java.util.concurrent 之七：先下载完成的图片先显示

考虑以下场景：浏览网页时，浏览器用了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。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 serv = new ExecutorCompletionService(exec);
		
		for(int index = 0; index < 5; index++){
			final int NO = index;
			Callable downImg = new Callable(){
				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 task = serv.take();
			String img = (String)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 2
Downloaded Image 3
Downloaded Image 0
Downloaded Image 4
Downloaded Image 1
End