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   <title>JSR 166 Snapshot Introduction.</title>

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

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

To join a mailing list discussing this JSR, go to:
<A HREF="http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest"> http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest</A> .

 Disclaimer - This prototype is experimental code developed as part of
 JSR166 and 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.

Package java.util.concurrent contains utility classes that are
commonly useful in concurrent programming. Like package java.util, it
includes a few small standardized extensible frameworks, as well as
some classes that provide useful functionality and are otherwise
tedious or difficult to implement.  In this JSR, we have been
conservative in selecting only those APIs and implementations that are
useful enough to encourage nearly all concurrent programmers to use
routinely.  JSR 166 also includes a few changes and additions in
packages outside of java.util.concurrent: java.lang, to address timing
and uncaught exceptions, and java.util to better integrate queues, and
to make Timers conform to new frameworks. The API covers:

    <li> Queues
    <li> Executors
    <li> Locks
    <li> Condition variables
    <li> Atomic variables
    <li> Timing
    <li> Barriers
    <li> Concurrent Collections 
    <li> Uncaught Exception Handlers

The main rationale for JSR 166 is that threading primitives, such as
synchronized blocks, Object.wait and Object.notify, are insufficient
for many programming tasks.  Currently, developers can use only the
concurrency control constructs provided in the Java language
itself. These are too low level for some applications, and are
incomplete for others.  As a result, application programmers are often
forced to implement their own concurrency facilities, resulting in
enormous duplication of effort creating facilities that are
notoriously hard to get right and even harder to optimize.  Offering a
standard set of concurrency utilities will ease the task of writing a
wide variety of multithreaded applications and generally improve the
quality of the applications that use them.

Here are brief descriptions and rationales of the main components.
For details see the javadocs at <a


A basic (nonblocking) Queue interface that is compatatible with
java.util.Collections will be introduced into java.util. Also,
although it is at the borders of being in scope of JSR-166, 
java.util.LinkedList will be adapted to support Queue, and
a new non-thread-safe java.util.HeapPriorityQueue will be added.

Four implementations in java.util.concurrent support the extended
BlockingQueue interface, that defines blocking versions of put and
take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, and
PriorityBlockingQueue. Additionally, java.util.concurrent.LinkedQueue
supplies an efficient thread-safe non-blocking queue.
Since the target release is JDK1.5, and generics are slated to be in
1.5, Queues should be parametrized on element type. (Also some others
below.) We are ignoring this for now.


Executors provide a simple standardized interface for defining custom
thread-like subsystems, including thread pools, asynch-IO, and
lightweight task frameworks.  Executors also standardize ways of
calling threads that compute functions returning results, via
Futures. This is supported in part by defining java.lang.Callable, the
argument/result analog of Runnable.

<p> While the Executor framework is intended to be extensible the most
commonly used Executor will be ThreadExecutor, which can be configured
to act as all sorts of thread pools, background threads, etc. The
class is designed to be general enough to suffice for the vast
majority of usages, even sophisticated ones, yet also includes methods
and functionality that simplify routine usage.

A few methods will also be added to the java.util.Timer to support
Futures, and address other requests for enhancement.


The Lock interface supports locking disciplines that differ in
semantics (reentrant, semaphore-based, etc), and that can be used in
non-block-structured contexts including hand-over-hand and lock
reordering algorithms. This flexibility comes at the price of more
awkward syntax.  Implementations include Semaphore, ReentrantMutex
FIFOSemaphore, and CountDownLatch.

The Locks class additionally supports trylock-designs using builtin
locks without needing to use Lock classes.  This requires adding new
capabilities to builtin locks inside JVMs.

A ReadWriteLock interface similarly defines locks that may be shared
among readers but are exclusive to writers. For this release, only a
single implementation, ReentrantReadWriteLock, is planned, since it
covers all standard usage contexts. But programmers may create their
own implementations to cover nonstandard requirements.


A Condition class provides the kinds of condition variables associated
with monitors in other cocurrent languages, as well as pthreads
condvars.  Their support reduces the need for tricky and/or
inefficient solutions to many classic concurrent problems.  Conditions
also address the annoying problem that Object.wait(msecs) does not
return an indication of whether the wait timed out. This leads to
error-prone code. Since this method is in class Object, the problem is
basically unfixable.
To avoid compatibility problems, the names of Condition methods need
to be different than Object versions. The downside of this is that
people can make the mistake of calling cond.notify instead of
cond.signal. However, they will get IllegalMonitorState exceptions if
they do, so they can detect the error if they ever run the code.
The implementation requires VM magic to atomically suspend and release
lock. But it is unlikely to be very challenging for JVM providers,
since most layer Java monitors on top of posix condvars or similar
low-level functionality anyway.

<h2>Atomic variables</h2>

Classes AtomicInteger, AtomicLong, AtomicDouble, AtomicFloat, and
AtomicReference provide simple scalar variables supporting
compareAndSwap (CAS) and related atomic operations. These are
desparately needed by those performing low-level concurrent system
programming, but much less commonly useful in higher-level frameworks.


Java has always supported sub-millisecond versions of several native
time-out-based methods (such as Object.wait), but not methods to
actually perform timing in finer-grained units. We address this by
introducing java.lang.Clock, which provides multiple granularities for
both accessing time and performing time-out based operations.


Barriers (multiway synchronization points) are very common in some
styles of parallel programming, yet tricky to get right. The two most
useful flavors (CyclicBarriers and Exchangers) don't have much of an
interface in common, and only have one standard implementation each,
so these are simply defined as public classes rather than interfaces
and implementations.

<h2>Concurrent Collections</h2>

There are no new interfaces, but JSR 166 will supply a few Collection
implementations designed for use in multithreaded contexts:
ConcurrentHashTable, CopyOnWriteArrayList, and CopyOnWriteArraySet.

<h2>Uncaught Exception Handlers</h2>

The java.lang.Thread class will be modified to allow per-thread
installation of handlers for uncaught exceptions. Ths optionally
disassociates these handlers from ThreadGroups, which has proven to be
too inflexible in many multithreaded programs. (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.)
Additionally, Threads and ThreadLocals will now support a means to
clear and remove ThreadLocals, which is needed in some thread-pool and
worker-thread designs.

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

Doug Lea
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