java 面试题 -- 线程 按序 交替

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…… 依次递归？

package com.company;

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

/*
 * 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…… 依次递归
 */
public class TestABCAlternate {

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

        new Thread(() -> {

            for (int i = 1; i <= 10; i++) {
                ad.loopA(i);
            }

        }, "A").start();

        new Thread(() -> {

            for (int i = 1; i <= 10; i++) {
                ad.loopB(i);
            }

        }, "B").start();

        new Thread(() -> {

            for (int i = 1; i <= 10; i++) {
                ad.loopC(i);

            }

        }, "C").start();
    }

}

class AlternateDemo {

    private int number = 1; //当前正在执行线程的标记

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

    /**
     * @param totalLoop : 循环第几轮
     */
    void loopA(int totalLoop) {
        lock.lock();

        try {
            //1. 判断
            if (number != 1) {
                condition1.await();
            }

            //2. 打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
            }

            //3. 唤醒
            number = 2;
            condition2.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    void loopB(int totalLoop) {
        lock.lock();

        try {
            //1. 判断
            if (number != 2) {
                condition2.await();
            }

            //2. 打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
            }

            //3. 唤醒
            number = 3;
            condition3.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

    void loopC(int totalLoop) {
        lock.lock();

        try {
            //1. 判断
            if (number != 3) {
                condition3.await();
            }

            //2. 打印
            for (int i = 1; i <= 1; i++) {
                System.out.println(Thread.currentThread().getName() + "\t" + i + "\t" + totalLoop);
            }
            System.out.println("------------------");
            //3. 唤醒
            number = 1;
            condition1.signal();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }
    }

}

上面有一部分使用java1.8新特性写的，看下结果：

A    1    1
B    1    1
C    1    1
------------------
A    1    2
B    1    2
C    1    2
------------------
A    1    3
B    1    3
C    1    3
------------------
A    1    4
B    1    4
C    1    4
------------------
A    1    5
B    1    5
C    1    5
------------------
A    1    6
B    1    6
C    1    6
------------------
A    1    7
B    1    7
C    1    7
------------------
A    1    8
B    1    8
C    1    8
------------------
A    1    9
B    1    9
C    1    9
------------------
A    1    10
B    1    10
C    1    10
------------------