CountDownLatch - jdk1.5併發包

原創

2020-02-20 16:18

什麼是CountDownLatch

CountDownLatch是一種同步類，它運行一個線程在執行前等待一個或多個線程，這經常用於服務器端開發。通常當主線程調用await()方法後將會等待直到計數器到達0或者被其它線程中斷。其它線程通過調用countDown()方法將會在完成任務後將計數器減1。計數器爲0是主線程將開始運行。

CountDownLatch 例子

import java.util.concurrent.CountDownLatch;

public class CountDownLatchDemo{
	public static void main(String[] args) {
		final CountDownLatch latch = new CountDownLatch(3);
		Thread cacheService = new Thread(new Service("CacheService", 1000, latch));
       	Thread alertService = new Thread(new Service("AlertService", 1000, latch));
       	Thread validationService = new Thread(new Service("ValidationService", 1000, latch));

       	cacheService.start(); //separate thread will initialize CacheService
       	alertService.start(); //another thread for AlertService initialization
       	validationService.start();

       	try{
       		latch.await();
       		System.out.println("All services are up, Application is starting now");
       	}catch(InterruptedException ie){
       		ie.printStackTrace();
       	}
	}
}

class Service implements Runnable{
    private final String name;
    private final int timeToStart;
    private final CountDownLatch latch;
  
    public Service(String name, int timeToStart, CountDownLatch latch){
        this.name = name;
        this.timeToStart = timeToStart;
        this.latch = latch;
    }
  
    @Override
    public void run() {
        try {
            Thread.sleep(timeToStart);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
        System.out.println( name + " is Up");
        latch.countDown(); //reduce count of CountDownLatch by 1
    }
  
}<strong>
</strong>

output：

AlertService is Up
CacheService is Up
ValidationService is Up
All services are up, Application is starting now

知識點：

1.CountDownLatch 一旦計數器減到0將不能被重用，這是和CyclicBarrier主要的區別

2.主線程調用await()方法等待，其它線程調用countDown()方法來通知主線程已經完成

參考：

http://javarevisited.blogspot.hk/2012/07/countdownlatch-example-in-java.html