Java多线程循环打印ABC等问题（三种方式）

矫情吗；* 2022-09-11 01:26 181阅读 0赞

## 前言 ##

有很多中写法，希望可以慢慢都写出来。

### 1、线程共享类对象：依靠AtomicInteger是线程安全的 ###

将 AtomicInteger currentCount 作为线程共享。

import java.util.concurrent.LinkedBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.atomic.AtomicInteger;
    
    /**
     * 三个线程按次序轮流打印a,b,c AtomicInteger 实现
     *
     */
    public class AtomicIntegerThread implements Runnable {
        private AtomicInteger currentCount = new AtomicInteger(0);
    
        private static final Integer MAX_COUNT = 6;
    
        private static String[] chars = {"a", "b", "c"};
    
        private String name;
    
        public AtomicIntegerThread(String name,AtomicInteger currentCount) {
            this.name = name;
            this.currentCount = currentCount;
        }
    
        @Override
        public void run() {
            while (currentCount.get() < MAX_COUNT) {
                if (this.name.equals(chars[currentCount.get() % 3])) {
                    printAndPlusOne(this.name);
                }
            }
        }
    
        public void printAndPlusOne(String content) {
            System.out.print(content);
            currentCount.getAndIncrement();
        }
    
        public static void main(String[] args) {
            
            AtomicInteger currentCount = new AtomicInteger(0);
    
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(3, 3, 20, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
            threadPoolExecutor.execute(new AtomicIntegerThread("a",currentCount));
            threadPoolExecutor.execute(new AtomicIntegerThread("b",currentCount));
            threadPoolExecutor.execute(new AtomicIntegerThread("c",currentCount));
            threadPoolExecutor.shutdown();
            
        }
    }

这里一定要注意有一个坑，如果线程池核心线程数少于3个，1个或者2个，上述代码会有问题，因为线程池的队列是无界队列，多余核心线程数的任务会被放到队列中，这样循环打印的时候，有一些线程不是在运行中的，是在队列里的，运行中的线程会被阻塞，导致相互等待，死锁问题出现。所以线程池的核心线程数必须等于大于3个，这样保证有3个线程一直在运行。

### 2、volatile静态变量控制 ###

还有一种就是将某个变量设置为静态，这样线程本身也共享该变量，则可以对其操作。但要注意给这个静态变量加上volatile字段，这个字段可以帮助多线程中的可见性。代码如下：

import java.util.concurrent.LinkedBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    
    /**
     * 三个线程按次序轮流打印a,b,c
     *
     */
    public class StaticVarThread implements Runnable {
    
            private static volatile Integer currentCount = 0;
    
    
        private static final Integer MAX_COUNT = 6;
    
        private static String[] chars = {"a", "b", "c"};
    
        private String name;
    
        public StaticVarThread(String name) {
            this.name = name;
        }
    
        @Override
        public void run() {
            while (currentCount < MAX_COUNT) {
                try {
                    while (this.name.equals(chars[currentCount % 3]) && currentCount < MAX_COUNT) {
                        printAndPlusOne(this.name);
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                } finally {
                }
            }
        }
    
        public void printAndPlusOne(String name) {
            System.out.println(name + "\t" + currentCount);
            currentCount++;
        }
    
        public static void main(String[] args) {
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(3, 3, 20, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
            threadPoolExecutor.execute(new StaticVarThread("a"));
            threadPoolExecutor.execute(new StaticVarThread("b"));
            threadPoolExecutor.execute(new StaticVarThread("c"));
            threadPoolExecutor.shutdown();
        }
    }

### 3、synchronized对象加锁 ###

利用一个计数来除线程个数得到余数从而控制线程输出，并且利用synchronized、一个Object的wait()和notify()方法来对线程的阻塞和唤醒，这里的技巧可以看看。

*  wait()  
    阻塞当前线程，等待被释放，继续执行当前线程
 *  notify()  
    唤醒监视该对象的第一个线程，notifyAll()唤醒监视该对象的所有线程。

import org.junit.Test;
    
    import java.util.concurrent.LinkedBlockingQueue;
    import java.util.concurrent.ThreadPoolExecutor;
    import java.util.concurrent.TimeUnit;
    
    public class SynchronizedThread {
    
        static volatile int state; // 线程共有，判断所有的打印状态
        static final Object t = new Object(); // 线程共有，加锁对象
    
        /**
         * 改装了一下
         */
        @Test
        public void mainSix() {
            state = 0;
            // 需要循环的次数
            int n = 10;
            int num = 2;
    
            ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2, 2, 20, TimeUnit.MINUTES, new LinkedBlockingQueue<>());
            threadPoolExecutor.execute(new MyThread("A", 0, n, num));
            threadPoolExecutor.execute(new MyThread("B", 1,  n, num));
            threadPoolExecutor.shutdown();
        }
    
    
        class MyThread implements Runnable {
            String name;
            int which; // 标示符：0：打印A；1：打印B；2：打印C
            int n; // 0：打印次数
            int num; // 循环周期
    
    
            public MyThread(String name,int which,int n,int num) {
                this.name = name;
                this.which = which;
    
                this.n = n;
                this.num = num;
            }
    
            @Override
            public void run() {
                for (int i = 0; i < n; i++) {
                    synchronized (t) {
                        while (state % num != which) {
                            try {
                                t.wait();
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        System.out.print(name); // 执行到这里，表明满足条件，打印
                        state++;
                        t.notify(); // 调用notifyAll方法
                    }
                }
            }
        }
    }

到此 Java多线程循环打印ABC等问题介绍完成。