java 多线程 实现多个线程的顺序执行

场景

编写一个程序,启动三个线程,三个线程的name分别是A,B,C;,每个线程将自己的ID值在屏幕上打印5遍,打印顺序是ABCABC...

使用 synchronized 实现 

public class MyService
{
    private int flag = 1;
    
    public synchronized void printA(){
        
        while (flag != 1)
        {
            try
            {
                this.wait();
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
        }
        System.out.print(Thread.currentThread().getName());
        flag = 2;
        this.notifyAll();
    }
    public synchronized void printB(){
        while (flag != 2)
        {
            try
            {
                this.wait();
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
        }
        System.out.print(Thread.currentThread().getName());
        flag = 3;
        this.notifyAll();
    }
    public synchronized void printC(){
        while (flag != 3)
        {
            try
            {
                this.wait();
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
        }
        System.out.print(Thread.currentThread().getName());
        flag = 1;
        this.notifyAll();
    }
}
View Code

这里的判断条件中用的是 while 而不是 if , 这两者之间有什么区别呢? 线程从 wait 状态被唤醒,并且获得锁以后会继续往下执行,比如 A 调用nofityAll() 唤醒 B,C,这时 B与C谁会先获得锁是不确定的。如果是C先获得了锁,那么C就继续往下执行打印,这与我们的期望的不符。所以这里我们使用了一个 while,当C获得锁以后再去判断一下flag,如果这时还不是它执行的时候,它就再次进入wait状态。此时A与C都是wait状态,获得锁的一定是B,从而实现我们期望的顺序打印。

测试类

package testABC;

public class TestMain
{
    public static void main(String[] args)
    {
//编写一个程序,启动三个线程,三个线程的ID分别是A,B,C;,每个线程将自己的ID值在屏幕上打印5遍,打印顺序是ABCABC...
//        MyService service = new MyService();
        MyService2 service = new MyService2();
        
        Thread A = new Thread(new Runnable()
        {
            @Override
            public void run()
            {
                for (int i = 0; i < 5; i++)
                {
                    service.printA();
                }
            }
        });
        A.setName("A");
        Thread B = new Thread(new Runnable()
        {
            @Override
            public void run()
            {
                for (int i = 0; i < 5; i++)
                {
                    service.printB();
                }
            }
        });
        B.setName("B");
        Thread C = new Thread(new Runnable()
        {
            @Override
            public void run()
            {
                for (int i = 0; i < 5; i++)
                {
                    service.printC();
                }
            }
        });
        C.setName("C");
        
        A.start();
        B.start();
        C.start();
    }
}
View Code

 使用 Lock 实现

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class MyService2
{
    private int flag = 1;
    private Lock lock = new ReentrantLock();
    private Condition conditionA = lock.newCondition();
    private Condition conditionB = lock.newCondition();
    private Condition conditionC = lock.newCondition();

    public void printA()
    {
        try
        {
            lock.lock();
            if (flag != 1)
            {
                try
                {
                    conditionA.await();
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
            System.out.print(Thread.currentThread().getName());
            flag = 2;
            conditionB.signal();
        }
        finally
        {
            lock.unlock();
        }

    }

    public void printB()
    {
        try
        {
            lock.lock();
            if (flag != 2)
            {
                try
                {
                    conditionB.await();
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
            System.out.print(Thread.currentThread().getName());
            flag = 3;
            conditionC.signal();
        }
        finally
        {
            lock.unlock();
        }

    }

    public void printC()
    {
        try
        {
            lock.lock();
            if (flag != 3)
            {
                try
                {
                    conditionC.await();
                }
                catch (InterruptedException e)
                {
                    e.printStackTrace();
                }
            }
            System.out.print(Thread.currentThread().getName());
            flag = 1;
            conditionA.signal();
        }
        finally
        {
            lock.unlock();
        }
    }
}
View Code

当使用LOCK时可以不使用while因为condition可以唤醒指定的线程。同时注意必须先调用 conditionA.signal(); 再调用 lock.unlock(); ,否则会抛 java.lang.IllegalMonitorStateException 异常。因为在调用unlock之后,当前线程已不是此监视器对象condition的持有者。也就是说要在此线程持有锁定对象时,才能使用此锁定对象。

关于此异常的博文:关于java.lang.IllegalMonitorStateException

 

posted @ 2019-04-30 11:21  Hoonick  阅读(23075)  评论(0编辑  收藏  举报