14 countDownlanch实现原理

vvEcho 2025-03-01 14:11:31
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总结
CountDownLatch是基于AQS实现，内部维护了一个state的计数器，await() 会在state不为 0 时阻塞线程，countDown() 通过 CAS 减少 state，当 state 为 0 时，通过 AQS 唤醒所有等待线程；

底层实现为提供一个常量类继承AQS实现的

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public class CountDownLatch {
    /**
     * Synchronization control For CountDownLatch.
     * Uses AQS state to represent count
     */
    private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;

        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }
    // 其他代码省略
}

例子：

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/**
 * countdownlatch 让线程同时执行
 * @auther: echo
 * @date: 2025/3/1
 */
public class CountDownLatchTest {

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(1);
        for (int i = 0; i < 3; i++) {
            new Thread(() -> {
                try {
                    countDownLatch.await();
                    System.out.println(System.currentTimeMillis());

                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }).start();
        }
        countDownLatch.countDown();
    }
}
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