Introducing Enterprise Java Application Architecture and Design, Page 4
Figure 6 depicts the relationship between the three components. The events triggered by any user action are intercepted by the controller. Depending on the action, the controller invokes the model to apply suitable business rules that modify application data. The controller then selects a view component to present the modified application data to the end user. Thus, you see that MVC provides guidelines for a clean separation of responsibilities in an application. Because of this separation, multiple views and controllers can work with the same model.
Figure 6. Model-View-Controller
Java EE Architecture with MVC
The MVC concept can be easily applied to form the basis for Java EE application architecture. Java EE servlet technology is ideally suited as a controller component. Any browser request can be transferred via HTTP to a servlet. A servlet controller can then invoke EJB model components, which encapsulate business rules and also retrieve and modify the application data. The retrieved and/or altered enterprise data can be displayed using JSP. This is an oversimplified representation of real-life enterprise Java architecture, although it works for a small-scale application. But this has tremendous implications for application development. Risks can be reduced and productivity increased if you have specialists in the different technologies working together. Moreover, one layer can be transparently replaced and new features easily added without adversely affecting others (see Figure 7).
Figure 7. Layered multitier Java EE application architecture based on MVC
Layers in a Java EE Application
It is evident from Figure 7 that layered architecture is an extension of the MVC architecture. In the traditional MVC architecture, the data access or integration layer was assumed to be part of the business layer. However, in Java EE, it has been reclaimed as a separate layer. This is because enterprise Java applications integrate and communicate with a variety of external information system for business data—relational database management systems (RDBMSs), mainframes, SAP ERP, or Oracle e-business suites, to name just a few. Therefore, positioning integration services as a separate layer helps the business layer concentrate on its core function of executing business rules.
The benefits of the loosely coupled layered Java EE architecture are similar to those of MVC. Since implementation details are encapsulated within individual layers, they can be easily modified without deep impact on neighboring layers. This makes the application flexible and easy to maintain. Since each layer has its own defined roles and responsibilities, it is simpler to manage, while still providing important services.
Java EE Application Design
In the past few sections I laid the foundation for exploring Java EE application design in greater detail. However, the design of Java EE software is a huge subject in itself, and many books have been written about it. My intention in this article is to simplify Java EE application design and development by applying patterns and best practices through the Spring Framework. Hence, in keeping with the theme and for the sake of brevity, I will cover only those topics relevant in this context. This will enable me to focus on only those topics that are essential for understanding the subject.
Some developers and designers are of the opinion that Java EE application design is essentially OO design. This is true, but Java EE application design involves a lot more than traditional object design. It requires finding the objects in the problem domain and then determining their relationships and collaboration. The objects in individual layers are assigned responsibilities, and interfaces are laid out for interaction between layers. However, the task doesn't finish here. In fact, it gets more complicated. This is because, unlike traditional object design, Java EE supports distributed object technologies such as EJB for deploying business components. The business components are developed as remotely accessible session Enterprise JavaBeans. JMS and message-driven beans (MDB) make things even complex by allowing distributed asynchronous interaction of objects.
The design of distributed objects is an immensely complicated task even for experienced professionals. You need to consider critical issues such as scalability, performance, transactions, and so on, before drafting a final solution. The design decision to use a coarse-grained or fine-grained session EJB facade can have serious impact on the overall performance of a Java EE application. Similarly, the choice of the correct method on which transactions will be imposed can have critical influence on data consistency.