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;;
;; from Dennis: http://www.blogjava.net/killme2008/archive/2010/08/archive/2010/07/17/326389.html
;;  使用atom实现一个计数器，和使用java.util.concurrent.AtomicInteger做计数器，
;;  做一个性能比较，各启动100个线程，每个线程执行100万次原子递增，计算各自的耗时

(ns countdownlatch_usage)
(import 'java.util.concurrent.atomic.AtomicInteger)
(import 'java.util.concurrent.CountDownLatch)

(def a (AtomicInteger. 0))
(def b (atom 0))

;;为了性能，给java加入type hint
(defn java-inc [#^AtomicInteger counter] (.incrementAndGet counter))
(defn countdown-latch [#^CountDownLatch latch] (.countDown latch))

;;单线程执行缓存次数
(def max_count 1000000)
;;线程数 
(def thread_count 100)

;; 让多个线程一起开始
;; 这里直接用的Thread新起一个进程， 而不是Future
(defn benchmark [fun]
  (let [ latch (CountDownLatch. thread_count)  ;;关卡锁
         start (System/currentTimeMillis) ]     ;;启动时间
       (dotimes [_ thread_count] (.start (Thread. #(do (dotimes [_ max_count] (fun)) (countdown-latch latch))))) 
       (.await latch);; 阻塞等待全部结束. 
       (- (System/currentTimeMillis) start)))
         

(println "atom:" (benchmark #(swap! b inc)))
(println "AtomicInteger:" (benchmark #(java-inc a)))

(println (.get a))
(println @b)

package countdownlatch_usage;//
// from here: https://blog.csdn.net/u013136708/article/details/49444459
// 比如一个跑步比赛，有五个选手参加，有两点需要注意，第一我们必须确保这5个
// 选手都准备就绪了，才能宣布比赛开始，第二只有当5个选手都完成比赛了才能宣布比赛结束。

/**
CountDownLatch类是一个同步计数器,构造时传入int参数,该参数就是计数器的初始值，每调用一次countDown()方法，计数器减1,计数器大于0 时，await()方法会阻塞程序继续执行
CountDownLatch如其所写，是一个倒计数的锁存器，当计数减至0时触发特定的事件。利用这种特性，可以让主线程等待子线程的结束。下面以一个模拟运动员比赛的例子加以说明。
*/
  import java.util.concurrent.CountDownLatch;
  import java.util.concurrent.Executor;
  import java.util.concurrent.ExecutorService;
  import java.util.concurrent.Executors;
 
  public class CountDownLatchDemo {
     private static final int PLAYER_AMOUNT = 5;
     public CountDownLatchDemo() {
         // TODO Auto-generated constructor stub    
      }
     /**
      * @param args
      */
     public static void main(String[] args) {
         // TODO Auto-generated method stub
         //begin为了确保每个运动员都准备就绪了才能开始比赛，所以每个运动员都会持有一个begin，并且调用begin.await()方法进入等待。
         CountDownLatch begin = new CountDownLatch(1);
         //对于整个比赛，所有运动员结束后才算结束。主线程会持有end，确保所有的运动员都到终点才能宣布比赛结束。同时每个运动员到达终点的时候都会调用，end.countdown()方法。
         CountDownLatch end = new CountDownLatch(PLAYER_AMOUNT);
         Player[] plays = new Player[PLAYER_AMOUNT];
         
         for(int i=0;i<PLAYER_AMOUNT;i++)
             plays[i] = new Player(i+1,begin,end);
         
         //设置特定的线程池，大小为5
         ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
         for(Player p:plays)
        	//分配线程，每个运动员都准备就绪，但是没有开始赛跑，因为他们持有的begin都调用了begin.await()
             exe.execute(p);            
         System.out.println("Race begins!");//宣布比赛开始
         //所有运动员开始赛跑，begin的参数减少1，变为0，持有begin.await的线程开始运行。
         begin.countDown();
         try{
        	//等待所有的运动员都到达终点，比赛结束。
             end.await();            
         }catch (InterruptedException e) {
             // TODO: handle exception
             e.printStackTrace();
         }finally{
             System.out.println("Race ends!");//宣布比赛结束
         }
         exe.shutdown();
     }
 }

package countdownlatch_usage;
//
// from here: https://stackoverflow.com/questions/17827022/how-is-countdownlatch-used-in-java-multithreading
//

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class Processor implements Runnable {
    private CountDownLatch latch;

    public Processor(CountDownLatch latch) {
        this.latch = latch;
    }

    public void run() {
        System.out.println("Started.");

        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        latch.countDown();
    }
}

class App {

    public static void main(String[] args) {

        CountDownLatch latch = new CountDownLatch(3); // coundown from 3 to 0

        ExecutorService executor = Executors.newFixedThreadPool(3); // 3 Threads in pool

        for(int i=0; i < 3; i++) {
            executor.submit(new Processor(latch)); // ref to latch. each time call new Processes latch will count down by 1
        }

        try {
            latch.await();  // wait until latch counted down to 0
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Completed.");
    }

}