从头认识多线程-2.14 由同步的synchronized (newobject()) 引起的异步现象和脏读

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这一章节我们来讨论一下由同步的synchronized (newobject()) 引起的异步现象和脏读。

1.代码清单

package com.ray.deepintothread.ch02.topic_14;/** *  * @author RayLee * */public class AsynchOfSynchWithUnSameObject {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 Object object1;private Object object2;public MyService() {object1 = new Object();object2 = new Object();}public void updateA() throws InterruptedException {synchronized (object1) {long startTime = System.currentTimeMillis();System.out.println("updateA startTime:" + startTime);Thread.sleep(1000);long endTime = System.currentTimeMillis();System.out.println("updateA endTime:" + endTime);}}public void updateB() throws InterruptedException {synchronized (object2) {long startTime = System.currentTimeMillis();System.out.println("updateB startTime:" + startTime);Thread.sleep(1000);long endTime = System.currentTimeMillis();System.out.println("updateB endTime:" + endTime);}}}

输出：

updateA startTime:1462628803229
updateB startTime:1462628803229
updateB endTime:1462628804229
updateA endTime:1462628804229

2.结论

从输出可以看见，updateA和updateB基本同时执行，然后同时结束，虽然这两个方法都是同步了相应的代码块，但是由于不同线程同时方法两个方法，而两个方法之间是不同步，只是方法内的某些语句同步，这就造成了上面的这种现象。

3.思考

有上面异步的结果推断，上面的方法虽然是同步了某些代码，但是还是会出现脏读的情况。

4.证明

package com.ray.deepintothread.ch02.topic_14;/** *  * @author RayLee * */public class DirtyRead {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 Object object1;private Object object2;private int id = 0;public MyService2() {object1 = new Object();object2 = new Object();}public void updateA() throws InterruptedException {synchronized (object1) {for (int i = 0; i < 5; i++) {System.out.println(Thread.currentThread().getName() + " " + id++);Thread.sleep(50);}}}public 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-0 0
Thread-1 1
Thread-0 2
Thread-1 4
Thread-0 3
Thread-0 5
Thread-0 6
Thread-1 6
Thread-1 7
Thread-1 8

从输出可以看见，如果真的是同步，应该每一个Thread都是书序的往下增加，但是上面显然不是，而且在id=2的位置thread-1出现了脏读。

总结：这一章节主要讨论了由同步的synchronized (newobject()) 引起的异步现象和脏读，以及证明的过程。

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

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