//Main thread start
//Create CountDownLatch for N threads  
//Create and start N threads
//Main thread wait on latch （待执行的程序先执行，这里通过CountDownLatch的值来判断前面的线程是否执行完毕，如果没有执行完毕会一直卡着）
//N threads completes there tasks are returns
//Main thread resume execution

//Constructs a CountDownLatch initialized with the given count.
public void CountDownLatch(int count) {...}

package countDown;

import java.util.concurrent.CountDownLatch;

public abstract class BaseHealthChecker implements Runnable {
     
    private CountDownLatch _latch;
    private String _serviceName;
    private boolean _serviceUp; //用来检测线程任务是否都成功执行完毕
 
    //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 { //不管程序有无异常，都会执行一次countDown()
            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();
}

package countDown;

import java.util.concurrent.CountDownLatch;

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");
    }
}

package countDown;

import java.util.concurrent.CountDownLatch;

public class DatabaseHealthChecker extends BaseHealthChecker {

     public DatabaseHealthChecker (CountDownLatch latch)  {
            super("Database 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");
        }
    }

package countDown;

import java.util.concurrent.CountDownLatch;

public class CacheHealthChecker extends BaseHealthChecker {

     public CacheHealthChecker (CountDownLatch latch)  {
            super("Cache 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");
        }
    }

package countDown;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
/**
 * 
 * @author prd-lxw
 * 启动类通过CountDownLatch来等待其它线程执行完毕
 *
 */
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()
    {
    }
 
    //单例启动类
    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
        //任何一个service没有正确结束，mian最后都不能返回正确的结果
        for(final BaseHealthChecker v : _services)
        {
            if( ! v.isServiceUp())
            {
                return false;
            }
        }
        return true;
    }
    
    
    
}

package countDown;

public class Main {
    public static void main(String[] args){
        boolean result = false; //检测所有结果是否正确的执行
        try {
            result = ApplicationStartupUtil.checkExternalServices();
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println("External services validation completed !! Result was :: "+ result);
    }
}

/**
 * Project Name:Spring0725
 * File Name:Sample.java
 * Package Name:work1128.singleton
 * Date:2017年11月28日下午5:15:38
 * Copyright (c) 2017, 深圳金融电子结算中心 All Rights Reserved.
 *
*/

package work1128.singleton;

import java.util.concurrent.CountDownLatch;

/**
 * ClassName: Sample <br/>
 * Function: 演示CountDownLatch的执行过程
 * date: 2017年11月28日 下午5:16:40 <br/>
 *
 * @author prd-lxw
 * @version 1.0
 * @since JDK 1.7
 */
public class Sample {
    /**
     * 计数器，用来控制线程
     * 传入参数2，表示计数器计数为2
     */
    private final static CountDownLatch mCountDownLatch = new CountDownLatch(2);

    /**
     * 示例工作线程类
     */
    private static class WorkingThread extends Thread {
        private final String mThreadName; //线程名称
        private final int mSleepTime; //睡眠时间
        public WorkingThread(String name, int sleepTime) {
            mThreadName = name;
            mSleepTime = sleepTime;
        }
        
        @Override
        public void run() {
            System.out.println("[" + mThreadName + "] started!");
            try {  
                    Thread.sleep(mSleepTime);  
            } catch (InterruptedException e) {  
                    e.printStackTrace();  
            }
            mCountDownLatch.countDown();
            System.out.println("[" + mThreadName + "] end!"); 
        }
    }
    
    /**
     * 示例线程类
     */
    private static class SampleThread extends Thread {
        
        @Override
        public void run() {
            System.out.println("[SampleThread] started!");
            try {
                // 会阻塞在这里等待 mCountDownLatch 里的count变为0；
                // 也就是等待另外的WorkingThread调用countDown()
                mCountDownLatch.await();
            } catch (InterruptedException e) {
                
            }
            System.out.println("[SampleThread] end!");
        }
    }
    
    public static void main(String[] args) throws Exception {
        // 最先run SampleThread ，后面线程执行完毕后，才会回来执行SampleThread这个线程
        new SampleThread().start();
        // 运行两个工作线程
        // 工作线程1运行5秒
        new WorkingThread("WorkingThread1", 5000).start();
        // 工作线程2运行2秒
        new WorkingThread("WorkingThread2", 2000).start();
    }
}