The J2EE patterns described in this book are based on our collective experience of working on the J2EE platform with Sun Java Center clients around the world. The Sun Java Center, a part of Sun Professional Services, is a consulting organization focused on building Java technology-based solutions for customers. We have been creating solutions for the J2EE platform since the platform's inception, focusing on achieving Quality of Service goals such as scalability,availability, and performance.
During the early days, as we designed, developed, and implemented various systems on the J2EE platform, we started documenting our experiences in an informal way as design considerations, ideas, and notes. As the knowledge base grew, we recognized a need for a slightly more formal documentation to capture and communicate this knowledge. We transitioned to documenting these ideas as patterns, since patterns are ideally suited to capturing and communicating knowledge related to recurring problems and solutions.
The first order of business was to sort out the level of abstraction with which the patterns were to be documented. Some problems and solutions overlapped others in that the core of the problem was the same, but the solution was implemented in a different manner. To address this overlap, we had to tackle the issue of the level of abstraction and the granularity with which we defined each pattern. As you will see in the J2EE pattern catalog, we eventually settled on a level of abstraction that hovers somewhere between design pattern and architectural pattern. This allows us to describe each pattern at a higher level of abstraction and at the same time discuss the implementation details.
Each pattern has been named and renamed many times. Additionally, each pattern has been rewritten many times, based on community feedback. Needless to say, these patterns, like all patterns, are subject to continuous improvement and will certainly evolve as the technology and specifications change.
The J2EE pattern catalog currently includes 15 patterns.
Table lists the patterns included in the catalog.
Table: Patterns in the J2EE Pattern Catalog
How to Use the J2EE Pattern Catalog
One of the challenges when using any set of patterns is understanding how to best use the patterns in combination. As Christopher Alexander says in his book, A Pattern Language:
In short, no pattern is an isolated entity. Each pattern can exist in the world, only to the extent that is supported by other patterns: the larger patterns in which it is embedded, and the patterns of the same size that surround it, and the smaller patterns which are embedded in it.
The patterns in the J2EE pattern catalog are no exception to this rule.
The pattern relationships diagram describes how each pattern is supported by other patterns in the catalog. It also provides a roadmap to the J2EE pattern catalog, presented in tabular form, with common J2EE design and architecture-related questions paired with pattern or refactoring references, providing solutions to each question. To gain the maximum benefit from using these patterns, it is recommended that the pattern relationships and the pattern roadmap be well understood.
As you study each pattern in detail, you will see the patterns and strategies that are embedded within it, in which it is contained, and which it supports. Sometimes the pattern builds on other patterns from the J2EE pattern catalog or from other patterns described in well-known literature such as Design Patterns: Elements of Reusable Object-Oriented Software or Patterns of Software Architecture.
For each bad practice that has been listed, we provide links to refactorings or patterns that offer solutions to alleviate the problems created by that bad practice. We present refactorings that describe the steps involved in moving from a less optimal solution to a preferred one. The mechanics section of each refactoring provides references to patterns and design considerations that influence the direction of the refactoring.
Finally we demonstrate an example of an application based on the J2EE patterns. We present some use cases to show how these patterns interact and work together to help realize a use case.
Benefits of Using Patterns
You can use the J2EE patterns in this book to improve your system design, and you can apply them at any point in a project life cycle. The patterns in the catalog are documented at a relatively high level of abstraction and will provide great benefit when applied early in a project. Alternatively, if you apply a pattern during the implementation phase, you may have to rework existing code. In this case, the refactorings may prove quite useful.
Patterns are often quite simple to use, though not always easy to understand. However, patterns can be difficult and time consuming to document, since this effort requires an examination of the essence of what constitutes a good practice. Recognizing good practices is typically a long-term effort. It involves distilling a large volume of knowledge down to its basics and putting it into words. We have tried to ensure that our documentation is clear and that it relates well to real world issues. At the same time, we recognize that this effort will continue to evolve and to be refined and improved over time.
What are the benefits of using patterns? We describe in the following sections some of the benefits of using and applying patterns in a project. In brief, patterns
Leverage a Proven Solution
A pattern is documented based on the fact that the solution it offers has been used over and over again to solve similar problems at different times in different projects. Thus, patterns provide a powerful mechanism for reuse, helping developers and architects to avoid reinventing the wheel.
Patterns provide software designers with a common vocabulary. As designers, we use patterns not only to help us leverage and duplicate successful designs, but also to help us convey a common vocabulary and format to developers.
A designer who does not rely on patterns needs to expend more effort to communicate his design to other designers or developers. Software designers use the pattern vocabulary to communicate effectively. This is similar to the real world, where we use a common vocabulary to communicate and exchange ideas. Just as in the real world, developers can build their vocabulary by learning and understanding patterns, increasing their design vocabulary as new patterns are documented.
Once you start to use these patterns, you'll notice that you'll quickly begin to incorporate the pattern names into your vocabulary—and that you use the names of the patterns to replace lengthy descriptions. For example, suppose your problem solution entails use of a Value Object pattern. At first, you might describe the problem without putting a label on it. You may describe the need for your application to exchange data with enterprise beans, the need to maximize performance given the network overhead with remote invocations, and so forth. Later, once you've learned how to apply the Value Object pattern to the problem, you may refer to a similar situation in terms of a “Value Object” solution and build from there.
To understand the impact of the pattern vocabulary, consider this exercise after you and another team member are familiar with the pattern catalog. Without using pattern names, try to explain what can be conveyed by simple sentences such as the following, in which the pattern names from the J2EE pattern catalog are italicized:
Constrains Solution Space
Pattern application introduces a major design component—constraints. Using a pattern constrains or creates boundaries within a solution space to which a design and implementation can be applied. Thus, a pattern strongly suggests to a developer boundaries to which an implementation might adhere. Going outside of these boundaries breaks the adherence to the pattern and design, and may lead to the unwanted introduction of an anti-pattern.
However, patterns do not stifle creativity. Instead, they describe a structure or shape at some level of abstraction. Designers and developers still have many options open to them for implementing the patterns within these boundaries.
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