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In this article, I will focus on the Business Delegate enterprise design pattern. This pattern describes a relatively new application of an old concept that facilitates decoupling between application tiers and various business services. I will describe the pattern in detail with an example of real enterprise usage. In addition, I will talk about key concepts and the evolution of this pattern. The idea of hiding complex implementations from different software clients or tiers is not new, GoF* book describes several patterns that make it possible. For example, a Façade [GoF] pattern provides a single high-level interface for a set of interfaces in a system and therefore hides the original implementation from the client subsystems. An Adapter [GoF] pattern or Broker pattern provide coupling for disparate systems, thereby also concealing the implementation of the parent system from the subsystems that use it.

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Since the publication of the renowned book Design Patterns: Elements of Reusable Object-Oriented Software by Gamma, Helm, Johnson, and Vlissides, also known as the Gang of Four (GoF) book in 1995, the wave of design patterns in an enterprise world has gained enough momentum to reach the mainstream developers and IT managers. Something that once was only invented to be used and shared by uber-developers to simplify reusability and maintenance of their projects was now accepted as a de facto way to do software engineering. The "epidemic" of design patterns has reached its tipping point and spread like wildfire. The book served as a catalyst for the most famous and original patterns, which were at that time mostly in C++. The patterns focused on the object-oriented aspects of software development. They were categorized into three main categories: creational, structural, and behavioral.

In this article, I will discuss the enterprise Intercepting Filter pattern. The Intercepting Filter pattern can be classified as a presentation tier pattern because it deals with requests from the presentation tier. This pattern is Web-application specific, and generally cannot be applied in other types of applications, but its concepts are generic and it is related to the structural Decorator (Wrapper) pattern, which I will discuss in this article as well. The Servlet specification 2.3, which is now finalized, incorporates this pattern, and I will focus on the differences between the Servlet and custom implementation methods. I also will describe the reasoning behind this pattern and how can it be reused throughout Web applications.

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This article will deal with two often overlooked or underused design patterns: one creational Builder pattern and one a behavioral Command pattern. I will explore their application in the software architecture process, describe the purpose of these patterns, and show how they can be used in real-world Java projects.

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