public class CountDownLatch {}

private final Sync sync;public CountDownLatch(int count) {if (count < 0) throw new IllegalArgumentException("count < 0");this.sync = new Sync(count);
}

public void await() throws InterruptedException {sync.acquireSharedInterruptibly(1);
}public boolean await(long timeout, TimeUnit unit)throws InterruptedException {return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}

public void countDown() {sync.releaseShared(1);
}

private static final class Sync extends AbstractQueuedSynchronizer {private static final long serialVersionUID = 4982264981922014374L;// 构造器，初始化 AQS 的 state 变量Sync(int count) {setState(count);}int getCount() {return getState();}// 尝试获取资源的操作// 只有当 state 变量为 0 的时候才能获取成功（返回 1）protected int tryAcquireShared(int acquires) {return (getState() == 0) ? 1 : -1;}// 尝试释放资源的操作    protected boolean tryReleaseShared(int releases) {// Decrement count; signal when transition to zerofor (;;) {int c = getState();if (c == 0)return false;// 该操作就是尝试把 state 变量减去 1int nextc = c-1;if (compareAndSetState(c, nextc))return nextc == 0;}}
}

public void test() throws InterruptedException {int count = 5;// CountDownLatch 的初始化计数器为 5// 注意线程数和计数器保持一致CountDownLatch countDownLatch = new CountDownLatch(count);for (int i = 0; i < count; i++) {int finalI = i;new Thread(() -> {try {TimeUnit.SECONDS.sleep(finalI);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + " is working ..");// 每个线程执行结束时执行 countDown
            countDownLatch.countDown();}).start();}// 主线程进入等待状态（尝试获取资源，成功后才能继续执行）
    countDownLatch.await();System.out.println(Thread.currentThread().getName() + " go on ..");
}/*  输出结果：Thread-0 is working ..Thread-1 is working ..Thread-2 is working ..Thread-3 is working ..Thread-4 is working ..main go on ..
*/

private static volatile List<Integer> list = new ArrayList<>();private static void test() {CountDownLatch countDownLatch = new CountDownLatch(1);new Thread(() -> {if (list.size() != 5) {try {// list 的大小为 5 时再继续执行，否则等待// 等待 state 减到 0
        countDownLatch.await();} catch (InterruptedException e) {e.printStackTrace();}}System.out.println(Thread.currentThread().getName() + " start..");}).start();new Thread(() -> {for (int i = 0; i < 10; i++) {list.add(i);System.out.println(Thread.currentThread().getName() + " add " + i);if (list.size() == 5) {// 满足条件时将 state 减 1
        countDownLatch.countDown();}try {TimeUnit.SECONDS.sleep(1);} catch (InterruptedException e) {e.printStackTrace();}}}).start();
}/*  输出结果：Thread-1 add 0Thread-1 add 1Thread-1 add 2Thread-1 add 3Thread-1 add 4Thread-0 start..Thread-1 add 5Thread-1 add 6Thread-1 add 7Thread-1 add 8Thread-1 add 9
*/