CountDownLatch学习

看了几篇博客，说用CountDownLatch实现一个先执行完子线程，再执行主线程的例子。因此写一篇博客总结一下。

一、CountDownLatch了解

1.CountDownLatch继承了AbstractQueuedSynchronizer类,很多方法都封装了在这个类里面，感兴趣可以看一下。内部写了一个Sync类，对其进行各种操作，以完成各个功能。

2.其原理是实现了一个计数器，通过对计数器操作，来实现对各个子线程的监控

二、例子

1.在启动服务前，需要对各个组件进行健康检查，因此设计一个方案，对各个组件进行检查，环境都ok后，再执行主线程。

2.先定义一个抽象类，该类实现了Runnable，用来进行健康检查

 1 public abstract class BaseHealthChecker implements Runnable {
 2     //Counter
 3     private CountDownLatch counter;
 4     private String serviceName;
 5     private boolean isServiceOk;
 6 
 7     public abstract void checkHealth();
 8 
 9     public String getServiceName() {
10         return serviceName;
11     }
12 
13     public boolean isServiceOk() {
14         return isServiceOk;
15     }
16 
17     public BaseHealthChecker(String serviceName, CountDownLatch countDownLatch) {
18         this.counter = countDownLatch;
19         this.serviceName = serviceName;
20         this.isServiceOk = false;
21     }
22 
23     @Override
24     public void run() {
25         try {
26             checkHealth();
27             isServiceOk = true;
28         } catch (Throwable t) {
29             t.printStackTrace();
30             isServiceOk = false;
31         } finally {
32             //Always count down counter
33             if (counter != null) {
34                 counter.countDown();
35             }
36         }
37     }
38 }

 

3.再定义一个HealthChecker. 

 1 package countdownlatch;
 2 
 3 import java.util.concurrent.CountDownLatch;
 4 
 5 /**
 6  * Created by adrian.wu on 2018/12/18.
 7  */
 8 public class HealthChecker extends BaseHealthChecker{
 9     public HealthChecker(String serviceName, CountDownLatch counter){
10         super(serviceName, counter);
11     }
12 
13     @Override
14     public void checkHealth(){
15         System.out.println("Check " + this.getServiceName());
16         try {
17             Thread.sleep(7000);
18         }catch (InterruptedException e){
19             e.printStackTrace();
20         }
21         System.out.println(this.getServiceName() + " is Ok");
22     }
23 }

 

4.设定要检查哪些service

 1 package countdownlatch;
 2 
 3 import java.util.ArrayList;
 4 import java.util.List;
 5 import java.util.concurrent.CountDownLatch;
 6 import java.util.concurrent.Executor;
 7 import java.util.concurrent.Executors;
 8 
 9 /**
10  * Created by adrian.wu on 2018/12/18.
11  */
12 public class ApplicationStartupUtil {
13     private static List<BaseHealthChecker> services;
14 
15     private static CountDownLatch latch;
16 
17     private ApplicationStartupUtil() {
18     }
19 
20     private final static ApplicationStartupUtil INSTANCE = new ApplicationStartupUtil();
21 
22     public static ApplicationStartupUtil getInstance() {
23         return INSTANCE;
24     }
25 
26     public static boolean checkExternalServices() throws Exception {
27         //Initialize the service checkers
28         latch = new CountDownLatch(4);
29 
30         services = new ArrayList<>();
31 
32         services.add(new HealthChecker("Flink Cluster", latch));
33 
34         services.add(new HealthChecker("Hbase Cluster", latch));
35 
36         services.add(new HealthChecker("Spark Cluster", latch));
37 
38         services.add(new HealthChecker("Kafka Cluster", latch));
39 
40         Executor executor = Executors.newFixedThreadPool(services.size());
41 
42         for (final BaseHealthChecker v : services) {
43             executor.execute(v);
44         }
45 
46         //wait all threads okay
47         latch.await();
48 
49         for (final BaseHealthChecker v : services) {
50             if (!v.isServiceOk()) {
51                 return false;
52             }
53         }
54 
55         return true;
56     }
57 }

 

5.检查服务

 1 package countdownlatch;
 2 
 3 /**
 4  * Created by adrian.wu on 2018/12/18.
 5  */
 6 public class Check {
 7     public static void main(String[] args) {
 8         boolean result = false;
 9         try {
10             result = ApplicationStartupUtil.checkExternalServices();
11         } catch (Exception e) {
12             e.printStackTrace();
13         }
14         System.out.println("Services check is ok. Result: " + result);
15     }
16 }

 

参考：http://www.importnew.com/15731.html

 

posted @ 2018-12-18 15:47  ylxn  阅读(248)  评论(0)  编辑  收藏  举报