JEP 266: More Concurrency Updates

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. Publishers produce items consumed by one or more Subscribers, 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 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.

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.