jcmd 用法

$ jcmd -help
Usage: jcmd <pid | main class> <command ...|PerfCounter.print|-f file>
   or: jcmd -l
   or: jcmd -h

  command must be a valid jcmd command for the selected jvm.
  Use the command "help" to see which commands are available.
  If the pid is 0, commands will be sent to all Java processes.
  The main class argument will be used to match (either partially
  or fully) the class used to start Java.
  If no options are given, lists Java processes (same as -p).

  PerfCounter.print display the counters exposed by this process
  -f  read and execute commands from the file
  -l  list JVM processes on the local machine
  -h  this help

参数描述

pid：接收诊断命令请求的进程ID。
main class ：接收诊断命令请求的进程的main类。匹配进程时，main类名称中包含指定子字符串的任何进程均是匹配的。如果多个正在运行的Java进程共享同一个main类，诊断命令请求将会发送到所有的这些进程中。
command：接收诊断命令请求的进程的main类。匹配进程时，main类名称中包含指定子字符串的任何进程均是匹配的。如果多个正在运行的Java进程共享同一个main类，诊断命令请求将会发送到所有的这些进程中。
注意: 如果任何参数含有空格，你必须使用英文的单引号或双引号将其包围起来。此外，你必须使用转义字符来转移参数中的单引号或双引号，以阻止操作系统shell处理这些引用标记。当然，你也可以在参数两侧加上单引号，然后在参数内使用双引号(或者，在参数两侧加上双引号，在参数中使用单引号)。
Perfcounter.print：打印目标Java进程上可用的性能计数器。性能计数器的列表可能会随着Java进程的不同而产生变化。
-f file：从文件file中读取命令，然后在目标Java进程上调用这些命令。在file中，每个命令必须写在单独的一行。以"#"开头的行会被忽略。当所有行的命令被调用完毕后，或者读取到含有stop关键字的命令，将会终止对file的处理。
-l：查看所有的进程列表信息。
-h：查看帮助信息。（同 -help）

查看进程 jcmd -l

命令：jcmd -l
描述：查看当前机器上所有的 jvm 进程信息。

jcmd
jcmd -l
jps -m

这三个命令的效果是一样的

查看性能统计

命令：jcmd <pid> PerfCounter.print
描述：查看指定进程的性能统计信息。

$ jcmd 22912 PerfCounter.print
22912:
java.ci.totalTime=45473763
java.cls.loadedClasses=3350
java.cls.sharedLoadedClasses=0
java.cls.sharedUnloadedClasses=0
java.cls.unloadedClasses=0
java.property.java.class.path="D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/jps-launcher.jar;D:/Program Files/Java/jdk1.8.0_161/lib/tools.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/optimizedFileManager.jar"
java.property.java.endorsed.dirs=""""
java.property.java.ext.dirs="D:\Program Files\Java\jdk1.8.0_161\jre\lib\ext;C:\WINDOWS\Sun\Java\lib\ext"
java.property.java.home="D:\Program Files\Java\jdk1.8.0_161\jre"
...

列出当前运行的 java 进程可以执行的操作

命令：jcmd <pid> help

$ jcmd 22912 help
22912:
The following commands are available:
JFR.stop
JFR.start
JFR.dump
JFR.check
VM.native_memory
VM.check_commercial_features
VM.unlock_commercial_features
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

查看具体命令的选项

如果想查看命令的选项，比如想查看 JFR.dump 命令选项，可以通过如下命令: jcmd <pid> help JFR.dump

$ jcmd 22912 help JFR.dump
22912:
JFR.dump
Copies contents of a JFR recording to file. Either the name or the recording id must be specified.

Impact: Low

Permission: java.lang.management.ManagementPermission(monitor)

Syntax : JFR.dump [options]

Options: (options must be specified using the <key> or <key>=<value> syntax)
        name : [optional] Recording name, e.g. \"My Recording\" (STRING, no default value)
        recording : [optional] Recording number, use JFR.check to list available recordings (JLONG, -1)
        filename :  Copy recording data to file, i.e \"C:\Users\user\My Recording.jfr\" (STRING, no default value)
        compress : [optional] GZip-compress "filename" destination (BOOLEAN, false)

JFR 相关命令

JFR 功能跟 jmc.exe 工具的飞行记录器的功能一样的。
要使用 JFR 相关的功能，必须使用 VM.unlock_commercial_features 参数取消锁定商业功能。

$ jcmd 22912 JFR.start
22912:
Java Flight Recorder not enabled.

Use VM.unlock_commercial_features to enable.

jmc.exe 显示的提示

启动JFR
执行命令：jcmd $PID JFR.start name=abc,duration=120s
Dump JFR
等待至少duration（本文设定120s）后，执行命令：jcmd PID JFR.dump name=abc,duration=120s filename=abc.jfr（注意，文件名必须为.jfr后缀）
检查JFR状态
执行命令：jcmd $PID JFR.check name=abc,duration=120s
停止JFR
执行命令：jcmd $PID JFR.stop name=abc,duration=120s
JMC分析
切回开发机器，下载步骤3中生成的abc.jfr，打开jmc，导入abc.jfr即可进行可视化分析

VM.uptime

命令：jcmd <pid> VM.uptime
描述：查看 JVM 的启动时长。

$ jcmd 22912 VM.uptime
22912:
266194.602 s

GC.class_histogram

命令：jcmd <pid> GC.class_histogram
描述：查看系统中类统计信息。

$ jcmd 22912 GC.class_histogram
22912:

 num     #instances         #bytes  class name
----------------------------------------------
   1:           649       17000784  [B
   2:         19299        1799336  [C
   3:          7474         475320  [Ljava.lang.Object;
   4:         19116         458784  java.lang.String
   5:          3603         402048  java.lang.Class
   6:          6979         223328  java.util.concurrent.ConcurrentHashMap$Node
   7:          4405         140960  java.util.HashMap$Node
   8:          2372         113856  gnu.trove.THashMap
   9:          6447         103152  java.lang.Object
  10:          1428          98088  [I
  11:            47          58064  [Ljava.util.concurrent.ConcurrentHashMap$Node;

这里和jmap -histo pid的效果是一样的，可以查看每个类的实例数量和占用空间大小。

Thread.print

命令：jcmd <pid> Thread.print
描述：查看线程堆栈信息。

$ jcmd 22912 Thread.print
22912:
2019-05-13 13:23:42
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.161-b12 mixed mode):

"NettythreadDeathWatcher-2-1" #17 daemon prio=1 os_prio=-2 tid=0x00000000178f0800 nid=0x8190 waiting on condition [0x0000000018faf000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at io.netty.util.ThreadDeathWatcher$Watcher.run(ThreadDeathWatcher.java:152)
        at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:138)
        at java.lang.Thread.run(Thread.java:748)

"DestroyJavaVM" #15 prio=5 os_prio=0 tid=0x00000000178f1800 nid=0x67d4 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"JPS event loop" #10 prio=5 os_prio=0 tid=0x0000000016ab4800 nid=0x1a80 runnable [0x00000000170ae000]
   java.lang.Thread.State: RUNNABLE
        at sun.nio.ch.WindowsSelectorImpl$SubSelector.poll0(Native Method)
        at sun.nio.ch.WindowsSelectorImpl$SubSelector.poll(WindowsSelectorImpl.java:296)
        at sun.nio.ch.WindowsSelectorImpl$SubSelector.access$400(WindowsSelectorImpl.java:278)
        at sun.nio.ch.WindowsSelectorImpl.doSelect(WindowsSelectorImpl.java:159)
        at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
        - locked <0x00000000d5419030> (a io.netty.channel.nio.SelectedSelectionKeySet)
        - locked <0x00000000d5419020> (a java.util.Collections$UnmodifiableSet)
        - locked <0x00000000d5418fa0> (a sun.nio.ch.WindowsSelectorImpl)
        at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)
        at io.netty.channel.nio.SelectedSelectionKeySetSelector.select(SelectedSelectionKeySetSelector.java:62)
        at io.netty.channel.nio.NioEventLoop.select(NioEventLoop.java:752)
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:408)
        at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)
        at java.lang.Thread.run(Thread.java:748)

该命令同 jstack 命令。

GC.heap_dump

命令：jcmd <pid> GC.heap_dump FILE_NAME
描述：查看 JVM 的Heap Dump。

$ jcmd 22912 GC.heap_dump d:\dump.hprof
22912:
Heap dump file created

跟 jmap命令：jmap -dump:format=b,file=heapdump.phrof pid 效果一样。导出的 dump 文件，可以使用MAT 或者 Visual VM 等工具进行分析（如果只指定文件名，默认会生成在启动 JVM 的目录里）。

VM.system_properties

命令：jcmd <pid> VM.system_properties
描述：查看 JVM 的属性信息。

$ jcmd 22912 VM.system_properties
22912:
#Mon May 13 13:28:38 CST 2019
java.vendor=Oracle Corporation
preload.project.path=D\:/WorkSpace/hqev/rbac
sun.java.launcher=SUN_STANDARD
sun.management.compiler=HotSpot 64-Bit Tiered Compilers
sun.nio.ch.bugLevel=
idea.config.path=D\:/Program Files/JetBrains/IntelliJIdeaConfig/config
idea.paths.selector=IntelliJIdea2018.1
kotlin.daemon.client.alive.path="C\:\\Users\\Victor.Zxy\\AppData\\Local\\Temp\\kotlin-idea-3229673183181290493-is-running"
os.name=Windows 10
sun.boot.class.path=D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\lib\\resources.jar;D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\lib\\rt.jar;D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\lib\\sunrsasign.jar;D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\lib\\jsse.jar;D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\lib\\jce.jar;D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\lib\\charsets.jar;D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\lib\\jfr.jar;D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\classes
sun.desktop=windows
idea.plugins.path=D\:/Program Files/JetBrains/IntelliJIdeaConfig/config/plugins
java.vm.specification.vendor=Oracle Corporation
java.runtime.version=1.8.0_161-b12
io.netty.serviceThreadPrefix=Netty
user.name=Victor.Zxy
kotlin.incremental.compilation=true
idea.home.path=D\:\\Program Files\\JetBrains\\IntelliJ IDEA 2018.1
user.language=zh
jdt.compiler.useSingleThread=true
sun.boot.library.path=D\:\\Program Files\\Java\\jdk1.8.0_161\\jre\\bin
java.version=1.8.0_161
user.timezone=Asia/Shanghai

VM.flags

命令：jcmd <pid> VM.flags
描述：查看 JVM 的启动参数。

$ jcmd 22912 VM.flags
22912:
-XX:CICompilerCount=3 -XX:InitialHeapSize=335544320 
-XX:MaxHeapSize=734003200 -XX:MaxNewSize=244318208 
-XX:MinHeapDeltaBytes=524288 -XX:NewSize=111673344 
-XX:OldSize=223870976 -XX:-PrintGC -XX:+UseCompressedClassPointers 
-XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps 
-XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC

VM.command_line

命令：jcmd <pid> VM.command_line
描述：查看 JVM 的启动命令行。

$ jcmd 22912 VM.command_line
22912:
VM Arguments:
jvm_args: -Xmx700m -Djava.awt.headless=true -Djava.endorsed.dirs="" -Djdt.compiler.useSingleThread=true -Dpreload.project.path=D:/WorkSpace/hqev/rbac -Dpreload.config.path=D:/Program Files/JetBrains/IntelliJIdeaConfig/config/options -Dcompile.parallel=false -Drebuild.on.dependency.change=true -Djava.net.preferIPv4Stack=true -Dio.netty.initialSeedUniquifier=8246017585702392224 -Dfile.encoding=UTF-8 -Duser.language=zh -Duser.country=CN -Didea.paths.selector=IntelliJIdea2018.1 -Didea.home.path=D:\Program Files\JetBrains\IntelliJ IDEA 2018.1 -Didea.config.path=D:/Program Files/JetBrains/IntelliJIdeaConfig/config -Didea.plugins.path=D:/Program Files/JetBrains/IntelliJIdeaConfig/config/plugins -Djps.log.dir=D:/Program Files/JetBrains/IntelliJIdeaConfig/system/log/build-log -Djps.fallback.jdk.home=D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/jre64 -Djps.fallback.jdk.version=1.8.0_152-release -Dio.netty.noUnsafe=true -Djava.io.tmpdir=D:/Program Files/JetBrains/IntelliJIdeaConfig/system/compile-server/rbac_aca57a50/_temp_ -Djps.backward.ref.index.builder=true -Dkotlin.incremental.compilation=true -Dkotlin.daemon.enabled -Dkotlin.daemon.client.alive.path="C:\Users\Victor.Zxy\AppData\Local\Temp\kotlin-idea-3229673183181290493-is-running"
java_command: org.jetbrains.jps.cmdline.Launcher D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/jps-builders.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/annotations.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/trove4j.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/resources_en.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/protobuf-java-3.0.0.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/idea_rt.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/commons-logging-1.2.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/slf4j-api-1.7.10.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/asm-all.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/lz4-java-1.3.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/javac2.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/aether-1.1.0-all.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/httpcore-4.4.5.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/guava-21.0.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/jna.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/oro-2.0.8.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/jps-model.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/jps-builders-6.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/util.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/nanoxml-2.2.3.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/httpclient-4.5.2.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/jna-platform.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/forms_rt.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/aether-dependency-resolver.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/jgoodies-forms.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/snappy-in-java-0.5.1.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/maven-aether-provider-3.3.9-all.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/commons-cod
java_class_path (initial): D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/jps-launcher.jar;D:/Program Files/Java/jdk1.8.0_161/lib/tools.jar;D:/Program Files/JetBrains/IntelliJ IDEA 2018.1/lib/optimizedFileManager.jar
Launcher Type: SUN_STANDARD

GC.run_finalization

命令：jcmd <pid> GC.run_finalization
描述：对 JVM 执行 java.lang.System.runFinalization()

$ jcmd 22912 GC.run_finalization
22912:
Command executed successfully

执行一次finalization操作，相当于执行java.lang.System.runFinalization()

GC.run

命令：jcmd <pid> GC.run
描述：对 JVM 执行 java.lang.System.gc()

$ jcmd 22912 GC.run
22912:
Command executed successfully

告诉垃圾收集器打算进行垃圾收集，而垃圾收集器进不进行收集是不确定的。

PerfCounter.print

命令：jcmd <pid> PerfCounter.print
描述：查看 JVM 性能相关的参数。

$ jcmd 22912 PerfCounter.print
22912:
java.ci.totalTime=45606938
java.cls.loadedClasses=3363
java.cls.sharedLoadedClasses=0
java.cls.sharedUnloadedClasses=0
java.cls.unloadedClasses=9
...

VM.version

命令：jcmd <pid> VM.version
描述：查看目标jvm进程的版本信息。

$ jcmd 22912 VM.version
22912:
Java HotSpot(TM) 64-Bit Server VM version 25.161-b12
JDK 8.0_161

JVM 性能调优工具 jcmd