Summary
-------

An interoperable publish-subscribe framework, enhancements to the
`CompletableFuture` API, and various other improvements.


Motivation
----------

The continual evolution of uses of concurrency and parallelism in
applications requires continual evolution in library support.


Description
-----------

  1. Interfaces supporting the Reactive Streams publish-subscribe
     framework, nested within the new class `Flow`.  `Publisher`s produce 
     items consumed by one or more `Subscriber`s, each managed by a 
     `Subscription`.  Communication relies on a simple form of flow control 
     (method `Subscription.request`, for communicating back pressure) that 
     can be used to avoid resource management problems that may otherwise 
     occur in "push" based systems.  A utility class `SubmissionPublisher` is provided 
     that developers can use to create custom components.  
     These (very small) interfaces correspond to those
     defined with broad participation (from the
     [Reactive Streams][reactiveStreams] initiative) and support
     interoperability across a number of async systems running on
     JVMs.  Nesting the interfaces within a class is a conservative policy
     allowing their use across various short-term and long-term
     possibilities.  There are no plans to provide network- or I/O-based 
     `java.util.concurrent` components for distributed messaging, but it is possible 
     that future JDK releases will include such APIs in other packages.

  2. Enhancements to the `CompletableFuture` API
      - Time-based enhancements are added that enable a future to complete with 
      a value or exceptionally after a certain duration, see methods `orTimeout` and
       `completeTimeout`. 
      In addition, a complementary `Executor` returned by the static methods 
      named `delayedExecutor` allow a task to execute after a certain duration. 
      This may be combined with `Executor` receiving methods on `CompletableFuture` 
      to support operations with time-delays.
      - Subclass enhancements are added making it easier to extend from 
      `CompletableFuture`, such as to provide a subclass that supports an alternative 
      default executor.

  3. Numerous implementation improvements accumulated since JDK 8; many of them are
     small, but some include Javadoc spec rewordings.

[reactiveStreams]: http://www.reactive-streams.org/


Testing
-------

The continuing set of JSR 166 EG members provide functionality, TCK, and
performance tests for all components.


Risks and Assumptions
---------------------

Care will be taken to ensure any small but required differences in code between the JDK 9 repository and the 166 repository are retained.