从头认识多线程-2.20 synchronized同步方法的无限等待与解决方法

这一章节我们讨论一下synchronized同步方法的无限等待与解决方法。

1.synchronized同步方法的无限等待

package com.ray.deepintothread.ch02.topic_20;/** *  * @author RayLee * */public class InfiniteWait {public static void main(String[] args) throws InterruptedException {MyService myService = new MyService();ThreadOne threadOne = new ThreadOne(myService);Thread thread = new Thread(threadOne);thread.start();ThreadTwo threadTwo = new ThreadTwo(myService);Thread thread2 = new Thread(threadTwo);thread2.start();}}class ThreadOne implements Runnable {private MyService myService;public ThreadOne(MyService myService) {this.myService = myService;}@Overridepublic void run() {try {myService.updateA();} catch (InterruptedException e) {e.printStackTrace();}}}class ThreadTwo implements Runnable {private MyService myService;public ThreadTwo(MyService myService) {this.myService = myService;}@Overridepublic void run() {try {myService.updateB();} catch (InterruptedException e) {e.printStackTrace();}}}class MyService {private static int id = 0;// 死循环或者不停的长连接public synchronized void updateA() throws InterruptedException {while (true) {}}public synchronized void updateB() throws InterruptedException {for (int i = 0; i < 5; i++) {System.out.println(Thread.currentThread().getName() + " " + id++);Thread.sleep(100);}}}

输出：

（无）

上面的例子由于线程1一直持有对象的锁，因此线程2无论如何都进不去方法里面。

2.解决方法

（1）使用同步代码块

package com.ray.deepintothread.ch02.topic_20;/** *  * @author RayLee * */public class SolutionOfInfiniteWait {public static void main(String[] args) throws InterruptedException {MyService2 myService = new MyService2();ThreadThree threadThree = new ThreadThree(myService);Thread thread = new Thread(threadThree);thread.start();ThreadFour threadFour = new ThreadFour(myService);Thread thread2 = new Thread(threadFour);thread2.start();}}class ThreadThree implements Runnable {private MyService2 myService;public ThreadThree(MyService2 myService) {this.myService = myService;}@Overridepublic void run() {try {myService.updateA();} catch (InterruptedException e) {e.printStackTrace();}}}class ThreadFour implements Runnable {private MyService2 myService;public ThreadFour(MyService2 myService) {this.myService = myService;}@Overridepublic void run() {try {myService.updateB();} catch (InterruptedException e) {e.printStackTrace();}}}class MyService2 {private static int id = 0;private static Object object1 = new Object();private static Object object2 = new Object();public void updateA() throws InterruptedException {synchronized (object1) {while (true) {}}}public synchronized void updateB() throws InterruptedException {synchronized (object2) {for (int i = 0; i < 5; i++) {System.out.println(Thread.currentThread().getName() + " " + id++);Thread.sleep(100);}}}}

输出：

Thread-1 0
Thread-1 1
Thread-1 2
Thread-1 3
Thread-1 4

虽然这个代码清单里面线程2能够进去方法里面，但是线程1所造成的无限等待还是没有解决，而我们平常的服务器最怕就是这种长连接，因为它占掉服务器非常多的资源，不能够有效释放

（2）增加超时计数器

package com.ray.deepintothread.ch02.topic_20;/** *  * @author RayLee * */public class SolutionOfInfiniteWait2 {public static void main(String[] args) throws InterruptedException {MyService3 myService = new MyService3();ThreadFive threadFive = new ThreadFive(myService);Thread thread = new Thread(threadFive);thread.start();ThreadSix threadSix = new ThreadSix(myService);Thread thread2 = new Thread(threadSix);thread2.start();}}class ThreadFive implements Runnable {private MyService3 myService;public ThreadFive(MyService3 myService) {this.myService = myService;}@Overridepublic void run() {try {myService.updateA();} catch (InterruptedException e) {e.printStackTrace();}}}class ThreadSix implements Runnable {private MyService3 myService;public ThreadSix(MyService3 myService) {this.myService = myService;}@Overridepublic void run() {try {myService.updateB();} catch (InterruptedException e) {e.printStackTrace();}}}class MyService3 {private int id = 0;private Object object1 = new Object();private Object object2 = new Object();public void updateA() throws InterruptedException {//计数器int count = 0;synchronized (object1) {while (true) {count++;if (count > 3) {System.out.println(Thread.currentThread().getName() + " get out");break;} else {Thread.sleep(100);}}}}public synchronized void updateB() throws InterruptedException {synchronized (object2) {for (int i = 0; i < 5; i++) {System.out.println(Thread.currentThread().getName() + " " + id++);Thread.sleep(100);}}}}

输出：

Thread-1 0
Thread-1 1
Thread-1 2
Thread-0 get out
Thread-1 3
Thread-1 4

从输出我们可以看见，线程0已经在某个边界的时候，通过计数器的超时得到了释放。

总结：这一章节主要讨论了synchronized同步方法的无限等待与解决方法。

这一章节就到这里，谢谢

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我的github：https://github.com/raylee2015/DeepIntoThread

目录：http://blog.csdn.net/raylee2007/article/details/51204573