Java 并行流 forEach 完成

Question

I would like to clarify a behavior of Java parallel streams. If I were to use a parallel stream as shown below, can I take it as a guarantee that the line "Should happen at the end" shown in the code below will only print after all parallel tasks have executed?

public static void main(String[] args) {
    Stream.of(1, 2, 3, 5, 7, 8, 9, 10, 11, 23, 399, 939).parallel().forEach(
        integer -> System.out
            .println(Thread.currentThread().getName() + ", for number " + integer));
    System.out.println("Should happen at the end");
}

The repeated trials always print "Should happen at the end" predictably at the end, as shown below. This maybe happening because main thread is also used to process a few requests. Is it possible that in some scenario that main thread is not used and in that case "Should happen at the end" will print before all the ForkJoinPool.commonPool-worker have finished executing their tasks?

main, for number 7
main, for number 6
ForkJoinPool.commonPool-worker-9, for number 3
ForkJoinPool.commonPool-worker-9, for number 4
ForkJoinPool.commonPool-worker-4, for number 1
ForkJoinPool.commonPool-worker-4, for number 10
ForkJoinPool.commonPool-worker-2, for number 2
ForkJoinPool.commonPool-worker-11, for number 5
ForkJoinPool.commonPool-worker-9, for number 8
main, for number 9
Should happen at the end

Answer 1

Stream terminal operations are not asynchronous. It means that Java will give back control to the calling thread only after forEach is terminated . Therefore, what you print after forEach is necessarily printed after in console.

Note, however, that if you were to use java.util.Logger API instead of System.out , output ordering would not be as predictable, as logging API itself cannot (for performance reason mainly) guarantee ordering of outputs.

Further reading on stream operations: Oracle official documentation .