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Tomcat 6 Developer's Guide
A Valve is an atomic request processing component that is assembled into the Pipeline of a Container. The name Valve is a metaphor for a processing unit in a real world pipeline, where a valve controls and/or modifies what flows through it.

The org.apache.catalina.Valve interface defines methods for obtaining the next Valve in the pipeline; a backgroundProcess() method to execute a periodic task; and an invoke(Request, Response) method, which performs the actual processing associated with this valve.
A Valve may act on the request and response in a number of different ways, including:
Examining and/or modifying the properties of the incoming request or outgoing response.
Wrapping the request and/or response objects to supplement their functionality before passing them on.
Completing its task and invoking the next Valve in the sequence, if any, by calling getNext().invoke() with the incoming Catalina Request and Response.
Terminating request processing by generating and returning a complete...
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