多线程面试题之三线程按顺序交替打印ABC的方法

Posted 2021-12-01 ganbo

建立三个线程,线程名字分别为:A、B、C，要求三个线程分别打印自己的线程名字,但是要求三个线程同时运行，并且实现交替打印，即按照ABCABCABC的顺序打印。打印10轮，打印完毕控制台输出字符串:"打印完毕"

方法之一:使用Condition等待唤醒机制,代码如下：

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

/**
 * Created by gan on 2019/5/19 10:54.
 */
public class ConditionDemo {

    private static int num = 1; //打印“谁”标识符

    private static Lock lock = new ReentrantLock();
    private static final Condition conditionA = lock.newCondition();
    private static final Condition conditionB = lock.newCondition();
    private static final Condition conditionC = lock.newCondition();

    private static CountDownLatch countDownLatch = new CountDownLatch(10); //使用闭锁计数器


    public static void main(String[] args) throws Exception {

        long loop = countDownLatch.getCount(); //初始化打印轮数

        new Thread(() -> {
            for (int i = 1; i <= loop; i++) {
                try {
                    printA();
                } catch (InterruptedException e) {
                }
            }
        }, "A").start();

        new Thread(() -> {
            for (int i = 1; i <= loop; i++) {
                try {
                    printB();
                } catch (InterruptedException e) {
                }
            }
        }, "B").start();

        new Thread(() -> {
            for (int i = 1; i <= loop; i++) {
                try {
                    printC(i);
                } catch (InterruptedException e) {
                }
            }
        }, "C").start();

        countDownLatch.await();

        System.out.println("打印完毕.........");
    }

    //打印‘A’的方法
    public static void printA() throws InterruptedException {
        lock.lock();
        if (num != 1) { //标识符等于1的时候打印A
            conditionA.await();
        }
        System.out.print(Thread.currentThread().getName());
        num = 2;
        conditionB.signal();
        lock.unlock();
    }

    //打印‘B’的方法
    public static void printB() throws InterruptedException {
        lock.lock();
        if (num != 2) { //标识符等于2的时候打印B
            conditionB.await();
        }
        System.out.print(Thread.currentThread().getName());
        num = 3;
        conditionC.signal();
        lock.unlock();
    }

    //打印‘C’的方法
    public static void printC(int loop) throws InterruptedException {
        lock.lock();
        if (num != 3) { //标识符等于3的时候打印A
            conditionC.await();
        }
        System.out.print(Thread.currentThread().getName());
        System.out.print("[" + loop + "]"); //轮数间隔符号
        num = 1;
        conditionA.signal();
        countDownLatch.countDown();
        lock.unlock();
    }


}

运行结果如下: