一、CountDownLatch介绍
CountDownLatch是一种同步手段，允许一个或者更多的线程等待，直到在其他线程正在执行的一组操作完成。给定count数目后CountDownLatch被初始化。await()方法阻塞，直到由于调用countDown()方法，当前count值达到0，之后所有等待线程被释放，而任何后续await()方法的调用会立即返回。这个是只有一次的现场，即count值无法被重设。如果你需要一个能够重设count值的版本，不妨考虑使用CyclicBarrier。
二、CountDownLatch应用
CountDownLatch是一个通用的同步工具，可用于许多目的。一个用count值为1来初始化的CountDownLatch可用作一个开关或者门闩：所有的线程调用await()方法等待一个线程调用countDown()方法后把门打开。一个用count值为N来初始化的CountDownLatch可用作使得一个线程等待，直到N个线程完成各自的事情，或者一些action被完成N次等。CountDownLatch一个有用的特性是：它不需要线程调用countDown()方法等待计数达到零在继续之前，它只是阻止任何线程继续过去一个等待,直到所有线程可以通过。
1、示例应用一
这里有一对类，一组工作线程使用两个countdown latches的示例：
第一个是启动信号，阻止worker线程工作直到driver准备好；
第二个是完成信号，允许driver等到所有的workers工作完成。
代码如下：
package com.pengli.jdk;
import java.util.concurrent.CountDownLatch;
public class TestCountDownLatch {
class Driver {
void main() throws InterruptedException {
CountDownLatch startSignal = new CountDownLatch(1);
CountDownLatch doneSignal = new CountDownLatch(20);
for (int i = 0; i < 20; ++i) // create and start threads
new Thread(new Worker(startSignal, doneSignal)).start();
doSomethingElse(); // don't let run yet
startSignal.countDown(); // let all threads proceed
doSomethingElse();
doneSignal.await(); // wait for all to finish
}
void doSomethingElse() {
// ...
}
}
class Worker implements Runnable {
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal;
Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
this.startSignal = startSignal;
this.doneSignal = doneSignal;
}
public void run() {
try {
startSignal.await();
doWork();
doneSignal.countDown();
} catch (InterruptedException ex) {
} // return;
}
void doWork() {
// ...
}
}
}
2、示例应用二
另一个典型用法是将一个问题分成n份，在一个线程中定义并执行一份，并在latch中count down，然后将所有的线程放入一个队列。当所有的部分完成，协调线程将会通过await()方法，继续处理。当线程必须以这种方式反复count down时，使用CyclicBarrier。
代码如下：
package com.pengli.jdk;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
public class TestCountDownLatch2 {
class Driver2 { // ...
void main() throws InterruptedException {
CountDownLatch doneSignal = new CountDownLatch(20);
Executor e = Executors.newFixedThreadPool(20);
for (int i = 0; i < 20; ++i) // create and start threads
e.execute(new WorkerRunnable(doneSignal, i));
doneSignal.await(); // wait for all to finish
}
}
class WorkerRunnable implements Runnable {
private final CountDownLatch doneSignal;
private final int i;
WorkerRunnable(CountDownLatch doneSignal, int i) {
this.doneSignal = doneSignal;
this.i = i;
}
public void run() {
doWork(i);
doneSignal.countDown();
}
void doWork(int i) {
// ...
}
}
}
三、CountDownLatch实现分析
1、Sync
在CountDownLatch内部，有一个Sync的同步器，它继承自java.util.concurrent包中各种同步工具共用的AbstractQueuedSynchronizer，其实现如下：
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
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;
}
}
} 关于AbstractQueuedSynchronizer，有其它的文章进行专门的介绍。这里只分析下Sync的实现。其有一个需要入参int count的构造函数，设置AbstractQueuedSynchronizer的state。并覆写了tryAcquireShared()和tryReleaseShared()方法，其中tryReleaseShared()方法用于CountDownLatch的countDown()方法，这个tryReleaseShared()方法的逻辑如下：
在一个for循环内，首先通过getState()获取state值，如果为0，直接返回false，否则取state-1，并尝试CAS操作，修改state状态，并且state等于0，返回true，否则返回false。
tryAcquireShared()方法更简单，判断state（即count值），如果等于0，返回1，否则返回-1.
2、countDown()
countDown()方法的实现很简单，如下：
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
*
* <p>If the current count is greater than zero then it is decremented.
* If the new count is zero then all waiting threads are re-enabled for
* thread scheduling purposes.
*
* <p>If the current count equals zero then nothing happens.
*/
public void countDown() {
sync.releaseShared(1);
} 其核心处理就是减少latch中count值，如果cout值为0，释放所有的等待线程。它调用的是sync的releaseShared()方法，而这个方法是在AbstractQueuedSynchronizer中实现的，如下：
/**
* Releases in shared mode. Implemented by unblocking one or more
* threads if {@link #tryReleaseShared} returns true.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryReleaseShared} but is otherwise uninterpreted
* and can represent anything you like.
* @return the value returned from {@link #tryReleaseShared}
*/
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
} 先调用tryReleaseShared()方法，即上述Sync的同名方法，并且如果返回true的话，继续调用doReleaseShared()方法，返回true，否则返回false。即如果修改后state（即count）值为正，不做其他处理，否则调用doReleaseShared()方法。
3、await()
await()方法的核心作用是，让当前线程阻塞，直到latch的count值更改为0，或者当前线程被interrupted。如果count值为0，则await()方法直接返回。代码如下：
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>If the current count is zero then this method returns immediately.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of two things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
} 还是借助的sync，调用的其acquireSharedInterruptibly()方法，这个方法是在Sync的父类中实现的，代码如下：
/**
* Acquires in shared mode, aborting if interrupted. Implemented
* by first checking interrupt status, then invoking at least once
* {@link #tryAcquireShared}, returning on success. Otherwise the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted.
* @param arg the acquire argument
* This value is conveyed to {@link #tryAcquireShared} but is
* otherwise uninterpreted and can represent anything
* you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
} 先调用Sync的tryAcquireShared()方法，如果返回值为负值，则调用doAcquireSharedInterruptibly()方法。上面讲到了，如果state（即count）值为0，则返回1，方法直接返回，否则进入doAcquireSharedInterruptibly()方法，实现阻塞。
doReleaseShared()和doAcquireSharedInterruptibly()方法的介绍参见AbstractQueuedSynchronizer的分析文章。
