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    <title>Writing Checks</title>
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    <section name="Content">
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    <section name="Overview">

      <p>
        OK, so you have finally decided to write your own Check.  Welcome
        aboard, this is really a easy thing to do. Very basic Java knowledge is required
        to write a Check, it is good practice for even for student.
        There are actually two
        kinds of Checks, so before you can start, you have to find out
        which kind of Check you want to implement.
      </p>

      <p>
        The functionality of Checkstyle is implemented in modules that can
        be plugged into Checkstyle. Modules can be containers for other
        modules, i.e. they form a tree structure. The top level modules
        that are known directly to the Checkstyle kernel (which is also a
        module and forms the root of the tree) implement the
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/FileSetCheck.html">FileSetCheck</a>
        interface. These are pretty simple to grasp: they take a set of
        input files and fire violation messages.
      </p>

      <p>
        Checkstyle provides a few FileSetCheck implementations by default
        and one of them happens to be the
        <a href="apidocs/com/puppycrawl/tools/checkstyle/TreeWalker.html">TreeWalker</a>. A
        TreeWalker supports submodules that are derived from the
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html">AbstractCheck</a>
        class. The TreeWalker operates by separately transforming each of
        the Java input files into an abstract syntax tree and then handing
        the result over to each of the Check submodules which in turn have
        a look at a certain aspect of the tree.
      </p>

    </section>

    <section name="Writing Checks">

      <p>
        Most of the functionality of Checkstyle is implemented as
        Checks. If you know how to write your own Checks, you can extend
        Checkstyle according to your needs without having to wait for the
        Checkstyle development team. You are about to become a Checkstyle
        Expert.
      </p>

      <p>
        Suppose you have a convention that the number of methods in a
        class should not exceed a certain limit, say 30. This rule makes
        sense, a class should only do one thing and do it well. With a
        zillion methods chances are that the class does more than one
        thing. The only problem you have is that your convention is not
        checked by Checkstyle, so you&#39;ll have to write your own Check
        and plug it into the Checkstyle framework.
      </p>

      <p> This chapter is organized as a tour that takes you
      through the process step by step and explains both the theoretical
      foundations and the
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/package-summary.html">Checkstyle
      API</a> along the way.  </p>

    </section>

    <section name="Java Grammar">

      <p>
        Every Java Program is structured into files, and each of these
        files has a certain structure. For example, if there is a
        package statement then it is the first line of the file that is
        not comment or whitespace. After the package statement comes a
        list of import statements, which is followed by a class or
        interface definition, and so on.
      </p>
      <p>
        If you have ever read an introductory level Java book you probably
        knew all of the above. And if you have studied computer science,
        you probably also know that the rules that specify the Java language
        can be formally specified using a grammar (statement is simplified
        for didactic purposes).
      </p>
      <p>
        There are tools which read a grammar definition and produce a parser
        for the language that is specified in the grammar. In other
        words, the output of the tool is a program that can transform a
        stream of characters (a Java file) into a tree representation
        that reflects the structure of the file. Checkstyle uses the
        parser generator <a href="https://www.antlr.org">ANTLR</a> but
        that is an implementation detail you do not need to worry about
        when writing Checks, as well tested parser will parse Java file for you.
      </p>

    </section>
      <section name="Tool to print Java tree structure">
          <p>
              Checkstyle can print Abstract Syntax Tree for Java trees. You need to run checkstyle
              jar file with <b>-t</b> or <b>-T</b> argument, providing java file.
          </p>
          <p>For example, here is MyClass.java file:</p>
          <source><![CDATA[
/**
 * My <b>class</b>.
 * @see AbstractClass
 */
public class MyClass {

}
      ]]></source>
        <div class="wrapper">
          <table style="table-layout: fixed;">
              <tr>
                  <td>
                      1) Using -t as parameter.
                      <br />
                      java -jar checkstyle-X.XX-all.jar -t MyClass.java
                  </td>
                  <td>
                      2) Using -T as parameter.
                      <br />
                      java -jar checkstyle-X.XX-all.jar -T MyClass.java
                  </td>
              </tr>

              <tr>
                  <td>
                      <source><![CDATA[
CLASS_DEF -> CLASS_DEF [5:0]
|--MODIFIERS -> MODIFIERS [5:0]
|   `--LITERAL_PUBLIC -> public [5:0]
|--LITERAL_CLASS -> class [5:7]
|--IDENT -> MyClass [5:13]
`--OBJBLOCK -> OBJBLOCK [5:21]
    |--LCURLY -> { [5:21]
    `--RCURLY -> } [7:0]
          ]]></source>
                  </td>
                  <td>
                      <source><![CDATA[
CLASS_DEF -> CLASS_DEF [5:0]
|--MODIFIERS -> MODIFIERS [5:0]
|   |--BLOCK_COMMENT_BEGIN -> /* [1:0]
|   |   |--COMMENT_CONTENT -> *\n * My <b>class</b>.\n * @see AbstractClass\n  [1:2]
|   |   `--BLOCK_COMMENT_END -> */ [4:1]
|   `--LITERAL_PUBLIC -> public [5:0]
|--LITERAL_CLASS -> class [5:7]
|--IDENT -> MyClass [5:13]
`--OBJBLOCK -> OBJBLOCK [5:21]
    |--LCURLY -> { [5:21]
    `--RCURLY -> } [7:0]
          ]]></source>
                  </td>
              </tr>
          </table>
        </div>
          <p>
            As you can see very small java file transforms to a huge Abstract Syntax Tree, because
            that is the most detailed tree including all components of the java file: object block,
            comments, modifiers, etc.
          </p>
      </section>

    <section name="The Checkstyle SDK Gui">

      <p>
        Still with us? Great, you have mastered the basic theory so here
        is your reward - a GUI that displays the structure of a Java source file. To run it type
      </p>
      <div class="wrap-content">
        <source>
          java -cp checkstyle-${projectVersion}-all.jar com.puppycrawl.tools.checkstyle.gui.Main
        </source>
      </div>

      <p>
        on the command line. Click the button at the bottom of the frame
        and select a syntactically correct Java source file. The frame
        will be populated with a tree that corresponds to the structure
        of the Java source code.
      </p>

      <p>
        <span class="wrapper block">
          <img alt="screenshot" src="images/gui_screenshot.png"/>
        </span>
      </p>

      <p> In the leftmost column you can open and close branches
      of the tree, the remaining columns display information about each node
      in the tree.  The second column displays a token type for each node. As
      you navigate from the root of the tree to one of the leafs, you&#39;ll
      notice that the token type denotes smaller and smaller units of your
      source file, i.e. close to the root you might see the token type
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#CLASS_DEF">CLASS_DEF</a>
      (a node that represents a class definition) while you will see token
      types like IDENT (an identifier) near the leaves of the tree.  </p>

      <p>
       In the bottom of frame you can find buttons "Open File", "Reload File"
       and dropdown list with parse modes to choose. First one opens file
       choose window. After choosing file tree that corresponds to java source
       file builds in frame. Notice that only files with ".java" extension
       can be opened. Second one reloads chosen file from file system and
       rebuilds source code tree. Dropdown list allow to choose one of three
       parse modes: "PLAIN JAVA", "JAVA WITH COMMENTS", "JAVA WITH JAVADOC AND COMMENTS".
       "PLAIN JAVA" mode uses to show java source code without comments. In
       "JAVA WITH COMMENTS" you can see also comments blocks on the tree. When
       "JAVA WITH JAVADOC AND COMMENTS" chosen javadoc tree builds and attaches
       for every comment block, that contains javadoc.
      </p>

      <p>
        Notice: text of a tree node and its children is selected automatically
        after either pressing "Enter" or double-clicking on it, so there is no
        need to make selection manually.
      </p>

      <p>
        We&#39;ll get back to the details in the other columns later, they are important for
        implementing Checks but not for understanding the basic concepts. For now it is sufficient
        to know that the gui is a tool that lets you look at the structure of a Java file, i.e. you
        can see the Java grammar &#39;in action&#39;.
      </p>

      <p> If you use <a href="https://www.eclipse.org/">Eclipse</a> you can install
        <a href="https://github.com/sevntu-checkstyle/checkstyle-ast-eclipse-viewer">
          Checkstyle AST Eclipse Viewer</a>
        plugin to launch that application from context menu on any file in Eclipse.
      </p>

    </section>

    <section name="Understanding the visitor pattern">

      <p>
        Ready for a bit more theory? The last bit
        that is missing before you can start writing Checks is understanding
        the <a href="https://en.wikipedia.org/wiki/Visitor_pattern">Visitor pattern</a>.
      </p>

      <p>
        When working with <a href="https://en.wikipedia.org/wiki/Abstract_syntax_tree">
        Abstract Syntax Tree (AST)</a>, a simple approach to define check operations
        on them would be to add a <code>check()</code> method to the Class that defines
        the AST nodes. For example, our AST type could have a method
        <code>checkNumberOfMethods()</code>. Such an approach would suffer from a few
        serious drawbacks. Most importantly, it does not provide an extensible
        design, i.e. the Checks have to be known at compile time; there is no
        way to write plugins.
      </p>

      <p> Hence Checkstyle&#39;s AST classes do not have any
      methods that implement checking functionality. Instead,
      Checkstyle&#39;s
      <a href="apidocs/com/puppycrawl/tools/checkstyle/TreeWalker.html">TreeWalker</a>
      takes a set of objects that conform to a
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html">AbstractCheck</a>
      interface. OK, you&#39;re right - actually it&#39;s not an interface
      but an abstract class to provide some helper methods. A Check provides
      methods that take an AST as an argument and perform the checking process
      for that AST, most prominently
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#visitToken-com.puppycrawl.tools.checkstyle.api.DetailAST-">
              <code>visitToken()</code></a>.  </p>

      <p> It is important to understand that the individual
      Checks do no drive the AST traversal (it possible to traverse itself, but not recommended).
      Instead, the TreeWalker initiates
      a recursive descend from the root of the AST to the leaf nodes and calls
      the Check methods. The traversal is done using a
      <a href="https://en.wikipedia.org/wiki/Tree_traversal">tree traversal (depth-first)</a>
      algorithm.  </p>

      <p> Before any visitor method is called, the TreeWalker will call
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#beginTree-com.puppycrawl.tools.checkstyle.api.DetailAST-">
              <code>beginTree()</code></a> to give the Check a chance to do
      some initialization. Then, when performing the recursive descend from
      the root to the leaf nodes, the <code>visitToken()</code>
      method is called. Unlike the basic examples in the pattern book, there
      is a <code>visitToken()</code> counterpart called
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#leaveToken-com.puppycrawl.tools.checkstyle.api.DetailAST-">
              <code>leaveToken()</code></a>. The TreeWalker will call that
      method to signal that the subtree below the node has been processed and
      the TreeWalker is backtracking from the node. After the root node has
      been left, the TreeWalker will call
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#finishTree-com.puppycrawl.tools.checkstyle.api.DetailAST-">
              <code>finishTree()</code></a>.  </p>

    </section>

    <section name="Visitor in action">

      <p>
        Let&#39;s get back to our example and start writing code - that&#39;s why
        you came here, right?
        When you fire up the Checkstyle GUI and look at a few source
        files you&#39;ll figure out pretty quickly that you are mainly
        interested in the number of tree nodes of type METHOD_DEF. The
        number of such tokens should be counted separately for each
        CLASS_DEF / INTERFACE_DEF.
      </p>

      <p>
        Hence we need to register the Check for the token types
        CLASS_DEF and INTERFACE_DEF. The TreeWalker will only call
        visitToken for these token types. Because the requirements of
        our tasks are so simple, there is no need to implement the other
        fancy methods, like <code>finishTree()</code>, etc., so here is our first
        shot at our Check implementation:
      </p>

      <source>
package com.mycompany.checks;
import com.puppycrawl.tools.checkstyle.api.*;

public class MethodLimitCheck extends AbstractCheck
{
  private static final int DEFAULT_MAX = 30;
  private int max = DEFAULT_MAX;

  @Override
  public int[] getDefaultTokens()
  {
    return new int[]{TokenTypes.CLASS_DEF, TokenTypes.INTERFACE_DEF};
  }

  @Override
  public void visitToken(DetailAST ast)
  {
    // find the OBJBLOCK node below the CLASS_DEF/INTERFACE_DEF
    DetailAST objBlock = ast.findFirstToken(TokenTypes.OBJBLOCK);

    // count the number of direct children of the OBJBLOCK
    // that are METHOD_DEFS
    int methodDefs = objBlock.getChildCount(TokenTypes.METHOD_DEF);

    // report violation if limit is reached
    if (methodDefs > this.max) {
      String message = "too many methods, only " + this.max + " are allowed";
      log(ast.getLineNo(), message);
    }
  }
}
      </source>

    </section>

    <section name="Understanding token sets">

      <p>
        There are four methods in Check class to control the processed
        <a href="apidocs/index.html">TokenTypes</a> -
        one setter
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#setTokens-java.lang.String...-">
        setTokens()</a>, which is used to define a custom set (which is different
        from the default one) of the processed TokenTypes via config file and
        three getters, which have to be overridden:
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#getDefaultTokens--">
        getDefaultTokens()</a>,
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#getAcceptableTokens--">
        getAcceptableTokens()</a>,
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#getRequiredTokens--">
        getRequiredTokens()</a>.
      </p>

      <ul>

        <li>
          getDefaultTokens() - returns a set of TokenTypes which are processed in
          <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#visitToken-com.puppycrawl.tools.checkstyle.api.DetailAST-">
          visitToken()</a> method by default.
        </li>

        <li>
          getRequiredTokens() - returns a set of TokenTypes which Check must be subscribed to for
          a valid execution. If the user wants to specify a custom set of TokenTypes then
          this set must contain all the TokenTypes from RequiredTokens.
        </li>

        <li>
          getAcceptableTokens() - returns a set, which contains all the TokenTypes that
          can be processed by the check. Both DefaultTokens and RequiredTokens and any custom
          set of TokenTypes are subsets of AcceptableTokens.
        </li>

      </ul>

    </section>

    <section name="Comment Tokens in AST">

      <p>
        Before <a href="releasenotes.html#Release_6.0">Checkstyle 6.0</a>, there was no comments
        in AST tree as comments are not a code and ignored by compiler, so it was not a primary
        focus of Checkstyle.
        But more and more requests appear to do validation of code vs javadoc or comments.
        So there was a simple solution to receive plain text of file in Check, do own parsing and
        searching for required comments in file.
      </p>
      <p>
        Since 6.0, there is a new method in AbstractCheck class that allow you to see or not
        comment nodes in AST Tree -
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractCheck.html#isCommentNodesRequired--">
        isCommentNodesRequired()</a>. It should return TRUE if Check want to see comments in AST
        Tree.
      </p>
      <p>
        It is done as optional because it is change for a AST Tree structure, and non of existing
        Checks ready for this.
        Checkstyle does not do re-parse file one more time, comments were already in grammar and
        parsed, but skipped during AST nodes creation.
      </p>
      <p>
        Before execution, all Checks are divided into 2 groups (base on isCommentNodesRequired
        method): "java only Checks", "comments Checks".
        Checkstyle execute "java only Checks" first, as all them finish, append to AST Tree missed
        comment AST nodes and call "comments Checks".
        All Javadoc Checks that are child of AbstractJavadocCheck are "comments Checks".
      </p>
      <p>
        Comment tokens:
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#SINGLE_LINE_COMMENT">
            SINGLE_LINE_COMMENT</a>,
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#BLOCK_COMMENT_BEGIN">
            BLOCK_COMMENT_BEGIN</a>,
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#BLOCK_COMMENT_END">
            BLOCK_COMMENT_END</a>,
        <a href="apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#COMMENT_CONTENT">
            COMMENT_CONTENT</a>.
      </p>

    </section>

    <section name="Navigating the AST">

      <p>
        In the example above you already saw that the DetailsAST class
        provides utility methods to extract information from the tree,
        like <code>getChildCount()</code>. By now you have
        probably consulted the API documentation and found that
        <a href="https://github.com/checkstyle/checkstyle/blob/master/src/main/java/com/puppycrawl/tools/checkstyle/api/DetailAST.java">
            DetailsAST</a> additionally provides methods for navigating around
        in the syntax tree, like finding the next sibling of a node, the
        children of a node, the parent of a node, etc.
      </p>

      <p>
        These methods provide great power for developing complex
        Checks. Most of the Checks that Checkstyle provides by default
        use these methods to analyze the environment of the ASTs that
        are visited by the TreeWalker.  Don&#39;t abuse that feature for
        exploring the whole tree, though. Let the TreeWalker drive the
        tree traversal and limit the visitor to the neighbours of a
        single AST.
      </p>

    </section>

    <section name="Defining Check Properties">

      <p>

        OK Mr. Checkstyle, that&#39;s all very nice but in my company we
        have several projects, and each has another number of allowed
        methods. I need to control my Check through properties, so where
        is the API to do that?
      </p>

      <p>
        Well, the short answer is, there is no API. It&#39;s magic. Really!
      </p>

      <p>
        If you need to make something configurable, just add a setter method
        to the Check:
      </p>

      <source>
public class MethodLimitCheck extends AbstractCheck
{
  // code from above omitted for brevity
  public void setMax(int limit)
  {
    max = limit;
  }
}
      </source>

      <p>
        With this code added, you can set the property <code>max</code> for the
        MethodLimitCheck module in the configuration file. It doesn&#39;t get any
        simpler than that. The secret is that Checkstyle uses
        <a href="https://docs.oracle.com/javase/tutorial/reflect/member/fieldValues.html">
        JavaBean reflection to set the JavaBean properties</a>. That works for all
        primitive types like boolean, int, long, etc., plus Strings, plus arrays of these types.
      </p>

    </section>

    <section name="Logging violations">

      <p>
        Detecting violation is one thing, presenting them to the user is
        another. To do that, the Check base class provides several log
        methods, the simplest of them being <code>Check.log(String)</code>. In your
        Check you can simply use a verbatim violation string like in
        <code>log(&quot;Too many methods, only &quot; + mMax +
        &quot; are allowed&quot;);</code> as the argument. That will
        work, but it&#39;s not the best possible solution if your Check is
        intended for a wider audience.
      </p>

      <p>
        If you are not living in a country where people speak English,
        you may have noticed that Checkstyle writes internationalized
        violation messages, for example if you live in Germany the violation
        messages are German. The individual Checks don&#39;t have to do
        anything fancy to achieve this, it&#39;s actually quite easy and the
        Checkstyle framework does most of the work.
      </p>

      <p>
        To support internationalized violation messages, you need to create or reuse existing
        a messages.properties file alongside your Check class (
        <a href="https://github.com/checkstyle/checkstyle/blob/master/src/main/resources/com/puppycrawl/tools/checkstyle/checks/sizes/messages.properties">
          example</a>)
        , i.e. the
        Java file and the properties files should be in the same
        directory.  Add a symbolic violation code and an English
        representation to the messages.properties. The file should
        contain the following line: <code>too.many.methods=Too many methods, only {0} are
        allowed</code>.  Then replace the verbatim violation message with
        the symbolic representation and use one of the log helper
        methods to provide the dynamic part of the message (mMax in this
        case): <code>log(&quot;too.many.methods&quot;,
        mMax);</code>. Please consult the documentation of Java&#39;s
        <a href="https://docs.oracle.com/javase/8/docs/api/java/text/MessageFormat.html">
          MessageFormat</a>
        to learn about the syntax of format strings (especially about
        those funny numbers in the translated text).
      </p>

      <p>
        Supporting a new language is very easy now, simply create a new
        messages file for the language, e.g. messages_fr.properties to
        provide French violation messages. The correct file will be chosen
        automatically, based on the language settings of the user&#39;s
        operating system.
      </p>

    </section>

    <section name="Integrate your Check">

      <p>
        The great final moment has arrived, you are about to run your
        Check. To integrate your Check, add a new subentry under the
        TreeWalker module of your configuration file. Use the full
        classname of your Check class as the name of the module.
        Your configuration file <code>config.xml</code> should look something like this:
      </p>

      <source>
&lt;?xml version=&quot;1.0&quot;?&gt;
&lt;!DOCTYPE module PUBLIC
    &quot;-//Checkstyle//DTD Checkstyle Configuration 1.3//EN&quot;
    &quot;https://checkstyle.org/dtds/configuration_1_3.dtd&quot;&gt;
&lt;module name=&quot;Checker&quot;&gt;
  &lt;module name=&quot;TreeWalker&quot;&gt;
          &lt;!-- your standard Checks that come with Checkstyle --&gt;
    &lt;module name=&quot;UpperEll&quot;/&gt;
    &lt;module name=&quot;MethodLength&quot;/&gt;
          &lt;!-- your Check goes here --&gt;
    &lt;module name=&quot;com.mycompany.checks.MethodLimitCheck&quot;&gt;
      &lt;property name=&quot;max&quot; value=&quot;45&quot;/&gt;
    &lt;/module&gt;
  &lt;/module&gt;
&lt;/module&gt;
      </source>

      <p>
        To run the new Check on the command line compile your Check,
        create a jar that contains the classes and property files,
        e.g. <code>mycompanychecks.jar</code>. Then run
        (with the path separator
        <a href="https://docs.oracle.com/javase/tutorial/essential/environment/sysprop.html">
        system property ("path.separator")</a> adjusted to your platform:
        Linux/Unix - ":", Windows - ";"):
      </p>

      <p>For Linux/Unix OS:</p>
      <div class="wrap-content">
        <source>
          java -classpath mycompanychecks.jar:checkstyle-${projectVersion}-all.jar \
          &#xa0;&#xa0;&#xa0;&#xa0;com.puppycrawl.tools.checkstyle.Main -c config.xml myproject
        </source>
      </div>

      <p>For Windows OS:</p>
      <div class="wrap-content">
        <source>
          java -classpath mycompanychecks.jar;checkstyle-${projectVersion}-all.jar ^
          &#xa0;&#xa0;&#xa0;&#xa0;com.puppycrawl.tools.checkstyle.Main -c config.xml myproject
        </source>
      </div>

      <p>
        Did you see all those violations about &quot;too many methods&quot;
        flying over your screen? Congratulations. You can now consider
        yourself a Checkstyle expert. Go to your fridge. Have a beer.
      </p>

      <p>
        Please consult the <a href="config.html#Packages">Checkstyle
        configuration manual</a> to learn how to integrate your Checks
        into the package configuration so that you can use <code>MethodLimit</code> instead of the
        full class name.
      </p>

    </section>

    <section name="Limitations">
      <p>
        There are basically only a few limits for Checkstyle:
      </p>
      <ul>
        <li>Java code should be written with
        <a href="https://en.wikipedia.org/wiki/ASCII">ASCII</a> characters only, no UTF-8
        support.</li>
        <li>To get valid violations, code have to be compilable, in other case you can get not easy
        to understand parse errors.</li>
        <li>You cannot determine the type of an expression.
        Example: "getValue() + getValue2()"</li>
        <li>You cannot determine the full inheritance hierarchy of type.</li>
        <li>You cannot see the content of other files. You have content of one file only during all
        Checks execution. All files are processed one by one.</li>
      </ul>
      <p>
        This means that you cannot implement some of the code inspection
        features that are available in advanced IDEs like Eclipse,
        <a href="https://www.jetbrains.com/idea/">IntelliJ IDEA</a>,
        <a href="https://spotbugs.github.io/">SpotBugs</a>,
        <a href="https://www.sonarqube.org">Sonarqube</a>.
      </p>

      <p>
        For example you will not be able to implement:
        <br/>
        - a Check that finds redundant type casts or unused public methods.
        <br/>
        - a Check that validate that user custom Exception class inherited from
          java.lang.Exception class.
      </p>

    </section>

    <section name="Writing FileSetChecks">

      <p>Writing a FileSetCheck usually required when you do not need parse Java file
      to get inner structure, or you are going to validate non "*.java" files.
      </p>

      <p> Writing a FileSetCheck is pretty straightforward: Just
      inherit from
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractFileSetCheck.html">
              AbstractFileSetCheck</a>
      and override the abstract
      <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractFileSetCheck.html#processFiltered-java.io.File-com.puppycrawl.tools.checkstyle.api.FileText-">
              <code>processFiltered(java.io.File, java.util.List)</code></a> method and you&#39;re
      done. A very simple example could fire a violation if the number of files
      exceeds a certain limit. Here is a FileSetCheck that does just that:</p>

      <source>
package com.mycompany.checks;
import java.io.File;
import java.util.List;
import com.puppycrawl.tools.checkstyle.api.*;

public class LimitImplementationFiles extends AbstractFileSetCheck
{
  private static final int DEFAULT_MAX = 100;
  private int fileCount;
  private int max = DEFAULT_MAX;
  public void setMax(int aMax)
  {
    this.max = aMax;
  }

  @Override
  public void beginProcessing(String aCharset)
  {
    super.beginProcessing(aCharset);

    //reset the file count
    this.fileCount = 0;
  }

  @Override
  public void processFiltered(File file, List&lt;String&gt; aLines)
  {
    this.fileCount++;

    if (this.fileCount > this.max) {
      // log the message
      log(1, "max.files.exceeded", Integer.valueOf(this.max));
      // you can call log() multiple times to flag multiple
      // violations in the same file
    }
  }
}
      </source>

      <p>
        Note that the configuration via bean introspection also applies
        here.  By implementing the <code>setMax()</code>
        method the FileSetCheck automatically makes &quot;max&quot; a
        legal configuration parameter that you can use in the Checkstyle
        configuration file.
      </p>
      <p>
        There are virtually no limits what you can do in
        FileSetChecks, but please do not be crazy.
      </p>

    </section>

      <section name="Declare check's external resource locations">
       <p>
         Checkstyle can cache external configuration resources of any kind which are used by your
         check. If you want to do such a thing, you should implement
         <a href="apidocs/com/puppycrawl/tools/checkstyle/api/ExternalResourceHolder.html">
             ExternalResourceHolder</a>
         interface. Such module must declare external resource locations as a set of Strings
         which will be returned from
         <a href="apidocs/com/puppycrawl/tools/checkstyle/api/ExternalResourceHolder.html#getExternalResourceLocations--">
           getExternalResourceLocations</a>
         method. This will allow Checkstyle to invalidate (clear) cache when the content of at
         least one external configuration resource of your check is changed.
       </p>
       <p>
         ATTENTION!
       </p>
       <ul>
         <li>
           If
           <a href="apidocs/com/puppycrawl/tools/checkstyle/api/ExternalResourceHolder.html#getExternalResourceLocations--">
             getExternalResourceLocations</a>
           return null, there will be NullPointerException in
           <a href="apidocs/com/puppycrawl/tools/checkstyle/Checker.html">
             Checker</a>.
           Such behaviour will signal that your module (check or filter) is designed incorrectly.
         </li>
         <li>
           It makes sense to return an empty set from
           <a href="apidocs/com/puppycrawl/tools/checkstyle/api/ExternalResourceHolder.html#getExternalResourceLocations--">
             getExternalResourceLocations</a>
           only for composite modules like
           <a href="apidocs/com/puppycrawl/tools/checkstyle/TreeWalker.html">
             TreeWalker</a>.
         </li>
       </ul>
      </section>

      <section name="Huh? I can&#39;t figure it out!">
      <p>
        That&#39;s probably our fault, and it means that we have to provide
        better documentation. Please do not hesitate to ask questions on
        the user <a href="mailing-lists.html">
        mailing lists</a>, this will help us to improve this
        document.  Please ask your questions as precisely as possible.
        We will not be able to answer questions like &quot;I want to
        write a Check but I don&#39;t know how, can you help me?&quot;. Tell
        us what you are trying to do (the purpose of the Check), what
        you have understood so far, and what exactly you are getting stuck
        on.
      </p>

    </section>

    <section name="Contributing">

      <p>
        We need <em>your</em> help to keep improving Checkstyle.

        Whenever you write a Check or FileSetCheck that you think is
        generally useful, please consider
        <a href="contributing.html">contributing</a> it to the
        Checkstyle community and submit it for inclusion in the next
        release of Checkstyle.

      </p>
    </section>

  </body>
</document>

