JSR 166 Introduction.

by Doug Lea

This is maintenance repository of JSR-166 specifications. For further information, go to: http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest.

JSR-166 introduces package java.util.concurrent containing utility classes commonly useful in concurrent programming. Like package java.util, it includes a few small standardized extensible frameworks, as well as other classes that provide useful functionality and are otherwise tedious or difficult to implement.

JSR-166 focuses on breadth, providing critical functionality useful across a wide range of concurrent programming styles and applications, ranging from low-level atomic operations, to customizable locks and synchronization aids, to various concurrent data structures, to high-level execution agents including thread pools. This diversity reflects the range of contexts in which developers of concurrent programs have been found to require or desire support not previously available in J2SE, while also keeping the resulting package small; providing only functionality that has been found to be worthwhile to standardize.

Descriptions and brief motivations for the main components may be found in the associated package documentation. JSR-166 also includes a few changes and additions in packages outside of java.util.concurrent. Here are brief descriptions.

Queues

A basic (nonblocking) Queue interface extending Collection is introduced into java.util. Existing class LinkedList is adapted to support Queue, and a new non-thread-safe PriorityQueue is added.

Threads

Three minor changes are introduced to the Thread class:

• It now allows per-thread installation of handlers for uncaught exceptions. This optionally disassociates handlers from ThreadGroups, which has proven to be too inflexible. (Note that the combination of features in JSR-166 make ThreadGroups even less likely to be used in most programs. Perhaps they will eventually be deprecated.)
• Access checks are no longer required when a Thread interrupts itself. The interrupt method is the only way to re-assert a thread's interruption status (and in the case of self-interruption has no other effect than this). The check here previously caused unjustifiable and uncontrollable failures when restricted code invoked library code that must reassert interruption to correctly propagate status when encountering some InterruptedExceptions.
• The destroy method, which has never been implemented, has finally been deprecated. This is just a spec change, reflecting the fact that the reason it has never been implemented is that it was undesirable and unworkable.

Timing

Method nanoTime is added to System. It provides a high-precision timing facility that is distinct from and uncoordinated with System.currentTimeMillis.

Removing ThreadLocals

The ThreadLocal class now supports a means to remove a ThreadLocal, which is needed in some thread-pool and worker-thread designs.

Modules

Module	Description
java.base