CountDownLatch是在java1.5被引入的，存在于java.util.concurrent包下。CountDownLatch这个类能够使一个线程等待其他线程完成各自的工作后再执行。
CountDownLatch是通过一个计数器来实现的，计数器的初始值为线程的数量。每当一个线程完成了自己的任务后，计数器的值就会减1。当计数器值到达0时，它表示所有的线程已经完成了任务，然后在闭锁上等待的线程就可以恢复执行任务。
在这个例子中，我模拟了一个应用程序启动类，它开始时启动了n个线程类，这些线程将检查外部系统并通知闭锁，并且启动类一直在闭锁上等待着。一旦验证和检查了所有外部服务，那么启动类恢复执行。
BaseHealthChecker.java：这个类是一个Runnable，负责所有特定的外部服务健康的检测。它删除了重复的代码和闭锁的中心控制代码。
public abstract class BaseHealthChecker implements Runnable { private CountDownLatch _latch; private String _serviceName; private boolean _serviceUp; //Get latch object in constructor so that after completing the task, thread can countDown() the latch public BaseHealthChecker(String serviceName, CountDownLatch latch) { super(); this._latch = latch; this._serviceName = serviceName; this._serviceUp = false; } @Override public void run() { try { verifyService(); _serviceUp = true; } catch (Throwable t) { t.printStackTrace(System.err); _serviceUp = false; } finally { if(_latch != null) { _latch.countDown(); } } } public String getServiceName() { return _serviceName; } public boolean isServiceUp() { return _serviceUp; } //This methos needs to be implemented by all specific service checker public abstract void verifyService(); }NetworkHealthChecker.java：这个类继承了BaseHealthChecker，实现了verifyService()方法。DatabaseHealthChecker.java和CacheHealthChecker.java除了服务名和休眠时间外，与NetworkHealthChecker.java是一样的。
public class NetworkHealthChecker extends BaseHealthChecker { public NetworkHealthChecker (CountDownLatch latch) { super("Network Service", latch); } @Override public void verifyService() { System.out.println("Checking " + this.getServiceName()); try { Thread.sleep(7000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(this.getServiceName() + " is UP"); } }ApplicationStartupUtil.java：这个类是一个主启动类，它负责初始化闭锁，然后等待，直到所有服务都被检测完。
public class ApplicationStartupUtil { //List of service checkers private static List<BaseHealthChecker> _services; //This latch will be used to wait on private static CountDownLatch _latch; private ApplicationStartupUtil() { } private final static ApplicationStartupUtil INSTANCE = new ApplicationStartupUtil(); public static ApplicationStartupUtil getInstance() { return INSTANCE; } public static boolean checkExternalServices() throws Exception { //Initialize the latch with number of service checkers _latch = new CountDownLatch(3); //All add checker in lists _services = new ArrayList<BaseHealthChecker>(); _services.add(new NetworkHealthChecker(_latch)); _services.add(new CacheHealthChecker(_latch)); _services.add(new DatabaseHealthChecker(_latch)); //Start service checkers using executor framework Executor executor = Executors.newFixedThreadPool(_services.size()); for(final BaseHealthChecker v : _services) { executor.execute(v); } //Now wait till all services are checked _latch.await(); //Services are file and now proceed startup for(final BaseHealthChecker v : _services) { if( ! v.isServiceUp()) { return false; } } return true; } }测试代码去检测一下闭锁的功能
public class Main { public static void main(String[] args) { boolean result = false; try { result = ApplicationStartupUtil.checkExternalServices(); } catch (Exception e) { e.printStackTrace(); } System.out.println("External services validation completed !! Result was :: "+ result); } }
