多个线程争抢同一个monitor的lock会陷入阻塞。

看代码：

/**
 * @author: wang ming xin
 * @create: 2019-01-12 11:00
 */
public class ThisMonitor {
    public synchronized void method1(){
        System.out.println(Thread.currentThread().getName() +  " enter to method1");

        try {
            TimeUnit.MINUTES.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public synchronized void method2(){
        System.out.println(Thread.currentThread().getName() +  " enter to method2");

        try {
            TimeUnit.MINUTES.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args){
      ThisMonitor monitor = new ThisMonitor();

      new Thread(monitor::method1,"thread1").start();
      new Thread(monitor::method2,"thread2").start();

    }

}

运行结果：

thread1获取到monitor锁后，陷入沉睡

thread2阻塞

我们来用jtack命令分析下这两个线程争抢锁的情况

首先使用jps查看进程id

jstack -l 14460

很明显的看到，thread1获取了锁，并sleep

locked   <0x00000000d5f8b050>

thread2 BLOCKED 阻塞

waiting to lock <0x00000000d5f8b050>

等待和线程1相同的锁

再看一个死锁的例子，并同样用jstack分析：

public class DeadLock implements Runnable {

    public int flag;

    private static Object o1 = new Object();
    private static Object o2 = new Object();

    @Override
    public void run() {
        System.out.println("当前线程:"+Thread.currentThread().getName()+",flag=" + flag);
        if (flag == 1) {
            synchronized (o1) {
                try {
                    Thread.sleep(500);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                synchronized (o2) {
                    System.out.println("1");
                }
            }
        }
        if (flag == 0) {
            synchronized (o2) {
                try {
                    Thread.sleep(500);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                synchronized (o1) {
                    System.out.println("0");
                }
            }
        }
    }

    public static void main(String[] args) {

        DeadLock td1 = new DeadLock();
        DeadLock td2 = new DeadLock();
        td1.flag = 1;
        td2.flag = 0;

        new Thread(td1,"thread1").start();
        new Thread(td2,"thread2").start();

    }
}

运行结果：

jstack分析：

自己需要获取的锁被对方持有，

对方需要获取的锁被自己持有，

导致死锁

转载请注明：汪明鑫的个人博客 » jstack的使用