Introduction
Servlet
filtering was introduced with the servlet API v2.3 in 2001. Filtering
is a powerful technology for servlet developers, who can use it to
generate chains of Java classes that execute in sequence in response
to client requests.
Developers begin by creating one or more Java classes that implement
the javax.servlet.Filter
interface. These classes can
undertake a number of actions prior to a servlet's
request handling, creating a chain of actions before the request is
delivered to its destination (including blocking the request
altogether). These actions include, according to the
Filter API documentation:
Register a filter in the deployment descriptor, and then map the
registered filter to either servlet names or URL patterns in your
application's deployment descriptor. When the web
container starts up your web application, it creates an instance of
each filter that you have declared in the deployment descriptor. The
filters execute in the order that they are declared in the deployment
descriptor.
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