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Java and other IT related posts


Sometimes, as you probably know, there could be a situation when you would want to send some commands to a running JVM instance to get an idea what’s heppening in the VM. For example you might want to send a command to make a heap dump, print class histogram or something else. This is the case for jcmd utility. It can send a lot of diagnostic commands to a JVM throught a special diagnostic channel. And the only condition for it to be possible is that JVM should be running normally, it must not be freezed. For example when JVM makes a heap dump it can’t accept any diagnostic commands.

So, using jcmd is as simply as

jcmd PID <command>

jcmd can print a list of supported commands.

jcmd PID help

This post will cover commands:

  • JFR.stop
  • JFR.start
  • JFR.dump
  • GC.class_stats
  • GC.class_histogram
  • GC.heap_dump
  • GC.run_finalization
  • VM.native_memory
  • VM.uptime
  • VM.flags
  • VM.system_properties
  • VM.command_line
  • VM.version

Java Flight Recorder commands

There some commands that can control jfr recording.

  • JFR.stop - stops current recording
  • JFR.start - starts a recording
  • JFR.dump - save current recording to a file

GC commands

jcmd can help with memory issues by using these commands

  • GC.class_stats - prints statistics about loaded classes: how many methods, annotations, bytecode, etc every loaded class has
  • GC.class_histogram - prints histogram that shows how many objects are live in heap of every loaded class
  • GC.heap_dump - makes a heap dump
  • GC.run_finalization - runs finalization phase of GC
  • - runs FullGC

VM commands

Finally, there are some informational commands jcmd can send.

  • VM.native_memory - shows infomation about native memory usage
  • VM.uptime - shows JVM uptime
  • VM.flags - shows flags that were used to launch JVM
  • VM.system_properties - shows all system properties that are accessable within JVM
  • VM.command_line - shows full command line that was used to launch JVM
  • VM.version - shows JVM’s version