从头认识多线程-2.13 由同步的synchronized (newobject()) 引起的异步现象和脏读

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;
	}

	@Override
	public void run() {
		try {
			myService.updateA();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

class ThreadTwo implements Runnable {

	private MyService myService;

	public ThreadTwo(MyService myService) {
		this.myService = myService;
	}

	@Override
	public 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;
	}

	@Override
	public void run() {
		try {
			myService.updateA();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

class ThreadFour implements Runnable {

	private MyService2 myService;

	public ThreadFour(MyService2 myService) {
		this.myService = myService;
	}

	@Override
	public 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