CountDownLatch
AQS共享模式的实现。
/**
* A synchronization aid that allows one or more threads to wait until
* a set of operations being performed in other threads completes.
* 一种同步辅助工具，允许一个或多个线程等待，直到在其他线程中执行的一组操作完成。
*
* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
* The {@link #await await} methods block until the current count reaches
* zero due to invocations of the {@link #countDown} method, after which
* all waiting threads are released and any subsequent invocations of
* {@link #await await} return immediately. This is a one-shot phenomenon
* -- the count cannot be reset. If you need a version that resets the
* count, consider using a {@link CyclicBarrier}.
* CountDownLatch是使用一个给定的count来初始化。
* await 方法会阻塞，直到当前计数由于调用 countDown 方法而达到零，之后释放所有等待的线程，
* 后续任何调用 await 会立即返回。这是一种一次性现象——无法重置计数。
* 如果需要重置计数的版本，请考虑使用 CyclicBarrier。
*
* <p>A {@code CountDownLatch} is a versatile synchronization tool
* and can be used for a number of purposes. A
* {@code CountDownLatch} initialized with a count of one serves as a
* simple on/off latch, or gate: all threads invoking {@link #await await}
* wait at the gate until it is opened by a thread invoking {@link
* #countDown}. A {@code CountDownLatch} initialized to <em>N</em>
* can be used to make one thread wait until <em>N</em> threads have
* completed some action, or some action has been completed N times.
* CountDownLatch 是一种多用途的同步工具，可用于多种目的。
* CountDownLatch用一个计数初始化，用作简单的开/关 门闩或闸门：
* 所有线程调用 await 方法等待闸门，直到一个线程调用countDown方法来打开闸门。
* CountDownLatch的计数被初始化为N，可以让一个线程等待，直到N个线程完成某些动作，
* 或者某些动作被完成N次。
*
* <p>A useful property of a {@code CountDownLatch} is that it
* doesn't require that threads calling {@code countDown} wait for
* the count to reach zero before proceeding, it simply prevents any
* thread from proceeding past an {@link #await await} until all
* threads could pass.
* CountDownLatch一个有用的特性是，它不要求调用countDown的线程在继续之前要等待计数达到0，
* 它只是防止任何线程通过await，直到所有线程都可以通过。
*
* <p><b>Sample usage:</b> Here is a pair of classes in which a group
* of worker threads use two countdown latches:
* 示例用法：这里有一对类，其中一组工作线程使用两个CountDownLatch：
*
* <ul>
* <li>The first is a start signal that prevents any worker from proceeding
* until the driver is ready for them to proceed;
* 第一个是一个启动信号，它阻止任何工人继续工作，直到司机准备好让他们继续工作；
*
* <li>The second is a completion signal that allows the driver to wait
* until all workers have completed.
* 第二个是完成信号，允许司机等待所有工人完成。
* </ul>
*
* <pre> {@code
* class Driver { // ...
* void main() throws InterruptedException {
* CountDownLatch startSignal = new CountDownLatch(1);
* CountDownLatch doneSignal = new CountDownLatch(N);
*
* for (int i = 0; i < N; ++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
* }
* }
*
* 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() { ... }
* }}</pre>
*
* <p>Another typical usage would be to divide a problem into N parts,
* describe each part with a Runnable that executes that portion and
* counts down on the latch, and queue all the Runnables to an
* Executor. When all sub-parts are complete, the coordinating thread
* will be able to pass through await. (When threads must repeatedly
* count down in this way, instead use a {@link CyclicBarrier}.)
* 另一个典型用法是将问题划分为N个部分，用一个Runnable描述每个部分，
* Runnable执行该部分并对闩锁进行倒计时，然后将所有Runnable排队给一个执行器。
* 当所有子部件完成时，协调线程将能够通过等待。（当线程必须以这种方式重复倒计时时，请改用{@link CyclicBarrier}。）
*
* <pre> {@code
* class Driver2 { // ...
* void main() throws InterruptedException {
* CountDownLatch doneSignal = new CountDownLatch(N);
* Executor e = ...
*
* for (int i = 0; i < N; ++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() {
* try {
* doWork(i);
* doneSignal.countDown();
* } catch (InterruptedException ex) {} // return;
* }
*
* void doWork() { ... }
* }}</pre>
*
* <p>Memory consistency effects: Until the count reaches
* zero, actions in a thread prior to calling
* {@code countDown()}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* actions following a successful return from a corresponding
* {@code await()} in another thread.
*
* @since 1.5
* @author Doug Lea
*/
public class CountDownLatch {
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
* CountDownLatch的同步控制，使用AQS的state来表示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) {
// state==0则成功，否则都是失败
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;//只有state被减到0才算成功
}
}
}
private final Sync sync;
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
* 使当前线程等待，直到锁计数倒计时到零，除非该线程被中断。
*
* <p>If the current count is zero then this method returns immediately.
* 如果当前计数器时0则该方法会立即返回。
*
* <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:
* 如果当前计数大于零，则出于线程调度目的，当前线程将被禁用，并处于休眠状态，直到发生以下两种情况之一：
*
* <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>
* 计数通过调用countDown方法达到0；
* 或者其他线程中断当前线程。
*
* <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.
* 如果当前线程：
* 在进入该方法时已经有中断状态了；或者时在等待中被中断，则会抛出InterruptedException
* 异常，并清除当前线程的中断状态。
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
* 减少该latch的计数，当计数达到0时，释放所有等待的线程。
*
* <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.
* 如果当前计数大于0则将它递减。如果新的计数时0则处于线程调度目的所有等待的线程将重新可用。
*
* <p>If the current count equals zero then nothing happens.
* 如果当前计数等于0则什么都不发生。
*/
public void countDown() {
sync.releaseShared(1);
}
public long getCount() {
return sync.getCount();
}
}
总结：
- 基本用法，一个或多个线程await，然后等待计数减到0，到0时将所有等待中的线程唤醒。
- 只要count不为0，所有的tryAcquireShared就不会成功。
- 只要count没有减到0，所有的tryReleaseShared也不会成功，最后一个将count减到0的线程才成功，再执行doReleaseShared操作。
- 一个大任务分片处理，多个线程分片执行，都执行完了再继续。则是一个线程去wait，多个线程去countDown。
这种情况下不太能显示出共享模式的用法，只是最后一个countDown为0的线程会去release掉那一个wait的线程。没有体现出传播的特性。 - 还有一种用法是多个线程在wait，一个线程countDown后，所有wait的线程开始工作，这是就体现出了传播的特性。
多个线程wait的时候，AQS队列中会有多个节点，countDown后第一个节点会被head给unPark，这只是唤醒了一个线程，
其它几个线程可还在park中呢，第一个线程获取锁后，会通过setHeadAndPropagate方法将后面的一个唤醒，后面的再
通过setHeadAndPropagate将其后面的一个唤醒，依次逐个唤醒，就体现了传播的动作了。
