How many threads are spawned in parallelStream in Java 8?

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).

While @uraimo is correct, the answer depends on exactly what "Do Something" does. The parallel.streams API uses the CountedCompleter Class which has some interesting problems. Since the F/J framework does not use a separate object to hold results, long chains may result in an OOME. Also those long chains can sometimes cause a Stack Overflow. The answer to those problems is the use of the Paraquential technique as I pointed out in this article.

The other problem is excessive thread creation when using nested parallel forEach.