考慮以下場景：瀏覽網(wǎng)頁時(shí)，瀏覽器了5個(gè)線程下載網(wǎng)頁中的圖片文件，由于圖片大小、網(wǎng)站訪問速度等諸多因素的影響，完成圖片下載的時(shí)間就會有很大的不同。如果先下載完成的圖片就會被先顯示到界面上，反之，后下載的圖片就后顯示。
Java的并發(fā)庫的CompletionService可以滿足這種場景要求。該接口有兩個(gè)重要方法：submit()和take()。submit用于提交一個(gè)runnable或者callable，一般會提交給一個(gè)線程池處理；而take就是取出已經(jīng)執(zhí)行完畢runnable或者callable實(shí)例的Future對象，如果沒有滿足要求的，就等待了。 CompletionService還有一個(gè)對應(yīng)的方法poll，該方法與take類似，只是不會等待，如果沒有滿足要求，就返回null對象。
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");
// 關(guān)閉線程池
exec.shutdown();
}
} |
運(yùn)行結(jié)果:
Show web content
Downloaded Image 1
Downloaded Image 2
Downloaded Image 4
Downloaded Image 0
Downloaded Image 3
End
