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线程工具类 - CountDownLatch
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"); } }
线程工具类 - CountDownLatch