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E++: A pattern language for J2EE applications, Part 1

Build better J2EE applications with a high-level pattern language

By Bin Yang, JavaWorld.com, 04/20/01

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We'll learn how to solve this problem using the E++ pattern language in the following sections.

The pattern language in detail and practice

The E++ pattern language, as depicted in Figure 2, follows a treelike topology. Each circle denotes a pattern, with arrows representing the relationships among patterns. Architectural patterns are green-colored to indicate the beginning of the language. With E++, building a J2EE framework becomes as easy as following the tree network from the root to the leaves.

Figure 2: The E++ pattern language
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The Layered Architecture discusses the whys and how-tos of partitioning application functionality into tiers; it is a general guideline for arranging large-scale applications. In light of the Layered Architecture, the MVC architecture shows that classes and objects of the B2C portion can be effectively organized into three major functionality subsystems: model, view, and control. On the B2B side, the REAI architectural and related design patterns deal with distributed integrations. Before delving into the language details, let us work through the language to show how the patterns work together.

B2C flow

In our scenario, a customer accesses the TSP Website using a Web browser, hits the items of interest, and generates HTTP requests to the Web server, which directs all the requests to the Application Mediator, with the mediator working as a single entry point to the application. To respond to the requests, the mediator dispatches the customer's requests to the JSP Template pattern, which serves as a layout manager and produces all the required JSP/HTML files for the customer's browsing. As the name suggests, the Template Method combines with JSP Tags and JSP Pages to handle the presentation scenario. Following that, it independently forwards the customer's requests to the Event Processor pattern.

If the requests require changing application models, such as ordering, accounting, and billing HTTP requests, the processor interprets the requests to Java events using the Event pattern and then delegates the corresponding tasks to the EJB tier through the EJB Facade by sending these events. The EJB Facade pattern acts as single entry point to the EJBs, as well as controls and synchronizes concurrent access to the EJB layer. Note that the Event pattern is essentially the Java delegation event model, which allows indirect method invocations. The Event pattern has been widely used as a better application control alternative in Java 1.1 and later versions.

The EJB Facade forwards all the events to the EJB Controller, a stateful session EJB representing the customer at the EJB tier. The EJB Controller simplifies the processing and leverages the State Machine pattern to change application state. The machine then uses the Command pattern to fulfill the event processing and create or change the Model EJB or Session EJB Facade. The Model EJB encapsulates business logic, while the Session EJB Facade offers an effective way, in terms of network traffic, to update database tables. The Value Object pattern improves response time to access coarse-grained data, while the Flyweight Object pattern reduces the number of objects in memory. The Data Access Strategy and Connection Factory patterns access relational databases in a flexible and extensible way. Finally, the Data Iterator pattern provides a practical way to fetch large chucks of read-only data. Through these patterns, the application model at the EJB tier is changed or updated.

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