OpenJDK之CountDownLatch
OpenJDK8，本人看的是openJDK。以前就看过，只是经常忘记，所以记录下
图1
CountDownLatch是Doug Lea在JDK1.5中引入的，作用就不详细描述了，
- await()方法，如果还有线程在执行，那么当前线程阻塞，如果最后一个线程执行完成，则被唤醒
- countDown()方法，如果不是最后一个线程，则将状态值减去1，如果是最后一个，则将被阻塞的线程唤醒
List-1
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;
}
}
}CountDownLatch里面有个内部类Sync继承了AbstractQueuedSynchronizer——这个类在java.util.concurrent.locks包下。
CountDownLatch的构造方法调用Sync的构造方法，设置state的值，这个起到一个类似计数器的作用。
1.await()
List-2
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}对CountDownLatch调用await()时，调用的是Sync的acquireSharedInterruptibly()——这个方法是AbstractQueuedSynchronizer里面的，如下List-3，tryAcquireShared方法在类Sync中实现，如List-1中，当还有线程还未执行完，tryAcquireShared会返还-1，这样就调用doAcquireSharedInterruptibly()如List-4，这个方法是AbstractQueuedSynchronizer的，会将当前当前线程加入到CLH队列中，这样就实现了将当前线程阻塞，后面让最后一个来唤醒。
List-3
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}List-4
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) {
int r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}2.countDown()
List-5
public void countDown() {
sync.releaseShared(1);
}如List-5，内部调用了Sync的releaseShared(1)，即释放1，如下List-6，先用tryReleaseShared方法来尝试是否能减少一个状态值，如List-7，在for循环中将状态值减去1，之后用CAS，更新状态值，如果此时状态值位0则返回true，如果不是则返回false。
tryReleaseShared()如果返还true，则说明是最后一个线程了，此时用doReleaseShared()唤醒阻塞的线程，底层上使用LockSupport.unpark(Thread).
List-6
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}List-7
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;
}
}CyclicBarrier内部是使用ReentrantLock来实现，而CountDownLatch内部是使用AQS内部类来实现。