I'm in the process of (tentatively) adding reactive-stream APIs in java.util.concurrent (targeting JDK9). The current somewhat unusual plan is nest all four interfaces inside new class Async, that will also include some other new async support stuff. (None of the standard alternatives (creating new package or adding all four into j.u.c itself) work out very well.

A snapshot of the draft reactive-stream parts of Async.java is temporarily at
http:/gee.cs.oswego.edu/dl/wwwtmp/Async.java

It is written in standard jdk javadoc style, intending to correspond (and cites) the reactive-stream specs. But please let me know about errors or suggestions for improvement.

Also, comments welcome on a draft snapshot of one of the associated upcoming utility classes, BufferedSubmissionPublisher, that can be used as an adaptor for item generators that want to be Publishers.
http:/gee.cs.oswego.edu/dl/wwwtmp/BufferedSubmissionPublisher.java