从头认识多线程-2.4 脏读（DirtyRead）

这一章节我们来讨论一下脏读（DirtyRead）。

1.为什么出现脏读？

因为代码没有做同步，虽然set方法同步，但是由于get方法一般都会忘了，导致读的值是被写过的

2.代码清单

（1）由于程序没有同步，并且执行太快，导致脏读

package com.ray.deepintothread.ch02.topic_4;

public class DirtyRead {
	public static void main(String[] args) throws InterruptedException {
		MyTestObjectOne myTestObjectOne = new MyTestObjectOne();
		ThreadOne threadTwo = new ThreadOne(myTestObjectOne);
		Thread thread = new Thread(threadTwo);
		thread.start();
		System.out.println("id:" + myTestObjectOne.getId() + " name:" + myTestObjectOne.getName());
	}
}

class ThreadOne implements Runnable {

	private MyTestObjectOne myTestObjectOne;

	public ThreadOne(MyTestObjectOne myTestObjectOne) {
		this.myTestObjectOne = myTestObjectOne;
	}

	@Override
	public void run() {
		myTestObjectOne.setId(1);
		myTestObjectOne.setName("ray");
	}
}

class MyTestObjectOne {
	private int id = 0;
	private String name = "init";

	public int getId() {
		return id;
	}

	public synchronized void setId(int id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public synchronized void setName(String name) {
		this.name = name;
	}
}

输出：

id:0 name:init

由于没有做同步，因此在set的线程启动时，已经get了初始化的值出来

（2）由于没有做同步，代码的执行先后顺序和执行时间无法控制，导致读取的值是已经修改过的

package com.ray.deepintothread.ch02.topic_4;

public class DirtyRead2 {
	public static void main(String[] args) throws InterruptedException {
		MyTestObjectTwo myTestObjectTwo = new MyTestObjectTwo();
		ThreadTwo threadTwo = new ThreadTwo(myTestObjectTwo);
		Thread thread = new Thread(threadTwo);
		thread.start();
		Thread.sleep(100);// 是否出现脏读，这里程序的执行时间，跟下面的执行时间有密切关系
		System.out.println("id:" + myTestObjectTwo.getId() + " name:" + myTestObjectTwo.getName());
	}
}

class ThreadTwo implements Runnable {

	private MyTestObjectTwo myTestObjectTwo;

	public ThreadTwo(MyTestObjectTwo myTestObjectTwo) {
		this.myTestObjectTwo = myTestObjectTwo;
	}

	@Override
	public void run() {
		myTestObjectTwo.setId(1);
		try {
			Thread.sleep(1000);// 代码执行时间
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		myTestObjectTwo.setName("ray");
	}
}

class MyTestObjectTwo {
	private int id = 0;
	private String name = "init";

	public int getId() {
		return id;
	}

	public synchronized void setId(int id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public synchronized void setName(String name) {
		this.name = name;
	}
}

输出：

id:1 name:init

从输出很明显的可以看到，id已经被修改了，但是还没有修改name的时候，就已经开始读取了。

3.解决方案-同步，而且是有技巧的同步

package com.ray.deepintothread.ch02.topic_4;

public class DirtyRead3 {
	public static void main(String[] args) throws InterruptedException {
		MyTestObjectThree myTestObjectThree = new MyTestObjectThree();
		ThreadThree threadThree = new ThreadThree(myTestObjectThree);
		Thread thread = new Thread(threadThree);
		thread.start();
		ThreadFour threadFour = new ThreadFour(myTestObjectThree);
		Thread thread1 = new Thread(threadFour);
		thread1.start();
	}
}

class ThreadThree implements Runnable {

	private MyTestObjectThree myTestObjectThree;

	public ThreadThree(MyTestObjectThree myTestObjectThree) {
		this.myTestObjectThree = myTestObjectThree;
	}

	@Override
	public void run() {
		myTestObjectThree.setValue(1, "ray");
	}
}

class ThreadFour implements Runnable {

	private MyTestObjectThree myTestObjectThree;

	public ThreadFour(MyTestObjectThree myTestObjectThree) {
		this.myTestObjectThree = myTestObjectThree;
	}

	@Override
	public void run() {
		myTestObjectThree.getValue();
	}
}

class MyTestObjectThree {
	private int id = 0;
	private String name = "init";

	public synchronized void setValue(int id, String name) {
		this.id = id;
		this.name = name;
	}

	public synchronized void getValue() {
		System.out.println("id:" + id + " name:" + name);
	}
}

输出：

id:1 name:ray

同步的确能够解决上面的问题，但是有一点需要注意的是，上面的set方法，是把两个属性一起设置的，如果分开了，就不行的，同理，get的方法同时得到两者，分开的那种不行。

总结：这一章节我们讨论了脏读的形成与解决方案。

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