Java多线程系列--“JUC锁”09之 CountDownLatch原理和示例
概要
前面对"独占锁"和"共享锁"有了个大致的了解；本章，我们对CountDownLatch进行学习。和ReadWriteLock.ReadLock一样，CountDownLatch的本质也是一个"共享锁"。本章的内容包括：
CountDownLatch简介
CountDownLatch数据结构
CountDownLatch源码分析(基于JDK1.7.0_40)
CountDownLatch示例
转载请注明出处：http://www.cnblogs.com/skywang12345/p/3533887.html
CountDownLatch简介
CountDownLatch是一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。
CountDownLatch和CyclicBarrier的区别
(01) CountDownLatch的作用是允许1或N个线程等待其他线程完成执行；而CyclicBarrier则是允许N个线程相互等待。
(02) CountDownLatch的计数器无法被重置；CyclicBarrier的计数器可以被重置后使用，因此它被称为是循环的barrier。
关于CyclicBarrier的原理，后面一章再来学习。
CountDownLatch函数列表
CountDownLatch(int count) 构造一个用给定计数初始化的 CountDownLatch。 // 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断。 void await() // 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断或超出了指定的等待时间。 boolean await(long timeout, TimeUnit unit) // 递减锁存器的计数，如果计数到达零，则释放所有等待的线程。 void countDown() // 返回当前计数。 long getCount() // 返回标识此锁存器及其状态的字符串。 String toString()
CountDownLatch数据结构
CountDownLatch的UML类图如下：
CountDownLatch的数据结构很简单，它是通过"共享锁"实现的。它包含了sync对象，sync是Sync类型。Sync是实例类，它继承于AQS。
CountDownLatch源码分析(基于JDK1.7.0_40)
CountDownLatch完整源码(基于JDK1.7.0_40)
/*
* 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.*;
import java.util.concurrent.atomic.*;
/**
* A synchronization aid that allows one or more threads to wait until
* a set of operations being performed in other threads completes.
*
* <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}.
*
* <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.
*
* <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.
*
* <p><b>Sample usage:</b> Here is a pair of classes in which a group
* of worker threads use two countdown latches:
* <ul>
* <li>The first is a start signal that prevents any worker from proceeding
* until the driver is ready for them to proceed;
* <li>The second is a completion signal that allows the driver to wait
* until all workers have completed.
* </ul>
*
* <pre>
* 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}.)
*
* <pre>
* 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.
*/
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;
}
}
}
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}.
*
* <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);
}
/**
* 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.
*
* <p>If the current count is zero then this method returns immediately
* with the value {@code true}.
*
* <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 three 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; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If the count reaches zero then the method returns with the
* value {@code true}.
*
* <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.
*
* <p>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.
*
* @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.
*
* <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);
}
/**
* Returns the current count.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @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() + "]";
}
}
CountDownLatch是通过“共享锁”实现的。下面，我们分析CountDownLatch中3个核心函数: CountDownLatch(int count), await(), countDown()。
1. CountDownLatch(int count)
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
说明：该函数是创建一个Sync对象，而Sync是继承于AQS类。Sync构造函数如下：
Sync(int count) {
setState(count);
}
setState()在AQS中实现，源码如下：
protected final void setState(long newState) {
state = newState;
}
说明：在AQS中，state是一个private volatile long类型的对象。对于CountDownLatch而言，state表示的”锁计数器“。CountDownLatch中的getCount()最终是调用AQS中的getState()，返回的state对象，即”锁计数器“。
2. await()
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
说明：该函数实际上是调用的AQS的acquireSharedInterruptibly(1);
AQS中的acquireSharedInterruptibly()的源码如下：
public final void acquireSharedInterruptibly(long arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
说明：acquireSharedInterruptibly()的作用是获取共享锁。
如果当前线程是中断状态，则抛出异常InterruptedException。否则，调用tryAcquireShared(arg)尝试获取共享锁；尝试成功则返回，否则就调用doAcquireSharedInterruptibly()。doAcquireSharedInterruptibly()会使当前线程一直等待，直到当前线程获取到共享锁(或被中断)才返回。
tryAcquireShared()在CountDownLatch.java中被重写，它的源码如下：
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
说明：tryAcquireShared()的作用是尝试获取共享锁。
如果"锁计数器=0"，即锁是可获取状态，则返回1；否则，锁是不可获取状态，则返回-1。
private void doAcquireSharedInterruptibly(long arg)
throws InterruptedException {
// 创建"当前线程"的Node节点，且Node中记录的锁是"共享锁"类型；并将该节点添加到CLH队列末尾。
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
// 获取上一个节点。
// 如果上一节点是CLH队列的表头，则"尝试获取共享锁"。
final Node p = node.predecessor();
if (p == head) {
long r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
// (上一节点不是CLH队列的表头) 当前线程一直等待，直到获取到共享锁。
// 如果线程在等待过程中被中断过，则再次中断该线程(还原之前的中断状态)。
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
说明：
(01) addWaiter(Node.SHARED)的作用是，创建”当前线程“的Node节点，且Node中记录的锁的类型是”共享锁“(Node.SHARED)；并将该节点添加到CLH队列末尾。关于Node和CLH在"Java多线程系列--“JUC锁”03之 公平锁(一)"已经详细介绍过，这里就不再重复说明了。
(02) node.predecessor()的作用是，获取上一个节点。如果上一节点是CLH队列的表头，则”尝试获取共享锁“。
(03) shouldParkAfterFailedAcquire()的作用和它的名称一样，如果在尝试获取锁失败之后，线程应该等待，则返回true；否则，返回false。
(04) 当shouldParkAfterFailedAcquire()返回ture时，则调用parkAndCheckInterrupt()，当前线程会进入等待状态，直到获取到共享锁才继续运行。
doAcquireSharedInterruptibly()中的shouldParkAfterFailedAcquire(), parkAndCheckInterrupt等函数在"Java多线程系列--“JUC锁”03之 公平锁(一)"中介绍过，这里也就不再详细说明了。
3. countDown()
public void countDown() {
sync.releaseShared(1);
}
说明：该函数实际上调用releaseShared(1)释放共享锁。
releaseShared()在AQS中实现，源码如下：
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
说明：releaseShared()的目的是让当前线程释放它所持有的共享锁。
它首先会通过tryReleaseShared()去尝试释放共享锁。尝试成功，则直接返回；尝试失败，则通过doReleaseShared()去释放共享锁。
tryReleaseShared()在CountDownLatch.java中被重写，源码如下：
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
// 获取“锁计数器”的状态
int c = getState();
if (c == 0)
return false;
// “锁计数器”-1
int nextc = c-1;
// 通过CAS函数进行赋值。
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
说明：tryReleaseShared()的作用是释放共享锁，将“锁计数器”的值-1。
总结：CountDownLatch是通过“共享锁”实现的。在创建CountDownLatch中时，会传递一个int类型参数count，该参数是“锁计数器”的初始状态，表示该“共享锁”最多能被count给线程同时获取。当某线程调用该CountDownLatch对象的await()方法时，该线程会等待“共享锁”可用时，才能获取“共享锁”进而继续运行。而“共享锁”可用的条件，就是“锁计数器”的值为0！而“锁计数器”的初始值为count，每当一个线程调用该CountDownLatch对象的countDown()方法时，才将“锁计数器”-1；通过这种方式，必须有count个线程调用countDown()之后，“锁计数器”才为0，而前面提到的等待线程才能继续运行！
以上，就是CountDownLatch的实现原理。
CountDownLatch的使用示例
下面通过CountDownLatch实现："主线程"等待"5个子线程"全部都完成"指定的工作(休眠1000ms)"之后，再继续运行。
1 import java.util.concurrent.CountDownLatch;
2 import java.util.concurrent.CyclicBarrier;
3
4 public class CountDownLatchTest1 {
5
6 private static int LATCH_SIZE = 5;
7 private static CountDownLatch doneSignal;
8 public static void main(String[] args) {
9
10 try {
11 doneSignal = new CountDownLatch(LATCH_SIZE);
12
13 // 新建5个任务
14 for(int i=0; i<LATCH_SIZE; i++)
15 new InnerThread().start();
16
17 System.out.println("main await begin.");
18 // "主线程"等待线程池中5个任务的完成
19 doneSignal.await();
20
21 System.out.println("main await finished.");
22 } catch (InterruptedException e) {
23 e.printStackTrace();
24 }
25 }
26
27 static class InnerThread extends Thread{
28 public void run() {
29 try {
30 Thread.sleep(1000);
31 System.out.println(Thread.currentThread().getName() + " sleep 1000ms.");
32 // 将CountDownLatch的数值减1
33 doneSignal.countDown();
34 } catch (InterruptedException e) {
35 e.printStackTrace();
36 }
37 }
38 }
39 }
运行结果：
main await begin. Thread-0 sleep 1000ms. Thread-2 sleep 1000ms. Thread-1 sleep 1000ms. Thread-4 sleep 1000ms. Thread-3 sleep 1000ms. main await finished.
结果说明：主线程通过doneSignal.await()等待其它线程将doneSignal递减至0。其它的5个InnerThread线程，每一个都通过doneSignal.countDown()将doneSignal的值减1；当doneSignal为0时，main被唤醒后继续执行。
