对于以下程序,我试图弄清楚为什么使用2个不同的流并行处理任务,并使用相同的流并在Completable future上调用join / get会使它们花费更长的时间,就好像它们被顺序处理一样。
public class HelloConcurrency {
private static Integer sleepTask(int number) {
System.out.println(String.format("Task with sleep time %d",number));
try {
TimeUnit.SECONDS.sleep(number);
} catch (InterruptedException e) {
e.printStackTrace();
return -1;
}
return number;
}
public static void main(String[] args) {
List<Integer> sleepTimes = Arrays.asList(1,2,3,4,5,6);
System.out.println("WITH SEPARATE STREAMS FOR FUTURE AND JOIN");
ExecutorService executorService = Executors.newFixedThreadPool(6);
long start = System.currentTimeMillis();
List<CompletableFuture<Integer>> futures = sleepTimes.stream()
.map(sleepTime -> CompletableFuture.supplyAsync(() -> sleepTask(sleepTime),executorService)
.exceptionally(ex -> { ex.printStackTrace(); return -1; }))
.collect(Collectors.toList());
executorService.shutdown();
List<Integer> result = futures.stream()
.map(CompletableFuture::join)
.collect(Collectors.toList());
long finish = System.currentTimeMillis();
long timeElapsed = (finish - start)/1000;
System.out.println(String.format("done in %d seconds.",timeElapsed));
System.out.println(result);
System.out.println("WITH SAME STREAM FOR FUTURE AND JOIN");
ExecutorService executorService2 = Executors.newFixedThreadPool(6);
start = System.currentTimeMillis();
List<Integer> results = sleepTimes.stream()
.map(sleepTime -> CompletableFuture.supplyAsync(() -> sleepTask(sleepTime),executorService2)
.exceptionally(ex -> { ex.printStackTrace(); return -1; }))
.map(CompletableFuture::join)
.collect(Collectors.toList());
executorService2.shutdown();
finish = System.currentTimeMillis();
timeElapsed = (finish - start)/1000;
System.out.println(String.format("done in %d seconds.",timeElapsed));
System.out.println(results);
}
}
输出
WITH SEPARATE STREAMS FOR FUTURE AND JOIN
Task with sleep time 6
Task with sleep time 5
Task with sleep time 1
Task with sleep time 3
Task with sleep time 2
Task with sleep time 4
done in 6 seconds.
[1,6]
WITH SAME STREAM FOR FUTURE AND JOIN
Task with sleep time 1
Task with sleep time 2
Task with sleep time 3
Task with sleep time 4
Task with sleep time 5
Task with sleep time 6
done in 21 seconds.
[1,6]