1.1 Design Patterns

A design pattern is a recurring solution to a recurring problem. From a programming perspective, a pattern provides a set of specific interactions that can be applied to generic objects to solve a known problem. Good patterns strike a balance between the size of the problem they solve and the specificity with which they address the problem.

The simplest patterns may be summed up in no more than a sentence or two. Using a database to store information for a web site is a pattern, albeit a fairly high-level and obvious one. More complex patterns require more explanation, perhaps including the use of modeling languages or a variety of other forms of longer description.

Design patterns originated outside the computer industry, originally showing up in conventional (as opposed to computer systems) architecture. Architects of buildings and architects of software have more in common than one might initially think. Both professions require attention to detail, and each practitioner will see their work collapse around them if they make too many mistakes.

The book to read, if you're interested in the architectural origins of design patterns, is A Pattern Language: Towns, Buildings, Construction by Christopher Alexander (Oxford University Press). Widespread acceptance of design patterns in software began with the publication of Design Patterns: Elements of Reusable Object Oriented Software (Addison-Wesley), by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, also known as the "Gang of Four."

Design patterns are discovered as much as created. Some of the most effective patterns emerged from two decades of object-oriented (OO) design theory and the fewer but highly intensive years of enterprise Java. In many cases, the challenge has been to move from best practices and gut feelings to clear, defined sets of activities that can form the basis for communication between developers. It's amazing how much easier it is to suggest that someone use an Actor-Observer pattern (which allows one piece of code to register interest in the activities of another and receive notifications of key events) than to explain how to implement an event model.

We hope that readers will have two reactions as they explore the patterns in this book. The first is to say, "That's cool" (or, if the boss is around, "That's useful!"). The second is to say, "So, what's the big deal?" Both reactions mean that we're doing our job—describing approaches that work. Design patterns in enterprise applications can be surprisingly simple on the surface. Of course, it's that generic aspect that makes them effective patterns in the first place.

Any developer who has built anything with J2EE, or even with significant components of J2EE, has probably discovered at least one or two of these patterns already. In fact, some of the patterns in this book originated before J2EE and influenced its development, and they can also be applied in other languages and environments. Using strong patterns makes for a better programmer, designer, or architect, regardless of the underlying language.

1.1.1 The Anatomy of a Pattern

Software design patterns represent a small fraction of the many types of patterns in the world. We mentioned already that design patterns started in the world of architecture. There are also business patterns, networking patterns, and even social patterns that describe how people should interact. So what makes a pattern a pattern?

To describe a general pattern, we'll take an example from city planning. Building a city is a lot like building a complicated application. There are buildings (objects) that encapsulate certain functions (police, fire, merchant banking, baseball stadiums, etc.). These buildings are largely independent, interacting only via the goods, services, and people (messages) that flow in and out of them on roads (interfaces). In a city, a design pattern might be "building in blocks."

There are several ways of describing a pattern, but they all have several aspects in common. The first part of the "building in blocks" pattern description is its name. The name is important because it is a simple way for city planners to uniquely refer to the pattern. Any time one planner refers to "building in blocks," all the other planners should know what he is talking about. If the pattern has an alternate name—for instance, "grid layout"—it should be mentioned also.

The second part of the pattern description is the goal. This describes the problem solved by the pattern, or what we hope to achieve by implementing the pattern. The goal is sometimes described in terms of the forces that apply to the pattern. One of the forces acting on the "building in blocks" pattern is that it is difficult to navigate in a large city where the roads do not run parallel. Another is that buildings are easier to build when they are square, a process that requires perpendicular intersections.

The most important aspect of a pattern description is the description itself. This element tells how the pattern works. It is usually phrased in terms of participants (the objects that interact in the pattern). The description of a pattern summarizes the characteristics of the participants, as well as how they interact. Note that the participants must be generic enough that the pattern can be reused, but specific enough to solve a particular problem: a balance that changes depending on the goal of the pattern.

For "building in blocks," we would likely define buildings, streets, avenues, vehicles, and people as participants. People and vehicles interact by navigating between buildings on the streets, which are laid out in a grid. However, we can also treat this pattern as something more generic. Rather than describing buildings, pedestrians, streets, and so forth, we can talk about components (buildings) connected by pathways (streets) and navigated by travelers (people and cars). This allows us to apply the same pattern to the layout of an office (cubes, hallways, workers), or of a restaurant (tables, aisles, waiters), as well as the layout of a city.

Pattern descriptions typically have an implementation section. Implementation covers any specific information that goes into making a pattern work in practice. In software patterns, the implementation section is where you find actual code—or, at least, code fragments. The code is a sample implementation that demonstrates the central ideas of the pattern. For the "building in blocks" pattern, we might mention in the implementation section that streets are often named by numbers, while avenues are usually referred to by name. Since this pattern doesn't address the variety or ratios of building types a city might require, we wouldn't emphasize how many police stations and convention centers the city might need. That would be a different pattern.

Finally, it is often helpful to show cases where the pattern has been applied successfully. Not only does this information help readers decide if a pattern is applicable to their problem, it also provides a reference for finding more detailed information. New York City, Washington, D.C., and Philadelphia all use the "building in blocks" pattern. Most of Boston does not, and as a result requires the largest public works project in history to add more capacity to the roads.^[1]

^[1] See http://www.bigdig.com/ for a detailed look at Boston's traffic woes and the world's largest refactoring activity.

1.1.2 Presenting Patterns

In this book, we take a somewhat unorthodox approach to working with design patterns. There are several books available, including the Gang of Four's, that present lists of patterns, sometimes organized according to a guiding principle or two. These are generally referred to as pattern catalogs. Rather than present another pattern catalog, we're going to discuss different aspects of J2EE development and introduce the patterns in context. The goal is to give the reader a set of patterns that build on each other. By presenting the patterns in the larger context of J2EE applications, we hope to foster a more complete understanding of effective Java Enterprise architecture.

For reference purposes, Appendix A contains an abbreviated form of a conventional pattern catalog, providing quick overviews of the problems dealt with by each pattern discussed in the book, and references back to the more detailed discussion.