J2EE™ Tutorial, The

Filtering Requests and Responses

A filter is an object that can transform the header or content or both of a request or response. Filters differ from Web components in that they usually do not themselves create a response. Instead, a filter provides functionality that can be “attached” to any kind of Web resource. As a consequence, a filter should not have any dependencies on a Web resource for which it is acting as a filter, so that it can be composable with more than one type of Web resource. The main tasks that a filter can perform are as follows:

• Query the request and act accordingly.

• Block the request-and-response pair from passing any further.

• Modify the request headers and data. You do this by providing a customized version of the request. ...