CountDownLatch官方使用手册：http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CountDownLatch.html
一、原理
CountDownLatch是一个非常实用的多线程控制工具类。Count Down在英文中意为倒计时，Latch意为门闩，可以简单的将CountDownLatch称为倒计时器。门闩的含义是：把门锁起来，不让里面的线程跑出来。因此，这个工具通常用来控制线程等待，它可以让一个线程等待知道倒计时结束，再开始执行。
CountDownLatch内部维护着一个count计数，只不过对这个计数器的操作都是原子操作，同时只能有一个线程去操作这个计数器。CountDownLatch通过构造函数传入一个初始计数值，调用者可以通过调用CounDownLatch对象的countDown()方法，来使计数减1；如果调用对象上的await()方法，那么调用者就会一直阻塞在这里，直到别人通过countDown方法，将计数减到0，才可以继续执行。
CountDownLatch的一种典型应用场景是火箭发射。在火箭发射前，为了保证万无一失，往往要进行各项设备、仪器的检查。只有等所有的检查完毕后，引擎才能点火。这种场景就非常适合使用CountDownLatch。它可以使得点火线程等待所有检查线程全部完成后再执行。
二、API
主要方法
- public CountDownLatch(int count);
构造方法参数指定了计数的次数
- public void countDown();
当前线程调用此方法，则计数减一
- public void await() throws InterruptedException
调用此方法会一直阻塞当前线程，直到计时器的值为0
三、Demo
CountDownLatch的一种典型用法是：
a group of worker threads use two countdown latches(一组工作线程使用两个CountDownLatch)。
Sample usage: a group of worker threads use two countdown latches(一组工作线程使用两个CountDownLatch):
- 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
下面以一个Demo来说明CountDownLatch的使用。
程序功能：模拟多线程下载，合并线程等到所有下载任务完成便开始合并。
/**
* CountDownLatch的典型用法2
* 代码功能：使用多个下载线程实现下载，等到所有任务完成后合并线程开始合并。
*
* @author lp
*
*/
public class CountDownLatchDemo2 {
public static void main(String[] args) throws InterruptedException {
int N = 8;
CountDownLatch startSignal = new CountDownLatch(1);//startSignal控制开始
CountDownLatch doneSignal = new CountDownLatch(N);//
ExecutorService executor = Executors.newFixedThreadPool(N + 1);// 8个下载线程，1个合并线程
// let all threads proceed
startSignal.countDown();
// N个下载任务开始执行
for (int i = 0; i < N; ++i) {
executor.execute(new DownloadRunnable(startSignal,doneSignal, i));
}
// wait for all to finish
doneSignal.await();//阻塞，直到计数器为0
// 执行合并任务
executor.execute(new MergeRunnable());
executor.shutdown();
}
}
/**
* 下载线程
*/
class DownloadRunnable implements Runnable {
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal;
private final int i;
public DownloadRunnable(CountDownLatch startSignal, CountDownLatch doneSignal, int i) {
this.startSignal = startSignal;
this.doneSignal = doneSignal;
this.i = i;
}
public void run() {
try {
startSignal.await();//当前线程等待
doDownload(i);
doneSignal.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private void doDownload(int i) {
System.out.println("Thead:" + i + "下载完成");
}
}
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 CyclicBarrier.)
另外一种典型的用法是：将一个问题分解成N部分，用Runnable来描述每一部分任务，每个Runnable执行完后将latch数减1，而且要将所有的Runnable都加入到Executor中。当所有子部分任务完成时，协调线程经过await后能够开始执行。(当线程必须可重复性的倒计时时，请使用CyclicBarrier代替)
/**
* CountDownLatch的典型用法1
* 代码功能：模拟多线程下载功能-使用多个下载线程实现下载，等到所有任务完成后合并线程开始合并。
*
* @author lp
*
*/
public class CountDownLatchDemo {
public static void main(String[] args) throws InterruptedException {
int N = 8;
CountDownLatch doneSignal = new CountDownLatch(N);// 计数器从N开始倒数
ExecutorService executor = Executors.newFixedThreadPool(N + 1);// N个下载线程，1个合并线程
// N个下载任务开始执行
for (int i = 0; i < N; ++i) {
executor.execute(new DownloadRunnable(doneSignal, i));
}
// wait for all to finish
doneSignal.await();//阻塞，直到计数器为0
// 执行合并任务
executor.execute(new MergeRunnable());
executor.shutdown();
}
}
/**
* 下载线程
*/
class DownloadRunnable implements Runnable {
private final CountDownLatch doneSignal;
private final int i;
DownloadRunnable(CountDownLatch doneSignal, int i) {
this.doneSignal = doneSignal;
this.i = i;
}
public void run() {
doDownload(i);
doneSignal.countDown();
}
private void doDownload(int i) {
System.out.println("Thead:" + i + "下载完成");
}
}
/**
* 合并线程
*/
class MergeRunnable implements Runnable{
@Override
public void run() {
doMerge();
}
private void doMerge(){
System.out.println("合并线程完成合并");
}
}