How many threads are spawned in parallelStream in Java 8?

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The Oracle’s implementation[1] of parallel stream uses the current thread and in addition to that, if needed, also the threads that compose the default fork join pool ForkJoinPool.commonPool(), which has a default size equal to one less than the number of cores of your CPU.

That default size of the common pool can be changed with this property:

-Djava.util.concurrent.ForkJoinPool.common.parallelism=8

Alternatively, you can use your own pool:

ForkJoinPool myPool = new ForkJoinPool(8);
myPool.submit(() ->
    list.parallelStream().forEach(/* Do Something */);
).get();

Regarding the order, jobs will be executed as soon as a thread is available, in no specific order.

As correctly pointed out by @Holger this is an implementation specific detail (with just one vague reference at the bottom of a document), both approaches will work on Oracle’s JVM but are definitely not guaranteed to work on JVMs from other vendors, the property could not exist in a non-Oracle implementation and Streams could not even use a ForkJoinPool internally rendering the alternative based on the behavior of ForkJoinTask.fork completely useless (see here for details on this).

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