Java parallel stream forEach completion

user_8996761 0
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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
java-stream multithreading java
1 Answers

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.

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