"jcmd 0 help" - Listing JVM Diagnostic Commands

Java Tools Tutorials - Herong's Tutorial Examples

∟"jcmd 0 help" - Listing JVM Diagnostic Commands

This section provides a tutorial example on how to use 'jcmd 0 help' command to get a list of diagnostic commands that are supported in each running JVM.

If you want to know which diagnostic command can be fired to a specific JVM, you can run the "jcmd <pid> help" command to get a list of valid commands for the given JVM.

Or you can run the "jcmd 0 help" command to a list of valid command for each every JVM on the local machine. For example, if I have 2 JVMs running: "java LongSleep" and "jshell", I will get the following output from "jcmd 0 help":

herong> jcmd 0 help

10848:
The following commands are available:
JFR.configure
JFR.stop
JFR.start
JFR.dump
JFR.check
VM.log
VM.native_memory
VM.check_commercial_features
VM.unlock_commercial_features
ManagementAgent.status
ManagementAgent.stop
ManagementAgent.start_local
ManagementAgent.start
Compiler.directives_clear
Compiler.directives_remove
Compiler.directives_add
Compiler.directives_print
VM.print_touched_methods
Compiler.codecache
Compiler.codelist
Compiler.queue
VM.classloader_stats
Thread.print
JVMTI.data_dump
JVMTI.agent_load
VM.stringtable
VM.symboltable
VM.class_hierarchy
VM.systemdictionary
GC.class_stats
GC.class_histogram
GC.heap_dump
GC.finalizer_info
GC.heap_info
GC.run_finalization
GC.run
VM.info
VM.uptime
VM.dynlibs
VM.set_flag
VM.flags
VM.system_properties
VM.command_line
VM.version
help

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

6024:
The following commands are available:
JFR.configure
JFR.stop
JFR.start
JFR.dump
JFR.check
VM.log
...

Wow! It looks like we can:

Control the JFR (Java Flight Recordings) feature in the JVM.
Get information from the running JVM, like uptime, runtime flags, command line arguments, etc.
Get GC (Garbage Collection) information and perform a GC.
Get JIT Compiler information and manage compiler directives.