Core J2EE Patterns

You have components and services that need access to system information.
You want to decouple application components and services from the protocol specifics of system information.
You want to expose only the relevant APIs within a context.

Solution

Use a Context Object to encapsulate state in a protocol-independent way to be shared throughout your application.

Class Diagram

Sequence Diagram

Strategies

Request Context Strategies
- Request Context Map Strategy
- Request Context POJO Strategy
- Request Context Validation Strategy
Configuration Context Strategies
- JSTL Configuration Strategy
- Security Context Strategy
General Context Object Strategies
- Context Object Factory Strategy
- Context Object Auto-Population Strategy

Consequences

Improves reusability and maintainability
Improves testability
Reduces constraints on evolution of interfaces
Reduces performance

Related Patterns

Intercepting Filter
An Intercepting Filter can use a ContextFactory to create a Context Object during web request handling.
Front Controller
A Front Controller can use a ContextFactory to create a Context Object during web request handling
Application Controller
An Application Controller can use a ContextFactory to create a Context Object during web request handling
Transfer Object
A Transfer Object is used specifically to transfer state across remote tiers to reduce network communication, while a Context Object is used to hide implementation details, improving reuse and maintainability.

Excerpted from Core J2EE Patterns, 2nd Edition. See the book for full description of the pattern including class diagrams, sequence diagrams and sample code.