A filter is a servlet that can transform the content of HTTP requests, responses, and header information. Filters do not generally create a response or respond to a request as servlets do, rather they modify or adapt the requests for a resource, and modify or adapt responses from a resource. Filters can act on dynamic or static content.

Filters are typically used for the following:

• Accessing of a resource before a request to it is invoked.
• Processing of the request for a resource before it is invoked.
• Modification of request headers and data by wrapping the request in customized versions of the request object.
• Modification of response headers and response data by providing customized versions of the response object.
• Interception of an invocation of a resource after its call.
• Actions on a servlet, on groups of servlets, or static content by zero, one, or more filters in a specifiable order.