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< h1 class = "post-title-main" > Writing XPath rules< / h1 >
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< div class = "summary" > Writing XPath rules for PMD< / div >
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< a target = "_blank" href = "https://github.com/pmd/pmd/blob/master/docs/pages/pmd/userdocs/extending/writing_xpath_rules.md" class = "btn btn-default githubEditButton" role = "button" > < i class = "fa fa-github fa-lg" > < / i > Edit me< / a >
< h1 id = "xpath-rule-tutorial" > XPath Rule tutorial< / h1 >
< div class = "alert alert-info" role = "alert" > < i class = "fa fa-info-circle" > < / i > < b > Note:< / b > For a translation to Georgian, see < a href = "http://webhostinggeeks.com/science/xpath-sourceforge-ka" > webhostinggeeks.com/science/xpath-sourceforge-ka< / a > < / div >
< p > Writing PMD rules with XPath can be a bit easier than writing rules with Java code. Here’ < / p >
< h2 id = "introduction" > Introduction< / h2 >
< p > PMD provides a very handy method for writing rules by writing an XPath query. When the XPath query finds a match, a violation is added to the report. This document focuses on XPath rules. You can go < a href = "howtowritearule.html" > here< / a > for more information about writing a rule.< / p >
< h2 id = "what-is-the-abstract-syntax-tree-ast" > What is the Abstract Syntax Tree (AST)?< / h2 >
< p > From < a href = "http://foldoc.org/abstract+syntax+tree" > FOLDOC< / a > an AST is< / p >
< blockquote >
< p > A data structure representing something which has been parsed, often used as a compiler or interpreter’ < / p >
< / blockquote >
< p > In our context, this means that we basically have a tree representation of the Java source file. This tree can viewed as a structured document - just like XML. And since it’ < / p >
< h2 id = "using-designer" > Using Designer< / h2 >
< p > PMD comes with a handy tool that you will love if you want to write an XPath rule. Designer, runnable from a script in < code class = "highlighter-rouge" > bin/< / code > , is a very simple and useful utility for writing rules.< / p >
< p > The basic steps involved in writing XPath rules are these:< / p >
< ol >
< li > Write a simple Java example source snippet in Designer< / li >
< li > See the AST for the class you wrote< / li >
< li > Write an XPath expression that matches the violation you are searching< / li >
< li > Modify the Java class and go back to previous step to refine the XPath expression< / li >
< / ol >
< h2 id = "simple-xpath-expressions" > Simple XPath expressions< / h2 >
< p > This section provides hands-on examples of XPath queries over the AST. You will probably find this section more useful if you follow it with Designer and copy/paste the examples.< / p >
< p > Copy the following Java source code to Designer:< / p >
< div class = "language-java highlighter-rouge" > < div class = "highlight" > < pre class = "highlight" > < code > < span class = "kd" > public< / span > < span class = "kd" > class< / span > < span class = "nc" > a< / span > < span class = "o" > {< / span >
< span class = "kt" > int< / span > < span class = "n" > fOne< / span > < span class = "o" > ;< / span >
< span class = "kt" > int< / span > < span class = "n" > fTwo< / span > < span class = "o" > ;< / span >
< span class = "kd" > private< / span > < span class = "kt" > void< / span > < span class = "nf" > run< / span > < span class = "o" > ()< / span > < span class = "o" > {< / span >
< span class = "kt" > int< / span > < span class = "n" > one< / span > < span class = "o" > ;< / span >
< span class = "kt" > int< / span > < span class = "n" > two< / span > < span class = "o" > ;< / span >
< span class = "o" > }< / span >
< span class = "o" > }< / span >
< / code > < / pre > < / div > < / div >
< p > Let’ < code class = "highlighter-rouge" > @Image< / code > attribute. So you try an XPath expression as follows:< / p >
< p > < code class = "highlighter-rouge" > //VariableDeclaratorId< / code > < / p >
< p > If you try this expression you’ < / p >
< p > < code class = "highlighter-rouge" > //FieldDeclaration< / code > < / p >
< p > In a similar way, you can match only local variables with this expression< / p >
< p > < code class = "highlighter-rouge" > //LocalVariableDeclaration< / code > < / p >
< p > With local variables we need to be more careful. Consider the following class:< / p >
< div class = "language-java highlighter-rouge" > < div class = "highlight" > < pre class = "highlight" > < code > < span class = "kd" > public< / span > < span class = "kd" > class< / span > < span class = "nc" > a< / span > < span class = "o" > {< / span >
< span class = "kd" > private< / span > < span class = "kt" > void< / span > < span class = "nf" > run< / span > < span class = "o" > ()< / span > < span class = "o" > {< / span >
< span class = "kd" > final< / span > < span class = "kt" > int< / span > < span class = "n" > one< / span > < span class = "o" > ;< / span >
< span class = "kt" > int< / span > < span class = "n" > two< / span > < span class = "o" > ;< / span >
< span class = "o" > {< / span >
< span class = "kt" > int< / span > < span class = "n" > a< / span > < span class = "o" > ;< / span >
< span class = "o" > }< / span >
< span class = "o" > }< / span >
< span class = "o" > }< / span >
< / code > < / pre > < / div > < / div >
< p > Local variable declarations will match ‘ ’ ’ < / p >
< p > < code class = "highlighter-rouge" > //MethodDeclaration//LocalVariableDeclaration< / code > < / p >
< p > You’ < / p >
< p > < code class = "highlighter-rouge" > //LocalVariableDeclaration[name(../../..) = 'MethodDeclaration']< / code > < / p >
< h2 id = "matching-variables-by-name" > Matching variables by name< / h2 >
< p > Let’ ’ ‘ ’ < / p >
< p > < code class = "highlighter-rouge" > //VariableDeclarator[../Type/ReferenceType/ClassOrInterfaceType[@Image='Logger']]< / code > < / p >
< p > Finding a class with more than one logger is quite easy now. This expression matches the classes we are looking for.< / p >
< div class = "language-xpath highlighter-rouge" > < div class = "highlight" > < pre class = "highlight" > < code > < span class = "nt" > TypeDeclaration< / span > < span class = "p" > [< / span > < span class = "nf" > count< / span > < span class = "p" > (< / span > < span class = "o" > //< / span > < span class = "nt" > VariableDeclarator< / span > < span class = "p" > [..< / span > < span class = "o" > /< / span > < span class = "nt" > Type< / span > < span class = "o" > /< / span > < span class = "nt" > ReferenceType< / span > < span class = "o" > /< / span > < span class = "nt" > ClassOrInterfaceType< / span > < span class = "p" > [< / span > < span class = "na" > @Image< / span > < span class = "err" > =< / span > < span class = "s1" > 'Logger'< / span > < span class = "p" > ]])< / span > < span class = "o" > > < / span > < span class = "m" > 1< / span > < span class = "w" >
< / span > < / code > < / pre > < / div > < / div >
< p > But let’ < / p >
< div class = "language-java highlighter-rouge" > < div class = "highlight" > < pre class = "highlight" > < code > < span class = "kd" > public< / span > < span class = "kd" > class< / span > < span class = "nc" > a< / span > < span class = "o" > {< / span >
< span class = "n" > Logger< / span > < span class = "n" > log< / span > < span class = "o" > =< / span > < span class = "kc" > null< / span > < span class = "o" > ;< / span >
< span class = "n" > Logger< / span > < span class = "n" > log< / span > < span class = "o" > =< / span > < span class = "kc" > null< / span > < span class = "o" > ;< / span >
< span class = "kt" > int< / span > < span class = "n" > b< / span > < span class = "o" > ;< / span >
< span class = "kt" > void< / span > < span class = "nf" > myMethod< / span > < span class = "o" > ()< / span > < span class = "o" > {< / span >
< span class = "n" > Logger< / span > < span class = "n" > log< / span > < span class = "o" > =< / span > < span class = "kc" > null< / span > < span class = "o" > ;< / span >
< span class = "kt" > int< / span > < span class = "n" > a< / span > < span class = "o" > ;< / span >
< span class = "o" > }< / span >
< span class = "kd" > class< / span > < span class = "nc" > c< / span > < span class = "o" > {< / span >
< span class = "n" > Logger< / span > < span class = "n" > a< / span > < span class = "o" > ;< / span >
< span class = "n" > Logger< / span > < span class = "n" > a< / span > < span class = "o" > ;< / span >
< span class = "o" > }< / span >
< span class = "o" > }< / span >
< / code > < / pre > < / div > < / div >
< p > With this class we will only be matching one violation, when we probably would have wanted to produce two violations (one for each class). The following refined expression matches classes that contain more than one logger.< / p >
< div class = "language-xpath highlighter-rouge" > < div class = "highlight" > < pre class = "highlight" > < code > < span class = "o" > //< / span > < span class = "nt" > ClassOrInterfaceBodyDeclaration< / span > < span class = "p" > [< / span > < span class = "nf" > count< / span > < span class = "p" > (< / span > < span class = "o" > //< / span > < span class = "nt" > VariableDeclarator< / span > < span class = "p" > [..< / span > < span class = "o" > /< / span > < span class = "nt" > Type< / span > < span class = "o" > /< / span > < span class = "nt" > ReferenceType< / span > < span class = "o" > /< / span > < span class = "nt" > ClassOrInterfaceType< / span > < span class = "p" > [< / span > < span class = "na" > @Image< / span > < span class = "err" > =< / span > < span class = "s1" > 'Logger'< / span > < span class = "p" > ]])< / span > < span class = "o" > > < / span > < span class = "m" > 1< / span > < span class = "p" > ]< / span > < span class = "w" >
< / span > < / code > < / pre > < / div > < / div >
< p > Let’ ’ < / p >
< div class = "language-java highlighter-rouge" > < div class = "highlight" > < pre class = "highlight" > < code > < span class = "kd" > public< / span > < span class = "kd" > class< / span > < span class = "nc" > a< / span > < span class = "o" > {< / span >
< span class = "n" > Factory< / span > < span class = "n" > f1< / span > < span class = "o" > ;< / span >
< span class = "kt" > void< / span > < span class = "nf" > myMethod< / span > < span class = "o" > ()< / span > < span class = "o" > {< / span >
< span class = "n" > Factory< / span > < span class = "n" > f2< / span > < span class = "o" > ;< / span >
< span class = "kt" > int< / span > < span class = "n" > a< / span > < span class = "o" > ;< / span >
< span class = "o" > }< / span >
< span class = "o" > }< / span >
< / code > < / pre > < / div > < / div >
< p > The following expression does the magic we need:< / p >
< div class = "language-xpath highlighter-rouge" > < div class = "highlight" > < pre class = "highlight" > < code > < span class = "o" > //< / span > < span class = "nt" > VariableDeclarator< / span > < span class = "w" >
< / span > < span class = "p" > [..< / span > < span class = "o" > /< / span > < span class = "nt" > Type< / span > < span class = "o" > /< / span > < span class = "nt" > ReferenceType< / span > < span class = "o" > /< / span > < span class = "nt" > ClassOrInterfaceType< / span > < span class = "w" >
< / span > < span class = "p" > [< / span > < span class = "na" > @Image< / span > < span class = "w" > < / span > < span class = "err" > =< / span > < span class = "w" > < / span > < span class = "s1" > 'Factory'< / span > < span class = "p" > ]< / span > < span class = "w" > < / span > < span class = "ow" > and< / span > < span class = "w" > < / span > < span class = "p" > ..[< / span > < span class = "na" > @Final< / span > < span class = "err" > =< / span > < span class = "s1" > 'false'< / span > < span class = "p" > ]]< / span > < span class = "w" >
< / span > < / code > < / pre > < / div > < / div >
< p > We recommend at this point that you experiment with Designer putting the final modifier to the Factory and verifying that the results produced are those expected.< / p >
< h2 id = "creating-a-new-rule-definition" > Creating a new rule definition< / h2 >
< p > To actually use your new XPath rule, it needs to be in a ruleset. You can create a new custom ruleset which just
contains your new XPath rule. You can use the following template. Just make sure, to replace the < code class = "highlighter-rouge" > xpath< / code > property,
the example code and give your rule a useful name and message.< / p >
< div class = "language-xml highlighter-rouge" > < div class = "highlight" > < pre class = "highlight" > < code > < span class = "cp" > < ?xml version="1.0"?> < / span >
< span class = "nt" > < ruleset< / span > < span class = "na" > name=< / span > < span class = "s" > "Custom Rules"< / span >
< span class = "na" > xmlns=< / span > < span class = "s" > "http://pmd.sourceforge.net/ruleset/2.0.0"< / span >
< span class = "na" > xmlns:xsi=< / span > < span class = "s" > "http://www.w3.org/2001/XMLSchema-instance"< / span >
2018-10-27 20:57:46 +00:00
< span class = "na" > xsi:schemaLocation=< / span > < span class = "s" > "http://pmd.sourceforge.net/ruleset/2.0.0 https://pmd.sourceforge.io/ruleset_2_0_0.xsd"< / span > < span class = "nt" > > < / span >
2018-06-16 20:39:36 +00:00
< span class = "nt" > < description> < / span >
Custom rules
< span class = "nt" > < /description> < / span >
< span class = "nt" > < rule< / span > < span class = "na" > name=< / span > < span class = "s" > "My Rule"< / span >
< span class = "na" > language=< / span > < span class = "s" > "java"< / span >
< span class = "na" > message=< / span > < span class = "s" > "violation message"< / span >
< span class = "na" > class=< / span > < span class = "s" > "net.sourceforge.pmd.lang.rule.XPathRule"< / span > < span class = "nt" > > < / span >
< span class = "nt" > < description> < / span >
Rule Description
< span class = "nt" > < /description> < / span >
< span class = "nt" > < priority> < / span > 3< span class = "nt" > < /priority> < / span >
< span class = "nt" > < properties> < / span >
< span class = "nt" > < property< / span > < span class = "na" > name=< / span > < span class = "s" > "xpath"< / span > < span class = "nt" > > < / span >
< span class = "nt" > < value> < / span > < span class = "cp" > < ![CDATA[
--- here comes your XPath expression
]]> < / span > < span class = "nt" > < /value> < / span >
< span class = "nt" > < /property> < / span >
< span class = "nt" > < /properties> < / span >
< span class = "nt" > < example> < / span >
< span class = "cp" > < ![CDATA[
public class ExampleCode {
public void foo() {
}
}
]]> < / span >
< span class = "nt" > < /example> < / span >
< span class = "nt" > < /rule> < / span >
< span class = "nt" > < /ruleset> < / span >
< / code > < / pre > < / div > < / div >
< p > Finally, for many more details on writing XPath rules, pick up < a href = "http://pmdapplied.com/" > PMD Applied< / a > !< / p >
< 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 >
< / div >
< / div >
< hr class = "shaded" / >
< footer >
< div class = "row" >
< div class = "col-lg-12 footer" >
© 2018 PMD Open Source Project. All rights reserved. < br / >
2018-12-08 10:05:09 +00:00
< span > Page last updated:< / span > July 3, 2016< br / > Site last generated: Dec 8, 2018 < br / >
2018-06-16 20:39:36 +00:00
< p > < img src = "images/pmd-logo-small.png" alt = "Company logo" / > < / p >
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< / footer >
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