用jstack诊断java应用故障

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公司的生产系统大多是java应用，特别是java消费者，前段时间频繁出现故障。消息队列会突然堵塞，查看消费者日志，发现处理延时明显延长，而此时网络无丢包。最后多方排查，是java消费者运行出现死锁。

这种问题可以通过jstack来dump获取java stack和native stack信息查明问题。

如：

jstack [-l] pid

提供一个测试的例子：

/**
* 死锁例子
* @author crane.ding
* @since 2011-3-20
*/
public class DeadLock {

    public static void main(String[] args) {
        final Object obj_1 = new Object(), obj_2 = new Object();

        Thread t1 = new Thread("t1"){
            @Override
            public void run() {
                synchronized (obj_1) {
                    try {
                        Thread.sleep(3000);
                    } catch (InterruptedException e) {}

                    synchronized (obj_2) {
                        System.out.println("thread t1 done.");
                    }
                }
            }
        };

        Thread t2 = new Thread("t2"){
            @Override
            public void run() {
                synchronized (obj_2) {
                    try {
                        Thread.sleep(3000);
                    } catch (InterruptedException e) {}

                    synchronized (obj_1) {
                        System.out.println("thread t2 done.");
                    }
                }
            }
        };
        t1.start();
        t2.start();
    }
}

保存为DeadLock.java。以上DeadLock类是一个死锁的例子,假使在我们不知情的情况下,运行DeadLock后,发现等了N久都没有在屏幕打印线程完成信息。这个时候我们就可以使用ps –ef|grep xxx查看该程序的pid值和使用jstack来生产堆栈结果问题。

javac DeadLock.java

jar cvf deadlock.jar DeadLock*.class

java -cp deadlock.jar DeadLock &

ps –ef|grep deadlock.jar #获取deadlock.jar进程的pid为1448

jstack -l 1448 > deadlock.jstack

结果文件deadlock.jstack内容类似如下：

2012-01-20 20:15:20
Full thread dump Java HotSpot(TM) Client VM (19.1-b02 mixed mode, sharing):

"DestroyJavaVM" prio=6 tid=0x00316800 nid=0x9fc waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"t2" prio=6 tid=0x02bcf000 nid=0xc70 waiting for monitor entry [0x02f6f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.demo.DeadLock$2.run(DeadLock.java:40)
    - waiting to lock <0x22a297a8> (a java.lang.Object)
    - locked <0x22a297b0> (a java.lang.Object)

   Locked ownable synchronizers:
    - None

"t1" prio=6 tid=0x02bce400 nid=0xba0 waiting for monitor entry [0x02f1f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.demo.DeadLock$1.run(DeadLock.java:25)
    - waiting to lock <0x22a297b0> (a java.lang.Object)
    - locked <0x22a297a8> (a java.lang.Object)

   Locked ownable synchronizers:
    - None

"Low Memory Detector" daemon prio=6 tid=0x02bb9400 nid=0xa6c runnable [0x00000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"CompilerThread0" daemon prio=10 tid=0x02bb2800 nid=0xcb8 waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"Attach Listener" daemon prio=10 tid=0x02bb1000 nid=0x7f4 waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"Signal Dispatcher" daemon prio=10 tid=0x02bd2800 nid=0xd80 runnable [0x00000000]
   java.lang.Thread.State: RUNNABLE

   Locked ownable synchronizers:
    - None

"Finalizer" daemon prio=8 tid=0x02bab000 nid=0xe1c in Object.wait() [0x02d3f000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x229e1148> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)
    - locked <0x229e1148> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

   Locked ownable synchronizers:
    - None

"Reference Handler" daemon prio=10 tid=0x02ba6800 nid=0xbe0 in Object.wait() [0x02cef000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x229e1048> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:485)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
    - locked <0x229e1048> (a java.lang.ref.Reference$Lock)

   Locked ownable synchronizers:
    - None

"VM Thread" prio=10 tid=0x02b6a400 nid=0x568 runnable

"VM Periodic Task Thread" prio=10 tid=0x02bc8400 nid=0x75c waiting on condition

JNI global references: 878


Found one Java-level deadlock:
=============================
"t2":
waiting to lock monitor 0x02baaeec (object 0x22a297a8, a java.lang.Object),
which is held by "t1"
"t1":
waiting to lock monitor 0x02baa2bc (object 0x22a297b0, a java.lang.Object),
which is held by "t2"

Java stack information for the threads listed above:
===================================================
"t2":
    at com.demo.DeadLock$2.run(DeadLock.java:40)
    - waiting to lock <0x22a297a8> (a java.lang.Object)
    - locked <0x22a297b0> (a java.lang.Object)
"t1":
    at com.demo.DeadLock$1.run(DeadLock.java:25)
    - waiting to lock <0x22a297b0> (a java.lang.Object)
    - locked <0x22a297a8> (a java.lang.Object)

Found 1 deadlock.

从这个结果文件我们一看到发现了一个死锁,具体是线程t2在等待线程t1,而线程t1在等待线程t2造成的,同时也记录了线程的堆栈和代码行数,通过这个堆栈和行数我们就可以去检查对应的代码块,从而发现问题和解决问题。

文章基本引用自：http://crane-ding.iteye.com/blog/968862

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