4.8 Summary

If you're trying to decouple that service from your model and you feel like you're standing half-dressed after pulling a single thread a little too far, take heart. If you put the effort into building transparency into your application, you're likely to get where you intended to go, fully dressed and on time.

Some other decoupling techniques don't go far enough. Inheriting a service leads to awkward models that are tough to extend in other ways. Hardwiring services has a place, but it starts to be limiting as an application grows in scope. New programming models such as AOP may help you some day, but others (like heavyweight invasive containers) can kill you.

Instead, if you've got an exceptional need to decouple a service from your model, strive for transparency. Effective frameworks seem to be divided across three camps. All have relative strengths and weaknesses.

• Reflection is the tool of choice if you're building a lightweight transparent service. It's relatively easy to use and doesn't require any changes to your build process. The downside is performance but if you don't overuse it, reflection is fast enough for many applications.

• Enhancement techniques directly modify the byte code in your application to perform the appropriate task. They do change the build process, and may theoretically be difficult to debug. In practice, though, it's a high-performance technique that's growing in popularity. Some frameworks to provide persistence, obfuscation, and aspect-oriented weavers all use byte code enhancement, either at runtime or build time.

• Code generators are relatively easy to understand and use. They merge a template and data to give you working code. Frameworks like XDoclet use code generation less for transparency than to eliminate repetition, but you can use other code generation frameworks like MiddleGen to create services and transparent models, often without any user intervention beyond creating a few JavaDoc tags or an XML file.

Use these techniques to build transparent services. Don't forget that complete transparency can often burn you, as with CORBA. Instead, create transparent services to handle coarse-grained interfaces, such as façades, and fine-grained services, such as your model. Keep an eye firmly fixed on the future. While AOP languages and environments may still be a ways off, AOP techniques such as lightweight containers and interceptors will creep into the mainstream very quickly.

In this chapter, our focus was on using transparency to decouple services from your model. In the next chapter, you'll learn to choose the right tool for the job. It may sound like a trivial detail, but using poor technologies or abusing good ones has sunk thousands of Java projects.