死锁

死锁是指两个或者两个以上的线程在执行过程中，因争夺资源而造成的一种互相等待的现象，若无外力干涉那么它们都将无法推进下去

模拟死锁

class HoldLockThread extends Thread {

    private String lockA;
    private String lockB;

    public HoldLockThread(String lockA, String lockB) {
        this.lockA = lockA;
        this.lockB = lockB;
    }

    @Override
    public void run() {
        synchronized (lockA) {
            System.out.println("线程" + Thread.currentThread().getName() + "持有锁" + lockA + ",尝试获取锁" + lockB);
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (Exception e) {
                e.printStackTrace();
            }
            synchronized (lockB) {
                System.out.println("线程" + Thread.currentThread().getName() + "持有锁" + lockB);
            }
        }
    }
}

public class DeadLockDemo {
    public static void main(String[] args) {
        String lockA = "lockA";
        String lockB = "lockB";
        new Thread(new HoldLockThread(lockA, lockB), "A").start();
        new Thread(new HoldLockThread(lockB, lockA), "B").start();
    }
}

 如何定位死锁问题？

1、先通过jps命令找到java进程pid

22208 Launcher
22209 DeadLockDemo
75993 
22570 Jps

2、运行jstack pid，会自动识别可能的死锁

Found one Java-level deadlock:
=============================
"B":
  waiting to lock monitor 0x00007fca8f813f68 (object 0x000000076accf6b0, a java.lang.String),
  which is held by "A"
"A":
  waiting to lock monitor 0x00007fca8f816958 (object 0x000000076accf6e8, a java.lang.String),
  which is held by "B"

Java stack information for the threads listed above:
===================================================
"B":
        at com.study.demo.HoldLockThread.run(DeadLockDemo.java:25)
        - waiting to lock <0x000000076accf6b0> (a java.lang.String)
        - locked <0x000000076accf6e8> (a java.lang.String)
        at java.lang.Thread.run(Thread.java:748)
"A":
        at com.study.demo.HoldLockThread.run(DeadLockDemo.java:25)
        - waiting to lock <0x000000076accf6e8> (a java.lang.String)
        - locked <0x000000076accf6b0> (a java.lang.String)
        at java.lang.Thread.run(Thread.java:748)

Found 1 deadlock.