三线程打印 ABC，循环十次的 N 种实现方式

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这是一个创建于 1790 天前的主题，其中的信息可能已经有所发展或是发生改变。

编写一个程序，开启 3 个线程 A,B,C，这三个线程的输出分别为 A、B、C，每个线程将自己的输出在屏幕上打印 10 遍，要求输出的结果必须按顺序显示。如：ABCABCABC....

核心在于多线程同步

完整代码： https://github.com/mightofcode/javacc

方法 1，轮询 AtomicInteger

缺点是轮询白耗 CPU，性能很差

package com.mocyx.javacc.printer;

import org.springframework.stereotype.Component;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * 轮询 AtomicInteger 实现交替输出 ABC
 * @author Administrator
 */
@Component
public class PollingPrinter implements Printer {

    private final AtomicInteger atomicInteger = new AtomicInteger(0);

    class Worker implements Runnable {

        private String pstr;
        private int index;
        private int gap;
        private int max;

        public Worker(String pstr, int index, int gap, int max) {
            this.pstr = pstr;
            this.index = index;
            this.gap = gap;
            this.max = max;
        }

        @Override
        public void run() {
            while (true) {
                int v = atomicInteger.get();
                if (v == max) {
                    return;
                } else {
                    if (v % gap == index) {
                        System.out.print(pstr);
                        atomicInteger.set(v + 1);
                    }
                }
            }
        }
    }


    @Override
    public void print() {
        List<Thread> threads = new ArrayList<>();
        threads.add(new Thread(new Worker("A", 0, 3, 30)));
        threads.add(new Thread(new Worker("B", 1, 3, 30)));
        threads.add(new Thread(new Worker("C", 2, 3, 30)));
        for (Thread t : threads) {
            t.start();
        }
        try {
            for (Thread t : threads) {
                t.join();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

方法 2，使用 Lock & Condition 同步

使用 Lock & Condition 要注意： 1 检查条件谓词，避免信号丢失和过早唤醒 2 注意在 finally 中进行 unlock，否则出现异常会 hang 住

package com.mocyx.javacc.printer;

import org.springframework.stereotype.Component;

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

/**
 * 使用 Lock and Condition 进行同步
 *
 * @author Administrator
 */
@Component
public class SignalPrinter implements Printer {
    private final Lock lock = new ReentrantLock();
    private volatile int counter = 0;

    class Worker implements Runnable {

        Condition curCondition;
        Condition nextCondition;
        String pstr;
        int max;
        int index;
        int gap;

        public Worker(String pstr, int index, int gap, int max, Condition curCondition, Condition nextCondition) {
            this.pstr = pstr;
            this.max = max;
            this.curCondition = curCondition;
            this.nextCondition = nextCondition;
            this.index = index;
            this.gap = gap;
        }

        private boolean isMyTurn() {
            return counter % gap == index;
        }

        @Override
        public void run() {
            while (true) {
                lock.lock();
                try {
                    while (!isMyTurn()) {
                        try {
                            curCondition.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    if (counter < max) {
                        System.out.print(pstr);
                    }
                    counter += 1;
                    nextCondition.signalAll();
                } finally {
                    lock.unlock();
                }
                if (counter >= max) {
                    return;
                }
            }
        }
    }

    @Override
    public void print() {
        List<Thread> threads = new ArrayList<>();
        List<Condition> conditions = new ArrayList<>();

        conditions.add(lock.newCondition());
        conditions.add(lock.newCondition());
        conditions.add(lock.newCondition());

        threads.add(new Thread(new Worker("A", 0, 3, 30, conditions.get(0), conditions.get(1))));
        threads.add(new Thread(new Worker("B", 1, 3, 30, conditions.get(1), conditions.get(2))));
        threads.add(new Thread(new Worker("C", 2, 3, 30, conditions.get(2), conditions.get(0))));

        for (Thread t : threads) {
            t.start();
        }

        try {
            for (Thread t : threads) {
                t.join();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

方法 3，使用 Semphore 进行同步

相比 Lock & Condition，使用 Semphore 代码比较简洁，不容易出错

package com.mocyx.javacc.printer;

import org.springframework.stereotype.Component;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Semaphore;

/**
 * @author Administrator
 */
@Component
public class SemaphorePrinter implements Printer {



    class Worker implements Runnable {
        private String pstr;

        private Semaphore curSemphore;
        private Semaphore nextSemphore;
        private int count = 0;

        Worker(String pstr, int count, Semaphore curSemphore, Semaphore nextSemphore) {
            this.pstr = pstr;
            this.count = count;
            this.curSemphore = curSemphore;
            this.nextSemphore = nextSemphore;
        }

        @Override
        public void run() {
            for (int i = 0; i < count; i++) {
                try {
                    curSemphore.acquire(1);
                    System.out.print(pstr);
                    nextSemphore.release(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    @Override
    public void print() {
        List<Thread> threads = new ArrayList<>();
        List<Semaphore> semaphores = new ArrayList<>();

        semaphores.add(new Semaphore(0));
        semaphores.add(new Semaphore(0));
        semaphores.add(new Semaphore(0));

        threads.add(new Thread(new Worker("A", 10, semaphores.get(0), semaphores.get(1))));
        threads.add(new Thread(new Worker("B", 10, semaphores.get(1), semaphores.get(2))));
        threads.add(new Thread(new Worker("C", 10, semaphores.get(2), semaphores.get(0))));

        for (Thread t : threads) {
            t.start();
        }

        semaphores.get(0).release(1);

        try {
            for (Thread t : threads) {
                t.join();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

方法 4，使用 BlockingQueue 进行同步

思路跟 go channel 类似，通过 BlockingQueue 传递信息

package com.mocyx.javacc.printer;


import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

/**
 * 使用阻塞队列进行同步
 *
 * @author Administrator
 */
public class QueuePrinter implements Printer {


    static class Msg {
        public static final Msg PRINT_SUCCESS = new Msg();
        public static final Msg PRINT = new Msg();
        public static final Msg QUIT = new Msg();
    }

    class Channel {
        BlockingQueue<Msg> inQueue = new ArrayBlockingQueue<Msg>(100);
        BlockingQueue<Msg> outQueue = new ArrayBlockingQueue<Msg>(100);
    }

    class Worker implements Runnable {

        Channel inChannel;
        String pstr;

        Worker(String pstr, Channel inChannel) {
            this.inChannel = inChannel;
            this.pstr = pstr;
        }

        @Override
        public void run() {
            while (true) {
                try {
                    Msg msg = inChannel.inQueue.take();
                    if (msg == Msg.PRINT) {
                        System.out.print(pstr);
                        inChannel.outQueue.put(Msg.PRINT_SUCCESS);
                    } else if (msg == Msg.QUIT) {
                        return;
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    @Override
    public void print() {
        List<Thread> threads = new ArrayList<>();

        List<Channel> channels = new ArrayList<>();
        channels.add(new Channel());
        channels.add(new Channel());
        channels.add(new Channel());


        threads.add(new Thread(new Worker("A", channels.get(0))));
        threads.add(new Thread(new Worker("B", channels.get(1))));
        threads.add(new Thread(new Worker("C", channels.get(2))));

        for (Thread t : threads) {
            t.start();
        }

        for (int i = 0; i < 30; i++) {
            try {
                channels.get(i % channels.size()).inQueue.put(Msg.PRINT);
                channels.get(i % channels.size()).outQueue.take();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        for (Channel c : channels) {
            c.inQueue.add(Msg.QUIT);
        }

        try {
            for (Thread t : threads) {
                t.join();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }


}

方法 0，Sleep 同步法

这个方法确实能工作，但是可能会被面试官打

package com.mocyx.javacc.printer;


import java.util.ArrayList;
import java.util.List;

/**
 * sleep and sleep
 *
 * @author Administrator
 */
public class SleepPrinter implements Printer {

    static class Worker implements Runnable {
        private String printStr;
        private int sleepGap;
        private int delay;
        private int count;

        public Worker(String printStr,  int delay, int sleepGap, int count) {
            this.printStr = printStr;
            this.sleepGap = sleepGap;
            this.delay = delay;
            this.count = count;
        }

        @Override
        public void run() {
            try {
                Thread.sleep(delay);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            for (int i = 0; i < count; i++) {
                try {
                    Thread.sleep(sleepGap);
                    System.out.print(printStr);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }



    @Override
    public void print() {
        List<Thread> threads = new ArrayList<>();
        threads.add(new Thread(new Worker("A", 00, 30, 10)));
        threads.add(new Thread(new Worker("B", 10, 30, 10)));
        threads.add(new Thread(new Worker("C", 20, 30, 10)));
        for (Thread t : threads) {
            t.start();
        }
        try {
            for (Thread t : threads) {
                t.join();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

10 条回复 • 2020-01-09 17:07:02 +08:00

lihongjie0209

2020-01-01 16:24:33 +08:00

道理我都懂，但是你在一个要求严格执行顺序的地方使用多线程的原因是什么？有什么实际应用场景吗？

HuHui

2020-01-01 17:06:14 +08:00 via Android

@lihongjie0209 面试就挺实际

mightofcode

2020-01-01 18:24:14 +08:00

@lihongjie0209 工作中没机会用，拿面试题练练手

inwar

2020-01-01 18:58:51 +08:00 via Android

应该把打印改成一个耗时任务,结果顺序输出,这样更实际一点

1194129822

2020-01-02 11:49:12 +08:00

现在所有 Java 类型的赋值操作是原子性的，所以 volatile 加 yield 不香吗？这样即使是轮询也查不到相当于阻塞了，CPU 消耗很少。

mightofcode

2020-01-02 16:37:32 +08:00

@1194129822 为啥轮询查不到相当于阻塞了？ yield 并不一定导致线程切换吧

luxinfl

2020-01-03 14:52:53 +08:00

认真的说，我一个都不会

luxinfl

2020-01-03 16:19:28 +08:00

while (true){
lock.lock();
if(in % 3 != index){
lock.unlock();
continue;
}
System.out.print(Thread.currentThread().getName());
in++;
if(in >31){
lock.unlock();
return;
}
lock.unlock();
}

这样写会不会很 ugly

mightofcode

2020-01-03 21:09:09 +08:00

@luxinfl 还好，本质上跟方案 1 用 AtomicInteger 是一个策略：轮询

kkkkkrua

2020-01-09 17:07:02 +08:00

用 condition，优雅点