JDK1.8源码阅读与翻译：CountDownLatch

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

/**
 * A synchronization aid that allows one or more threads to wait until
 * a set of operations being performed in other threads completes.
 * CountDownLatch是一个同步辅助类，允许一个或多个线程等待，直到其他线程
 * 进行的一系列操作完成。
 *
 * 
A {@code CountDownLatch} is initialized with a given count.
 * 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使用指定的count进行初始化。await()会一直阻塞，直到通过
 * 调用countDown()使count达到0。达到0后所有阻塞的线程都会释放并且await的
 * 后续调用会立即执行。这个过程是一次性的--count值不能重新设置。如果您需
 * 要count可以重设的版本，可以考虑使用CyclicBarrier。
 *
 * 
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 N
 * can be used to make one thread wait until N threads have
 * completed some action, or some action has been completed N times.
 * CountDownLatch是一个多功能的同步工具可以被用于多种目的。一个用count=1
 * 初始化的CountDownLatch可以用作简单的开关或门禁：所有调用await()的线程
 * 都在门禁处等待，直到一个其他线程通过countDown()打开门禁。一个用count=N
 * 初始化的CountDownLatch可以用于让一个线程等待，直到N个其他线程完成某些
 * 动作，或某些动作被完成N次。
 *
 * 
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的一个有用的属性是它不要求调用countDown()的线程等待
 * count到0。它只是阻止任何线程通过await()的阻塞，直到所有线程都可以
 * 通过。
 *
 * 
Sample usage: Here is a pair of classes in which a group
 * of worker threads use two countdown latches:
 * 

 * 
The first is a start signal that prevents any worker from proceeding
 * until the driver is ready for them to proceed;
 * 
The second is a completion signal that allows the driver to wait
 * until all workers have completed.
 * 
 * 示例：这里有两个类，一组工作线程使用两个CountDownLatch：
 * 第一个CountDownLatch是一个开始信号，避免任何工人在司机准备好之前就进行工作。
 * 第二个CountDownLatch是一个完成信号，允许司机等待所有工人工作完成。
 *
 *  
 {@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() { ... }
 * }}
 *
 * 
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}.)
 * 另一个典型的用法是将一个问题分割成N个部分，每个部分用一个Runnable来
 * 执行并进行countDown()，所有的Runnable放入Executor。当所有的部分都完成时，
 * 线程才可以通过await()。（当线程必须重复countDown时，用CyclicBarrier来代替）
 *
 *  
 {@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() { ... }
 * }}
 *
 * 
Memory consistency effects: Until the count reaches
 * zero, actions in a thread prior to calling
 * {@code countDown()}
 * happen-before
 * actions following a successful return from a corresponding
 * {@code await()} in another thread.
 * 内存一致效果：直到count减到0，之前调用了countDown()的线程的动作，happen-before
 * 调用await()的线程成功返回后的动作。（这里的含义个人理解是countDown会比await先返回，
 * 因为AQS中的state是被volatile修饰的，java的“先行发生”原则保证了countDown对state的
 * 更新操作先于await中对state的读操作）
 *
 * @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（AbstractQueuedSynchronizer）的状态（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) {
            // 尝试获取共享锁，当state（就是count）为0时，返回1成功，否则-1失败
            // 获取锁失败时，AQS内会LockSupport.park()，当前线程进入阻塞（waiting）。
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            // count减一，当减到0时发出信号，其实就是返回true时AQS会将阻塞线程全部unPark
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

    private final Sync sync;

    /**
     * Constructs a {@code CountDownLatch} initialized with the given count.
     *
     * @param count the number of times {@link #countDown} must be invoked
     *        before threads can pass through {@link #await}
     * @throws IllegalArgumentException if {@code count} is negative
     */
    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}.
     * 调用本方法将使当前线程进入等待状态，直到锁计数器减为0，除非这个线程被
     * 中断（interrupted）。
     *
     * 
If the current count is zero then this method returns immediately.
     * 如果当前的count就是0,，本函数将立即返回。
     *
     * 
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count大于0，则当前线程因线程调度进入休眠，直到以下两种情况发生：
     *
     * 

     * 
The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * 
Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread.
     * 
     * count由于countDown被调用减到0；或其他线程将当前线程中断。
     *
     * 
If the current thread:
     * 

     * 
has its interrupted status set on entry to this method; or
     * 
is {@linkplain Thread#interrupt interrupted} while waiting,
     * 
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     * 如果当前线程：
     * 进入本方法前就设置了中断状态，或在waiting时调用中断函数Thread.interrupt()，
     * 则本函数将抛出InterruptedException，并清理掉当前线程的interrupted状态。
     *
     * @throws InterruptedException if the current thread is interrupted
     *         while waiting
     */
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * Causes the current thread to wait until the latch has counted down to
     * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
     * or the specified waiting time elapses.
     * 调用本方法将使当前线程进入等待状态，直到锁计数器减为0，除非这个线程被
     * 中断（interrupted）,或者超出了指定的等待时间。
     *
     * 
If the current count is zero then this method returns immediately
     * with the value {@code true}.
     *
     * 
If the current count is greater than zero then the current
     * thread becomes disabled for thread scheduling purposes and lies
     * dormant until one of three things happen:
     * 

     * 
The count reaches zero due to invocations of the
     * {@link #countDown} method; or
     * 
Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     * 
The specified waiting time elapses.
     * 
     *
     * 
If the count reaches zero then the method returns with the
     * value {@code true}.
     *
     * 
If the current thread:
     * 

     * 
has its interrupted status set on entry to this method; or
     * 
is {@linkplain Thread#interrupt interrupted} while waiting,
     * 
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * 
If the specified waiting time elapses then the value {@code false}
     * is returned.  If the time is less than or equal to zero, the method
     * will not wait at all.
     * 如果超出了指定的等待时长，将会返回false。如果指定时长不大于0，本方法
     * 将不会进入等待。
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the {@code timeout} argument
     * @return {@code true} if the count reached zero and {@code false}
     *         if the waiting time elapsed before the count reached zero
     * @throws InterruptedException if the current thread is interrupted
     *         while waiting
     */
    public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    /**
     * Decrements the count of the latch, releasing all waiting threads if
     * the count reaches zero.
     * 将锁计数器count递减，当减到0时释放所有的等待线程。
     *
     * 
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.
     * 当前count大于0则将会递减。如果递减后新的count是0，则所有的等待线程将
     * 重新启用以进行线程调度。
     *
     * 
If the current count equals zero then nothing happens.
     * 如果当前count为0，本函数不会触发任何操作。
     */
    public void countDown() {
        sync.releaseShared(1);
    }

    /**
     * Returns the current count.
     * 返回当前的count值。
     *
     * 
This method is typically used for debugging and testing purposes.
     * 本函数通常用于调试和测试。
     *
     * @return the current count
     */
    public long getCount() {
        return sync.getCount();
    }

    /**
     * Returns a string identifying this latch, as well as its state.
     * The state, in brackets, includes the String {@code "Count ="}
     * followed by the current count.
     *
     * @return a string identifying this latch, as well as its state
     */
    public String toString() {
        return super.toString() + "[Count = " + sync.getCount() + "]";
    }
}