java thread dump 分析死锁

喜欢ヅ旅行 2022-07-14 12:00 288阅读 0赞

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 DeadlockerThread2.run(Deadlocker.java:47)−waitingtolock<0x00000007d6f97ec8>(a\[I)−locked<0x00000007d6f97ee0>(a\[I)“Thread−0”:atDeadlockerThread1.run(Deadlocker.java:28)  
\- waiting to lock <0x00000007d6f97ee0> (a \[I)  
\- locked <0x00000007d6f97ec8> (a \[I)

Found 1 deadlock.

上面的红色部分便也可以清楚查询到死锁的位置了；

当然据说第二部分只有java5 以上版本才有的（未验证）。

[原帖见 ][Link 1]

[Link 1]: http://caoyaojun1988-163-com.iteye.com/blog/1697395