Intercepting Filter

See Core J2EE Patterns, 2nd Edition for full description of this pattern and its strategies.

Problem

You want to intercept and manipulate a request and a response before and after the request is processed.

Forces

• You want centralized, common processing across requests, such as checking the data-encoding scheme of each request, logging information about each request, or compressing an outgoing response.
• You want pre and postprocessing components loosely coupled with core request-handling services to facilitate unobtrusive addition and removal.
• You want pre and postprocessing components independent of each other and self contained to facilitate reuse.

Solution

Use an Intercepting Filter as a pluggable filter to pre and postprocess requests and responses. A filter manager combines loosely coupled filters in a chain, delegating control to the appropriate filter. In this way, you can add, remove, and combine these filters in various ways without changing existing code.

Class Diagram

Sequence Diagram

Strategies

• Standard Filter Strategy
• Custom Filter Strategy
• Base Filter Strategy
• Template Filter Strategy
• Web Service Message Handling Strategies
  • Custom SOAP Filter Strategy
  • JAX RPC Filter Strategy

Consequences

• Centralizes control with loosely coupled handlers
• Improves reusability
• Declarative and flexible configuration
• Information sharing is inefficient

Related Patterns

• Front Controller
  The controller solves some similar problems, but is better suited to handling core processing.
• Decorator [GoF]
  The Intercepting Filter is related to the Decorator, which provides for dynamically pluggable wrappers.
• Template Method [GoF]
  The Template Method is used to implement the Template Filter strategy.
• Interceptor [POSA2]
  The Intercepting Filter is related to the Interceptor, which allows services to be added transparently and triggered automatically.
• Pipes and Filters [POSA1]
  The Intercepting Filter is related to Pipes and Filters.

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