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.

Fyi, .parallelStream() will make use of the ForkJoinPool.commonPool(). — Solomon Slow
– Solomon Slow, Commented Oct 16, 2015 at 20:49
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. — Louis Wasserman
– Louis Wasserman, Commented Oct 16, 2015 at 21:22

Makoto · Accepted Answer · 2015-10-16 20:38:23Z

25

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()));

answered Oct 16, 2015 at 20:38

community wiki

Makoto

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1 Comment

otoomey Over a year ago

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)

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