Java 8 Stream multithreading

Question

mylist.stream()
      .filter(m -> m.isokay() != null)
      .forEach(m -> m.dosomething()));

For this code, is it running on multiple threads? If not, how can I do it? I want each m.dosomething() to run on seperate threads to speed this work up.

Solution

Use parallelStream() to accomplish this. Note that the documentation says that it is "possibly parallel", so there's a chance that you could return a non-parallel stream. I would imagine those cases are rare, but be aware that it is in fact a limitation.

mylist.parallelStream()
      .filter(m -> m.isokay() != null)
      .forEach(m -> m.dosomething()));

Comments

[Tunaki]

I think you are looking for parallelStream().

[Solomon Slow]

Fyi, .parallelStream() will make use of the ForkJoinPool.commonPool().

[Louis Wasserman]

Be aware that multithreading can, in many cases, slow your code down. See e.g. this page written by one of the authors of streams.

[otoomey]

Oracle doc specifies "When a stream executes in parallel, the Java runtime partitions the stream into multiple substreams". In this case, "possibly parallel" probably means that you get a sequential stream if it just doesn't make sense to make a parallel one (say if the stream is only one item in size, or threads are currently in short supply)