面试题:有A,B,C三个线程,如何保证三个线程同时执行?如何在并发情况下保证三个线程依次执行?如何保证二个线程有序交错进行?

面试题:有A,B,C三个线程,如何保证三个线程同时执行?如何在并发情况下保证三个线程依次执行?如何保证二个线程有序交错进行?

面试题:有A,B,C三个线程,如何保证三个线程同时执行?如何在并发情况下保证三个线程依次执行?如何保证二个线程有序交错进行?

这三个题目都是控制多个线程执行的顺序。这一类题目除了通过一些判断来完成,大多情况下是通过锁来控制顺序的。下面我们就分别来讨论下这三个题目的实现办法。

有A,B,C三个线程,如何保证三个线程同时执行?

保证线程同时执行可以用于并发测试。可以使用倒计时锁CountDownLatch实现让三个线程同时执行。代码如下所示:

        ExecutorService executorService = Executors.newCachedThreadPool();
        CountDownLatch countDownLatch = new CountDownLatch(1);

        executorService.submit(()->{
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println("线程A执行,执行时间:" + System.currentTimeMillis());
        });

        executorService.submit(()->{
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println("线程B执行,执行时间:" + System.currentTimeMillis());
        });

        executorService.submit(()->{
            try {
                countDownLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println("线程C执行,执行时间:" + System.currentTimeMillis());
        });

        countDownLatch.countDown();

打印内容如下,通过时间可以证明三个线程是同时执行的。

线程A执行,执行时间:1617811258309
线程C执行,执行时间:1617811258309
线程B执行,执行时间:1617811258309

当然让三个线程同时进行,也可以使用循环栅栏CyclicBarrier来实现,当三个线程都到达栅栏处,才开始执行。

有A,B,C三个线程,如何在并发情况下保证三个线程依次执行?

要保证三个线程依次执行,就是说,A线程执行完之后,才执行B线程,B线程执行完之后才执行C线程。
如下代码所示,分别用两个方法实现了分别用volatile和倒计时锁CountDownLatch实现顺序执行。

package com.wangguitang.freedom.interview.concurrent;

import org.junit.jupiter.api.Test;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * 让ABC三个线程循序打印
 *
 * @author freedom wang
 * @date 2021-04-08 22:48:33
 */
public class ThreeThreadsOrderPrint {

    private volatile int count = 0;

    /**
     * 使用一个变量进行判断执行哪个线程。没有轮到的线程在不停循环,没有停止线程。
     *
     * @author freedom wang
     * @date 2021-04-08 22:57:29
     */
    @Test
    public void testUseVolatile() {
        ExecutorService executorService = Executors.newCachedThreadPool();
        executorService.submit(() -> {
            while (true) {
                if (count == 0) {
                    for (int i = 0; i < 10; i++) {
                        System.out.println("A - " + i);
                    }

                    count = 1;
                    break;
                }
            }
        });

        executorService.submit(() -> {
            while (true) {
                if (count == 1) {
                    for (int i = 0; i < 10; i++) {
                        System.out.println("B - " + i);
                    }

                    count = 2;
                    break;
                }
            }
        });
        executorService.submit(() -> {
            while (true) {
                if (count == 2) {
                    for (int i = 0; i < 10; i++) {
                        System.out.println("C - " + i);
                    }

                    count = 3;
                    break;
                }
            }
        });

    }

    /**
     * 使用倒计时锁来进行控制
     *
     * @author freedom wang
     * @date 2021-04-08 22:57:56
     */
    @Test
    public void testUseCountDownLatch() {
        ExecutorService executorService = Executors.newCachedThreadPool();
        CountDownLatch aLatch = new CountDownLatch(1);
        CountDownLatch bLatch = new CountDownLatch(1);
        CountDownLatch cLatch = new CountDownLatch(1);

        executorService.submit(() -> {
            try {
                aLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            for (int i = 0; i < 10; i++) {
                System.out.println("A - " + i);
            }

            bLatch.countDown();
        });

        executorService.submit(() -> {
            try {
                bLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            for (int i = 0; i < 10; i++) {
                System.out.println("B - " + i);
            }

            cLatch.countDown();
        });

        executorService.submit(() -> {
            try {
                cLatch.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            for (int i = 0; i < 10; i++) {
                System.out.println("C - " + i);
            }
        });

        aLatch.countDown();
    }
}

有A,B,C三个线程,如何保证三个线程有序交错进行?

实现三个线程交错打印,可以使用ReentrantLock以及3个Condition来实现,代码如下所示:

package com.wangguitang.freedom.interview.concurrent;

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

/**
 * 实现三个线程交替打印
 *
 * @author freedom wang
 * @date 2021-02-22 21:42:44
 */
public class ThreeThreadsAlternatePrint {

    private static final ReentrantLock lock = new ReentrantLock();
    private static Condition c1 = lock.newCondition();
    private static Condition c2 = lock.newCondition();
    private static Condition c3 = lock.newCondition();

    /**
     * 解题思路:使用显示锁
     *
     * @author freedom wang
     * @date 2021-02-22 21:47:36
     */
    public static void main(String[] args) {

        new Thread(() -> {
            try {
                lock.lock();

                for (int i = 0; i < 10; i++) {
                    System.out.println("A - " + i);
                    c2.signal();
                    c1.await();
                }

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

        new Thread(() -> {
            try {
                lock.lock();

                for (int i = 0; i < 10; i++) {
                    System.out.println("B - " + i);
                    c3.signal();
                    c2.await();
                }

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

        new Thread(() -> {
            try {
                lock.lock();

                for (int i = 0; i < 10; i++) {
                    System.out.println("C - " + i);
                    c1.signal();
                    c3.await();
                }

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

    }
}

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