J2EE Design Pattern

1. Function
2. Non- Functional (Maintainablity,scalability,realiabilty,decoupling)

You should not start an application immediately but plan a solution for the problem. The Solution plan is called a design.

A Good design is very important for successful execution of projects.

When designing and developing different applications, we come across various problems. As a result, we end up finding new solutions for similar problems. To save time and effort, we should keep some proven solution to common problem areas.Such common soltutions are called Design patterns.

why do need to use design patterns?
Design patterns have a number of advantages like
1. Once described, any level engineer can use the pattern.
2. They allow for reuse without having to reinvent in every a project.
3. They allow to better define system structure.
4. They provide a design vocabulary.
5. They provide reusable artifacts.
6. Patterns can form frameworks that can then be used for implementations.

J2EE design patterns are grouped under five heads:
1. Client Tier
2. Presentation Tier Patterns
3. Business Tier Patterns
4. Integration Tier Patterns
5. Resource Tier

Client Tier :

This tier represents all device or system clients accessing the system or the application. A client can be a Web browser, a Java or other application, a Java applet, a WAP phone, a network application, or some device introduced in the future. It could even be a batch process.

Presentation Tier :

This tier encapsulates all presentation logic required to service the clients that access the system. The presentation tier intercepts the client requests, provides single sign-on, conducts session management, controls access to business services, constructs the responses, and delivers the responses to the client. Servlets and JSPs reside in this tier. Note that servlets and JSPs are not themselves UI elements, but they produce UI elements.

Business Tier :

This tier provides the business services required by the application clients. The tier contains the business data and business logic. Typically, most business processing for the application is centralized into this tier. It is possible that, due to legacy systems, some business processing may occur in the resource tier. Enterprise bean components are the usual and preferred solution for implementing the business objects in the business tier.

Integration Tier :

This tier is responsible for communicating with external resources and systems such as data stores and legacy applications. The business tier is coupled with the integration tier whenever the business objects require data or services that reside in the resource tier. The components in this tier can use JDBC, J2EE connector technology, or some proprietary middleware to work with the resource tier.

Resource Tier :

This is the tier that contains the business data and external resources such as mainframes and legacy systems, business-to-business (B2B) integration systems, and services such as credit card authorization.

Presentation Tier Patterns
Following is the list of Presentation Tier Design Patterns.
1. Intercepting Filter
2. Front Controller
3. Context Object
4. Application Controller
5. View Helper
6. Composite View
7. Service to worker
8. Dispatcher View

Business Tier Patterns
Following is the list of Business Tier Design Patterns.
1. Business Delegate
2. Service Locator
3. Session Façade
4. Application Service
5. Business Object
6. Composite Entity
7. Transfer Object
8. Transfer Object Assembler
9. Value List Handler

Integration Tier Patterns
Following is the list of Integration Tier Design Patterns.
1. Data Access Object
2. Service Activator
3. Domain Store
4. Web Service Broker