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Concurrent Programming with J2SE 5.0

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Java is a multithreaded programming language that makes programming with threads easier, by providing built-in language support for threads. The built-in primitives however, such as synchronized blocks, Object.wait(), and Object.notify() are insufficient for many programming tasks. This leads developers to implement their own high-level synchronization facilities, but given the difficulty of concurrency issues, their implementations may not be correct, efficient, or high quality.

The Java 2 Platform, Standard Edition release 5.0 (J2SE 5.0), which is also known as Tiger, has provided a new path to multithreading in the Java programming language. The original mechanisms for coordinating threads with wait() and notify() are now enhanced with new and sophisticated mechanisms for working with threads. The new mechanisms are part of the java.util.concurrent package, which aims to offer a standard set of concurrency utilities that will ease the task of developing multithreaded applications and servers. In addition, such standards will improve the quality of such applications. The package has been defined through the Java Community Process as JSR 166: Concurrency Utilities.

This article provides an overview and an introductory tutorial to the new concurrency mechanisms that have been added to J2SE 5.0. It helps developers get started using the new java.util.concurrent package and its subpackages.
Limitations of Built-in Synchronization in J2SE 1.4.x
When writing multithread applications, the issues that may create difficulties are related to data synchronization; these are the errors that make design harder, and such errors are hard to detect. Built-in synchronization (methods and blocks) are fine for many lock-based applications, but they do have their own limitations, such as:

  • No way to back off from an attempt to acquire a lock that is already held, or to give up after waiting for a specified period of time, or to cancel a lock attempt after an interrupt.
  • No way to alter the semantics of a lock, for example, with respect to reentrancy, read versus write protection, or fairness.
  • No access control for synchronization. Any method can perform synchronized(obj) for any accessible object.
  • Synchronization is done within methods and blocks, thus limiting use to strict block-structured locking. In other words, you cannot acquire a lock in one method and release it in another.
Such problems can be overcome by using utility classes to control locking, such as Mutex, which is another term for a lock. A mutex, however, doesn't nest like synchronization methods or blocks.

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