CountDownLatch门闩

门闩是concurrent包中定义的一个类型，是用于多线程通讯的一个辅助类型。
门闩相当于在一个门上加多个锁，当线程调用await方法时，会检查门闩数量，如果门
闩数量大于0，线程会阻塞等待。当线程调用countDown时，会递减门闩的数量，当门闩数量为0时，await阻塞线程可执行。
例

/**
 * CountDownLatch 门闩
 */
package com.sxt.concurrent.t02;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;

public class Test_03 {
    public static void main(String[] args) {
        final Test_03_Container t = new Test_03_Container();
        final CountDownLatch latch = new CountDownLatch(1);

        new Thread(new Runnable(){
            @Override
            public void run() {
                if(t.size() != 5){
                    try {
                        latch.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("size = 5");
            }
        }).start();
        
        new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i = 0; i < 10; i++){
                    System.out.println("add Object to Container " + i);
                    t.add(new Object());
                    if(t.size() == 5){
                        latch.countDown();
                    }
                    try {
                        TimeUnit.SECONDS.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }
}

class Test_03_Container{
    volatile List<Object> container = new ArrayList<>();
    
    public void add(Object o){
        this.container.add(o);
    }
    
    public int size(){
        return this.container.size();
    }
}