使用多线程出现脏读，得到的结果不是设计时想要的怎么办呢？

直接看示例：

出现脏读的程序

DealThread.java

public class DealThread implements Runnable {

private String username;

public void setUsername(String username) {

this.username = username;

}

@Override

public void run() {

System.out.println(" in run " + Thread.currentThread().getName() + " " + username);

try {

Thread.sleep(2000);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println(" out run " + Thread.currentThread().getName() + " " + username);

}

}

Run2_2.java

public class Run2_2 {

public static void main(String[] args) {

DealThread d1 = new DealThread();

d1.setUsername("aa");

Thread t1 = new Thread(d1,"A");

t1.start();

try {

Thread.sleep(200);

} catch (InterruptedException e) {

e.printStackTrace();

}

d1.setUsername("bb");

Thread t2 = new Thread(d1,"B");

t2.start();

}

}

输出结果是：

in run A  aa

in run B  bb

out run A  bb

out run B  bb

加同步锁和使用不同的DealThread对象都能解决这个问题，但是同步锁加得不对还是不能解决问题的，比如修改DealThread.java

修改方法1：(不能解决)

public class DealThread implements Runnable {

private String username;

public void setUsername(String username) {

this.username = username;

}

@Override

public void run() {

synchronized (this) {

System.out.println(" in run " + Thread.currentThread().getName() + " " + username);

try {

Thread.sleep(2000);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println(" out run " + Thread.currentThread().getName() + " " + username);

}

}

}

输出结果为：

in run A  aa

out run A  bb

in run B  bb

out run B  bb

还是没达到效果，这样已经做对一半了，再给 setUsername加上synchronized就可以了

修改方法2：

public class DealThread implements Runnable {

private String username;

public synchronized void setUsername(String username) {

this.username = username;

}

@Override

public void run() {

synchronized (this) {

System.out.println(" in run " + Thread.currentThread().getName() + " " + username);

try {

Thread.sleep(2000);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println(" out run " + Thread.currentThread().getName() + " " + username);

}

}

}

这样输出结果就为：

in run A  aa

out run A  aa

in run B  bb

out run B  bb

另外一种改法就是修改Run2_2.java  为每个线程分配一个DealThread对象，这样就不会出现脏读

修改方法3：

public class Run2_2 {

public static void main(String[] args) {

DealThread d1 = new DealThread();

d1.setUsername("aa");

Thread t1 = new Thread(d1,"A");

t1.start();

try {

Thread.sleep(200);

} catch (InterruptedException e) {

e.printStackTrace();

}

DealThread d2 = new DealThread();

d2.setUsername("bb");

Thread t2 = new Thread(d2,"B");

t2.start();

}

}

这样的话两个线程会异步打印出各自的内容，互不影响，效率也会快一点，因为不用等上一个线程释放锁。

结合前面两篇关于多线程synchronized还可以这样改(个人觉得这样比较符合实际开发)

修改方法4：

新建merch.java

public class merch {

private String username;

public String getUsername() {

return username;

}

public synchronized void setUsername(String username) {

this.username = username;

}

}

修改DealThread.java

public class DealThread implements Runnable {

private merch merchA;

DealThread (merch merchA) {

this.merchA = merchA;

}

@Override

public void run() {

synchronized (merchA) {

System.out.println(" in run " + Thread.currentThread().getName() + " " + merchA.getUsername());

try {

Thread.sleep(2000);

} catch (InterruptedException e) {

e.printStackTrace();

}

System.out.println(" out run " + Thread.currentThread().getName() + " " + merchA.getUsername());

}

}

}

修改Run2_2.java

public class Run2_2 {

public static void main(String[] args) {

merch merchA = new merch();

merchA.setUsername("aa");

DealThread d1 = new DealThread(merchA);

Thread t1 = new Thread(d1,"A");

t1.start();

try {

Thread.sleep(200);

} catch (InterruptedException e) {

e.printStackTrace();

}

merchA.setUsername("bb");

Thread t2 = new Thread(d1,"B");

t2.start();

}

}

这样运行结果还是：

in run A  aa

out run A  aa

in run B  bb

out run B  bb

原理还是给 setUsername 和 输出的代码块加锁

个人即兴写的程序和临时想到的解决方法，如果更优的解决方法请留下高见，谢谢。