Java多线程-7-死锁

七、死锁

1、问题引出

class DeadLock {
    
    
    private final Object LOCK_A = new Object();
    private final Object LOCK_B = new Object();

    public void methodA() {
    
    
        synchronized (LOCK_A) {
    
    
            try {
    
     Thread.sleep(1000); } catch (InterruptedException e) {
    
     e.printStackTrace(); }

            System.out.println(Thread.currentThread().getName() + "获取锁A，尝试获取锁B");
            synchronized (LOCK_B) {
    
    
            }
        }
    }

    public void methodB() {
    
    
        synchronized (LOCK_B) {
    
    
            try {
    
     Thread.sleep(1000); } catch (InterruptedException e) {
    
     e.printStackTrace(); }

            System.out.println(Thread.currentThread().getName() + "获取锁B，尝试获取锁A");
            synchronized (LOCK_A) {
    
    
            }
        }
    }

}

public class DeadLockTest {
    
    

    public static void main(String[] args) {
    
    
        DeadLock dl = new DeadLock();

        new Thread(dl::methodA, "线程A").start();
        new Thread(dl::methodB, "线程B").start();
    }

}

执行代码，可以看到程序阻塞，无法执行完毕

2、问题原因

多个已经获取到锁的线程，当尝试获取其他线程已持有的锁时，就会发生死锁现象

3、如何排查

使用Java自带的两个命令：jps和jstack

首先执行命令：jps -l，获取正在执行的Java程序的进程ID

再执行命令：jstack -l 进程ID，即可看到以下类似的信息

"线程B" #12 prio=5 os_prio=0 tid=0x000000001657a800 nid=0x65c waiting for monitor entry [0x0000000016d8f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at DeadLock.methodB(DeadLockTest.java:21)
        - waiting to lock <0x00000000f57e2568> (a java.lang.Object)
        - locked <0x00000000f57e2578> (a java.lang.Object)
        at DeadLockTest$$Lambda$2/205797316.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:
        - None

"线程A" #11 prio=5 os_prio=0 tid=0x0000000016579800 nid=0x4f4 waiting for monitor entry [0x0000000016c8e000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at DeadLock.methodA(DeadLockTest.java:11)
        - waiting to lock <0x00000000f57e2578> (a java.lang.Object)
        - locked <0x00000000f57e2568> (a java.lang.Object)
        at DeadLockTest$$Lambda$1/1637070917.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

   Locked ownable synchronizers:
        - None



Found one Java-level deadlock:
=============================
"线程B":
  waiting to lock monitor 0x0000000002ffde38 (object 0x00000000f57e2568, a java.lang.Object),
  which is held by "线程A"
"线程A":
  waiting to lock monitor 0x0000000002ffb658 (object 0x00000000f57e2578, a java.lang.Object),
  which is held by "线程B"

Java stack information for the threads listed above:
===================================================
"线程B":
        at DeadLock.methodB(DeadLockTestjava:21)
        - waiting to lock <0x00000000f57e2568> (a java.lang.Object)
        - locked <0x00000000f57e2578> (a java.lang.Object)
        at DeadLockTest$$Lambda$2/205797316.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)
"线程A":
        at DeadLock.methodA(DeadLockTestjava:11)
        - waiting to lock <0x00000000f57e2578> (a java.lang.Object)
        - locked <0x00000000f57e2568> (a java.lang.Object)
        at DeadLockTest$$Lambda$1/1637070917.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:748)

Found 1 deadlock.

清楚地标识出了死锁的位置