JSP Architecture

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There are two basic ways of using the JSP technology. They are the client/server (page-centric) 2-tier approach and the N-tier approach (dispatcher).

The Page-Centric Approach

Applications built using a client-server (2-tier) approach consist of one or more application programs running on client machines and connecting to a server-based application to work. With the arrival of Servlets technology, 2-tier applications could also be developed using Java programming language. This model allows JSPs or Servlets direct access to some resource such as database or legacy application to service a client's request. The JSP page is where the incoming request is intercepted, processed and the response sent back to the client. JSPs differ from Servlets in this scenario by providing clean code, separating code from the content by placing data access in EJBs. Even though this model makes application development easier, it does not scale up well for a large number of simultaneous clients as it entails a significant amount of request processing to be performed and each request must establish or share a potentially scarce/expensive connection to the resource in question.

Page-view - This basic architecture involves direct request invocations to a server page with embedded Java code, and markup tags which dynamically generate output for substitution within the HTML. This approach has been blessed a number of benefits. It is very straightforward and is a low-overhead approach from a developerment perspective. All the Java code may be embedded within the HTML, so changes are confined to a very limited area, reducing complexity drastically.

The big trade-off here is in the level of sophistication. As the scale of the system grows, some limitations begin to surface, such as bloating of business logic code in the page instead of factoring forward to a mediating Servlet or factoring back to a worker bean. It is a fact that utilizing a Servlet and helper beans helps to separate developer roles more cleanly and improves the potential for code reuse.

Page-view with bean - This pattern is used when the above architecture becomes too cluttered with business-related code and data access code. The Java code representing the business logic and simple data storage implementation in the previous model moves from the JSP to the JavaBean worker. This refactoring leaves a much cleaner JSP with limited Java code, which can be comfortably owned by an individual in a web-production role, since it encapsulates mostly markup tags.

The Dispatcher Approach

In this approach, a Servlet or JSP acts as a mediator or controller, delegating requests to JSP pages and JavaBeans. There are three different architectures. They are mediator-view, mediator-composite view and service to workers.

In an N-tier application, the server side of the architecture is broken up into multiple tiers. In this case, the application is composed of multiple tiers, where the middle tier, the JSP, interacts with the back end resources via another object or EJBs component. The Enterprise JavaBeans server and the EJB provide managed access to resources, support transactions and access to underlying security mechanisms, thus addressing the resource sharing and performance issues of the 2-tier approach.

The first step in N-tiered application design should be identifying the correct objects and their interaction and the second step is identifying the JSPs or Servlets. These are divided into two categories.

Front end JSPs or Servlets manage application flow and business logic evaluation. They act as a point to intercept the HTTP requests coming from the users. They provide a single entry point to an application, simplifying security management and making application state easier to maintain.

Presentation JSPs or Servlets generate HTML or XML with their main purpose in life being presentation of dynamic content. They contain only presentation and rendering logic.

These categories resemble to the Modal-View design pattern, where the front-end components is the model and the presentation component the view. In this approach, JSPs are used to generate the presentation layer and either JSPs or Servlets to perform process-intensive tasks. The front-end component acts as the controller and is in charge of the request processing and the creation of any beans or objects used by the presentation JSP, as well as deciding, depending on the user's actions, which JSP to forward this request to. There is no processing logic within the presentation JSP itself and it simply responsible for retrieving any objects or beans that may have been previously created by the Servlet and extracting the dynamic content for insertion within static templates.