通过一道面试题了解Condition线程通信

Condition

• Condition接口描述了可能会与锁有关联的条件变量。这些变量在用法与使用Object.wait访问的隐式监视器类似，但提供了更强大的功能。需要特别指出的是，单个Lock可能与多个Condition对象关联。为了避免兼容性问题，Condition方法的名称与对应的Object版本中的不同；
• 在Condition对象中，与wait、notify和notifyAll方法对应的分别是await、signal和signalAll；
• Condition实例实质上被绑定到一个锁上。要为特定Lock实例获得Condition实例，请使用器newCondition()方法。

面试题

package com.ccfdod.juc;

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

/**
* 题目：编写一个程序，开启 3 个线程，这三个线程的 ID 分别为 A、B、C，每个线程将自己的 ID 在屏幕上打印 10 遍，要求输出的结果必须按顺序显示。
* 如：ABCABCABC……
*/
class AlternateDemo {
    //标志当前由哪一个线程输出，1代表A，2代表B，3代表C
    private int number = 1;

    Lock lock = new ReentrantLock();

    //Condition的强大之处在于它可以为多个线程间建立不同的Condition
    Condition condition1 = lock.newCondition();
    Condition condition2 = lock.newCondition();
    Condition condition3 = lock.newCondition();

    // loopNum:当前循环轮数
    public void loopA(int loopNum) {
        //上锁
        lock.lock();
        try {
            while (number != 1) {
                //等待
                condition1.await();
            }

            System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
            number = 2;
            //唤醒
            condition2.signal();

        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            //确保释放锁
            lock.unlock();
        }
    }

    public void loopB(int loopNum) {
        lock.lock();
        try {
            while (number != 2) {
                condition2.await();
            }

            System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
            number = 3;
            condition3.signal();

        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    public void loopC(int loopNum) {
        lock.lock();
        try {
            while (number != 3) {
                condition3.await();
            }

            System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
            number = 1;
            condition1.signal();

        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }
}

public class TestABCAlternate {

    public static void main(String[] args) {
        AlternateDemo ad = new AlternateDemo();

        new Thread(new Runnable() {

            @Override
            public void run() {
                for (int i = 0; i < 10; i++)
                    ad.loopA(i);
            }
        }, "A").start();

        new Thread(new Runnable() {

            @Override
            public void run() {
                for (int i = 0; i < 10; i++)
                    ad.loopB(i);
            }
        }, "B").start();

        new Thread(new Runnable() {

            @Override
            public void run() {
                for (int i = 0; i < 10; i++)
                    ad.loopC(i);
            }
        }, "C").start();
    }
}