/*
* 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() + "]";
}
}