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<section><a id="Overview"></a>
<h1>Overview</h1>

<div class="toc-panel">
<input type="checkbox" id="toc-toggle" class="toc-toggle-checkbox" checked="checked" />
<label for="toc-toggle" class="toc-toggle-arrow" title="Collapse">&#160;</label>

<div class="toc-content">

<p class="toc-heading">On This Page</p>

<ul class="toc-list">

<li><a href="#Overview">Overview</a></li>

<li><a href="#Writing_Filters">Writing Filters</a></li>

<li><a href="#Writing_TreeWalkerFilters">Writing TreeWalkerFilters</a></li>

<li><a href="#Using_TreeWalkerFilters">Using TreeWalkerFilters</a></li>

<li><a href="#Declare_check.27s_external_resource_locations"> Declare check's external resource locations</a></li>

<li><a href="#Using_Filters">Using Filters</a></li>

<li><a href="#Huh.3F_I_can.27t_figure_it_out.21">Huh? I can't figure it out!</a></li>

<li><a href="#Contributing">Contributing</a></li>
</ul>
</div>
</div>

<p>
A <code>Checker</code> has a set of <code>Filter</code>s that decide which audit events the <code>Checker</code> reports through its listeners. Interface <code>Filter</code> and class <code>FilterSet</code> are intended to support general <code>AuditEvent</code> filtering using a set of <code>Filter</code>s. </p>


<p>
A <code>Filter</code> has <code>boolean</code> method <code>accept(AuditEvent)</code> that returns <code>true</code> if the <code>Filter</code> accepts the <code>AuditEvent</code> parameter and returns <code>false</code> if the <code>Filter</code> rejects it. </p>


<p>
A <code>FilterSet</code> is a particular <code>Filter</code> that contains a set of <code>Filter</code>s. A <code>FilterSet</code> accepts an <code>AuditEvent</code> if and only if all <code>Filter</code>s in the set accept the <code>AuditEvent</code>. </p>


<p>
Here is a UML diagram for interface <code>Filter</code> and class <code>FilterSet</code>. </p>


<div class="wrapper">
<img src="images/Filter.png" alt="Filter UML diagram" /> </div>


<p>
Checkstyle also provides the <code>TreeWalkerFilter</code> interface for filters that must live inside the <code>TreeWalker</code> module. See <a href="#Writing_TreeWalkerFilters">Writing TreeWalkerFilters</a> below. </p>
</section>

<section><a id="Writing_Filters"></a>
<h1>Writing Filters</h1>

<p>
The <code>Filter</code> that we demonstrate here rejects audit events for files whose name matches a <a href="property-types.html#Pattern">Pattern</a>. In order to enable the specification of the file name pattern as a property in a configuration file, the <code>Filter</code> is an <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AutomaticBean.html">AutomaticBean</a> with mutator method <code>setFiles(String)</code> that receives the file name pattern. An <code>AutomaticBean</code> uses JavaBean introspection to set JavaBean properties such as <code>files</code>. </p>


<div class="wrapper">
<pre class="prettyprint"><code class="language-java">
package com.mycompany.filters;

import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.regex.PatternSyntaxException;

import com.puppycrawl.tools.checkstyle.api.AuditEvent;
import com.puppycrawl.tools.checkstyle.api.AutomaticBean;
import com.puppycrawl.tools.checkstyle.api.Filter;
import com.puppycrawl.tools.checkstyle.api.Utils;

public class FilesFilter
    extends AutomaticBean
    implements Filter
{
  private Pattern mFileRegexp;

  public FilesFilter()
      throws PatternSyntaxException
  {
    setFiles(&quot;^$&quot;);
  }

  public boolean accept(AuditEvent aEvent)
  {
    final String fileName = aEvent.getFileName();
    return ((fileName == null) || !mFileRegexp.matcher(fileName).find());
  }

  public void setFiles(String aFilesPattern)
      throws PatternSyntaxException
  {
    mFileRegexp = Utils.getPattern(aFilesPattern);
  }
}
      </code></pre></div>
</section>

<section><a id="Writing_TreeWalkerFilters"></a>
<h1>Writing TreeWalkerFilters</h1>

<p>
A <code>TreeWalkerFilter</code> is like a <code>Filter</code> but must be declared as a child of the <code>TreeWalker</code> module instead of <code>Checker</code>. It receives a <code>TreeWalkerAuditEvent</code> instead of a plain <code>AuditEvent</code>, which additionally provides access to the file's <code>FileContents</code> (including comments) and the root <code>DetailAST</code> of the parsed syntax tree. </p>


<p>
The <code>TreeWalkerFilter</code> that we demonstrate here suppresses violations on any line that contains a single-line comment matching a configurable pattern. Like the built-in <code>SuppressionCommentFilter</code>, it extends <code>AbstractAutomaticBean</code> so its property can be set from the configuration file. </p>


<div class="wrapper">
<pre class="prettyprint"><code class="language-java">
package com.mycompany.filters;

import java.util.regex.Pattern;

import com.puppycrawl.tools.checkstyle.AbstractAutomaticBean;
import com.puppycrawl.tools.checkstyle.TreeWalkerAuditEvent;
import com.puppycrawl.tools.checkstyle.TreeWalkerFilter;
import com.puppycrawl.tools.checkstyle.api.FileContents;
import com.puppycrawl.tools.checkstyle.api.TextBlock;

public class CommentAnnotationFilter
    extends AbstractAutomaticBean
    implements TreeWalkerFilter
{
    private Pattern commentPattern = Pattern.compile(&quot;@SuppressMyCheck&quot;);

    public void setCommentPattern(String pattern)
    {
        commentPattern = Pattern.compile(pattern);
    }

    @Override
    protected void finishLocalSetup() {}

    @Override
    public boolean accept(TreeWalkerAuditEvent event)
    {
        if (event.violation() == null) {
            return true;
        }
        final FileContents contents = event.fileContents();
        final TextBlock comment =
            contents.getSingleLineComments().get(event.getLine());
        return comment == null
            || !commentPattern.matcher(comment.getText()[0]).find();
    }
}
      </code></pre></div>
</section>

<section><a id="Using_TreeWalkerFilters"></a>
<h1>Using TreeWalkerFilters</h1>

<p>
To incorporate a <code>TreeWalkerFilter</code>, declare it as a child of the <code>TreeWalker</code> module in the configuration file. For example: </p>


<div class="wrapper">
<pre class="prettyprint"><code class="language-xml">
&lt;module name=&quot;Checker&quot;&gt;
  &lt;module name=&quot;TreeWalker&quot;&gt;
    &lt;module name=&quot;com.mycompany.filters.CommentAnnotationFilter&quot;&gt;
      &lt;property name=&quot;commentPattern&quot; value=&quot;@SuppressMyCheck&quot;/&gt;
    &lt;/module&gt;
    &lt;module name=&quot;ConstantName&quot;/&gt;
  &lt;/module&gt;
&lt;/module&gt;
      </code></pre></div>


<p>
Note that a <code>TreeWalkerFilter</code> only applies to checks that are also children of the <code>TreeWalker</code> module. To filter non-TreeWalker checks, use a regular <code>Filter</code> under <code>Checker</code> instead. </p>
</section>

<section><a id="Declare_check.27s_external_resource_locations"></a>
<h1>Declare check's external resource locations</h1>

<p>
See <a href="writing-checks.html#Declare_check.27s_external_resource_locations"> Declare check's external resource locations</a>. </p>
</section>

<section><a id="Using_Filters"></a>
<h1>Using Filters</h1>

<p>
To incorporate a <code>Filter</code> in the filter set for a <code>Checker</code>, include a module element for the <code>Filter</code> in the <a href="config.html#Filters">configuration file</a>. For example, to configure a <code>Checker</code> so that it uses custom filter <code>FilesFilter</code> to prevent reporting of audit events for files whose name contains &quot;Generated&quot;, include the following module in the configuration file: </p>


<div class="wrapper">
<pre class="prettyprint"><code class="language-xml">
&lt;module name=&quot;com.mycompany.filters.FilesFilter&quot;&gt;
  &lt;property name=&quot;files&quot; value=&quot;Generated&quot;/&gt;
&lt;/module&gt;
      </code></pre></div>
</section>

<section><a id="Huh.3F_I_can.27t_figure_it_out.21"></a>
<h1>Huh? I can't figure it out!</h1>

<p>
That's probably our fault, and it means that we have to provide better documentation. Please do not hesitate to ask questions on the user mailing list, this will help us to improve this document. Please ask your questions as precisely as possible. We will not be able to answer questions like &quot;I want to write a filter but I don't know how, can you help me?&quot;. Tell us what you are trying to do (the purpose of the filter), what you have understood so far, and what exactly you are getting stuck on. </p>
</section>

<section><a id="Contributing"></a>
<h1>Contributing</h1>

<p>
We need <em>your</em> help to keep improving Checkstyle. Whenever you write a filter that you think is generally useful, please consider <a href="contributing.html">contributing</a> it to the Checkstyle community and submit it for inclusion in the next release of Checkstyle. </p>
</section>

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