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What does all this mean for developing web applications, specifically Spring MVC Simply put, you will find that your web applications will become simpler as your business logic is unencumbered by the framework. The web framework s role in your application will be minimized to providing translation layer between the HTTP world and your business domain world. You will write more POJOs, and you will build more successful object hierarchies and domain models. You will write more tests because it will be easier to isolate pieces of code. This will increase the quality of your code, and thus the application. Having a strong test suite will increase the confidence you have in the application functioning correctly. This confidence will allow you to refactor more, take risks, and react to evolving business requirements faster.

Summary

We reintroduced the Spring Framework and pointed out the three concepts we consider most important to understand. Dependency Injection, a type of Inversion of Control, is a mechanism to insert dependencies into application code. The application code is a passive participant, allowing it to remain free of framework-specific code. By relying on Dependency Injection, the application does not manage dependency creation or have to actively locate it. The application code becomes very testable. The ApplicationContext is Spring s main object registry and integration point. It is usually configured via an XML file in which beans and their dependencies are declared. The ApplicationContext has many features, but its central role is object creation and Dependency Injection. In most cases, the ApplicationContext will be a transparent piece of our applications, freeing our application logic from Spring-specific integration. Finally, all of this has been leading up to an emphasis on developing systems with plain old Java objects. The POJO is a simple concept with powerful repercussions. Using strong object-oriented development techniques, systems developed with POJOs are testable and flexible. Most importantly, they allow the developer to concentrate on business logic and the problem domain instead of how to deal with the framework. In the next chapter, we will cover principles for web application design, including typical layers and where the Spring Framework fits in.

efore we begin our exploration of the internals of Spring MVC, it is important to discuss how a typical Spring MVC is built. In this chapter, we will answer such questions as, Where should the business logic live and What are the correct levels of abstractions We will present the entire picture of a Spring MVC web application to better explain the individual roles that Spring MVC plays and where it fits in the overall architecture. Mastering a new framework requires more than studying its APIs. Examining and understanding the architecture of a complete Spring MVC web application provides you with clues and motivations for the design of the framework itself. This allows for a higher level of understanding, allowing you to make more contextually sound choices when building your application.

Spring MVC applications are broken down into a series of layers. We consider a layer to be a discrete, orthogonal area of concern within an application. For instance, all of the persistence code is considered a separate layer from the view rendering code. Layers are abstractions within an application, and interfaces provide the contract by which layers interact. Some layers might be well hidden, used only by the layer immediately above it. In contrast, the most important layer (the domain model itself) spans nearly all the other layers in the system. Layers are conceptual boundaries and are not necessarily physically isolated. More often than not, all of the layers will be located within the same virtual machine for a web application. For a good discussion on application distribution, consult Rod Johnson s Expert One-on-One J2EE Design and Development (Wrox, 2002).

Note Are layers the same thing as tiers Many people use the two terms interchangeably, but separating the two helps when discussing the application and its deployment. A layer is a logical abstraction within an application. A tier is best thought of as a physical deployment of the layers. Thinking in layers can help the software developer, while thinking in tiers can assist the system administrator. Layers are mapped onto tiers.