先看一个例子，来说明下CountDownLatch运用场景
public class SpiderDemo {
private static Logger logger = LoggerFactory.getLogger(SpiderDemo.class);
private static ExecutorService spiderPool = Executors.newFixedThreadPool(5);
public static void main(String[] args) {
CountDownLatch countDown = new CountDownLatch(5);
for (int i=0;i<5;i++) {
spiderPool.execute(new SpiderThread(countDown, i));
}
try {
countDown.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("spider end");
spiderPool.shutdown();
}
static class SpiderThread implements Runnable{
private CountDownLatch countDown;
private int index;
public SpiderThread(CountDownLatch countDown, int index) {
this.countDown = countDown;
this.index = index;
}
@Override
public void run() {
// TODO Auto-generated method stub
logger.info("spider"+index+" data begin.. ");
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
logger.info("spider"+index+" data complete.. ");
countDown.countDown();
}
}
}
我们来看看上面的例子，这里创建了一个CountDownLatch对象，传入一个数值，使用5个线程去执行SpiderThread，结束后使用countDown()方法将计数器递减，最后使用await()方法堵塞，直到所有的线程执行完成。
所以CountDownLatch的使用场景是可以异步去执行多个任务，主线程可以等待所有任务执行完成后，再继续执行。
下面从源码方面分析下CountDownLatch的实现原理
首先查看构造方法CountDownLatch(int count )
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
Sync(int count) {
setState(count);
}
当count小于0的时候，直接抛出异常，大于0则创建了一个Sync对象。Sync构造函数则是直接设置了一个变量值state来保存count数组
再看看countDown()方法，很显然它的目的是将count数组-1
public void countDown() {
sync.releaseShared(1);
}
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
它里面是直接调用了releaseShared(1)方法，而releaseShared又调用了tryReleaseShared方法，如果尝试执行成功，则再调用doReleaseShared()方法
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;
}
}
tryReleaseShared是干什么的呢？从上面的例子看它的作用就是将count值-1，如果当前count值就是0了，那就啥都不干，直接返回false了，否则使用CAS算法尝试将count值-1，最后判断当前的count值是否为0，若为0则返回true，返回true就需要执行doReleaseShared方法了
private void doReleaseShared() {
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
doReleaseShared又干了些啥？doReleaseShared的作用就是唤醒主线程继续执行，这里调用了unparkSuccessor方法
最后来看下await()方法
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
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);
}
}
可以看到最终会调用doAcquireSharedInterruptibly（若中途线程有被打断，则抛出异常了）
首先会添加一个Node节点（Node.SHARED看源码实质就是new Node()），这个节点就是保存所有调用await()方法的线程（因为可能会有多个线程调用了await()方法），所以这里每调用一次就保存下来一个节点，最后一个一个的释放，如果这里判断count不为0，则会使用park方法将线程挂起。
