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A Checkstyle configuration file is a description of how a Java source file should be analyzed. They are written in XML and consist of principally of module elements.

The root element of every configuration is a module with a name of "Checker".

The most important element inside of the root element is a module with a name of "TreeWalker" which, itself, contains the actual checks. It's name indicates that the checks it contains involve traversing the parse tree of the Java source file.

So, for example, a typical configuration file would be something like the following:

<module name="Checker">
  <module name="TreeWalker">
 
  <!-- Check Elements -->
 
  </module>
</module>

The "TreeWalker" element contains module elements that define the checks to perform. (It can also contain property elements that dictate things like the file type to use to identify Java source files.)

For example, the following configuration file instructs Checkstyle to check if source files inappropriately use the * wildcard in imports.

<module name="Checker">
  <module name="TreeWalker">
    <module name="AvoidStarImport"/>
  </module>
</module>

Many checks have property elements that control how they behave. For example, the following check restricts the number of logical operators that can be included in a Boolean expression to seven.

    <module name="BooleanExpressionComplexity">
      <property name="max" value="7"/>
    </module>

For more information about the kinds of checks that can be performed, see the on-line documentation .

Style guidelines don't always apply to all files. For example, many organizations do not require that style guideline be followed for JUnit tests. In such cases, one can include a "BeforeExecutionFileFilter" element inside of the "Checker" element.

For more information see the on-line documentation.