CountDownLatch在Jdk1.5中引入，通过内部实现继承了AbstractQueuedSynchronizer的Sync类，并实现了tryAcquireShared(arg)、tryReleaseShared方法，使用方式如下
public class CountDownLatchDemo {
public static void main(String[] args) throws InterruptedException {
CountDownLatch latch=new CountDownLatch(3);
for (int i=0;i<3;i++){
new Thread(()->{
System.out.println("thread "+Thread.currentThread().getName()+" entry");
latch.countDown();
System.out.println("thread "+Thread.currentThread().getName()+" exit");
},"T"+i).start();
}
latch.await();
System.out.println("main thread end");
}
}
创建了一个初始值为3的CountDownLatch对象latch，然后创建了3个线程，每个线程执行时都会执行latch.countDown()使计数器的值减1，而主线程在执行到latch.await()时会等待直到计数器的值为0。输出的结果如下：
thread T0 entry
thread T2 entry
thread T1 entry
thread T2 exit
thread T0 exit
thread T1 exit
main thread end
await方法：
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
这是countDownLatch实现的方法，通过调用sync中的acquireSharedInterruptibly方法，实际调用了AQS框架中的acquireSharedInterruptibly，而coundoownlatch本身只需要实现tryAcquireShared方法即可。
acquireSharedInterruptibly方法：
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
coundownlatch自己实现的tryAcquireShared方法
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
根据状态来判断，如果state等于0说明计数器为0了，返回1表示成功，否则返回-1表示失败，需要放入队列中继续等待state变为0
当state！=0时，执行doAcquireSharedInterruptibly方法：
1.创建共享的node节点，并加入等待队列
2.获取前置节点p，如果等于head节点，则尝试获取锁，如果state==0（r>=0），则将node节点设置为head，并向后面节点传播
3.如果不为head,则自旋，找到安全点后park自己，并在唤醒后返回是否被中断
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);
}
}
private void setHeadAndPropagate(Node node, int propagate) {
Node h = head; // Record old head for check below
setHead(node);
/*
* Try to signal next queued node if:
* Propagation was indicated by caller,
* or was recorded (as h.waitStatus either before
* or after setHead) by a previous operation
* (note: this uses sign-check of waitStatus because
* PROPAGATE status may transition to SIGNAL.)
* and
* The next node is waiting in shared mode,
* or we don't know, because it appears null
*
* The conservatism in both of these checks may cause
* unnecessary wake-ups, but only when there are multiple
* racing acquires/releases, so most need signals now or soon
* anyway.
*/
CDL如果获取锁成功，则propagate=1
h.waitStatus>=0表示线程取消或刚初始化
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
if (s == null || s.isShared())
doReleaseShared();
}
}
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
如果当前节点ws为-1,表明需要唤醒后继节点
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
ws==0,表明节点初始化状态，需要设置为Node.PRoPagate状态
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
如果节点发生变化，自旋
if (h == head) // loop if head changed
break;
}
}
CDL的countDown()方法
//CountDownLatch
public void countDown() {
sync.releaseShared(1);
}
//AQS
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
//CDL中重写了tryReleaseShared
1.自旋的进行CAS操作
2.当state==0时，去唤醒后继节点
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;
}
}
}
await(long time,TimeUtil unit)
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
//如果获取到锁返回true（state==0?1:-1）
//否则进行入队，自旋获取锁，park等过程
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
}
//添加了自旋阈值的控制spinForTimeoutThreshold
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
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 true;
}
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
调用await时
共享锁获取失败（计数器还不为0），则将该线程封装为一个Node对象放入队列中，并阻塞该线程；
共享锁获取成功（计数器为0），则从第一个节点开始依次唤醒后继节点，实现共享状态的传播。
调用countDown时
如果计数器不为0，则释放，继续阻塞，并把state的值减1；
如果计数器为0，则唤醒节点，解除线程的阻塞状态。
共享和独占的区别：
共享锁在节点成为头节点获取到锁之后，立马唤醒后继节点，实现节点传播
独占锁在获取锁且未释放锁之前，其他节点一致阻塞
【参考博客】
http://www.ideabuffer.cn/2017/03/19/深入理解AbstractQueuedSynchronizer（二）
