定义了parallelStream终止吗？

Question

I find I struggle slightly with the consistency guarantees offered by the parallel streams:

1. myList.parallelStream().map(mymapper).forEach(myFn)
2. // Is myFn guaranteed to be called for each element in myList here ?

In other words, can I be sure that all the forkJoinTasks that will execute myFn are completed at line 2 ?

Answer 1

Yes. The parallel tasks within a complete stream pipeline are executed synchonously. Line 2 will be executed only after all the tasks have completed.

The package documentation for java.util.stream states,

In almost all cases, terminal operations are eager, completing their traversal of the data source and processing of the pipeline before returning.

Note however that any side effects accomplished by the forEach tasks might not be visible to other threads. In practice, forEach can only operate by having its action perform side effects. As mentioned in the Stream.forEach javadoc , the action must use proper synchronization or other constructs in order to ensure thread-safe operation and proper visibility of side effects.