玩转java多线程(wait和notifyAll的正确使用姿势)

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敲代码谁都会，关键敲出性能好而又简洁易懂易维护的代码并不是每个程序员都能做到，关键是要找好姿势，这样才能避免很多坑。

对了，今天聊的是wait和notifyAll的正确使用姿势。(继上一篇玩转Java多线程(乒乓球比赛）https://www.cnblogs.com/haibiscuit/p/11094348.html)

贴出代码前，首先先讲一下程序的逻辑规则:

1.  6个线程，3个线程执行++操作，3个线程执行--操作，保证线程的同步和并发要求

2.  每个线程循环n次操作,默认是20次

3.  执行++操作时数据不能超出20,执行--操作时数据不能低于0

贴出代码:

package Thread;

import java.util.concurrent.CountDownLatch;

/**
* wait方法和Notify方法写在实体类对象中
* wait代表退出当前线程对某实体对象的操作，释放锁暂时退出当前线程的操作，由其他线程调用该对象的notify方法来释放前一个线程，继续对该对象进行操作
*/
public class AwaitTest {
public static void main(String[] args)
{
CountDownLatch countDownLatch = new CountDownLatch(6);
NumberHolder numberHolder = new NumberHolder();

Thread t1 = new IncreaseThread(numberHolder,countDownLatch);
Thread t2 = new DecreaseThread(numberHolder,countDownLatch);

Thread t3 = new IncreaseThread(numberHolder,countDownLatch);
Thread t4 = new DecreaseThread(numberHolder,countDownLatch);

Thread t5 = new IncreaseThread(numberHolder,countDownLatch);
Thread t6 = new DecreaseThread(numberHolder,countDownLatch);


t1.start();
t2.start();

t3.start();
t4.start();

t5.start();
t6.start();

try {
countDownLatch.await();
System.out.println("主线程执行结束!");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

}

}

class NumberHolder
{ //**********非常重要
private int number; //这里number执行increase的次数和执行decrease的次数要相同，
//否则可能造成线程一直处于wait状态，除非将下面的while改成if
private int i; //标记执行的次数，测试CountDownLatch类

public synchronized void increase()
{
while (20 == number) //当前的number为10的时候，对象当前的线程进入等待的状态
{
try
{
this.wait(500);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}

// 能执行到这里说明已经被唤醒
// 并且number为0
number++;
System.out.println("执行次数"+(++i));
System.out.println(number);

// 通知在等待的线程
this.notifyAll();
}

public synchronized void decrease()
{
while (0 == number) //当前对象的number等于0时，进入等待的状态
{
try
{
this.wait(500);
}
catch (InterruptedException e)
{
e.printStackTrace();
}

}

// 能执行到这里说明已经被唤醒
// 并且number不为0
number--;
System.out.println("执行次数"+(++i));
System.out.println(number);
this.notifyAll();
}

}
//线程类：用于控制实体类下面的数据相加
class IncreaseThread extends Thread
{
private NumberHolder numberHolder;
private CountDownLatch countDownLatch;

public IncreaseThread(NumberHolder numberHolder,CountDownLatch countDownLatch)
{
this.numberHolder = numberHolder;
this.countDownLatch = countDownLatch;
}

@Override
public void run()
{
for (int i = 0; i < 20; ++i)
{
// 进行一定的延时
try
{
Thread.sleep((long) Math.random() * 1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}

// 进行增加操作

numberHolder.increase();
}
countDownLatch.countDown();
}

}
//线程类，完成对实体类的操作
class DecreaseThread extends Thread
{
private NumberHolder numberHolder;
private CountDownLatch countDownLatch;

public DecreaseThread(NumberHolder numberHolder,CountDownLatch countDownLatch)
{
this.numberHolder = numberHolder;
this.countDownLatch = countDownLatch;
}

@Override
public void run()
{
for (int i = 0; i < 20; ++i)
{
// 进行一定的延时
try
{
Thread.sleep((long) Math.random() * 1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}

// 进行减少操作
numberHolder.decrease();
}
countDownLatch.countDown();
}

}

总结:

1.wait方法要在synchronized方法体内使用,另外如果有限制条件需要结合while使用   (不建议结合if使用)

2.notifyAll()方法需要在synchronized方法内使用    (不建议使用notify方法)