写一个java死锁的例子

一，死锁是什么

死锁是指两个或两个以上的线程在执行过程中，由于竞争资源或者由于彼此通信而造成的一种阻塞的现象，若无外力作用，它们都将无法推进下去。

相当于两台并行前进的车，左边的车想变道右边，右边的车想变道左边，谁也不让谁。

死锁代码：

public class DeadLock {
    public static class MyThread implements Runnable {
        Object obj = new Object();
        public void run() {
            System.out.println( Thread.currentThread().getName() + "run prepare");
                synchronized(obj) {// obj锁
                    System.out.println( Thread.currentThread().getName() + "run start");
                    doSomething(); // this锁
                }


        }


        public  void doSomething() {  // this锁
            System.out.println( Thread.currentThread().getName() + "doSomething start");
            try {
                Thread.sleep(15000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            synchronized(obj) { // obj锁
                System.out.println( Thread.currentThread().getName() + "doSomething synchronized");
            }
        }
    }

    public static void main(String[] args) {
        MyThread t = new MyThread();
        Thread th = new Thread(t);
        Thread th2 = new Thread(t);

        th.start();
        th2.start();
    }
}

结果：

.jar:/Users/lucy6/Documents/maven/Spring2/out/production/lock DeadLock

Thread-1run prepare
Thread-0run prepare
Thread-1run start
Thread-1doSomething start
Thread-1doSomething synchronized
Thread-0run start
Thread-0doSomething start
Thread-0doSomething synchronized

Process finished with exit code 0

分析：

线程1首先执行，打印Thread-1run prepare

线程0马上抢到执行权，打印Thread-0run prepare

线程1有抢到执行权，进入同步代码块，拿到obj锁，打印：

     Thread-1doSomething start、

     Thread-1doSomething start、

    Thread-1doSomething synchronized

然后线程进入冻结状态，相当于放弃cpu执行权。

显然线程0会拿到cpu执行权，接下来线程0应该进入同步代码块，但是什么都没有发生，因为obj被线程1拿着没释放，出现死锁。

当线程1冻结时间到，重新拿回cpu执行权，执行完所有代码，释放锁，然后线程0得以继续执行。

二，一个更好的例子

public class DeadLock2 {
    static Object objA = new Object();
    static Object objB = new Object();

    public static class MyThread implements Runnable {
        public void run() {
            System.out.println(Thread.currentThread().getName() + "，run prepare");
            synchronized (objA) {// obj锁
                System.out.println(Thread.currentThread().getName() + ",拿到objA锁");
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (objB) {
                    System.out.println(Thread.currentThread().getName() + ",拿到objA锁");

                }
            }


        }
    }

    public static class MyThread2 implements Runnable {
        public void run() {
            System.out.println(Thread.currentThread().getName() + "run prepare");
            synchronized (objB) {// obj锁
                System.out.println(Thread.currentThread().getName() + "，拿到 objB锁");
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (objA) {
                    System.out.println(Thread.currentThread().getName() + "，拿到 objA锁");


                }
            }


        }
    }

    public static void main(String[] args) {
        MyThread t = new MyThread();
        MyThread2 t2 = new MyThread2();
        Thread th = new Thread(t);
        Thread th2 = new Thread(t2);

        th.start();
        th2.start();
    }
}

三，死锁产生的条件

 

1，多线程

2，多个锁

3，锁嵌套