Easy JVM Observability with the JCMD CLI

Ha! Dr. David Syer just taught me about something that's been in Java forever: jmcd.

The Bellsoft website has a good explanation:

jcmd is the command line executed with JVM diagnostic tools shipped with OpenJDK. Before the introduction of jcmd, there were multiple tools to run live and post mortem diagnostics for JVM. jcmd became a to-go utility for live diagnostics of JVM executed from the command line (e.g., dumping threads or inspecting JVM configuration). The post mortem diagnostics are performed with other tools.

It's like a one-stop shop for all your JVM observability needs. The JVM exports certain signals, or commands, that it will respond to if you invoke them from jcmd. So, suppose you have a Spring Boot application that's up and running.

Run just jcmd to see which JVM applications are running. It'll give you process identifiers (PIDs):

jcmd

Alternatively, Spring Boot will print the PID in the console when the application starts up:

... Starting JcmdApplication using Java 17.0.5 with PID 66488 ...

The PID for my application is 66488. So, run jmcd <pid> and you'll get a number of commands to which the JVM will respond for that application.

> jcmd 66488

66488:
The following commands are available:
Compiler.CodeHeap_Analytics
Compiler.codecache
Compiler.codelist
Compiler.directives_add
Compiler.directives_clear
Compiler.directives_print
Compiler.directives_remove
Compiler.queue
GC.class_histogram
GC.finalizer_info
GC.heap_dump
GC.heap_info
GC.run
GC.run_finalization
JFR.check
JFR.configure
JFR.dump
JFR.start
JFR.stop
JVMTI.agent_load
JVMTI.data_dump
ManagementAgent.start
ManagementAgent.start_local
ManagementAgent.status
ManagementAgent.stop
Thread.print
VM.cds
VM.class_hierarchy
VM.classloader_stats
VM.classloaders
VM.command_line
VM.dynlibs
VM.events
VM.flags
VM.info
VM.log
VM.metaspace
VM.native_memory
VM.print_touched_methods
VM.set_flag
VM.stringtable
VM.symboltable
VM.system_properties
VM.systemdictionary
VM.uptime
VM.version
help

For more information about a specific command use 'help <command>'.

Then you can invoke the signal with jcmd, like this:

> jcmd 66488 VM.version

66488:
OpenJDK 64-Bit Server VM version 17.0.5+8-jvmci-22.3-b08
JDK 17.0.5

Super useful, eh?