Some examples of this type of design pattern are the Model-View-Controller (MVC) pattern, the Value Object J2EE pattern, and the Data Access Object (DAO) J2EE pattern.

Isolated use refers to design patterns for which the use of the design pattern is a hidden detail. Some qualities of this type of design pattern are:

Some examples of this type of design pattern are the Singleton GoF pattern or the Intercepting Filter J2EE pattern.

Dividing design patterns into these categories provides a quick approach for understanding the scope of the design pattern. Understanding scope makes it easy to assess the impact of the design pattern. Is it a design pattern that can be used and discarded? Is it a design pattern that once adopted will impact the design of the application? Is it a design pattern that affects multiple parts of my application and possibly other applications? Understanding these impacts up front provides guidance for adopting the design pattern.

Design Pattern Adoption AntiPatterns
As design patterns grew in popularity, another type of patterns emerged called AntiPatterns. Whereas design patterns offer expert knowledge on repeatable best approaches, AntiPatterns describe often repeated behavior that should be avoided. AntiPatterns prove there is as much to be learned from people who do things wrong as those who do things right.

In this section, we'll look at AntiPatterns that apply to the adoption of design patterns. Understanding these AntiPatterns should help you avoid the pitfalls of design pattern adoption. Like design patterns, the AntiPatterns here may be entirely new to you, in which case they provide some foresight; or they may be situations you are somewhat familiar with, in which case they can add color to your experiences and let you know that you're not alone. If you are interested in reading more AntiPatterns, see the resource list at the end of this article.

AntiPattern Checklist
Design Patterns? Yes, We Have All of Them
Problem: Determining which design patterns to include on a project.

Applies To: Both broad exposure and isolated use design patterns.

Context: A developer has been introduced to design patterns and wants to use them on a project.

Forces: Two forces are usually the source for this AntiPattern. One is the desire of the developer to improve the project by including best practices from design patterns. The other is that innate curiosity drives the developer to use the project for research on design patterns.

Supposed Solution: All known design patterns are used on the project. A checklist is formed from the design pattern book and the motivating goal is to be able to check off all of the design patterns.

Resulting Context: The project team and the delivered application suffer from the unnatural introduction of too many design patterns. This results in designs and development that are overly complex. This unnecessary complexity can impact the development effort in terms of the amount of work that has to be done, the ability of the development team to understand what is happening, and the actual performance and functional correctness of the application.

Design Rationale: Design patterns are a great source of expert knowledge. If using them is good, then using all of them is mistakenly thought of as better.

Real Solution: The description of a design pattern includes the context for which the pattern is targeted. Thought must be given to ensure that the design pattern is a match for the project. Second, design patterns are often incorporated not from someone reading a design pattern book and asking "where can I use this design pattern?" but from someone looking for a solution to a problem they have discovered.

Implementation per Developer/Project AntiPattern
(Also known as: Design pattern xyz? Yeah, we have 10 of those.)

Problem: Controlling the implementation of design patterns within a project and across projects.

Applies To: Both broad exposure and isolated use design patterns benefit from addressing this situation. However, broad exposure design patterns should definitely have controlled implementations.

Context: The development team is incorporating design patterns into the project. The team consists of many experienced developers who understand when to use design patterns. The correct design patterns are being used. If multiple projects are involved, there is no design pattern - implementation sharing between projects.

Forces: There are tight deadlines and the team is very efficient individually. Team efficiency might be impacted if they have to reuse implementations. It is assumed that since they are experts, their implementation will be good anyway. In the case of multiple projects, the cross-team communication and code sharing either is not considered or is dismissed as a potential impact on schedules.

Supposed Solution: The team can include and implement the design patterns they need as individuals.

Resulting Context: Even though the correct design patterns were used, they were implemented in many different ways. There are incompatibilities between the implementations that restrict integration and reuse efforts. More time and effort than necessary will have to be spent maintaining, debugging, and extending the various implementations. Eventually the various implementations will have to be consolidated.

Design Rationale: A team of experts should be allowed to work alone. Including design patterns is good enough, there is no reason to share implementations.

Real Solution: The development team should coordinate their use of design patterns. Sharing a common implementation of a design pattern can lower costs in the future, but more important, it should result in cross-developer compatibility today. If necessary, this sharing could be limited to design patterns that fall into the broad exposure category we discussed earlier. Reusing implementations is also valuable across projects, especially if they are expected to integrate in the future.

Role of IDEs in Design Pattern Adoption
IDEs continue to evolve and offer more functionality. Originally IDEs consisted of an editing environment and some debugging facilities. Now they often include a design environment, auditing facilities, integration with configuration management systems, and more. As IDEs expand their reach, their role in implementing design patterns needs to be identified. Of course, design patterns are implemented in a development language and IDEs can be used for editing the source code. But what other roles can IDEs play?

Some IDEs have pull-down menus that allow you to select design patterns to include in your application. While this can expedite the inclusion of a design pattern, it might just result in writing bad code faster. There are a couple of factors to remember in evaluating this feature.

One is that design patterns describe problems in the abstract and require some interpretation to arrive at the correct implementation. However, they often include a "sample implementation" and it is this sample class structure that the IDEs insert into your application. This is probably not the implementation that you need and just dropping it into your application will introduce more confusion and require more editing and refactoring than just putting thought into your own original implementation.

Another problem with the IDE drag-and-drop design pattern approach is the two AntiPatterns we already discussed. Expediting the implementation of design patterns is more likely to result in rampant inclusion of design patterns and multiple versions of the same design pattern than it is to solve any problems.

The challenge with design patterns isn't just in getting a quick implementation, but making sure you are using a correct implementation, and ideally an implementation that already exists in your organization.

BEA WebLogic Workshop 8.1 and Design Patterns
Since you are probably a BEA customer if you are reading this article, you may be wondering how the new BEA WebLogic Workshop 8.1 impacts your design pattern considerations. First of all, WebLogic Workshop is an IDE so the previous section on IDEs applies. The two additional aspects of Workshop that are interesting for this discussion are Controls and pre-implemented design patterns.

WebLogic Workshop Controls are a way to package functionality so that it can be easily included in your application using the Workshop IDE. The packaging includes the visual elements required for the IDE, the runtime behavior, the configuration required, and more. How do Controls impact your inclusion of design patterns? Remember the earlier segregation of design patterns into isolated use and broad exposure? Design patterns that fall into the isolated use category may be candidates to be packaged as Workshop Controls. Packaging the design pattern as a Control would allow other users of the Workshop IDE to share an implementation, avoiding the Implementation per Developer/Project AntiPattern.

You may be wondering why the broad exposure category cannot be implemented as Controls. The reason is that the broad exposure design patterns result in the creation of many other classes or in some dependence with other applications. This situation does not lend itself to the plug-and-play aspect of Controls. Adoption of the broad exposure design patterns should be done with much thought and consideration, and once adopted cannot be undone easily. These requirements do not match with the WebLogic Workshop Control goals.

WebLogic Workshop also comes with many pre-implemented design patterns, such as Pageflow and the user interface structure. From within Workshop, you can create JSPs and define Pageflows to control the navigation between the pages of your Web application. Under the hood, WebLogic Workshop uses the popular Apache Struts presentation layer framework. This aspect of Workshop (which uses Struts) provides a Model-View-Controller (MVC) design pattern implementation, which means you won't have to create your own MVC implementation. Workshop includes other functionality that will probably replace your own design pattern implementations. While it is nice to get some of these design patterns implemented out-of-the-box, you should validate that you are comfortable with not only the implementation but with any dependency on WebLogic Workshop that the implementation creates.

Three Steps for Successful Design Pattern Adoption
So how do you merge the adoption of design patterns with the tight deadlines, scaled back budgets, and limited team resources that exist in many companies today? The following are three steps to follow to make your design pattern experience successful.

Strong Communication and Education
Many organizations have technical leadership, possibly formalized as an architects forum or informally as the recognized experts. These leaders should promote open communication around design pattern adoption and should educate the teams about specific design patterns. The communication should be across development teams and projects to proactively prevent adoption silos and multiple unique implementations (remember the Implementation per Developer/Project AntiPattern). The education can take the form of informal internal lunch-and-learns, formalized internal classes, or maybe even sending a few people to external training. Any of these education options should promote correct applications of design patterns. If only a limited audience can attend the training, the best candidates are ones who feel comfortable educating their peers upon return.

Mentored Design Pattern Adoption
Design patterns can be used for the benefit of the project but they are also easy to misuse to the detriment of the application. Their use should be encouraged but their adoption should be reviewed and verified. Design patterns can be included during both design and development. In either case, design patterns' use should be identified and verified by the reviewer. Also during this review, situations may be encountered where additional design patterns may be applicable that weren't originally included. Even if official reviews are not done in your environment, this step could be accomplished through peer reviews or team discussions. The reviewers in this step should be either part of the leadership team or in open communication with them.

Mentoring adoption is especially critical for the broad exposure category of design patterns. These are the design patterns that have more associated risk because they will create dependencies. These dependencies may be within a class of objects, such as data access objects (DAOs) that only work within the scope of a broader DAO design pattern implementation, or across application boundaries, such as use of the Value Object design pattern to communicate data between applications and application layers. These design patterns are also ones that may have been implemented by somebody else on the project or by a different project, and that implementation should be reused versus creating another unique implementation.

Reuse Implementations, Not Just Design Patterns
While there is a certain satisfaction that comes in creating your own implementation of a design pattern, there is more benefit to the team and company when reuse takes place at the code level and not just the design idea level. The initial benefit design patterns provide to an organization is improved implementations. The real goal, however, should be to reuse implementations. Reusing implementations will result in: a) other reusable classes (that depend on the common implementation); b) lower development time and costs; c) lower maintenance time and costs; and d) easier integration both within and between applications.

This reuse is more important (and sometimes essential) for the broad exposure category. These design patterns are the ones that create external dependencies (where integration would benefit from a common implementation) or result in a whole library of custom classes (that would be reusable if there was a common foundation). The isolated use category would benefit from reuse but they might be more difficult to reuse if they were strongly customized to their specific situation.

Something you may ask yourself is, "If reuse is better, why don't design patterns come with a reusable implementation?" We touched on this when we talked about how design patterns are provided for the benefit of the largest audience. Achieving this goal of broad applicability would be difficult, if not impossible, if it came with a predetermined implementation. However, once the design patterns are applied to a particular problem domain or technical infrastructure, the resulting implementation may be reusable within that environment.

Design Patterns Through Architecture
It may seem a daunting task to understand how design patterns apply to your situation, build a quality implementation, and then encourage the reuse of that implementation. One approach to this is the introduction of application architecture into your environment. Application architecture provides the structure required for your application so that the development team can focus on the domain logic of the application. This includes implemented design patterns. Instead of reusing design pattern concepts or individual implementations, the architecture can be reused across projects and applications. This shared common implementation ensures compatibility and provides a lower cost alternative to developing and maintaining many different implementations. The compatibility provides the required technical foundation for reuse efforts to be possible. There is not enough space here to go into the other valuable qualities of architecture, such as runtime monitoring and management, configurable application logic, and adaptable behavior. You can learn more about architecture from the Carnegie Mellon Software Engineering Institute (www.sei.cmu.edu/ata/ata_init.html).

Summary
Design patterns are an amazing resource that should be used to your advantage. While design patterns provide reusable concepts, the challenges come in determining which design patterns to use and working towards a reusable implementation. By understanding some of the risks involved with design pattern adoption, you can hopefully avoid them as you continue to learn and implement more design patterns.

Following the steps outlined in this article will put a process in place that can be used to promote the successful adoption of design patterns on your team and within your organization.

Resources