并行流看起来并不像完全并行工作

Question

1. Set's parallelStream doesn't use enough thread.

Java8 parallelStream doesn't working exactly parallel. In my computer, java8 set's parallelStream is not using enough thread when task's count is smaller than processor's count.

public class ParallelStreamSplitTest {
    @Test
    public void setStreamParallelTest() {
        System.out.printf("Total processor count : %d \n", Runtime.getRuntime().availableProcessors());
        long start = System.currentTimeMillis();
        IntStream.range(1, 8).boxed().collect(Collectors.toCollection(HashSet::new)).parallelStream().forEach((index) -> {
            System.out.println("Starting " + Thread.currentThread().getName() + ",    index=" + index + ", " + new Date());
            try {
                Thread.sleep(1000);
            } catch (Exception e) {
            }
        });
        long end = System.currentTimeMillis();
        System.out.println(Thread.currentThread().getName() + "'s elapsed time : " + (end - start));
    }

    @Test
    public void intStreamParallelTest() {
        System.out.printf("Total processor count : %d \n", Runtime.getRuntime().availableProcessors());
        long start = System.currentTimeMillis();
        IntStream.range(1, 8).parallel().forEach(index -> {
            System.out.println("Starting " + Thread.currentThread().getName() + ",    index=" + index + ", " + new Date());
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
        });
        long end = System.currentTimeMillis();
        System.out.println(Thread.currentThread().getName() + "'s elapsed time : " + (end - start));
    }
}

In my code, setStreamParallelTest takes 4 seconds whereas intStreamParallelTest takes 1 second.

I expect that setStreamParallelTest also done in 1 seconds. Is it bug?

2. Is it okay to use parallel stream to call another api in web application? If it is wrong, why?

My web application need to call another api server in parallel. So I use parallel stream to call api.

Sets.newHashSet(api1, api2, api3, api4).parallelStream().forEach(api -> callApiSync(api))

I think all requests bound for my server share a fork-join pool. so, It looks dangerous when one of api's response is slow.

Is it correct?

Answer 1

The contract for parallelStream says:

Returns a possibly parallel Stream with this collection as its source. It is allowable for this method to return a sequential stream.

If you want to invoke several tasks in parallel, use an ExecutorService .