title: jvm工具_jcmd
date: 2017-02-15 09:18:05
tags:
category: JVM

jcmd，1.7发布新的jvm相关信息的诊断工具，Oracle官网建议的替代jmap的工具。注意其使用条件是只能在被诊断的JVM同台sever上，并且具有相同的用户和组(user and group).

jcmd最简单的用法是列出机器上启动的java进程。直接输出jcmd，在我的机器上显示如下：

25571 org.jetbrains.jps.cmdline.Launcher /Applications/IntelliJ IDEA.app/Contents/lib/javac2.jar:/Applications/IntelliJ IDEA.app/Contents/lib/snappy-in-java-0.5.1.jar:/Applications/IntelliJ IDEA.app/Contents/lib/jna.jar:/Applications/IntelliJ IDEA.app/Contents/lib/openapi.jar:/Applications/IntelliJ IDEA.app/Contents/lib/oromatcher.jar:/Applications/IntelliJ IDEA.app/Contents/lib/trove4j.jar:/Applications/IntelliJ IDEA.app/Contents/lib/jps-builders.jar:/Applications/IntelliJ IDEA.app/Contents/lib/nanoxml-2.2.3.jar:/Applications/IntelliJ IDEA.app/Contents/lib/jgoodies-forms.jar:/Applications/IntelliJ IDEA.app/Contents/lib/jdom.jar:/Applications/IntelliJ IDEA.app/Contents/lib/asm-all.jar:/Applications/IntelliJ IDEA.app/Contents/lib/protobuf-2.5.0.jar:/Applications/IntelliJ IDEA.app/Contents/lib/rt/jps-plugin-system.jar:/Applications/IntelliJ IDEA.app/Contents/lib/jna-platform.jar:/Applications/IntelliJ IDEA.app/Contents/lib/annotations.jar:/Applications/IntelliJ IDEA.app/Contents/lib/forms_rt.jar:/Applications/Intelli
21156 /Applications/SmartGit.app/Contents/Resources/Java/bootloader.jar
25542 org.jetbrains.idea.maven.server.RemoteMavenServer
25622 sun.tools.jcmd.JCmd
23880 
25485 
23647 org.apache.catalina.startup.Bootstrap start
24223 

jdk还有个类似的工具“jps",也具有此功能。

jcmd指令后面通常接某一java进程的ID或者mainclass 加上各种指令，得到相关诊断信息。

而jcmd提供了相当丰富的指令，正常人都不可能记住……输入help指令，可以显示可选指令的列表。

tobideMacBook-Pro:hexoBlog tobi$ jcmd 23647 help
23647:
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

前面几个相关的功能,官网文档说明是可以线上动态诊断jvm信息的工具，但是要商用协议，没细研究，略过. 光看指令的名称，任然只能大概知道他的意思，如果想进一步了解该指令，继续在help 后面加上指令，可以看见详细说明，如下此指令的意思是打印jvm中线程的情况，impact表示对运行中的JVM的影响，permission，需要的权限，使用的语法：

jcmd 23647 help Thread.print
23647:
Thread.print
Print all threads with stacktraces.

Impact: Medium: Depends on the number of threads.

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

Syntax : Thread.print [options]

Options: (options must be specified using the <key> or <key>=<value> syntax)
    -l : [optional] print java.util.concurrent locks (BOOLEAN, false)

打印jvm中的线程细节

jcmd 23647 Thread.print -l

Thread.print -l
23647:
2017-02-15 10:27:29
Full thread dump Java HotSpot(TM) 64-Bit Server VM (25.112-b16 mixed mode):

"Attach Listener" #1598 daemon prio=9 os_prio=31 tid=0x00007f9ec22e4000 nid=0x441b waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"RMI TCP Connection(6)-127.0.0.1-EventThread" #1593 daemon prio=5 os_prio=31 tid=0x00007f9ebc8b0800 nid=0x7fbb waiting on condition [0x0000700011190000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x000000077ad4d708> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
    at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:494)

   Locked ownable synchronizers:
    - None

"RMI TCP Connection(10.18.19.28:12181)" #1592 daemon prio=5 os_prio=31 tid=0x00007f9eb887e000 nid=0x8f5f runnable [0x0000700010d84000]
   java.lang.Thread.State: RUNNABLE
    at sun.nio.ch.KQueueArrayWrapper.kevent0(Native Method)
    at sun.nio.ch.KQueueArrayWrapper.poll(KQueueArrayWrapper.java:198)
    at sun.nio.ch.KQueueSelectorImpl.doSelect(KQueueSelectorImpl.java:117)
    at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
    - locked <0x000000077ad4bfa0> (a sun.nio.ch.Util$3)
    - locked <0x000000077ad4bf90> (a java.util.Collections$UnmodifiableSet)
    - locked <0x000000077ad4be70> (a sun.nio.ch.KQueueSelectorImpl)
    at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)
    at org.apache.zookeeper.ClientCnxnSocketNIO.doTransport(ClientCnxnSocketNIO.java:349)
    at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1081)

   Locked ownable synchronizers:
    - None

"Curator-Framework-0-EventThread" #1591 daemon prio=5 os_prio=31 tid=0x00007f9ebf060000 nid=0xa563 waiting on condition [0x0000700010978000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x000000077f1aaa50> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
    at org.apache.zookeeper.ClientCnxn$EventThread.run(ClientCnxn.java:494)

   Locked ownable synchronizers:
    - None

如上，可以导出jvm中的线程细节。指令中的-l是打印线程所拥有的锁的情况。

另外jstack和jcmd Thread.print都具有发现死锁的功能。

2个线程分别持有锁A和锁B,去取对方的锁，java代码如下：

public class DeadLock {
    public static void main(String[] args) throws InterruptedException {
        ReentrantLock lockA = new ReentrantLock();
        ReentrantLock lockB = new ReentrantLock();
        Condition conditionA = lockA.newCondition();
        Condition conditionB = lockB.newCondition();
        new Thread(
                () -> {
                    try {
                        lockA.lock();
                        TimeUnit.SECONDS.sleep(2L);
                        lockB.lock();

                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } finally {
                        lockA.unlock();
                        lockB.unlock();
                    }
                }
        ).start();

        new Thread(
                () -> {
                    try {
                        lockB.lock();
                        TimeUnit.SECONDS.sleep(2L);
                        lockA.lock();

                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } finally {
                        lockA.unlock();
                        lockB.unlock();
                    }
                }
        ).start();
        TimeUnit.DAYS.sleep(1L);
    }
}

运行起来后，使用jcmd pid Thread.print -l，会发现多了一些显示如下:

Found one Java-level deadlock:
=============================
"Thread-1":
  waiting for ownable synchronizer 0x000000076abd6b20, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
  which is held by "Thread-0"
"Thread-0":
  waiting for ownable synchronizer 0x000000076abd6b50, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
  which is held by "Thread-1"

Java stack information for the threads listed above:
===================================================
"Thread-1":
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x000000076abd6b20> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
    at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209)
    at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285)
    at lazyguy.yyf.exercise.DeadLock.lambda$main$1(DeadLock.java:39)
    at lazyguy.yyf.exercise.DeadLock$$Lambda$2/363771819.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:745)
"Thread-0":
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x000000076abd6b50> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
    at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209)
    at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285)
    at lazyguy.yyf.exercise.DeadLock.lambda$main$0(DeadLock.java:23)
    at lazyguy.yyf.exercise.DeadLock$$Lambda$1/668386784.run(Unknown Source)
    at java.lang.Thread.run(Thread.java:745)

Found 1 deadlock.

jcmd自动为你发现了死锁~~

GC.class_stats和GC.class_histogram

显示jvm meta class的统计信息，impact high。

GC.heap_dump

导出堆的快照文件，可以配合jvisualm使用。

GC.run_finalization和GC.run

调用 java.lang.System.runFinalization().方法和java.lang.System.gc()方法。

VM.uptime

显示JVM启动的时间

VM.flags

打印虚拟机相关参数

VM.system_properties

打印当前jvm读到的system_properties,配合grep工具快速查看

VM.command_line

打印”启动“JVM的时候使用了那些指令

事实上jcmd是原来的

jps(java 进程查找)+jstack(线程信息)+jmap(堆和meta data内存信息)

这3样工具的集合加强版。