording and has the option to discard the recording data. For example:

    jcmd 7060 JFR.stop

  • Dump a recording

    The JFR.dump diagnostic command stops a running recording and has the option to dump recordings to a file. For example:

    jcmd 7060 JFR.dump name=MyRecording filename=C:\TEMP\myrecording.jfr

  • Create a heap dump

    The preferred way to create a heap dump is

    jcmd <pid> GC.heap_dump filename=Myheapdump

  • Create a heap histogram

    The preferred way to create a heap histogram is

    jcmd <pid> GC.class_histogram filename=Myheaphistogram

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