6.5 Summary

Building an application that's easy to extend does not happen by accident. It takes patience, forethought, and effort. You've got to design your external interfaces well so programmers can extend your application. You've got to consider an effective configuration strategy and a plug-in model so administrators and users can extend your solution after deployment. You also have to make a deliberate decision about whether investing in extension makes sense at all.

You saw the basic toolkit for extension, including open standards, a configuration strategy, and class loading. I suggested three tools for configuration: Java Preferences, Apache Digester, and Spring. Each of these tools has strong support for the core features that you need, and each targets a different market. Preferences is the simplest configuration API, and is most appropriate on the client. Digester uses a SAX-based XML engine, and lets you specify patterns that fire rules. It's simple and you can understand it quickly. Spring is a framework with a unified configuration strategy. You need not use all of Spring to take advantage of its services. These basic tools combine nicely to support an extension model called the plug-in. You saw that plug-ins can use two different models: the service locator or dependency injection. I favor dependency injection, though some experts, including Martin Fowler, prefer the service locator. Each has strengths and weaknesses.

Finally, effective configuration and extension depends on an intimate knowledge of your customer. Your extension paradigm needs to reflect your cycle time and the needs of your end customer.

You've now seen each of the five principles for better, faster, lighter Java:

• Keep it simple

• Do one thing, and do it well

• Strive for transparency

• You are what you eat

• Allow for extension

I don't want to leave you hanging: in the next chapters, you'll see these principles in action. First, you'll see two of my favorite open source frameworks, written by third-party developers that I know and trust. Next, you'll see how to build a sophisticated open source service, using these principles. Along the way, we'll examine a fantastic variety of traditional enterprise web applications, a lightweight container, a persistence engine, and a web crawler. As you internalize these principles you'll see that they can be used for many different purposes.