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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 }
View Code
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。
更多内容
2. Java多线程系列–“JUC锁”02之 互斥锁ReentrantLock
6. Java多线程系列–“JUC锁”06之 Condition条件
7. Java多线程系列–“JUC锁”07之 LockSupport
8. Java多线程系列–“JUC锁”08之 共享锁和ReentrantReadWriteLock
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