一道多线程通信实例分析

程序如下：

public static void main(String[] args) throws  Exception{

    final List list = new ArrayList();

    final Object lock = new Object();

    Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {

            synchronized (lock){

                for(int i = 0 ; i < 10 ; i++){

                    list.add(i);
                    if(list.size() == 5){
                        lock.notify();
                        System.out.println(Thread.currentThread().getName() + "发出通知!");
                    }
                }

            }
            System.out.println(Thread.currentThread().getName() + "execute over!");

        }
    });

    Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {

            synchronized (lock){

                if(list.size() != 5){
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println(Thread.currentThread().getName() + " 收到通知!");
            }

            System.out.println(Thread.currentThread().getName() + "execute over!");

        }
    });

    t2.start();

    Thread.sleep(1000);

    t1.start();
}

分析：

程序的意图本是利用多线程之间的通信，利用wait/notify实现，可是运行的结果是虽然线程T1发出了通知，但是线程T2并没有立即收到通知进行执行，这是为什么呢？ 因为只有线程T1执行完毕释放了锁，T2才能执行，那么也就是说wait/notify并不是实时的（wait释放了锁，而notify没有释放锁导致的），那么线程之间实时的通信该怎么做呢？可以利用CountDownLatch来实现。

对程序的改进：

public static void main(String[] args) throws  Exception{

    final List list = new ArrayList();

    final Object lock = new Object();

    final CountDownLatch countDownLatch = new CountDownLatch(1);

    Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {

                for(int i = 0 ; i < 10 ; i++){

                    list.add(i);
                    if(list.size() == 5){
                        countDownLatch.countDown();
                        System.out.println(Thread.currentThread().getName() + "发出通知!");
                    }
                }

            System.out.println(Thread.currentThread().getName() + "execute over!");

        }
    });

    Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {

                if(list.size() != 5){
                    try {
                        countDownLatch.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                System.out.println(Thread.currentThread().getName() + " 收到通知!");
            }

            System.out.println(Thread.currentThread().getName() + "execute over!");

        }
    });

    t2.start();

    Thread.sleep(1000);

    t1.start();
}