# Writing XPath rules

This page describes XPath rule support in more details
Table of Contents

This page describes some points of XPath rule support in more details. See also the tutorial about how to write a first (XPath) rule.

## XPath version

PMD uses XPath 3.1 for its XPath rules since PMD 7. Before then, the default version was XPath 1.0, with opt-in support for XPath 2.0.

See the Saxonica documentation for an introduction to new features in XPath 3.1.

The property `version` of `XPathRule` is deprecated and has been removed with PMD 7.

## DOM representation of ASTs

XPath rules view the AST as an XML-like DOM, which is what the XPath language is defined on. Concretely, this means:

- Every AST node is viewed as an XML element
  - The element has for local name the value of `getXPathNodeName` for the given node
- Some Java getters are exposed as XML attributes on those elements
  - This means, that documentation for attributes can be found in our Javadocs. For example, the attribute `@SimpleName` of the Java node `EnumDeclaration` is backed by the Java getter `getSimpleName`.

### Value conversion

To represent attributes, we must map Java values to XPath Data Model (XDM) values. In the following table we refer to the type conversion function as `conv`, a function from Java types to XDM types.

Java type `T` | XSD type `conv(T)`
--- | ---
`int` | `xs:integer`
`long` | `xs:integer`
`double` | `xs:decimal`
`float` | `xs:decimal`
`boolean` | `xs:boolean`
`String` | `xs:string`
`Character` | `xs:string`
`Enum<E>` | `xs:string` (uses `Object::toString`)
`Collection<E>` | `conv(E)*` (a sequence type)

The same `conv` function is used to translate rule property values to XDM values.

Additionally, PMD’s own `net.sourceforge.pmd.lang.document.Chars` is also translated to a `xs:string`

## Rule properties

See Defining rule properties

## PMD extension functions

PMD provides some language-specific XPath functions to access semantic information from the AST.

The namespace of custom PMD functions must be explicitly mentioned.

### All languages

Functions available to all languages are in the namespace `pmd`.

Function name | Description (click for details) |
--- | --- | ---
fileName | Returns the simple name of the current file |
#### pmd: fileName() as xs:string Returns the current simple file name, without path but including the extension. This can be used to write rules that check file naming conventions. Since PMD 6.38.0 Remarks The requires the context node to be an element Examples `//b[pmd:fileName() = 'Foo.xml']` Matches any `<b>` tags in files called `Foo.xml`.
startLine | Returns the start line of the given node |
#### pmd: startLine(xs:element) as xs:int Returns the line where the node starts in the source file. Line numbers are 1-based. Since PMD 6.44.0 Remarks The function is not context-dependent, but takes a node as its first parameter. Parameters element as xs:element Any element node Examples `//b[pmd:startLine(.) > 5]` Matches any `<b>` node which starts after the fifth line.
endLine | Returns the end line of the given node |
#### pmd: endLine(xs:element) as xs:int Returns the line where the node ends in the source file. Line numbers are 1-based. Since PMD 6.44.0 Remarks The function is not context-dependent, but takes a node as its first parameter. Parameters element as xs:element Any element node Examples `//b[pmd:endLine(.) == pmd:startLine(.)]` Matches any `<b>` node which doesn't span more than one line.
startColumn | Returns the start column of the given node (inclusive) |
#### pmd: startColumn(xs:element) as xs:int Returns the column number where the node starts in the source file. Column numbers are 1-based. The start column is inclusive. Since PMD 6.44.0 Remarks The function is not context-dependent, but takes a node as its first parameter. Parameters element as xs:element Any element node Examples `//b[pmd:startColumn(.) = 1]` Matches any `<b>` node which starts on the first column of a line
endColumn | Returns the end column of the given node (exclusive) |
#### pmd: endColumn(xs:element) as xs:int Returns the column number where the node ends in the source file. Column numbers are 1-based. The end column is exclusive. Since PMD 6.44.0 Remarks The function is not context-dependent, but takes a node as its first parameter. Parameters element as xs:element Any element node Examples `//b[pmd:startLine(.) = pmd:endLine(.) and pmd:endColumn(.) - pmd:startColumn(.) = 1]` Matches any `<b>` node which spans exactly one character

### Java

Java functions are in the namespace `pmd-java`.

Function name | Description (click for details) |
--- | --- | ---
nodeIs | Tests the runtime type of the node instance |
#### pmd-java: nodeIs(xs:string) as xs:boolean Returns true if the runtime type of the AST node is a subtype of the given class. Contrary to typeIs, this tests the type of the AST node. For example, the AST node for a literal (e.g. `5d`) has type ASTNumericLiteral, and this function will ignore the static type of the expression (double) Remarks Parameters nodeClassName as xs:string Simple name of a class or interface in package `net.sourceforge.pmd.lang.java.ast`, without the 'AST' prefix Examples `//*[pmd-java:nodeIs("Expression")]` Matches all nodes that implement `ASTExpression` `//*[pmd-java:nodeIs("TypeDeclaration")]` Matches all nodes that implement `ASTTypeDeclaration` `//*[pmd-java:nodeIs("Foo")]` Runtime error, there's no class ASTFoo in the package
typeIs | Tests a node's static type |
#### pmd-java: typeIs(xs:string) as xs:boolean Returns true if the context node's static Java type is a subtype of the given type. This tests for the resolved type of the Java construct, not the type of the AST node. For example, the AST node for a literal (e.g. `5d`) has type ASTNumericLiteral, however this function will compare the type of the literal (eg here, `double`) against the argument. Remarks The context node must be a `TypeNode` Parameters javaQualifiedName as xs:string The qualified name of a Java class, possibly with pairs of brackets to indicate an array type. Can also be a primitive type name. Examples `//FormalParameter[pmd-java:typeIs("java.lang.String[]")]` Matches formal parameters of type `String[]` (including vararg parameters) `//VariableId[pmd-java:typeIs("java.lang.List")]` Matches variable declarators of type `List` or any of its subtypes (including e.g. `ArrayList`)
typeIsExactly | Tests a node's static type, ignoring subtypes |
#### pmd-java: typeIsExactly(xs:string) as xs:boolean Returns true if the context node's static type is exactly the given type. In particular, returns false if the context node's type is a subtype of the given type. Remarks The context node must be a `TypeNode` Parameters javaQualifiedName as xs:string The qualified name of a Java class, possibly with pairs of brackets to indicate an array type. Can also be a primitive type name. Examples `//VariableId[pmd-java:typeIsExactly("java.lang.List")]` Matches variable declarators of type `List` (but not e.g. `ArrayList`)
metric | Computes and returns the value of a metric |
#### pmd-java: metric(xs:string) as xs:decimal? Returns the value of the metric as evaluated on the context node. If the metric cannot be computed on that node, returns an empty sequence (which is falsy). Remarks Parameters metricKey as xs:string The name of a metric in `JavaMetrics` (or an alias thereof). Examples `//ClassDeclaration[metric('NCSS') > 200]` `//MethodDeclaration[metric('CYCLO') > 10 and metric('NCSS') > 20]` `//TypeParameter[metric('idontexist') > 50]` Error: no such metric
hasAnnotation | Tests whether an annotation is present on the node |
#### pmd-java: hasAnnotation(xs:string) as xs:boolean Returns true if the node has an annotation with the given qualified name Remarks The context node must be an `Annotatable`, otherwise this returns false Parameters annotationClassName as xs:string Canonical name of an annotation type Examples `//MethodDeclaration[pmd-java:hasAnnotation("java.lang.Override")]` Matches all method declarations that are annotated with @Override
modifiers | Produce the effective modifiers of a node |
#### pmd-java: modifiers() as xs:string* Returns a sequence of the effective modifiers of a node as strings. This is documented on `getEffectiveModifiers`. Remarks The context node must be an `ModifierOwner`, otherwise this returns an empty sequence Examples `//MethodDeclaration[pmd-java:modifiers() = "native"]` Matches native method declarations `//MethodDeclaration[pmd-java:modifiers() = ("native", "static")]` Matches method declarations that have a 'native' OR a 'static' modifier. This may be counter-intuitive. `//MethodDeclaration[pmd-java:modifiers() = "public"]` Matches method declarations that have a 'public' modifier, explicit or implicit. For example, this would match methods in interfaces, which implicitly have the modifier. Use the `explicitModifiers` function if you don't want the implicit part. Also note that `@Visibility = 'public'` is a better use of the API, in this particular instance.
explicitModifiers | Produce the explicit modifiers of a node |
#### pmd-java: explicitModifiers() as xs:string* Returns a sequence of the explicit modifiers of a node as strings. This is documented on `getExplicitModifiers`. Remarks The context node must be an `ModifierOwner`, otherwise this returns an empty sequence Examples `//MethodDeclaration[pmd-java:explicitModifiers() = "public"]` Matches method declarations that have an explicit 'public' modifier.
matchesSig | Matches the signature called by a method or constructor call |
#### pmd-java: matchesSig(xs:string) as xs:boolean Uses an `TypeTestUtil.InvocationMatcher` to test the method signature called by the context node. The format of the parameter is described on that class. Remarks The context node must be an `InvocationNode`, otherwise this returns false Parameters sig as xs:string A signature, in the format described on `TypeTestUtil.InvocationMatcher` Examples `//MethodCall[pmd-java:matchesSig("_#equals(java.lang.Object)")]` Matches calls to the method `equals` on any receiver type `//MethodCall[pmd-java:matchesSig("java.lang.Enum#equals(java.lang.Object)")]` Matches calls to the method `equals` on receiver that is a subtype of Enum `//MethodCall[pmd-java:matchesSig("java.lang.String#toString()")]` Matches calls to the method `toString` on String receivers `//MethodCall[pmd-java:matchesSig("_#_(int,int)")]` Matches calls to any method with 2 `int` parameters (!= argument) `//ConstructorCall[pmd-java:matchesSig("java.util.ArrayList#new(int)")]` Matches constructors calls of ArrayList with a single int parameter

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