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<h1 class="post-title-main">Introduction to writing PMD rules</h1>
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<div class="post-content">
<div class="summary">Writing your own PMD rules</div>
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<a target="_blank" href="https://github.com/pmd/pmd/blob/master/docs/pages/pmd/userdocs/extending/writing_rules_intro.md" class="btn btn-default githubEditButton" role="button"><i class="fa fa-github fa-lg"></i> Edit me</a>
<p>PMD is a framework to perform code analysis. You can create your own rules to
check for patterns specific to your codebase, or the coding practices of your
team.</p>
<h2 id="how-rules-work-the-ast">How rules work: the AST</h2>
<p>Before running rules, PMD parses the source file into a data structure called an
<strong>abstract syntax tree (AST)</strong>. This tree represents the syntactic structure of the
code, and encodes syntactic relations between source code elements. For instance,
in Java, method declarations belong to a class: in the AST, the nodes representing
method declarations will be descendants of a node representing the declaration of
their enclosing class. This representation is thus much richer than the original
source code (which, for a program, is just a chain of characters), or the token
chain produced by a lexer (which is e.g. what Checkstyle works on). For example:</p>
<table>
<colgroup>
<col width="40%" />
<col width="70%" />
</colgroup>
<thead>
<tr class="header">
<th>Sample code (Java)</th>
<th>AST</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">class</span> <span class="nc">Foo</span> <span class="kd">extends</span> <span class="nc">Object</span> <span class="o">{</span>
<span class="o">}</span>
</code></pre></div> </div>
</td>
<td>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="err">└─</span> <span class="nc">CompilationUnit</span>
<span class="err">└─</span> <span class="nc">TypeDeclaration</span>
<span class="err">└─</span> <span class="nc">ClassOrInterfaceDeclaration</span> <span class="s">"Foo"</span>
<span class="err">├─</span> <span class="nc">ExtendsList</span>
<span class="err">│</span> <span class="err">└─</span> <span class="nc">ClassOrInterfaceType</span> <span class="s">"Object"</span>
<span class="err">└─</span> <span class="nc">ClassOrInterfaceBody</span>
</code></pre></div> </div>
</td>
</tr>
</tbody>
</table>
<p>Conceptually, PMD rules work by <strong>matching a “pattern” against the AST</strong> of a
file.
Rules explore the AST and find nodes that satisfy some conditions that are characteristic
of the specific thing the rule is trying to flag. Rules then report a violation on these nodes.</p>
<h3 id="discovering-the-ast">Discovering the AST</h3>
<p>ASTs are represented by Java classes deriving from <a href="https://docs.pmd-code.org/apidocs/pmd-core/6.30.0-SNAPSHOT/net/sourceforge/pmd/lang/ast/Node.html#"><code>Node</code></a>.
Each PMD language has its own set of such classes, and its own rules about how
these classes relate to one another, based on the grammar of the language. For
example, all Java AST nodes extend <a href="https://docs.pmd-code.org/apidocs/pmd-java/6.30.0-SNAPSHOT/net/sourceforge/pmd/lang/java/ast/JavaNode.html#"><code>JavaNode</code></a>.</p>
<p>The structure of the AST can be discovered through</p>
<ul>
<li>the <a href="pmd_userdocs_extending_designer_reference.html#ast-inspection">Rule Designer</a></li>
<li>the <a href="pmd_devdocs_experimental_ast_dump.html">AST dump feature</a></li>
</ul>
<h2 id="writing-new-rules">Writing new rules</h2>
<p>PMD supports two ways to define rules: using an <strong>XPath query</strong>, or using a
<strong>Java visitor</strong>. XPath rules are much easier to set up, since they’re defined
directly in your ruleset XML, and are expressive enough for nearly any task.</p>
<p>On the other hand, some parts of PMD’s API are only accessible from Java, e.g.
accessing the usages of a declaration. And Java rules allow you to do some
complicated processing, to which an XPath rule couldn’t scale.</p>
<p>In the end, choosing one strategy or the other depends on the difficulty of what
your rule does. I’d advise to keep to XPath unless you have no other choice.</p>
<h2 id="xml-rule-definition">XML rule definition</h2>
<p>New rules must be declared in a ruleset before they’re referenced. This is the
case for both XPath and Java rules. To do this, the <code class="language-plaintext highlighter-rouge">rule</code> element is used, but
instead of mentioning the <code class="language-plaintext highlighter-rouge">ref</code> attribute, it mentions the <code class="language-plaintext highlighter-rouge">class</code> attribute,
with the implementation class of your rule.</p>
<ul>
<li><strong>For Java rules:</strong> this is the class extending AbstractRule (transitively)</li>
<li><strong>For XPath rules:</strong> this is <code class="language-plaintext highlighter-rouge">net.sourceforge.pmd.lang.rule.XPathRule</code></li>
</ul>
<p>Example:</p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;rule</span> <span class="na">name=</span><span class="s">"MyJavaRule"</span>
<span class="na">language=</span><span class="s">"java"</span>
<span class="na">message=</span><span class="s">"Violation!"</span>
<span class="na">class=</span><span class="s">"com.me.MyJavaRule"</span> <span class="nt">&gt;</span>
<span class="nt">&lt;description&gt;</span>
Description
<span class="nt">&lt;/description&gt;</span>
<span class="nt">&lt;priority&gt;</span>3<span class="nt">&lt;/priority&gt;</span>
<span class="nt">&lt;/rule&gt;</span>
</code></pre></div></div>
<div class="alert alert-info" role="alert"><i class="fa fa-info-circle"></i> <b>Note:</b> In PMD 7, the <code class="language-plaintext highlighter-rouge">language</code> attribute will be required on all <code class="language-plaintext highlighter-rouge">rule</code>
elements that declare a new rule. Some base rule classes set the language implicitly in their
constructor, and so this is not required in all cases for the rule to work. But this
behavior will be discontinued in PMD 7, so missing <code class="language-plaintext highlighter-rouge">language</code> attributes are
reported beginning with PMD 6.27.0 as a forward compatibility warning.</div>
<h2 id="resource-index">Resource index</h2>
<p>To learn how to write a rule:</p>
<ul>
<li><a href="pmd_userdocs_extending_your_first_rule.html">Your First Rule</a>
introduces the basic development process of a rule with a running example</li>
<li><a href="pmd_userdocs_extending_writing_xpath_rules.html">Writing XPath Rules</a>
explains a bit more about XPath rules and our XPath API</li>
<li><a href="pmd_userdocs_extending_writing_java_rules.html">Writing Java Rules</a>
describes how to write a rule in Java</li>
</ul>
<p>To go further:</p>
<ul>
<li><a href="pmd_userdocs_extending_defining_properties.html">Defining Properties</a>
describes how to make your rules more configurable with rule properties</li>
<li><a href="pmd_userdocs_extending_testing.html">Testing your Rules</a> introduces
our testing framework and how you can use it to safeguard the quality of
your rule</li>
</ul>
<div class="tags">
<b>Tags: </b>
<a href="tag_extending.html" class="btn btn-default navbar-btn cursorNorm" role="button">extending</a>
<a href="tag_userdocs.html" class="btn btn-default navbar-btn cursorNorm" role="button">userdocs</a>
<a href="tag_getting_started.html" class="btn btn-default navbar-btn cursorNorm" role="button">getting_started</a>
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