Condition

       在上一篇博客《生产者消费者模式之Lock与Condition》中已经对Condition的使用有所了解了，下面再举一个之前在网上看过的例子作为Condition的补充。

 

    问题：假设有三个线程，一个主线程mainThread和两个子线程subThread1、subThread2，要求按顺序输出，mainThread先输出1——3，然后subThread1输出4——6，最后subThread2输出7——9，循环执行。

    问题分析：有三个线程要顺序执行，所以我们可以定义三个Condition分别表示这三个线程，然后定义一个判断标记flag，flag==0表示mainThread、flag==1表示subThread1、flag==2表示subThread2。

package com.gk.thread.condition.demo;

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


public class ConditionDemo {
	
	private static int count = 1;
	
	private Lock lock = new ReentrantLock();
	private Condition mainCondition = lock.newCondition();
	private Condition subCondition1 = lock.newCondition();
	private Condition subCondition2 = lock.newCondition();
	
	/*
	 * flag = 0代表mainCondition
	 * flag = 1代表subCondition1
	 * flag = 2代表subCondition2
	 */
	private int flag = 0;
	
	/**
	 * mainThread线程调的方法
	 */
	public void mainThread() {
			
		lock.lock();
		try {
			while(flag != 0) {	
				mainCondition.await();	// 如果flag != 0，mainThread线程等待
			}
			
			// 如果flag == 0，mainThread线程往下执行
			System.out.println("第" + count++ + "次输出");
			for(int i=1; i<=3; i++) {
				System.out.println("mainThread : " + i);
			}
			System.out.println();
			
			// 修改flag = 1，并唤醒subCondition1执行
			flag = 1;		
			subCondition1.signal();
			
		} catch (InterruptedException e) {
			throw new RuntimeException(e);
		}finally {
			
			lock.unlock();
		}
	}
		
	/**
	 * subThread1线程调的方法
	 */
	public void subThread1() {
			
		lock.lock();
			
		try {
			while(flag != 1) {
				subCondition1.await();
			}
			
			for(int i=4; i<=6; i++) {
				System.out.println("subThread1 : " + i);
			}
			System.out.println();
			
			flag = 2;
			subCondition2.signal();
				
		} catch (InterruptedException e) {
			throw new RuntimeException(e);
		}finally {
			lock.unlock();
		}
	}

	/**
	 * subThread2线程调的方法
	 */
	public void subThread2() {
			
		lock.lock();
			
		try {
			while(flag != 2) {
				subCondition2.await();
			}
				
			for(int i=7; i<=9; i++) {
				System.out.println("subThread2 : " + i);
			}
			
			System.out.println("\n=======================\n");
			flag = 0;
			mainCondition.signal();
		} catch (InterruptedException e) {
			throw new RuntimeException(e);
		}finally {
			lock.unlock();
		}
	
	}
		
}

    测试代码：

package com.gk.thread.condition.demo;

public class Test {
	
	public static void main(String[] args) {
		
		final int COUNT = 3;
		ConditionDemo cd = new ConditionDemo();
		
		for (int i=1; i<=COUNT; i++) {
			
			new Thread(new Runnable() {
				
				@Override
				public void run() {
					
					cd.mainThread();	// mainThread线程
				}	
			}).start();
			
			new Thread(new Runnable() {
				
				@Override
				public void run() {
					
					cd.subThread1();	// subThread1线程
				}
			}).start();

			new Thread(new Runnable() {
				
				@Override
				public void run() {
					
					cd.subThread2();	// subThread2线程
				}
			}).start();
			
		}
	}

}

    最后再友情提示一句：在API中有Condition实现阻塞队列的示例，读者可以体会其中的算法思想。