CountDownLatch 主要是作用是用来维护一个线程控制多个线程,内部是通过一个计数器实现的,当我们创建一个CountDownLatch对象的时候,就需要指定一个数值,这个数值就表示了线程的数量,每当一个线程任务执行完毕,计数器就会减 1,当计数器的值变为0时,就表示所有的线程都已经执行完毕,可以执行后续的程序。
实例分析
CountDownLatch内部依赖Sync实现,而Sync继承AQS。CountDownLatch仅提供了一个构造方法:
// count 指定计数值
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
sync为CountDownLatch的一个内部类,其定义如下:
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;
}
}
}
通过这个内部类Sync我们可以清楚地看到CountDownLatch是采用共享锁来实现的。
共享锁:可简单的理解为读锁去竞争资源,假如竞争到了,则跟它一起共享这个资源的队列的线程,都要被依次唤醒,共同查这个数据。
await()
CountDownLatch提供await()方法来使当前线程在锁存器倒计数至零之前一直等待,除非线程被中断,定义如下:
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
await其内部使用AQS的acquireSharedInterruptibly(int arg):
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
}
在内部类Sync中重写了tryAcquireShared(int arg)方法:
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
getState()获取同步状态,其值等于计数器的值,从这里我们可以看到如果计数器值不等于0,则会调用doAcquireSharedInterruptibly(int arg),该方法为一个自旋方法会尝试一直去获取同步状态:
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
final Node p = node.predecessor();
if (p == head) {
/**
* 对于CountDownLatch而言,如果计数器值不等于0,那么r 会一直小于0
*/
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
//等待
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}