dump线程让死锁无处可逃
编程技术 / houtizong 发布于 3年前 65
昨天看到有人问：”一个程序在运行的时候，如何知道它是否发生死锁，如果发生死锁，如何找到发生死锁的位置？“；便贴了一段dump线程的数据，今天又有人问我怎么从dump文件中分析死锁，随做此文：
1、首先构造死锁，代码如下：
public class Deadlocker { private static Object lock_1 = new int[1]; private static Object lock_2 = new int[1]; public class Thread1 extends Thread { @Override public void run() { System.out.println("thread 1 start"); synchronized (lock_1) { try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("thread 1 get lock 1 need lock 2"); synchronized (lock_2) { } } System.out.println("thread 1 end"); } } public class Thread2 extends Thread { @Override public void run() { System.out.println("thread 2 start"); synchronized (lock_2) { try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("thread 2 get lock 2 need lock 1"); synchronized (lock_1) { } } System.out.println("thread 2 end"); } } public static void main(String[] args) { Thread1 thread1 = new Deadlocker().new Thread1(); Thread2 thread2 = new Deadlocker().new Thread2(); thread1.start(); thread2.start(); }}
2、运行结果如下：
thread 1 start
thread 2 start
thread 1 get lock 1 need lock 2
thread 2 get lock 2 need lock 1
3、分析：
从打印的结果很容易分析线程1 和线程2 处于死锁。相互需要对方的持有的锁；
4、使用：jps 查看线程id，再使用：jstack 线程id > dumpthread 命令。部分结果如下（去除各种gc线程。。。）：
第一部分：
"Thread-1" prio=10 tid=0x0000000040a49800 nid=0x2d1e waiting for monitor entry [0x00007f50eda98000]
java.lang.Thread.State: BLOCKED (on object monitor)
at Deadlocker$Thread2.run(Deadlocker.java:47)
- waiting to lock <0x00000007d6f97ec8> (a [I)
- locked <0x00000007d6f97ee0> (a [I)
"Thread-0" prio=10 tid=0x0000000040a47800 nid=0x2d1d waiting for monitor entry [0x00007f50edb99000]
java.lang.Thread.State: BLOCKED (on object monitor)
at Deadlocker$Thread1.run(Deadlocker.java:28)
- waiting to lock <0x00000007d6f97ee0> (a [I)
- locked <0x00000007d6f97ec8> (a [I)
。。。。。各种其他线程
这里可以看出两个线程处于BLOCKED 状态，各自在等待各自的锁（ 0x00000007d6f97ec8 和0x00000007d6f97ee0 ）
第二部分：
Found one Java-level deadlock:
=============================
"Thread-1":
waiting to lock monitor 0x0000000040a33dd8 (object 0x00000007d6f97ec8, a [I),
which is held by "Thread-0"
"Thread-0":
waiting to lock monitor 0x0000000040a365e0 (object 0x00000007d6f97ee0, a [I),
which is held by "Thread-1"
这个一看就更明白了。。说发现一个java死锁，Thread-1在等待一个lock，而这个锁被Thread-0 持有，
而 Thread-0在等待另外一个lock，而这个锁被Thread-1 持有
Java stack information for the threads listed above:
===================================================
"Thread-1":
at Deadlocker$Thread2.run(Deadlocker.java:47)
- waiting to lock <0x00000007d6f97ec8> (a [I)
- locked <0x00000007d6f97ee0> (a [I)
"Thread-0":
at Deadlocker$Thread1.run(Deadlocker.java:28)
- waiting to lock <0x00000007d6f97ee0> (a [I)
- locked <0x00000007d6f97ec8> (a [I)
Found 1 deadlock.
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