最近面试被问到一个题目：编写一个程序，开启 3 个线程，这三个线程分别负责打印 A、B、C，要求循环输出10次ABC，且输出的结果必须按顺序显示。如：ABCABCABC……

使用ReentrantLock实现

参考文章：https://my.oschina.net/beauxie/blog/1529954

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public class TestThreadDemo  {
    // 通过JDK5中的锁来保证线程的访问的互斥
    private static Lock lock = new ReentrantLock();

    private static int state = 0;// 用state来判断轮到谁执行

    private static final int RUN_NUMBER=10;//表示循环的次数

    //A线程
    static class ThreadA extends Thread {

        @Override
        public void run() {
            for (int i = 0; i < RUN_NUMBER;) {
                lock.lock();//获取锁定
                if (state % 3 == 0) {
                    System.out.print("A");
                    state++;
                    i++;
                }
                lock.unlock();//释放锁定,不释放锁定，会被该线程一直保持
            }
        }
    }
    //B线程
    static class ThreadB extends Thread {

        @Override
        public void run() {
            for (int i = 0; i < RUN_NUMBER;) {
                lock.lock();
                if (state % 3 ==1) {
                    System.out.print("B");
                    state++;
                    i++;
                }
                lock.unlock();
            }
        }
    }
    //C线程
    static class ThreadC extends Thread {

        @Override
        public void run() {
            for (int i = 0; i < RUN_NUMBER;) {
                lock.lock();
                if (state % 3 == 2) {
                    System.out.print("C");
                    state++;
                    i++;
                }
                lock.unlock();
            }
        }
    }
    public static void main (String[] args){
        new ThreadA().start();
        new ThreadB().start();
        new ThreadC().start();
    }
}

使用synchronized实现

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public class TestThreadDemo2 {
    // 用于判断该输出那个字符 0输出A  1 输出B 2 输出C
    private volatile int flag = 0;

    public synchronized void printA() {
        for (int i = 0; i < 10; i++) {
            while (flag != 0){
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.print("A");
            flag = 1;
            notifyAll();
        }
    }

    /**
     * 输出B方法
     */
    public synchronized void printB(){
        for (int i = 0; i < 10; i++) {
            while (flag != 1){
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.print("B");
            flag = 2;
            notifyAll();
        }
    }
    public synchronized void printC() {
        for (int i = 0; i < 10; i++) {
            while (flag != 2){
                try {
                    wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            System.out.print("C");
            flag = 0;
            notifyAll();
        }

    }

    public static void main(String[] args) {
        final TestThreadDemo2 demo = new TestThreadDemo2();
        synchronized (demo){
            new Thread(()->demo.printA()).start();
            new Thread(()->demo.printB()).start();
            new Thread(()->demo.printC()).start();
        }
    }
}