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CHAPTER 3 SPRING MVC APPLICATION ARCHITECTURE
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Thinking in layers can help conceptualize the flow through an application. Visualizing the application s layers as a cake (layers of cake stacked one on another) is a common and convenient way to illustrate how the application is organized. Typical metaphors such as down into persistence and back up to the user interface refer to a cake, and denote a sense of vertical direction, reinforcing the metaphor. Figure 3-1 illustrates the common, highly generalized layers for web applications. Top Layer (User Interface, Web) Middle Layer (Service, Domain Model) Bottom Layer (Persistence) Figure 3-1. General, high-level layers in a web application Typically, any persistence functionality is at the bottom of the cake, while the user interface is at the top. What is found in the middle, and how it is organized, is the subject of this chapter. We will use this metaphor when explaining our architecture. Breaking it down further, typical Spring MVC applications have at least five layers of abstraction that you as a developer will code to. The layers are user interface web service domain object model persistence You might notice that common applications elements, such as transaction management or security, are not in the preceding list. If you are familiar with the Spring Framework and its extensive use of aspect-oriented programming (AOP), this won t come as a surprise. Transaction management, for instance, is considered a transparent aspect of a system, not a full layer. Figure 3-2 more specifically illustrates the relative placement of the different layers.
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User Interface Domain Model Web Service Persistence Figure 3-2. Spring MVC application layers
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CHAPTER 3 SPRING MVC APPLICATION ARCHITECTURE
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You will notice that the domain model vertically spans all the other layers. This is because all the other layers have a dependency on the domain model. It is the only layer that crosscuts all the rest.
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Layer Isolation
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Isolating problem domains, such as persistence, web navigation, and user interface, into separate layers creates a flexible and testable application. Implementations of each layer will vary independently, increasing the flexibility of the application. Decreasing the coupling between areas of the application will increase the testability, making it easier to test each part of the application in isolation. This isolation is accomplished by minimizing the amounts of dependencies between the layers. The fewer dependencies a layer has upon itself, the less costly it is to change that layer. It is a best practice to ensure that a layer is required only by one or two other layers. Avoid having one single layer required by many different parts of the application.1 You can avoid dependency explosion in at least two ways. If a layer begins to employ too many layers, consider creating a new layer of abstraction wrapping all the previous interactions. On the other hand, if you find that a layer has permeated throughout many layers, consider if that layer is itself an aspect of the system. If the functionality can be applied across great swaths of the system transparently, use Spring s AOP functionality to remove the explicit dependency from your code. The important point to remember is that one of the great benefits of layering an application is it creates a decoupled design. When you discover a layer or facet of the application that is too intrusive, refactor it to another abstraction or through AOP This will keep your applica. tion flexible and testable.
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Java Interface As Layer Contract
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The Java interface is the key enabler to building an application with layers. The interface is a contract for a layer, making it easy to keep implementations and their details hidden while enforcing correct layer usage. The benefits of low coupling provided by interfaces have been well known for some time. Their full benefits have been hindered because the instantiation of concrete types was still required. The promise of implementation abstraction wasn t quite realized this is, before Spring and other Dependency Injection frameworks. Spring helps interfaces truly shine, because it handles the creation of the objects instead of your application code. Treating an interface as a contract between layers is very helpful in large team settings. Coordinating the many resources often required by large projects is difficult, and it is rare that integration between layers happens precisely. Interfaces help to speed development between teams because of their lightweight nature. Developers program against the interface, while its implementation continues to be built and tested. On a practical level, the Spring Framework works especially well with interfaces. Its AOP facilities are built around JDK proxies,2 making it easy to extend an implementation of an interface with additional services.
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1. Exception to this rule is the domain model, which typically spans many layers. 2. Spring can weave aspects into classes without interfaces, but a few caveats are involved. For one, the use of cglib (http://cglib.sourceforge.net) is required.
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