Concurrency Utilities
Overview

The Java 2 platform includes a new package of concurrency utilities. These are classes that are designed to be used as building blocks in building concurrent classes or applications. Just as the collections framework simplified the organization and manipulation of in-memory data by providing implementations of commonly used data structures, the concurrency utilities 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 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 were carefully audited for deadlock, starvation, and race conditions.
• Improved maintainability. Programs that use standard library classes are easier to understand and maintain than those that 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 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 your program be clearer, shorter, faster, more reliable, more scalable, easier to write, easier to read, and easier to maintain.

The concurrency utilities includes:

• Task scheduling framework. The Executor interface standardizes invocation, scheduling, execution, and control of asynchronous tasks according to a set of execution policies. Implementations are provided that enable 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 that support arbitrary execution policies. The built-in implementations offer configurable policies such as queue length limits and saturation policy that can improve the stability of applications by preventing runaway resource use.
• Fork/join framework. Based on the ForkJoinPool class, this framework is an implementation of Executor. It is designed to efficiently run a large number of tasks using a pool of worker threads. A work-stealing technique is used to keep all the worker threads busy, to take full advantage of multiple processors.
• Concurrent collections. Several new collections classes were 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 information.
• Atomic variables. Utility classes are provided that atomically manipulate single variables (primitive types or references), providing high-performance atomic arithmetic and compare-and-set methods. The atomic variable implementations in the java.util.concurrent.atomic package offer higher performance than would be available by using synchronization (on most platforms), making them useful for implementing high-performance concurrent algorithms and conveniently implementing counters and sequence number generators.
• Synchronizers. General purpose synchronization classes, including semaphores, barriers, latches, phasers, and exchangers, facilitate coordination between threads.
• Locks. While locking is built into the Java language through the synchronized keyword, there are a number of 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, and it also supports specifying a timeout when attempting to acquire a lock, multiple condition variables per lock, nonnested ("hand-over-hand") holding of multiple locks, and support for interrupting threads that 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 that 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 the System.nanoTime method is platform-dependent.