【Java多线程实战设计】用三个线程按顺序循环打印abc三个字母，比如abcabcabc

目录

1 思路

2 synchronized/wait/notifyAll 机制

3 ReentrantLock/Condition.await()/Condition.signalAll() 机制

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1 思路

启动三个线程，采用多线程间等待/通知的机制来设计。维护一个全局不变数组printArr和变量索引index，printArr[index]就是接下来的线程要打印的字符。三个线程一开始就定好了打印的字符，当线程获取锁后，要判断printArr[index]与指定该线程的字符相等，相等就打印字符同时index = (index+1)%3，不相等表明还没轮到当前线程打印，当前线程等待。

2 synchronized/wait/notifyAll 机制

public class Test1 {

    private static char[] printArr = new char[]{'a','b','c'};
    private static volatile int index;                          // 保证index的可见性

    public static void main(String[] args) {
        ExecutorService threadPool = Executors.newFixedThreadPool(3);
        final Object lock = new Object();                       
        threadPool.execute(new MyThread(lock,printArr[index]));
        threadPool.execute(new MyThread(lock,printArr[index+1]));
        threadPool.execute(new MyThread(lock,printArr[index+2]));
        threadPool.shutdown();
    }

    private static class MyThread extends Thread{        // 静态内部打印类
        Object lock;        // 同步锁和等待/通知锁
        char printC;        // 指定的要打印的字符
        MyThread(final Object lock ,char printC){
            this.lock = lock;
            this.printC = printC;
        }

        @Override
        public void run(){
            while(true){
                synchronized (lock){
                    char c = printArr[index];
                    if (c == printC){
                        System.out.println(c);
                        index = (index+1)%printArr.length;    // index“+1”
                        lock.notifyAll();                    // 唤醒所有等待在lock上的线程
                    } else {
                        try {
                            lock.wait();                    
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
    }
}

3 ReentrantLock/Condition.await()/Condition.signalAll() 机制

public class Test2 {

    private static char[] printArr = new char[]{'a','b','c'};
    private static volatile int index;                          // 保证index的可见性

    public static void main(String[] args) {
        ExecutorService threadPool = Executors.newFixedThreadPool(3);
        ReentrantLock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        threadPool.execute(new MyThread(lock,condition,printArr[index]));
        threadPool.execute(new MyThread(lock,condition,printArr[index+1]));
        threadPool.execute(new MyThread(lock,condition,printArr[index+2]));
        threadPool.shutdown();
    }

    private static class MyThread extends Thread{        // 静态内部打印类
        ReentrantLock lock;
        Condition condition;
        char printC;                // 指定的要打印的字符

        MyThread(ReentrantLock lock ,Condition condition ,char printC){
            if (lock == null || condition == null)
                throw new NullPointerException("lock or condition is null");
            this.lock = lock;
            this.condition = condition;
            this.printC = printC;
        }

        @Override
        public void run(){
            while(true){
                lock.lock();
                try {
                    char c = printArr[index];
                    if (c == printC){
                        System.out.println(c);
                        index = (index+1)%printArr.length;
                        condition.signalAll();
                    } else {
                        try {
                            condition.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }finally {
                    lock.unlock();
                }
            }
        }
    }
}