jcmd

jcmd is a command-line tool included with the Java Development Kit (JDK) that allows users to send diagnostic commands to a running Java Virtual Machine (JVM). It communicates via the Attach API and is designed to assist in diagnosing performance, memory, threading, and other VM state issues. The tool targets a specific JVM, typically identified by its process identifier (PID), and executes commands through that target.

jcmd consolidates functionality from several older tools, providing a unified interface for operating on a JVM. It can work with a locally running process and, in some configurations, can connect to remote targets as well. Users invoke jcmd with the syntax: jcmd <pid> <command> [arguments].

Common usage and commands include: jcmd <pid> help to list available commands for the target JVM; jcmd <pid> GC.run to trigger a garbage collection; jcmd <pid> GC.class_histogram to produce a histogram of loaded classes; jcmd <pid> GC.heap_dump /path/file.hprof to dump the heap; jcmd <pid> Thread.print to print thread stacks; and JFR-related commands such as JFR.start, JFR.stop, or JFR.dump on newer JDKs. The exact command set varies by JDK version, and some commands may require appropriate permissions or specific product editions.

Availability and context: jcmd is included in Oracle JDK and OpenJDK distributions from Java 7 onward and is located in the JDK’s bin directory. It is a widely used tool for obtaining runtime diagnostics, gathering data for performance tuning, and producing diagnostic artifacts. As with any administrative tool, access should be restricted to authorized users.