> This is a draft proposal for how we could improve stream performance for the > case where the streams are empty. Empty collections are common-place. If we > iterate over them with an Iterator, we would have to create one small > Iterator object (which could often be eliminated) and if it is empty we are > done. However, with Streams we first have to build up the entire pipeline, > until we realize that there is no work to do. With this example, we change > Collection#stream() to first check if the collection is empty, and if it is, > we simply return an EmptyStream. We also have EmptyIntStream, EmptyLongStream > and EmptyDoubleStream. We have taken great care for these to have the same > characteristics and behaviour as the streams returned by Stream.empty(), > IntStream.empty(), etc. > > Some of the JDK tests fail with this, due to ClassCastExceptions (our > EmptyStream is not an AbstractPipeline) and AssertionError, since we can call > some methods repeatedly on the stream without it failing. On the plus side, > creating a complex stream on an empty stream gives us upwards of 50x increase > in performance due to a much smaller object allocation rate. This PR includes > the code for the change, unit tests and also a JMH benchmark to demonstrate > the improvement.
kabutz has updated the pull request incrementally with one additional commit since the last revision: Refactored empty stream implementations to reduce duplicate code and improved unordered() Added StreamSupport.empty for primitive spliterators and use that in Arrays.stream() ------------- Changes: - all: https://git.openjdk.java.net/jdk/pull/6275/files - new: https://git.openjdk.java.net/jdk/pull/6275/files/6e60ac6c..5813aaaa Webrevs: - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6275&range=01 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6275&range=00-01 Stats: 501 lines in 8 files changed: 217 ins; 166 del; 118 mod Patch: https://git.openjdk.java.net/jdk/pull/6275.diff Fetch: git fetch https://git.openjdk.java.net/jdk pull/6275/head:pull/6275 PR: https://git.openjdk.java.net/jdk/pull/6275
