Part I - Introduction to Frameworks

For all of my friends who are already familiar with development of web based applications with Java (J2EE) using JSP and servlets, and would like to start using Java Frameworks, here is a three part article that introduce the basics of modern java frameworks.

The concept of framework has been kicking around in software development for a long time in one form or another. In its simplest form, a framework is simply a body of tried and tested code that is reused in multiple software development projects. A framework in general, provides an implementation for the core and unvarying functions and includes mechanisms to allow developer to plug-in various functions or to extend the funtions.

Frameworks can be classified into 3 based on their scope, as follows:

1. System infrastructure frameworks - These frameworks simplify the development of portable and efficient system infrastructure such as operating system and communication frameworks, and frameworks for user interfaces and language processing tools. System infrastructure frameworks are primarily used internally within a software organization and are not sold to customers directly.

2. Middleware integration frameworks - These frameworks are commonly used to integrate distributed applications and components. Middleware integration frameworks are designed to enhance the ability of software developers to modularize, reuse, and extend their software infrastructure to work seamlessly in a distributed environment. There is a thriving market for Middleware integration frameworks, which are rapidly becoming commodities. Common examples include ORB frameworks, message-oriented middleware, and transactional databases.

3. Enterprise application frameworks - These frameworks address broad application domains (such as telecommunications, avionics, manufacturing, and financial engineering) and are the cornerstone of enterprise business activities. Relative to System infrastructure and Middleware integration frameworks, Enterprise frameworks are expensive to develop and/or purchase. However, Enterprise frameworks can provide a substantial return on investment since they support the development of end-user applications and products directly.

Regardless of their scope, frameworks can also be classified by the techniques used to extend them, which range along a continuum from whitebox frameworks to blackbox frameworks.

1. Whitebox frameworks rely heavily on OO language features like inheritance and dynamic binding to achieve extensibilty. Existing functionality is reused and extended by (1) inheriting from framework base classes and (2) overriding pre-defined hook methods using patterns like Template Method. Whitebox frameworks require application developers to have intimate knowledge of the frameworks' internal structure. Although whitebox frameworks are widely used, they tend to produce systems that are tightly coupled to the specific details of the framework's inheritance hierarchies.

2. Blackbox frameworks support extensibility by defining interfaces for components that can be plugged into the framework via object composition. Existing functionality is reused by (1) defining components that conform to a particular interface and (2) integrating these components into the framework using patterns like Strategy and Functor. Blackbox frameworks are structured using object composition and delegation more than inheritance. As a result, blackbox frameworks are generally easier to use and extend than whitebox frameworks. However, blackbox frameworks are more difficult to develop since they require framework developers to define interfaces and hooks that anticipate a wider range of potential use-cases.

Object-Oriented (OO) Application Frameworks

Object-oriented (OO) application frameworks are a promising technology for reifying proven software designs and implementations in order to reduce the cost and improve the quality of software. An OO application framework is a reusable, ``semi-complete'' application that can be specialized to produce custom applications. In contrast to earlier OO reuse techniques based on class libraries, frameworks are targeted for particular business units (such as data processing or cellular communications) and application domains (such as user interfaces or persistance).

The primary benefits of OO application frameworks stem from the modularity, reusability, extensibility, and inversion of control they provide to developers, as described below:

Modularity - Frameworks enhance modularity by encapsulating volatile implementation details behind stable interfaces. Framework modularity helps improve software quality by localizing the impact of design and implementation changes. This localization reduces the effort required to understand and maintain existing software.

Reusability - The stable interfaces provided by frameworks enhance reusability by defining generic components that can be reapplied to create new applications. Framework reusability leverages the domain knowledge and prior effort of experienced developers in order to avoid re-creating and re-validating common solutions to recurring application requirements and software design challenges. Reuse of framework components can yield substantial improvements in programmer productivity, as well as enhance the quality, performance, reliability and interoperability of software.

Extensibility - A framework enhances extensibility by providing explicit hook methods that allow applications to extend its stable interfaces. Hook methods systematically decouple the stable interfaces and behaviors of an application domain from the variations required by instantiations of an application in a particular context. Framework extensibility is essential to ensure timely customization of new application services and features.

Inversion of control - The run-time architecture of a framework is characterized by an ``inversion of control.'' This architecture enables canonical application processing steps to be customized by event handler objects that are invoked via the framework's reactive dispatching mechanism. When events occur, the framework's dispatcher reacts by invoking hook methods on pre-registered handler objects, which perform application-specific processing on the events. Inversion of control allows the framework (rather than each application) to determine which set of application-specific methods to invoke in response to external events (such as window messages arriving from end-users or packets arriving on communication ports).

Early object-oriented frameworks (such as MacApp and Interviews) originated in the domain of graphical user interfaces (GUIs). The Microsoft Foundation Classes (MFC) is a contemporary GUI framework that has become the de facto industry standard for creating graphical applications on PC platforms. Although MFC has limitations (such as lack of portability to non-PC platforms), its wide-spread adoption demonstrates the productivity benefits of reusing common frameworks to develop graphical business applications.

The next generation of OO application frameworks targeted at complex business and application domains. At the heart of this effort were the Object Request Broker (ORB) frameworks, which facilitate communication between local and remote objects. ORB frameworks eliminate many tedious, error-prone, and non-portable aspects of creating and managing distributed applications and reusable service components. This enables programmers to develop and deploy complex applications rapidly and robustly, rather than wrestling endlessly with low-level infrastructure concerns. Widely used ORB frameworks include CORBA, DCOM, and Java RMI.

In server-side development, a number of core tasks crop up over and over again. Such tasks can be pulled into a core framework, built and tested once, and reused across multiple projects. Utilizing this opportunity, many frameworks emerged that simplified the development of web based projects. As development of Web-based application servers and their applications expanded, so did the frameworks that supported these technologies. Currently, there are many software frameworks in the enterprise development space especially for the Java J2EE platform.

A good framework enhances the maintainability of software through API consistency, comprehensive documentation, and thorough testing. Some companies invest formally in frameworks and developers build up a library of components that they use often. Such actions reduce development time while improving delivered software quality - which means that developers can spend more time concentrating on the business-specific problem at hand rather than on the plumbing code behind it. There are also many mature frameworks available in the open source arena. Adopting such stable frameworks are more effective than going on to develop a framework from scratch.

End of Part I