--- jsr166/src/main/intro.html 2003/06/07 11:54:08 1.3 +++ jsr166/src/main/intro.html 2003/09/27 12:21:56 1.14 @@ -1,172 +1,97 @@ - JSR 166 Snapshot Introduction. + JSR 166 Introduction. -

JSR 166 Snapshot Introduction.

+

JSR 166 Introduction.

by Doug Lea

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

- - Disclaimer - This prototype is experimental code developed as part of - JCP 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 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. JSR166 also includes a few changes and -additions in packages outside of java.util.concurrent: java.lang, to -address uncaught exceptions, and java.util to better integrate with -collections. Since the target release is JDK1.5, many APIs use -generics to parameterize on types. Here are brief descriptions of the -main components. - -

Executors

- -{@link java.util.concurrent.Executor} is a simple standardized -interface for defining custom thread-like subsystems, including thread -pools, asynch-IO, and lightweight task frameworks. Depending on which -concrete Executor class is being used, tasks may execute in a newly -created thread, an existing task-execution thread, or the thread -calling execute(), and may execute sequentially or -concurrently. Executors also standardize ways of calling threads that -compute functions returning results, via a {@link -java.util.concurrent.Future}. This is supported in part by defining -interface {@link java.util.concurrent.Callable}, the argument/result -analog of Runnable. - -

{@link java.util.concurrent.ExecutorService} provides a more -complete framework for executing Runnables. An ExecutorService -manages queueing and scheduling of tasks, and allows controlled -shutdown. The two primary implementations of ExecutorService are -{@link java.util.concurrent.ThreadPoolExecutor}, a highly tunable and -flexible thread pool and {@link -java.util.concurrent.ScheduledExecutor}, which adds support for -delayed and periodic task execution. These, and other Executors can -be used in conjunction with a {@link FutureTask} to asynchronously -start a potentially long-running computation and query the FutureTask -to determine if its execution has completed, or cancel it. - -

The {@link java.util.concurrent.Executors} class provides factory -methods for the most common kinds and styles of Executors, as well as -a few utilities methods for using them. +This is an updated version of the specification submitted for JCP Public +Review. To check for further updates, access a preliminary prototype +release of main functionality, or join a mailing list discussing +JSR-166, go to: +http://altair.cs.oswego.edu/mailman/listinfo/concurrency-interest. + +

Note: The javadocs here includes some existing java.util +Collection interfaces and classes that are not part of the JSR-166 +spec, but are included because JSR-166 methods implement or inherit +from their specifications. + +

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 some classes that +provide useful functionality and are otherwise tedious or difficult to +implement. + +

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 functionality that it makes +sense 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) {@link java.util.Queue} interface extending -java.util.Collection is introduced into java.util. Existing class -java.util.LinkedList is adapted to support Queue, and a new -non-thread-safe {@link java.util.concurrent.java.util.PriorityQueue} -is added. The java.util.concurrent {@link -java.util.concurrent.LinkedQueue} class supplies an efficient -thread-safe non-blocking queue. - -

Five implementations in java.util.concurrent support the extended -{@link java.util.concurrent.BlockingQueue} interface, that defines -blocking versions of put and take: {@link -java.util.concurrent.LinkedBlockingQueue}, {@link -java.util.concurrent.ArrayBlockingQueue}, {@link -java.util.concurrent.SynchronousQueue}, {@link -java.util.concurrent.PriorityBlockingQueue}, and {@link DelayQueue}. - - -

Locks

- -The {@link java.util.concurrent.Lock} interface supports locking -disciplines that differ in semantics (reentrant, fair, 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 {@link -java.util.concurrent.ReentrantLock} and {@link -java.util.concurrent.FairReentrantLock}. - -

The {@link java.util.concurrent.Locks} class additionally supports -some common trylock-designs using builtin locks. - -

The {@link java.util.concurrent.ReadWriteLock} interface similarly -defines locks that may be shared among readers but are exclusive to -writers. Only a single implementation, {@link -java.util.concurrent.ReentrantReadWriteLock}, is provided, since it -covers all standard usage contexts. But programmers may create their -own implementations to cover nonstandard requirements. - -

Conditions

- -The {@link java.util.concurrent.Condition} interface describes the -kinds of condition variables associated with monitors in other -concurrent languages, as well as pthreads-style condvars. Their -support reduces the need for tricky and/or inefficient solutions to -many classic concurrent problems. To avoid compatibility problems, -the names of Condition methods are different than Object versions. - -

Atomics

- -The atomic subpackage includes a small library of classes, including -AtomicInteger, AtomicLong, and AtomicReference that support -compareAndSet (CAS) and related atomic operations. +{@link java.util.Collection} is introduced into +java.util. Existing class {@link java.util.LinkedList} is +adapted to support Queue, and a new non-thread-safe {@link +java.util.PriorityQueue} is added. + +

Threads

+ +Three minor changes are introduced to the {@link java.lang.Thread} +class: +

Timing

-The {@link java.util.concurrent.TimeUnit} class provides multiple -granularities (including nanoseconds) for both accessing time and -performing time-out based operations. - -

Synchronizers

- -Five classes aid common special-purpose synchronization idioms. -{@link java.util.concurrent.Semaphore} and {@link -java.util.concurrent.FairSemaphore} are classic concurrency tools. -{@link java.util.concurrent.CountDownLatch} is very simple yet very -common utility for blocking until a single signal, event, or condition -holds. A {link CyclicBarrier} is a resettable multiway -synchronization point common in some styles of parallel -programming. An {@link java.util.concurrent.Exchanger} allows two -threads to exchange objects at a rendezvous point. - -

Concurrent Collections

- -This package supplies a few Collection implementations designed for -use in multithreaded contexts: {@link -java.util.concurrent.ConcurrentHashMap}, {@link -java.util.concurrent.CopyOnWriteArrayList}, and {@link -java.util.concurrent.CopyOnWriteArraySet}. - -

Most concurrent Collection implementations (including most Queues) -differ from the usual java.util conventions in that their Iterators -provide weakly consistent rather than fast-fail traversal. A -weakly consistent iterator is thread-safe, but does not necessarily -freeze the collection while iterating, so it may (or may not) reflect -any updates since the iterator was created. - -

Uncaught Exception Handlers

- -The java.lang.Thread class is 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 JSR166 make ThreadGroups even less likely -to be used in most programs. Perhaps they will eventually be -deprecated.) +Method nanoTime is added to {@link java.lang.System}. It +provides a high-precision timing facility that is distinct from and +uncoordinated with System.currentTimeMillis. + +

Removing ThreadLocals

-

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


-
Doug Lea