JRun Overview of filters

http://livedocs.adobe.com/jrun/4/Programmers_Guide/filters2.htm
Filters process request objects before they get to the server or process response objects after they leave the server and before they are returned to the client. You can use filters to do the following tasks:

Take flow-control logic out of web application components.
Examine and modify the request object before the server receives it from the client.
Examine and modify the response object before the client receives it from the server.
Change the content of the response.
...

Specifically, filters are often implemented as:
Request dispatchers
User authenticators and authorizers
Request and response loggers and auditors
Form validators
Image converters
Data compressors and decompressors
Data encryptors and decryptors
Response output tokenizers
Triggers for resource access
XML transformers
Content localizers