1. Concurrency Utilities
Overview
Introduction
The Java 2 platform includes a new package of concurrency utilities. These are classes which are
designed to be used as building blocks in building concurrent classes or applications. Just as the
Collections Framework greatly simplified the organization and manipulation of in-memory data
by providing implementations of commonly used data structures, the Concurrency Utilities aims
to simplify the development of concurrent classes by providing implementations of building
blocks commonly used in concurrent designs. The Concurrency Utilities include a high-
performance, flexible thread pool; a framework for asynchronous execution of tasks; a host of
collection classes optimized for concurrent access; synchronization utilities such as counting
semaphores; atomic variables; locks; and condition variables.
Using the Concurrency Utilities, instead of developing components such as thread pools
yourself, offers a number of advantages:
Reduced programming effort. It is far easier to use a standard class than to develop it
yourself.
Increased performance. The implementations in the Concurrency Utilities were
developed and peer-reviewed by concurrency and performance experts; these
implementations are likely to be faster and more scalable than a typical implementation,
even by a skilled developer.
Increased reliability. Developing concurrent classes is difficult -- the low-level
concurrency primitives provided by the Java language ( synchronized, volatile,
wait(), notify(), and notifyAll()) are difficult to use correctly, and errors using
these facilities can be difficult to detect and debug. By using standardized, extensively
tested concurrency building blocks, many potential sources of threading hazards such as
deadlock, starvation, race conditions, or excessive context switching are eliminated. The
concurrency utilities have been carefully audited for deadlock, starvation, and race
conditions.
Improved maintainability. Programs which use standard library classes are easier to
understand and maintain than those which rely on complicated, homegrown classes.
Increased productivity. Developers are likely to already understand the standard library
classes, so there is no need to learn the API and behavior of ad-hoc concurrent
components. Additionally, concurrent applications are far simpler to debug when they are
built on reliable, well-tested components.
In short, using the Concurrency Utilities to implement a concurrent application can help you
make your program clearer, shorter, faster, more reliable, more scalable, easier to write, easier to
read, and easier to maintain.

2. The Concurrency Utilities includes:
Task Scheduling Framework - The Executor framework is a framework for
standardizing invocation, scheduling, execution, and control of asynchronous tasks
according to a set of execution policies. Implementations are provided that allow tasks to
be executed within the submitting thread, in a single background thread (as with events in
Swing), in a newly created thread, or in a thread pool, and developers can create
customized implementations of Executor supporting arbitrary execution policies. The
built-in implementations offer configurable policies such as queue length limits and
saturation policy which can improve the stability of applications by preventing runaway
resource consumption.
Concurrent Collections - Several new Collections classes have been added, including
the new Queue, BlockingQueue and BlockingDeque interfaces, and high-performance,
concurrent implementations of Map, List, and Queue. See the Collections Framework
Guide for more details.
Atomic Variables - Classes for atomically manipulating single variables (primitive types
or references), providing high-performance atomic arithmetic and compare-and-set
methods. The atomic variable implementations in java.util.concurrent.atomic offer
higher performance than would be available by using synchronization (on most
platforms), making them useful for implementing high-performance concurrent
algorithms as well as conveniently implementing counters and sequence number
generators.
Synchronizers - General purpose synchronization classes, including semaphores,
mutexes, barriers, latches, and exchangers, which facilitate coordination between threads.
Locks - While locking is built into the Java language via the synchronized keyword,
there are a number of inconvenient limitations to built-in monitor locks. The
java.util.concurrent.locks package provides a high-performance lock
implementation with the same memory semantics as synchronization, but which also
supports specifying a timeout when attempting to acquire a lock, multiple condition
variables per lock, non-nested ("hand-over-hand") holding of multiple locks, and support
for interrupting threads which are waiting to acquire a lock.
Nanosecond-granularity timing - The System.nanoTime method enables access to a
nanosecond-granularity time source for making relative time measurements, and methods
which accept timeouts (such as the BlockingQueue.offer, BlockingQueue.poll,
Lock.tryLock, Condition.await, and Thread.sleep) can take timeout values in
nanoseconds. The actual precision of System.nanoTime is platform-dependent.
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