在有效的Java第二项72中实现CountDownLatch的更好方法是什么？

Question

Effective Java Item 72 shows bad example of CountDownLatch implementation. But it doesn't show a right way to implement it. Do I have to use wait() and notify() instead of a while loop ?

Can anyone suggest me a good example of this item ?

Below is the bad code example:

public class SlowCountDownLatch {
    private int count;
    public SlowCountDownLatch(int count) {
        if (count < 0)
            throw new IllegalArgumentException(count + " < 0");
        this.count = count;
    }
    public void await() {
        while (true) {
            synchronized (this) {
                if (count == 0)
                    return;
            }
        }
    }
    public synchronized void countDown() {
        if (count != 0)
            count--;
    }
}

Answer 1

If you study carefully that item again , you will see this "Threads should not busy-wait, repeatedly checking a shared object waiting for something to happen" that is exactly what bad example is doing in while loop, that's why he has mentioned this also "Threads should not run if they aren't doing useful work".

See the correct CountDownLatch Implementation details as below:

public class CountDownLatch{

    private int count;
    /**
     * CountDownLatch is initialized with given count.
     * count specifies the number of events that must occur
     * before latch is released.
     */
    public CountDownLatch(int count) {
           this.count=count;
    }

    /**
     * Causes the current thread to wait until  one of the following things happens-
                  - latch count has down to reached 0, or
                  - unless the thread is interrupted.
     */
    public synchronized void await() throws InterruptedException {
           //If count is greater than 0, thread waits.
           if(count>0)
                  this.wait();
    }

    /**
     *  Reduces latch count by 1.
     *  If count reaches 0, all waiting threads are released.
     */
    public synchronized void countDown() {
           //decrement the count by 1.
           count--;

           //If count is equal to 0, notify all waiting threads.
           if(count == 0)
                  this.notifyAll();
    }
}