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<h1 class="topictitle1">Listeners</h1>
<div>
<p>Filters are new in the <cite>Sun Microsystems Java Servlet
2.3 Specification</cite>. They enable you to intercept to intercept a
request before it reaches a resource. In other words, a filter gives you
access to the HttpServletRequest and the HttpServletResponse objects
before they are passed on to a servlet.</p>
<p>Filters can be very useful. For example, you can write a filter
that records all incoming requests and logs the IP addresses of the
computers from which the requests originate. You also can use a filter
as an encryption and decryption device. Other uses include user
authentication, data compression, user input validation, and so on.</p>
<p>You also can put a set of filters in a chain. The first filter in
the chain will be called first and then pass control to the second
filter, and so on. Filter chaining ensures that you can write a filter
that does a specific task but adds some functionality in another filter.</p>
<div class="p">When writing a filter, you basically deal with the
following three interfaces in the javax.servlet package:
<ul>
<li><strong>javax.servlet.Filter:</strong> represents the
life-cycle of the filter.</li>
<li><strong>javax.servlet.FilterConfig:</strong> represents the
configuration of the filter.</li>
<li><strong>javax.servlet.FilterChain:</strong> object provided by
the servlet container to the developer giving a view into the
invocation chain of a filtered request for a resource.</li>
</ul>
</div>
<p>For more information about filters, refer to the <cite>Sun Microsystems Java™ Servlet
2.3 Specification</cite> at <samp class="codeph">java.sun.com/products/servlet/download.html</samp>.</p>
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<div><div class="relconcepts"><strong>Related concepts</strong><br />
<div><a href="../topics/cwservbn.html" title="">Servlets</a></div>
<div><a href="../topics/cwlistbn.html" title="">Listeners</a></div>
<div><a href="../topics/ccwtover.html" title="">Web application overview</a></div>
</div>
<div class="reltasks"><strong>Related tasks</strong><br />
<div><a href="../topics/twsrvwiz.html">Creating Servlets</a></div>
<div><a href="../topics/twlstwiz.html">Creating Listeners</a></div>
<div><a href="../topics/twfltwiz.html">Creating Filters</a></div>
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