//Main thread start
//Create CountDownLatch for N threads
//Create and start N threads
//Main thread wait on latch
//N threads completes there tasks are returns
//Main thread resume execution

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

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();
}

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");
    }
}

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;
    }
}

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);
    }
}

/**
 * Description: 赛跑时，等待所有人都准备好时，才起跑：
 *
 * @author shenlongguang
 * @date: 2017/4/27 下午1:50.
 */
public class CyclicBarrierTest {

    public static void main(String[] args) throws IOException, InterruptedException {
        //如果将参数改为4，但是下面只加入了3个选手，这永远等待下去
        //Waits until all parties have invoked await on this barrier.
        CyclicBarrier barrier = new CyclicBarrier(5);

        ExecutorService executor = Executors.newFixedThreadPool(5);
        executor.submit(new Thread(new Runner(barrier, " No.1")));
        executor.submit(new Thread(new Runner(barrier, " No.2")));
        executor.submit(new Thread(new Runner(barrier, " No.3")));
        executor.submit(new Thread(new Runner(barrier, " No.4")));
        executor.submit(new Thread(new Runner(barrier, " No.5")));

        executor.shutdown();
    }
}

class Runner implements Runnable {
    // 一个同步辅助类，它允许一组线程互相等待，直到到达某个公共屏障点 (common barrier point)
    private CyclicBarrier barrier;

    private String name;

    public Runner(CyclicBarrier barrier, String name) {
        super();
        this.barrier = barrier;
        this.name = name;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(1000 * (new Random()).nextInt(8));
            System.out.println(System.currentTimeMillis()+ name + " ready...");
            // barrier的await方法，在所有参与者都已经在此 barrier 上调用 await 方法之前，将一直等待。
            barrier.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (BrokenBarrierException e) {
            e.printStackTrace();
        }
        System.out.println(System.currentTimeMillis()+ name + " go！");
    }
}