Java線程：併發協作-死鎖

原創

LeleLJ

2020-02-22 17:45

Java線程：併發協作-死鎖

線程發生死鎖可能性很小，即使看似可能發生死鎖的代碼，在運行時發生死鎖的可能性也是小之又小。

發生死鎖的原因一般是兩個對象的鎖相互等待造成的。

在《Java線程：線程的同步與鎖》一文中，簡述死鎖的概念與簡單例子，但是所給的例子是不完整的，這裏給出一個完整的例子。

/**
* Java線程：併發協作-死鎖
*
* @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() + "獲取了resourceA的鎖！");
                        synchronized (resourceB) {
                                System.out.println("read():" + Thread.currentThread().getName() + "獲取了resourceB的鎖！");
                                return resourceB.value + resourceA.value;
                        }
                }
        }

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

下面死鎖的情況發生了，真是難得一見啊：