JSR 166 Snapshot 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 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 uncaught exceptions, and java.util to better integrate queues. The API covers:

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 http://gee.cs.oswego.edu/dl/concurrent/index.html


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.PriorityQueue will be added.

Five implementations in java.util.concurrent support the extended BlockingQueue interface, that defines blocking versions of put and take: LinkedBlockingQueue, ArrayBlockingQueue, SynchronousQueue, PriorityBlockingQueue, and DelayQueue. 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 are parametrized on element type. (Also some others below.)


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 interface Callable, the argument/result analog of Runnable.

Executors provide a framework for executing Runnables. The Executor manages queueing and scheduling of tasks, and creation and teardown of threads. 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.

Several concrete implementations of Executor are included in java.util.concurrent, including ThreadPoolExecutor, a flexible thread pool and ScheduledExecutor, which adds support for delayed and periodic task execution. Executor can be used in conjunction with FutureTask (which implements Runnable) to asynchronously start a potentially long-running computation and query the FutureTask to determine if its execution has completed.

The Executors class provides factory methods for all of the types of executors provided in java.util.concurrent.


The 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 ReentrantLock and FairReentrantLock.

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.

Atomic variables

The atomic subpackage includes a small library of classes, including AtomicInteger, AtomicLong, and AtomicReference that support variables performinf compareAndSet (CAS) and related atomic operations.


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 class TimeUnit, which provides multiple granularities for both accessing time and performing time-out based operations.


Five classes aid common special-purpose synchronization idioms. Semaphores and FairSemaphores are classic concurrency tools. CountDownLatches are very simple yet very common objects useful for blocking until a single signal, event, or condition holds. CyclicBarriers are resettable multiway synchronization points very common in some styles of parallel programming. Exchangers allow two threads to exchange objects at a rendezvous point.

Concurrent Collections

JSR 166 will supply a few Collection implementations designed for use in multithreaded contexts: ConcurrentHashMap, CopyOnWriteArrayList, and CopyOnWriteArraySet.

Uncaught Exception Handlers

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, ThreadLocals will now support a means to remove a ThreadLocal, which is needed in some thread-pool and worker-thread designs.

Doug Lea