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   <title>JSR 166 Community Review Draft Introduction.</title>

  <body bgcolor="#ffffee" vlink="#0000aa" link="#cc0000">
  <h1>JSR 166 Community Review Draft Introduction.</h1>

  by <a href="http://gee.cs.oswego.edu/dl">Doug Lea</a>

To check for updates to this draft, access a preliminary prototype
release of main functionality, or join a mailing list discussing this
JSR, go to: <A

<em> <b>Disclaimer</b>. The prototype implementation is experimental
code developed as part of JCP JSR-166 is made available to the
developer community for use as-is. It is not a supported product. Use
it at your own risk. The specification, language and implementation
are subject to change as a result of your feedback. Because these
features have not yet been approved for addition to the Java language,
there is no schedule for their inclusion in a product.  </em>

<p> <em> <b>Disclaimer</b>.  This draft specification was produced
using JDK1.4 tools plus some preprocessing. The resulting javadocs do
not yet correctly render other planned JDK1.5 constructs on which
JSR-166 relies, most notably the use of generic types. We are
releasing this version now (before the availability of JDK1.5-based
tools) because, even though they are misformatted and sometimes lack
proper cross-referencing, they otherwise convey the intended
specifications.  </em>

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

<p>JSR-166 focusses 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, which also keeping the
resulting package small; providing only that minimial support for
which it makes sense to standardize.

<p>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.


A basic (nonblocking) {@link java.util.Queue} interface extending
{@link java.util.Collection} is introduced into
<tt>java.util</tt>. Existing class {@link java.util.LinkedList} is
adapted to support Queue, and a new non-thread-safe {@link
java.util.PriorityQueue} is added.


Two minor changes are introduced to the {@link java.lang.Thread}
class: It now allows per-thread installation of handlers for uncaught
exceptions. Ths 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.) Secondly,
access checks are no longer required when a Thread interrupts
<em>itself</em>.  The <tt>interrupt</tt> 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


Method <tt>nanoTime</tt> is added to {@link java.lang.System}. It
provides a high-precision timing facility that is distinct from and
uncoordinated with <tt>System.currentTimeMillis</tt>.

<h2>Removing ThreadLocals</h2>

The {@link java.lang.ThreadLocal} class now supports a means to remove
a ThreadLocal, which is needed in some thread-pool and worker-thread

  <address><A HREF="http://gee.cs.oswego.edu/dl">Doug Lea</A></address>

Doug Lea
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