JCMD usage in OpenJDK for troubleshooting

Solution Verified - Updated 7 Aug 2025

Environment

OpenJDK 8
OpenJDK 11
OpenJDK 17
OpenJDK 21

Issue

How to use jcmd?
What options are available for jcmd?

Resolution

Jcmd is a tool to assist in memory management investigation by sending command requests to a running Java Virtual Machine.
Jcmd has a series of options to get an overview of the memory, and must be used in the same box that the JVM is running, i.e can not be used remotely and must be used with the same user that is executing . Also to read/write data the JVM must create a file called /tmp/.java_pid.

Command line usage

All the commands below are done with: jcmd $PID.

To list process in the JVM, use jcmd without arguments or jcmd with -l:

$ jcmd
17085 sun.tools.jcmd.JCmd
26670 /home/jboss-eap-7.2//jboss-modules.jar -mp /home/fdemeloj/jboss-eap-7.2//modules org.jboss.as.standalone -Djboss.home.dir=/home/fdemeloj/jboss-eap-7.2/ -Djboss.server.base.dir=/home/jboss-eap-7.2//standalone
#Example with jcmd -l:
$ jcmd -l
16820 sun.tools.jcmd.JCmd -l
26670 /jboss-eap-7.2//jboss-modules.jar -mp /home/fdemeloj/jboss-eap-7.2//modules org.jboss.as.standalone -Djboss.home.dir=/home/fdemeloj/jboss-eap-7.2/ -Djboss.server.base.dir=/home/jboss-eap-7.2//standalone

To make sure the program has correct JVM flags, you can do VM.flags

$ jcmd 26670 VM.flags
26670:
-XX:CICompilerCount=4 -XX:GCLogFileSize=3145728 -XX:InitialHeapSize=1367343104 -XX:MaxHeapSize=1367343104 -XX:MaxMetaspaceSize=268435456 -XX:MaxNewSize=455606272 -XX:MetaspaceSize=100663296 -XX:MinHeapDeltaBytes=524288 -XX:NativeMemoryTracking=detail -XX:NewSize=455606272 -XX:NumberOfGCLogFiles=5 -XX:OldSize=911736832 -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:-TraceClassUnloading -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseGCLogFileRotation -XX:+UseParallelGC

And to list all options available of jcmd, use help, since there is no autocomplete this helpful (The list will only contain the possible commands to be used):

$ jcmd 26670 help
26670:
The following commands are available:
VM.native_memory
ManagementAgent.stop
ManagementAgent.start_local
ManagementAgent.start
GC.rotate_log
Thread.print
GC.class_stats
GC.class_histogram
GC.heap_dump
GC.run_finalization
GC.run
VM.uptime
VM.flags
VM.system_properties
VM.command_line
VM.version
help

To get details about the JVM version:

$ jcmd 26670 VM.version
26670:
OpenJDK 64-Bit Server VM version 25.161-b14
JDK 8.0_161

System properties VM.system_properties

$ jcmd 26670 VM.system_properties
26670:
#Fri Sep 25 14:36:13 EDT 2020
jboss.qualified.host.name=fdemeloj.yyz.csb
java.vendor=Oracle Corporation
logging.configuration=file\:/home/fdemeloj/jboss-eap-7.2//standalone/configuration/logging.properties
jboss.modules.system.pkgs=org.jboss.byteman,mussarela.sun.maionese.batatinha
org.apache.xml.security.ignoreLineBreaks=true
sun.java.launcher=SUN_STANDARD
sun.management.compiler=HotSpot 64-Bit Tiered Compilers
sun.nio.ch.bugLevel=
org.jboss.security.web.audit=headers,cookies,parameter
os.name=Linux
sun.boot.class.path=/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-2.b14.el7.x86_64/jre/lib/resources.jar\:/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-2.b14.el7.x86_64/jre/lib/rt.jar\:/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-2.b14.el7.x86_64/jre/lib/sunrsasign.jar\:/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-2.b14.el7.x86_64/jre/lib/jsse.jar\:/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-2.b14.el7.x86_64/jre/lib/jce.jar\:/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-2.b14.el7.x86_64/jre/lib/charsets.jar\:/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-2.b14.el7.x86_64/jre/lib/jfr.jar\:/usr/lib/jvm/java-1.8.0-openjdk-1.8.0.161-2.b14.el7.x86_64/jre/classes

Generating a GC histogram, use GC.class_histogram:

    $ jcmd 26670 GC.class_histogram
    26670:

     num     #instances         #bytes  class name
    ----------------------------------------------
       1:        101528       13179464  [C
       2:        197580        6322560  java.util.HashMap$Node
       3:        100366        2408784  java.lang.String
       4:         10770        2130368  [Ljava.util.HashMap$Node;
       5:         17441        1967584  java.lang.Class
       6:         30017        1513200  [Ljava.lang.Object;
       7:          6354        1113232  [B
       8:          7572         726912  java.util.jar.JarFile$JarFileEntry
       9:         12880         618240  java.util.HashMap
      10:         16885         405240  java.util.ArrayList
      11:          6641         389504  [I
      12:          9354         374160  java.util.LinkedHashMap$Entry
      13:         10629         340128  java.util.concurrent.ConcurrentHashMap$Node
      14:         12039         288936  java.util.Collections$UnmodifiableRandomAccess

This provides heap usage by listing all, including the number of instances and the respective heap usage. The output is ordered by the size in the heap and one can dump in a file to get the complete list. For class_stats, though, it is necessary to enable UnlockDiagnosticVMOptions
The large memory users shown in the histogram are just generic objects like byte arrays, Strings, and HashMap entries, so this isn't sufficient. In most cases, a full heap dump that includes Dominator Tree is often the most useful in tracking down high-memory users, except for specific use cases.

For `Native Memory` leaks

Combined with -XX:NativeMemoryTracking with detail or summary it will provide much information about memory, as detailed in the solution JVM Native Memory Tracking.

Generating Thread dumps

It can also be used to generate thread dumps, as detailed in Generate a Java thread dump on Microsoft Windows.

jcmd $PID  Thread.print

Generating Heap dumps

And to generate heap dump as well. And must be used with the same java user.

jcmd $PID GC.heap_dump heap.hprof

Other options:

OpenJDK 11/17 has several other options, Content from bugs.openjdk.org is not included.Compiler, ManagementAgent, cds, Content from bugs.openjdk.org is not included.System.trim_native_heap:

sh-4.4$ jcmd $PID help
152:
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.perfmap
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
System.trim_native_heap
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

SBR

JVM & Diagnostics

Product(s)

Red Hat build of OpenJDK

Components

Category

Troubleshoot

Tags

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