考虑以下场景:浏览网页时,浏览器了5个线程下载网页中的图片文件,由于图片大小、网站访问速度等诸多因素的影响,完成图片下载的时间就会有很大的不同。如果先下载完成的图片就会被先显示到界面上,反之,后下载的图片就后显示。
Java的并发库的CompletionService可以满足这种场景要求。该接口有两个重要方法:submit()和take()。submit用于提交一个runnable或者callable,一般会提交给一个线程池处理;而take就是取出已经执行完毕runnable或者callable实例的Future对象,如果没有满足要求的,就等待了。 CompletionService还有一个对应的方法poll,该方法与take类似,只是不会等待,如果没有满足要求,就返回null对象。
package concurrent;
import java.util.concurrent.Callable; import java.util.concurrent.CompletionService; import java.util.concurrent.ExecutionException; import java.util.concurrent.ExecutorCompletionService; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future;
public class TestCompletionService { public static void main(String[] args) throws InterruptedException, ExecutionException { ExecutorService exec = Executors.newFixedThreadPool(10); CompletionService<String> serv = new ExecutorCompletionService<String>(exec);
for (int index = 0; index < 5; index++) { final int NO = index; Callable<String> downImg = new Callable<String>() { public String call() throws Exception { Thread.sleep((long) (Math.random() * 10000)); return "Downloaded Image " + NO; } }; serv.submit(downImg); }
Thread.sleep(1000 * 2); System.out.println("Show web content"); for (int index = 0; index < 5; index++) { Future<String> task = serv.take(); String img = task.get(); System.out.println(img); } System.out.println("End"); // 关闭线程池 exec.shutdown(); } } |
运行结果:
Show web content
Downloaded Image 1
Downloaded Image 2
Downloaded Image 4
Downloaded Image 0
Downloaded Image 3
End