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» Defining rule properties
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<h1 class="post-title-main">Defining rule properties</h1>
</div>
<div class="post-content" data-github-edit-url="https://github.com/pmd/pmd/blob/master/docs/pages/pmd/userdocs/extending/defining_properties.md">
<div class="summary">Learn how to define your own properties both for Java and XPath rules.</div>
<div id="inline-toc"><!-- empty, move TOC here when screen size too small --></div>
<p>Rule properties are a way to make your rules configurable directly from the
ruleset XML. Their usage is described on the <a href="pmd_userdocs_configuring_rules.html#rule-properties">Configuring Rules</a> page.</p>
<p>If you’re a rule developer, you may want to think about what would be useful for
a user of your rule to parameterise. It could be a numeric report level, a boolean
flag changing the behaviour of your rule… Chances are there <em>is</em> some detail
that can be abstracted away from your implementation, and in that case, this
page can help you squeeze that sweet flexibility out of your rule.</p>
<h2 id="overview-of-properties">Overview of properties</h2>
<p>The basic thing you need to do as a developer is to define a <strong>property descriptor</strong> and declare that your rule uses it. A property descriptor defines a number of attributes for your property:</p>
<ul>
<li>Its <em>name</em>, with which the user will refer to your property;</li>
<li>Its <em>description</em>, for documentation purposes;</li>
<li>Its <em>default value</em></li>
</ul>
<p>Don’t worry, all of these attributes can be specified in a single Java statement (or xml element for XPath rules).</p>
<h2 id="for-java-rules">For Java rules</h2>
<p>The procedure to define a property is quite straightforward:</p>
<ul>
<li>Create a property descriptor of the type you want, by using a
builder from <a href="https://docs.pmd-code.org/apidocs/pmd-core/7.0.0-SNAPSHOT/net/sourceforge/pmd/properties/PropertyFactory.html#"><code>PropertyFactory</code></a></li>
<li>Call <a href="https://docs.pmd-code.org/apidocs/pmd-core/7.0.0-SNAPSHOT/net/sourceforge/pmd/properties/PropertySource.html#definePropertyDescriptor(net.sourceforge.pmd.properties.PropertyDescriptor)"><code>definePropertyDescriptor(PropertyDescriptor)</code></a>` in the rule’s noarg constructor.</li>
</ul>
<p>You can then retrieve the value of the property at any time using <a href="https://docs.pmd-code.org/apidocs/pmd-core/7.0.0-SNAPSHOT/net/sourceforge/pmd/properties/PropertySource.html#getProperty(net.sourceforge.pmd.properties.PropertyDescriptor)"><code>getProperty(PropertyDescriptor)</code></a>.</p>
<h3 id="creating-a-descriptor">Creating a descriptor</h3>
<p>Properties can be built using type-specific <strong>builders</strong>, which can be obtained
from the factory methods of <a href="https://docs.pmd-code.org/apidocs/pmd-core/7.0.0-SNAPSHOT/net/sourceforge/pmd/properties/PropertyFactory.html#"><code>PropertyFactory</code></a>. For example, to build a
string property, you’d call</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nc">PropertyFactory</span><span class="o">.</span><span class="na">stringProperty</span><span class="o">(</span><span class="s">"myProperty"</span><span class="o">)</span>
<span class="o">.</span><span class="na">desc</span><span class="o">(</span><span class="s">"This is my property"</span><span class="o">)</span>
<span class="o">.</span><span class="na">defaultValue</span><span class="o">(</span><span class="s">"foo"</span><span class="o">)</span>
<span class="o">.</span><span class="na">build</span><span class="o">();</span>
</code></pre></div></div>
<p>This is fairly more readable than a constructor call, but keep in mind the description and the default value are not optional.</p>
<div class="alert alert-info" role="alert"><i class="fas fa-info-circle"></i> <b>Note:</b> As of version 6.10.0, all property concrete classes are deprecated for
removal in 7.0.0. See the <a href="pmd_release_notes_pmd7.html#properties-framework">detailed list of planned removals</a> for
information about how to migrate.</div>
<p>For <strong>numeric properties</strong>, you can add constraints on the range of acceptable values, e.g.</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nc">PropertyFactory</span><span class="o">.</span><span class="na">intProperty</span><span class="o">(</span><span class="s">"myIntProperty"</span><span class="o">)</span>
<span class="o">.</span><span class="na">desc</span><span class="o">(</span><span class="s">"This is my property"</span><span class="o">)</span>
<span class="o">.</span><span class="na">defaultValue</span><span class="o">(</span><span class="mi">3</span><span class="o">)</span>
<span class="o">.</span><span class="na">require</span><span class="o">(</span><span class="n">positive</span><span class="o">())</span>
<span class="o">.</span><span class="na">range</span><span class="o">(</span><span class="mi">0</span><span class="o">,</span> <span class="mi">100</span><span class="o">)</span>
<span class="o">.</span><span class="na">build</span><span class="o">();</span>
</code></pre></div></div>
<p>The <a href="https://docs.pmd-code.org/apidocs/pmd-core/7.0.0-SNAPSHOT/net/sourceforge/pmd/properties/constraints/NumericConstraints.html#positive()"><code>positive</code></a> method is part of
the <a href="https://docs.pmd-code.org/apidocs/pmd-core/7.0.0-SNAPSHOT/net/sourceforge/pmd/properties/constraints/NumericConstraints.html#"><code>NumericConstraints</code></a> class, which provides some
other constraints. The constraint mechanism will be completely unlocked with 7.0.0,
since we’ll be migrating our API to Java 8.</p>
<p><strong>Enumerated properties</strong> are a bit less straightforward to define, though they are
arguably more powerful. These properties don’t have a specific value type, instead,
you can choose any type of value, provided the values are from a closed set. To make
that actionable, you give string labels to each of the acceptable values, and the user
will provide one of those labels as a value in the XML. The property will give you back
the associated value, not the label. Here’s an example:</p>
<div class="language-java highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="kd">static</span> <span class="nc">Map</span><span class="o">&lt;</span><span class="nc">String</span><span class="o">,</span> <span class="nc">ModeStrategy</span><span class="o">&gt;</span> <span class="n">map</span> <span class="o">=</span> <span class="k">new</span> <span class="nc">HashMap</span><span class="o">&lt;&gt;();</span>
<span class="kd">static</span> <span class="o">{</span>
<span class="n">map</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="s">"easyMode"</span><span class="o">,</span> <span class="k">new</span> <span class="nc">EasyStrategy</span><span class="o">());</span>
<span class="n">map</span><span class="o">.</span><span class="na">put</span><span class="o">(</span><span class="s">"hardMode"</span><span class="o">,</span> <span class="k">new</span> <span class="nc">HardStrategy</span><span class="o">());</span>
<span class="o">}</span>
<span class="kd">static</span> <span class="nc">PropertyDescriptor</span><span class="o">&lt;</span><span class="nc">ModeStrategy</span><span class="o">&gt;</span> <span class="n">modeProperty</span>
<span class="o">=</span> <span class="nc">PropertyFactory</span><span class="o">.</span><span class="na">enumProperty</span><span class="o">(</span><span class="s">"modeProperty"</span><span class="o">,</span> <span class="n">map</span><span class="o">)</span>
<span class="o">.</span><span class="na">desc</span><span class="o">(</span><span class="s">"This is my property"</span><span class="o">)</span>
<span class="o">.</span><span class="na">defaultValue</span><span class="o">(</span><span class="k">new</span> <span class="nc">EasyStrategy</span><span class="o">())</span>
<span class="o">.</span><span class="na">build</span><span class="o">();</span>
</code></pre></div></div>
<h3 id="example">Example</h3>
<p>You can see an example of properties used in a PMD rule <a href="https://github.com/pmd/pmd/blob/d06b01785a712e61d33f366520f37c2473f5bd1a/pmd-java/src/main/java/net/sourceforge/pmd/lang/java/rule/design/SingularFieldRule.java#L43-L52">here</a>.
There are several things to notice here:</p>
<ul>
<li>The property descriptors are declared <code class="language-plaintext highlighter-rouge">static final</code>, which should generally be
the case, as descriptors are immutable and can be shared between instances of the same rule;</li>
<li>The property is declared using <a href="https://docs.pmd-code.org/apidocs/pmd-core/7.0.0-SNAPSHOT/net/sourceforge/pmd/properties/PropertySource.html#definePropertyDescriptor(net.sourceforge.pmd.properties.PropertyDescriptor)"><code>definePropertyDescriptor</code></a>` <em>in the constructor</em>,
which ensures the property gets recognised by PMD at the time the properties
are overridden (which happens before rule execution);</li>
<li>The value of the property is <em>not retrieved in the constructor</em>, but in one of
the <code class="language-plaintext highlighter-rouge">visit</code> methods (typically on the highest node in the tree, since the property
doesn’t change).</li>
</ul>
<h2 id="for-xpath-rules">For XPath rules</h2>
<p>XPath rules can also define their own properties. To do so, you must add a <code class="language-plaintext highlighter-rouge">property</code> element in the <code class="language-plaintext highlighter-rouge">properties</code> element of your rule, which <strong>declares the <code class="language-plaintext highlighter-rouge">type</code> attribute</strong>. This attribute conditions what type the underlying property has, and can have the following values:</p>
<table>
<thead>
<tr>
<th><code class="language-plaintext highlighter-rouge">type</code> attribute</th>
<th>XSD type</th>
</tr>
</thead>
<tbody>
<tr>
<td>Integer</td>
<td>xs:integer</td>
</tr>
<tr>
<td>Long</td>
<td>xs:integer</td>
</tr>
<tr>
<td>Double</td>
<td>xs:decimal</td>
</tr>
<tr>
<td>Boolean</td>
<td>xs:boolean</td>
</tr>
<tr>
<td>String</td>
<td>xs:string</td>
</tr>
<tr>
<td>Character</td>
<td>xs:string</td>
</tr>
<tr>
<td>Regex</td>
<td>xs:string</td>
</tr>
</tbody>
</table>
<div class="alert alert-info" role="alert"><i class="fas fa-info-circle"></i> <b>Note:</b> In XPath 1.0 mode, all values are actually represented as
string values, which is mostly fine as there is no type
checking. This is a problem when <a href="pmd_userdocs_extending_writing_xpath_rules.html#migrating-from-10-to-20">migrating from XPath 1.0
to 2.0</a> though</div>
<p>Note that enumerated properties are not available in XPath rules (yet?).</p>
<p>Properties defined in XPath also <em>must</em> declare the <code class="language-plaintext highlighter-rouge">description</code> attribute.
Numeric properties also expect the <code class="language-plaintext highlighter-rouge">min</code> and <code class="language-plaintext highlighter-rouge">max</code> attributes for now. Here are
a few examples to sum it up:</p>
<div class="language-xml highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"stringProp"</span> <span class="na">type=</span><span class="s">"Boolean"</span> <span class="na">value=</span><span class="s">"true"</span> <span class="na">description=</span><span class="s">"A BooleanProperty."</span><span class="nt">/&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"intProp"</span> <span class="na">type=</span><span class="s">"Integer"</span> <span class="na">value=</span><span class="s">"3"</span> <span class="na">min=</span><span class="s">"1"</span> <span class="na">max=</span><span class="s">"20"</span> <span class="na">description=</span><span class="s">"An IntegerProperty."</span><span class="nt">/&gt;</span>
</code></pre></div></div>
<p>You can then use the property in XPath with the syntax <code class="language-plaintext highlighter-rouge">$propertyName</code>, for 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">"MyXpathRule"</span> <span class="err">...</span><span class="nt">&gt;</span>
<span class="nt">&lt;properties&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"maxStatements"</span> <span class="na">type=</span><span class="s">"Integer"</span> <span class="na">value=</span><span class="s">"10"</span> <span class="na">min=</span><span class="s">"1"</span> <span class="na">max=</span><span class="s">"40"</span>
<span class="na">description=</span><span class="s">"Max number of statements per method"</span><span class="nt">/&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"xpath"</span><span class="nt">&gt;</span>
<span class="cp">&lt;![CDATA[
//MethodDeclaration/Block[count(//BlockStatement) &gt;</span> $maxStatements]
]]&gt;<span class="nt">&lt;/property&gt;</span>
<span class="nt">&lt;/properties&gt;</span>
<span class="nt">&lt;/rule&gt;</span>
</code></pre></div></div>
<h3 id="multivalued-properties">Multivalued properties</h3>
<p>Multivalued properties are also allowed and their <code class="language-plaintext highlighter-rouge">type</code> attribute has the form
<code class="language-plaintext highlighter-rouge">List[Boolean]</code> or <code class="language-plaintext highlighter-rouge">List[Character]</code>, with every above type allowed. These
properties <strong>require XPath 2.0</strong> to work properly, and make use of the
<strong>sequence datatype</strong> provided by that language. You thus need to set the
<code class="language-plaintext highlighter-rouge">version</code> property to <code class="language-plaintext highlighter-rouge">2.0</code> to use them. Properties can also declare the
<code class="language-plaintext highlighter-rouge">delimiter</code> attribute.</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">"MyXpathRule"</span> <span class="err">...</span><span class="nt">&gt;</span>
<span class="nt">&lt;properties&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"version"</span> <span class="na">value=</span><span class="s">"2.0"</span> <span class="nt">/&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"intProp"</span> <span class="na">type=</span><span class="s">"List[Integer]"</span> <span class="na">value=</span><span class="s">"1,2,5"</span> <span class="na">description=</span><span class="s">"An IntegerMultiProperty."</span> <span class="nt">/&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"reportedIdentifiers"</span> <span class="na">type=</span><span class="s">"List[String]"</span> <span class="na">value=</span><span class="s">"foo$bar"</span> <span class="na">delimiter=</span><span class="s">"$"</span>
<span class="na">description=</span><span class="s">"A StringMultiProperty."</span> <span class="nt">/&gt;</span>
<span class="nt">&lt;property</span> <span class="na">name=</span><span class="s">"xpath"</span><span class="nt">&gt;</span>
<span class="cp">&lt;![CDATA[
//VariableDeclaratorId[@Image = $reportedIdentifiers]
]]&gt;</span><span class="nt">&lt;/property&gt;</span>
<span class="nt">&lt;/properties&gt;</span>
<span class="nt">&lt;/rule&gt;</span>
</code></pre></div></div>
<p>Notice that in the example above, <code class="language-plaintext highlighter-rouge">@Image = $reportedIdentifiers</code> doesn’t test
<code class="language-plaintext highlighter-rouge">@Image</code> for equality with the whole sequence <code class="language-plaintext highlighter-rouge">('foo', 'bar')</code>, it tests whether
the sequence <em>contains</em> <code class="language-plaintext highlighter-rouge">@Image</code>. That is, the above rule will report all variables
named <code class="language-plaintext highlighter-rouge">foo</code> or <code class="language-plaintext highlighter-rouge">bar</code>. All other XPath 2.0 <a href="https://www.w3.org/TR/xpath-functions/#sequence-functions">functions operating on sequences</a>
are supported.</p>
<div class="alert alert-success" role="alert"><i class="fas fa-check-square-o"></i> <b>Tip:</b> You can also <a href="pmd_userdocs_extending_designer_reference.html#rule-properties">define properties directly in the designer</a></div>
<div class="tags">
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