JAVA并发（5）— 多线程顺序的打印出A，B，C（线程间的协作）

多线程顺序打印问题的本质就是线程间的互斥与协作。线程互斥执行后修改阻塞条件然后唤醒阻塞线程。

1. Synchronized关键字实现

Synchronzied关键字是对Object加锁，即Object对象头指向Object Monitor（Java管程）对象。正是依赖Object Monitor，线程才能同步(互斥)的访问临界区资源。

Object Monotor提供了wait和notify/notifyAll的方法。即阻塞和唤醒线程。线程可以使用上述方法完成协作。

public class SequentialPrinting {
    private int runCount = 0;
    /**
     * 打印机
     */
    private void printABC(int id, String name) {
        while (true) {
            synchronized (this) {
                //使用while循环保证唤醒时能够继续争夺锁
                while (runCount % 3 != id) {
                    try {
                       //将该线程放入到wait_set阻塞
                        this.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.print(name+" ");
                runCount++;
                this.notifyAll();
            }
        }
    }
    public static void main(String[] args) {
        SequentialPrinting sp=new SequentialPrinting ();
        new Thread(()->{
            sp.printABC(0,"A");
        }).start();
        new Thread(()->{
            sp.printABC(1,"B");
        }).start();
        new Thread(()->{
            sp.printABC(2,"C");
        }).start();
    }
}

2. Lock+Condition实现

public class PrintingByCondition {
    private ReentrantLock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();
    private int runCount;
    public void printABC(int id, String name) {
        while (true) {
            try {
                lock.lock();
                while (runCount % 3 != id) {
                    try {
                        condition.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.print(name+" ");
                runCount++;
                condition.signalAll();
            } finally {
                lock.unlock();
            }
        }
    }
    public static void main(String[] args) {
        PrintingByCondition printingByCondition=new PrintingByCondition();
        new Thread(()->{
            printingByCondition.printABC(0,"A");
        }).start();
        new Thread(()->{
            printingByCondition.printABC(1,"B");
        }).start();
        new Thread(()->{
            printingByCondition.printABC(2,"C");
        }).start();
    }
}

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