一、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 ，要求输出结果必须按顺序显示。

二、代码

package com.gf.demo;

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


public class TestABCAlternate {

    public static void main(String args[]) {

        Alternate ad = new Alternate();

        new Thread( new Runnable() {
            @Override
            public void run() {
                for (int i = 1 ; i <= 10 ; i ++) {
                    ad.loopA(i);
                }
            }
        } , "A").start();

        new Thread( new Runnable() {
            @Override
            public void run() {
                for (int i = 1 ; i <= 10 ; i ++) {
                    ad.loopB(i);
                }
            }
        } , "B").start();

        new Thread( new Runnable() {
            @Override
            public void run() {
                for (int i = 1 ; i <= 10 ; i ++) {
                    ad.loopC(i);
                }
            }
        } , "C" ).start();

    }

}

class Alternate {

    private int number = 1;

    private Lock lock = new ReentrantLock();
    private Condition condition1 = lock.newCondition();
    private Condition condition2 = lock.newCondition();
    private Condition condition3 = lock.newCondition();

    public void loopA(int totalLoop) {
        try {
            lock.lock();
            //1. 判断
            if (1 != number ) {
                try {
                    condition1.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //2. 打印
            System.out.println(Thread.currentThread().getName());
            number = 2;
            condition2.signal();
        } finally {
            lock.unlock();
        }

    }

    public void loopB(int totalLoop) {
        try {
            lock.lock();
            //1. 判断
            if (2 != number ) {
                try {
                    condition2.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //2. 打印
            System.out.println(Thread.currentThread().getName());

            number = 3;
            condition3.signal();
        } finally {
            lock.unlock();
        }

    }

    public void loopC(int totalLoop) {
        try {
            lock.lock();
            //1. 判断
            if (3 != number ) {
                try {
                    condition3.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            //2. 打印
            System.out.println(Thread.currentThread().getName());
            System.out.println("-------------------"+ "第 " + totalLoop + " 遍");
            number = 1;
            condition1.signal();
        } finally {
            lock.unlock();
        }

    }

}