CompletionService--实现并行获取future.get()结果

CompletionService实际上可以看做是Executor和BlockingQueue的结合体。CompletionService在接收到要执行的任务时，通过类似BlockingQueue的put和take获得任务执行的结果。

CompletionService的一个实现是ExecutorCompletionService，ExecutorCompletionService把具体的计算任务交给Executor完成。

异步下载图片，并实时渲染已下载图片的代码样例，通过将结果加入阻塞队列的方式来绕过异步结果Future.get()的等待问题

package net.jcip.examples;

import java.util.*;
import java.util.concurrent.*;
import static net.jcip.examples.LaunderThrowable.launderThrowable;

/**
 * Renderer
 * <p/>
 * Using CompletionService to render page elements as they become available
 *
 * @author Brian Goetz and Tim Peierls
 */
public abstract class Renderer {
    private final ExecutorService executor;

    Renderer(ExecutorService executor) {
        this.executor = executor;
    }

    void renderPage(CharSequence source) {
        final List<ImageInfo> info = scanForImageInfo(source);
        CompletionService<ImageData> completionService =
                new ExecutorCompletionService<ImageData>(executor);
        for (final ImageInfo imageInfo : info)
            completionService.submit(new Callable<ImageData>() {
                public ImageData call() {
                    return imageInfo.downloadImage();
                }
            });

        renderText(source);

        try {
            for (int t = 0, n = info.size(); t < n; t++) {
                Future<ImageData> f = completionService.take();
                ImageData imageData = f.get();
                renderImage(imageData);
            }
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        } catch (ExecutionException e) {
            throw launderThrowable(e.getCause());
        }
    }

    interface ImageData {
    }

    interface ImageInfo {
        ImageData downloadImage();
    }

    abstract void renderText(CharSequence s);

    abstract List<ImageInfo> scanForImageInfo(CharSequence s);

    abstract void renderImage(ImageData i);

}