//在完成一组操作之前允许一个或多个线程等待内部用的AQS
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;
}
}
}
//先看构造函数
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
//初始化sync.
this.sync = new Sync(count);
}
//阻塞当前线程
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
//阻塞指定时间超时返回false
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
//递减count.0则唤醒等待的所有线程。
public void countDown() {
sync.releaseShared(1);
}
//获取当前计数器数量
public long getCount() {
return sync.getCount();
}
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}