Java线程：并发协作-死锁

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Java线程：并发协作-死锁

线程发生死锁可能性很小，即使看似可能发生死锁的代码，在运行时发生死锁的可能性也是小之又小。

发生死锁的原因一般是两个对象的锁相互等待造成的。

在《Java线程：线程的同步与锁》一文中，简述死锁的概念与简单例子，但是所给的例子是不完整的，这里给出一个完整的例子。

/**
* Java线程：并发协作-死锁
*
* @author Administrator 2009-11-4 22:06:13
*/
public class Test {
        public static void main(String[] args) {
                DeadlockRisk dead = new DeadlockRisk();
                MyThread t1 = new MyThread(dead, 1, 2);
                MyThread t2 = new MyThread(dead, 3, 4);
                MyThread t3 = new MyThread(dead, 5, 6);
                MyThread t4 = new MyThread(dead, 7, 8);

                t1.start();
                t2.start();
                t3.start();
                t4.start();
        }

}

class MyThread extends Thread {
        private DeadlockRisk dead;
        private int a, b;

        MyThread(DeadlockRisk dead, int a, int b) {
                this.dead = dead;
                this.a = a;
                this.b = b;
        }

        @Override
        public void run() {
                dead.read();
                dead.write(a, b);
        }
}

class DeadlockRisk {
        private static class Resource {
                public int value;
        }

        private Resource resourceA = new Resource();
        private Resource resourceB = new Resource();

        public int read() {
                synchronized (resourceA) {
                        System.out.println("read():" + Thread.currentThread().getName() + "获取了resourceA的锁！");
                        synchronized (resourceB) {
                                System.out.println("read():" + Thread.currentThread().getName() + "获取了resourceB的锁！");
                                return resourceB.value + resourceA.value;
                        }
                }
        }

        public void write(int a, int b) {
                synchronized (resourceB) {
                        System.out.println("write():" + Thread.currentThread().getName() + "获取了resourceB的锁！");
                        synchronized (resourceA) {
                                System.out.println("write():" + Thread.currentThread().getName() + "获取了resourceA的锁！");
                                resourceA.value = a;
                                resourceB.value = b;
                        }
                }
        }
}

下面死锁的情况发生了，真是难得一见啊：

read():Thread-0获取了resourceA的锁！
read():Thread-0获取了resourceB的锁！
write():Thread-0获取了resourceB的锁！
read():Thread-3获取了resourceA的锁！

线程0的read方法获取A，B的锁之后，代码运行完成释放锁，write获取了B的锁，这个时候需要A的锁，线程3的read方法获取了A的锁，这个时候需要B的锁，结果线程0等待A的锁，线程3等待B的锁，互相等待，造成死锁。

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