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)
1 /*
2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
3 *
4 *
5 *
6 *
7 *
8 *
9 *
10 *
11 *
12 *
13 *
14 *
15 *
16 *
17 *
18 *
19 *
20 *
21 *
22 *
23 */
24
25 /*
26 *
27 *
28 *
29 *
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent;
37 import java.util.concurrent.locks.*;
38 import java.util.concurrent.atomic.*;
39
40 /**
41 * A synchronization aid that allows one or more threads to wait until
42 * a set of operations being performed in other threads completes.
43 *
44 * <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
45 * The {@link #await await} methods block until the current count reaches
46 * zero due to invocations of the {@link #countDown} method, after which
47 * all waiting threads are released and any subsequent invocations of
48 * {@link #await await} return immediately. This is a one-shot phenomenon
49 * -- the count cannot be reset. If you need a version that resets the
50 * count, consider using a {@link CyclicBarrier}.
51 *
52 * <p>A {@code CountDownLatch} is a versatile synchronization tool
53 * and can be used for a number of purposes. A
54 * {@code CountDownLatch} initialized with a count of one serves as a
55 * simple on/off latch, or gate: all threads invoking {@link #await await}
56 * wait at the gate until it is opened by a thread invoking {@link
57 * #countDown}. A {@code CountDownLatch} initialized to <em>N</em>
58 * can be used to make one thread wait until <em>N</em> threads have
59 * completed some action, or some action has been completed N times.
60 *
61 * <p>A useful property of a {@code CountDownLatch} is that it
62 * doesn't require that threads calling {@code countDown} wait for
63 * the count to reach zero before proceeding, it simply prevents any
64 * thread from proceeding past an {@link #await await} until all
65 * threads could pass.
66 *
67 * <p><b>Sample usage:</b> Here is a pair of classes in which a group
68 * of worker threads use two countdown latches:
69 * <ul>
70 * <li>The first is a start signal that prevents any worker from proceeding
71 * until the driver is ready for them to proceed;
72 * <li>The second is a completion signal that allows the driver to wait
73 * until all workers have completed.
74 * </ul>
75 *
76 * <pre>
77 * class Driver { // ...
78 * void main() throws InterruptedException {
79 * CountDownLatch startSignal = new CountDownLatch(1);
80 * CountDownLatch doneSignal = new CountDownLatch(N);
81 *
82 * for (int i = 0; i < N; ++i) // create and start threads
83 * new Thread(new Worker(startSignal, doneSignal)).start();
84 *
85 * doSomethingElse(); // don't let run yet
86 * startSignal.countDown(); // let all threads proceed
87 * doSomethingElse();
88 * doneSignal.await(); // wait for all to finish
89 * }
90 * }
91 *
92 * class Worker implements Runnable {
93 * private final CountDownLatch startSignal;
94 * private final CountDownLatch doneSignal;
95 * Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
96 * this.startSignal = startSignal;
97 * this.doneSignal = doneSignal;
98 * }
99 * public void run() {
100 * try {
101 * startSignal.await();
102 * doWork();
103 * doneSignal.countDown();
104 * } catch (InterruptedException ex) {} // return;
105 * }
106 *
107 * void doWork() { ... }
108 * }
109 *
110 * </pre>
111 *
112 * <p>Another typical usage would be to divide a problem into N parts,
113 * describe each part with a Runnable that executes that portion and
114 * counts down on the latch, and queue all the Runnables to an
115 * Executor. When all sub-parts are complete, the coordinating thread
116 * will be able to pass through await. (When threads must repeatedly
117 * count down in this way, instead use a {@link CyclicBarrier}.)
118 *
119 * <pre>
120 * class Driver2 { // ...
121 * void main() throws InterruptedException {
122 * CountDownLatch doneSignal = new CountDownLatch(N);
123 * Executor e = ...
124 *
125 * for (int i = 0; i < N; ++i) // create and start threads
126 * e.execute(new WorkerRunnable(doneSignal, i));
127 *
128 * doneSignal.await(); // wait for all to finish
129 * }
130 * }
131 *
132 * class WorkerRunnable implements Runnable {
133 * private final CountDownLatch doneSignal;
134 * private final int i;
135 * WorkerRunnable(CountDownLatch doneSignal, int i) {
136 * this.doneSignal = doneSignal;
137 * this.i = i;
138 * }
139 * public void run() {
140 * try {
141 * doWork(i);
142 * doneSignal.countDown();
143 * } catch (InterruptedException ex) {} // return;
144 * }
145 *
146 * void doWork() { ... }
147 * }
148 *
149 * </pre>
150 *
151 * <p>Memory consistency effects: Until the count reaches
152 * zero, actions in a thread prior to calling
153 * {@code countDown()}
154 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
155 * actions following a successful return from a corresponding
156 * {@code await()} in another thread.
157 *
158 * @since 1.5
159 * @author Doug Lea
160 */
161 public class CountDownLatch {
162 /**
163 * Synchronization control For CountDownLatch.
164 * Uses AQS state to represent count.
165 */
166 private static final class Sync extends AbstractQueuedSynchronizer {
167 private static final long serialVersionUID = 4982264981922014374L;
168
169 Sync(int count) {
170 setState(count);
171 }
172
173 int getCount() {
174 return getState();
175 }
176
177 protected int tryAcquireShared(int acquires) {
178 return (getState() == 0) ? 1 : -1;
179 }
180
181 protected boolean tryReleaseShared(int releases) {
182 // Decrement count; signal when transition to zero
183 for (;;) {
184 int c = getState();
185 if (c == 0)
186 return false;
187 int nextc = c-1;
188 if (compareAndSetState(c, nextc))
189 return nextc == 0;
190 }
191 }
192 }
193
194 private final Sync sync;
195
196 /**
197 * Constructs a {@code CountDownLatch} initialized with the given count.
198 *
199 * @param count the number of times {@link #countDown} must be invoked
200 * before threads can pass through {@link #await}
201 * @throws IllegalArgumentException if {@code count} is negative
202 */
203 public CountDownLatch(int count) {
204 if (count < 0) throw new IllegalArgumentException("count < 0");
205 this.sync = new Sync(count);
206 }
207
208 /**
209 * Causes the current thread to wait until the latch has counted down to
210 * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
211 *
212 * <p>If the current count is zero then this method returns immediately.
213 *
214 * <p>If the current count is greater than zero then the current
215 * thread becomes disabled for thread scheduling purposes and lies
216 * dormant until one of two things happen:
217 * <ul>
218 * <li>The count reaches zero due to invocations of the
219 * {@link #countDown} method; or
220 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
221 * the current thread.
222 * </ul>
223 *
224 * <p>If the current thread:
225 * <ul>
226 * <li>has its interrupted status set on entry to this method; or
227 * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
228 * </ul>
229 * then {@link InterruptedException} is thrown and the current thread's
230 * interrupted status is cleared.
231 *
232 * @throws InterruptedException if the current thread is interrupted
233 * while waiting
234 */
235 public void await() throws InterruptedException {
236 sync.acquireSharedInterruptibly(1);
237 }
238
239 /**
240 * Causes the current thread to wait until the latch has counted down to
241 * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
242 * or the specified waiting time elapses.
243 *
244 * <p>If the current count is zero then this method returns immediately
245 * with the value {@code true}.
246 *
247 * <p>If the current count is greater than zero then the current
248 * thread becomes disabled for thread scheduling purposes and lies
249 * dormant until one of three things happen:
250 * <ul>
251 * <li>The count reaches zero due to invocations of the
252 * {@link #countDown} method; or
253 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
254 * the current thread; or
255 * <li>The specified waiting time elapses.
256 * </ul>
257 *
258 * <p>If the count reaches zero then the method returns with the
259 * value {@code true}.
260 *
261 * <p>If the current thread:
262 * <ul>
263 * <li>has its interrupted status set on entry to this method; or
264 * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
265 * </ul>
266 * then {@link InterruptedException} is thrown and the current thread's
267 * interrupted status is cleared.
268 *
269 * <p>If the specified waiting time elapses then the value {@code false}
270 * is returned. If the time is less than or equal to zero, the method
271 * will not wait at all.
272 *
273 * @param timeout the maximum time to wait
274 * @param unit the time unit of the {@code timeout} argument
275 * @return {@code true} if the count reached zero and {@code false}
276 * if the waiting time elapsed before the count reached zero
277 * @throws InterruptedException if the current thread is interrupted
278 * while waiting
279 */
280 public boolean await(long timeout, TimeUnit unit)
281 throws InterruptedException {
282 return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
283 }
284
285 /**
286 * Decrements the count of the latch, releasing all waiting threads if
287 * the count reaches zero.
288 *
289 * <p>If the current count is greater than zero then it is decremented.
290 * If the new count is zero then all waiting threads are re-enabled for
291 * thread scheduling purposes.
292 *
293 * <p>If the current count equals zero then nothing happens.
294 */
295 public void countDown() {
296 sync.releaseShared(1);
297 }
298
299 /**
300 * Returns the current count.
301 *
302 * <p>This method is typically used for debugging and testing purposes.
303 *
304 * @return the current count
305 */
306 public long getCount() {
307 return sync.getCount();
308 }
309
310 /**
311 * Returns a string identifying this latch, as well as its state.
312 * The state, in brackets, includes the String {@code "Count ="}
313 * followed by the current count.
314 *
315 * @return a string identifying this latch, as well as its state
316 */
317 public String toString() {
318 return super.toString() + "[Count = " + sync.getCount() + "]";
319 }
320 }
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。