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<h2 title="Interface Filter" class="title">Interface Filter</h2>
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<pre>public interface <span class="typeNameLabel">Filter</span></pre>
<div class="block">A filter is an object that performs filtering tasks on either the request to
a resource (a servlet or static content), or on the response from a resource,
or both. <br>
<br>
Filters perform filtering in the <code>doFilter</code> method. Every Filter
has access to a FilterConfig object from which it can obtain its
initialization parameters, a reference to the ServletContext which it can
use, for example, to load resources needed for filtering tasks.
<p>
Filters are configured in the deployment descriptor of a web application
<p>
Examples that have been identified for this design are<br>
1) Authentication Filters <br>
2) Logging and Auditing Filters <br>
3) Image conversion Filters <br>
4) Data compression Filters <br>
5) Encryption Filters <br>
6) Tokenizing Filters <br>
7) Filters that trigger resource access events <br>
8) XSL/T filters <br>
9) Mime-type chain Filter <br></div>
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<dt><span class="simpleTagLabel">Since:</span></dt>
<dd>Servlet 2.3</dd>
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<caption><span id="t0" class="activeTableTab"><span>All Methods</span><span class="tabEnd"> </span></span><span id="t2" class="tableTab"><span><a href="javascript:show(2);">Instance Methods</a></span><span class="tabEnd"> </span></span><span id="t3" class="tableTab"><span><a href="javascript:show(4);">Abstract Methods</a></span><span class="tabEnd"> </span></span></caption>
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<td class="colFirst"><code>void</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="../../javax/servlet/Filter.html#destroy--">destroy</a></span>()</code></th>
<td class="colLast">
<div class="block">Called by the web container to indicate to a filter that it is being
taken out of service.</div>
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<tr id="i1" class="rowColor">
<td class="colFirst"><code>void</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="../../javax/servlet/Filter.html#doFilter-javax.servlet.ServletRequest-javax.servlet.ServletResponse-javax.servlet.FilterChain-">doFilter</a></span>​(<a href="../../javax/servlet/ServletRequest.html" title="interface in javax.servlet">ServletRequest</a> request,
<a href="../../javax/servlet/ServletResponse.html" title="interface in javax.servlet">ServletResponse</a> response,
<a href="../../javax/servlet/FilterChain.html" title="interface in javax.servlet">FilterChain</a> chain)</code></th>
<td class="colLast">
<div class="block">The <code>doFilter</code> method of the Filter is called by the container
each time a request/response pair is passed through the chain due to a
client request for a resource at the end of the chain.</div>
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<tr id="i2" class="altColor">
<td class="colFirst"><code>void</code></td>
<th class="colSecond" scope="row"><code><span class="memberNameLink"><a href="../../javax/servlet/Filter.html#init-javax.servlet.FilterConfig-">init</a></span>​(<a href="../../javax/servlet/FilterConfig.html" title="interface in javax.servlet">FilterConfig</a> filterConfig)</code></th>
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<div class="block">Called by the web container to indicate to a filter that it is being
placed into service.</div>
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<h4>init</h4>
<pre>void init​(<a href="../../javax/servlet/FilterConfig.html" title="interface in javax.servlet">FilterConfig</a> filterConfig)
throws <a href="../../javax/servlet/ServletException.html" title="class in javax.servlet">ServletException</a></pre>
<div class="block">Called by the web container to indicate to a filter that it is being
placed into service. The servlet container calls the init method exactly
once after instantiating the filter. The init method must complete
successfully before the filter is asked to do any filtering work.
<p>
The web container cannot place the filter into service if the init method
either:
<ul>
<li>Throws a ServletException</li>
<li>Does not return within a time period defined by the web
container</li>
</ul></div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>filterConfig</code> - The configuration information associated with the
filter instance being initialised</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code><a href="../../javax/servlet/ServletException.html" title="class in javax.servlet">ServletException</a></code> - if the initialisation fails</dd>
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<h4>doFilter</h4>
<pre>void doFilter​(<a href="../../javax/servlet/ServletRequest.html" title="interface in javax.servlet">ServletRequest</a> request,
<a href="../../javax/servlet/ServletResponse.html" title="interface in javax.servlet">ServletResponse</a> response,
<a href="../../javax/servlet/FilterChain.html" title="interface in javax.servlet">FilterChain</a> chain)
throws java.io.IOException,
<a href="../../javax/servlet/ServletException.html" title="class in javax.servlet">ServletException</a></pre>
<div class="block">The <code>doFilter</code> method of the Filter is called by the container
each time a request/response pair is passed through the chain due to a
client request for a resource at the end of the chain. The FilterChain
passed in to this method allows the Filter to pass on the request and
response to the next entity in the chain.
<p>
A typical implementation of this method would follow the following
pattern:- <br>
1. Examine the request<br>
2. Optionally wrap the request object with a custom implementation to
filter content or headers for input filtering <br>
3. Optionally wrap the response object with a custom implementation to
filter content or headers for output filtering <br>
4. a) <strong>Either</strong> invoke the next entity in the chain using
the FilterChain object (<code>chain.doFilter()</code>), <br>
4. b) <strong>or</strong> not pass on the request/response pair to the
next entity in the filter chain to block the request processing<br>
5. Directly set headers on the response after invocation of the next
entity in the filter chain.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>request</code> - The request to process</dd>
<dd><code>response</code> - The response associated with the request</dd>
<dd><code>chain</code> - Provides access to the next filter in the chain for this
filter to pass the request and response to for further
processing</dd>
<dt><span class="throwsLabel">Throws:</span></dt>
<dd><code>java.io.IOException</code> - if an I/O error occurs during this filter's
processing of the request</dd>
<dd><code><a href="../../javax/servlet/ServletException.html" title="class in javax.servlet">ServletException</a></code> - if the processing fails for any other reason</dd>
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<h4>destroy</h4>
<pre>void destroy()</pre>
<div class="block">Called by the web container to indicate to a filter that it is being
taken out of service. This method is only called once all threads within
the filter's doFilter method have exited or after a timeout period has
passed. After the web container calls this method, it will not call the
doFilter method again on this instance of the filter. <br>
<br>
This method gives the filter an opportunity to clean up any resources
that are being held (for example, memory, file handles, threads) and make
sure that any persistent state is synchronized with the filter's current
state in memory.</div>
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