参考原文
CountDownLatch是什么
CountDownLatch是一个同步工具类，它允许一个或多个线程一直等待，直到其他线程的操作执行完后再执行。
单词Latch的中文翻译是门闩，也就是有“门锁”的功能，所以当门没有打开时，N个人是不能进入屋内的，也就是N个线程是不能继续往下运行的，支持这样的特性可以控制线程执行任务的时机CountDownLatch是通过一个计数器来实现的，计数器的初始化值为线程的数量。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了。
CountDownLatch 常用API
构造方法：
public CountDown(int count){} //count初始化计数值，一旦count初始化完成后，就不可重新初始化或者修改CountDownLatch对象的内部计数器的值。 常用方法:
public void await() {}; //使当前线程挂起，直到计数值为0时，才继续往下执行。public boolean await(long timeout , TimeUnit timeUnit) throws InterruptExcetion {};public void countDown() {} //将count值减1 应用场景
CountDownLatch的一个非常典型的应用场景就是：有一个任务想要往下执行，但必须等到其他多个任务执行(如果是单个任务 顺序执行就好了)完毕后才进行继续往下执行。此时，我们可以在这个等待其他任务的线程里调用一个CountDownLatch对象的await()方法，其他的任务执行完自己的任务后调用同一个CountDownLatch对象上的countDown()方法，这个调用await()方法的任务将一直阻塞等待，直到这个CountDownLatch对象的计数值减到0为止。
import java.util.Arrays;import java.util.List;import java.util.concurrent.CountDownLatch;import java.util.concurrent.Executor;import java.util.concurrent.Executors;/** * 模拟程序启动时 相关环境检查 */public class CountDownLatchTest { public static void main(String[] args) { try { Boolean result = ApplicationStartupUtil.checkExtenalServoce(); if(result) { System.out.println("健康检查全部健康"); } else { throw new RuntimeException("程序依赖环境不健康"); } } catch (Exception e) { e.printStackTrace(); } }}abstract class BaseHealthChecker implements Runnable { private CountDownLatch countDownLatch ; protected boolean isHealth ; private String serviceName ; public BaseHealthChecker(String serviceName , CountDownLatch countDownLatch) { this.countDownLatch = countDownLatch; this.serviceName = serviceName ; isHealth = false ; } @Override public void run() { try { this.verifyService(); isHealth = true ; }catch (Exception e) { e.printStackTrace(); isHealth = false ; } finally { if(countDownLatch != null) { countDownLatch.countDown(); } } } abstract void verifyService() ; public boolean isHealth() { return isHealth; } public String getServiceName() { return serviceName; }}class NetWorkHealthChecker extends BaseHealthChecker { public NetWorkHealthChecker( CountDownLatch countDownLatch) { super( "NetWorkHealthChecker" , countDownLatch ) ; } @Override void verifyService() { System.out.println("starting Checking " + this.getServiceName()); try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( this.getServiceName() + " is Health"); isHealth = true ; }}class DatabaseHealthChecker extends BaseHealthChecker { public DatabaseHealthChecker( CountDownLatch countDownLatch) { super( "DatabaseHealthChecker" , countDownLatch ) ; } @Override void verifyService() { System.out.println("starting Checking " + this.getServiceName()); try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( this.getServiceName() + " is Health"); isHealth = true ; }}class ApplicationStartupUtil { private static List healthCheckerList ; private static CountDownLatch countDownLatch ; private ApplicationStartupUtil() { } public static boolean checkExtenalServoce() throws Exception { countDownLatch = new CountDownLatch(2) ; healthCheckerList = Arrays.asList(new DatabaseHealthChecker(countDownLatch) , new NetWorkHealthChecker(countDownLatch)) ; Executor executor = Executors.newFixedThreadPool(healthCheckerList.size()) ; healthCheckerList.forEach((num)-> executor.execute(num)); countDownLatch.await() ; System.out.println(countDownLatch.getCount()); //等待健康检查完毕 healthCheckerList.forEach((num) -> { System.out.println(num.isHealth); if(!num.isHealth()) { throw new RuntimeException(num.getServiceName() + "健康检查失败"); } }); return true ; }}