Mastering jcmd: Essential Java Debugging Commands and Examples

jcmd Overview

jcmd is a built‑in JDK tool introduced in JDK 7, offering powerful diagnostic capabilities that can replace tools such as jstack and jmap.

It sends diagnostic commands to a JVM; for security, the user must match the Java process’s user and group.

Syntax

jcmd [pid|main-class] command... | PerfCounter.print | -f filename

jcmd -l

jcmd -h

pid or main‑class identifies the target JVM; command specifies the operation.

-f filename – read commands from a file

-l – list JVMs not running in Docker

-h – help

Common Commands

List running JVMs jcmd -l Outputs all active JVM processes, similar to jps.

Print thread stack jcmd <pid> Thread.print -l The -l flag includes lock information from java.util.concurrent.

Example output shows thread details and locked synchronizers.

Heap information jcmd <pid> GC.heap_info Retrieves heap statistics.

Heap dump jcmd <pid> GC.heap_dump heap_dump.out Creates a heap dump file named heap_dump.out.

Class histogram jcmd <pid> GC.class_histogram Shows the number of instances and memory usage per class, useful for performance analysis.

Java Flight Recorder (JFR)

jcmd also supports JFR commands such as JFR.start, JFR.dump, JFR.stop, providing event recording capabilities inside the JVM.

Explore additional jcmd functions to enhance Java diagnostics.