Content

• Content
• What is Javadoc comment
• Limitations
• Tight-HTML rules
• How to create Javadoc Check
• Difference between Java Grammar and Javadoc comments Grammar
• Tool to print Javadoc tree structure
• Access Java AST from Javadoc Check
• HTML Code In Javadoc Comments
• Checkstyle SDK GUI
• Customize token types in Javadoc Checks
• Integrating new Javadoc Check
• Declare check's external resource locations
• Examples of Javadoc Checks
• Javadoc parser behavior for current HTML version and new HTML version

What is Javadoc comment

Javadoc comment is a multiline comment /* */ that starts with the * character and placed above the class definition, interface definition, enum definition, method definition or field definition. If an annotation precedes any of the definitions listed above, then the javadoc comment should be placed before the annotation. If several multiline comments with javadoc identifiers are placed sequentially, only the one closest to the definition, right above it, with the javadoc identifier will be used.

Javadoc comments should contain: a short summary (the first sentence), an optional documentation section, an optional tag section. The first sentence has a special meaning and should be clear, punchy, short, and is ended by a period symbol. Immediately after the first sentence, the main description could begin, which may be followed by the tag section. The tag section starts with the first block tag, which is defined by the first @ character that begins a line (ignoring leading asterisks, white space, and leading separator /**).

For example, here is java file:

/**
 * My <b>class</b>.
 *
 * @see Annotation
 */
public class MyClass {

  /** Not a javadoc (ignored). */

  /**
   * Doubles the value.
   * The long and detailed explanation what the method does.
   *
   * @param value for doubling.
   * @return double value.
   */

  /*
   Multiline comment (ignored).
  */
  @Annotation
  /** Extra javadoc (ignored). */
  // Single line comment (ignored).
  public int method(int value) {
    /** Inner javadoc (ignored). */
    return value * 2;
  }
}
      

Javadoc content for the MyClass will be:

    My <b>class</b>.

    @see Annotation
      

Javadoc content for the MyClass.method will be:

    Doubles the value.
    The long and detailed explanation what the method does.

    @param value for doubling.
    @return double value.
      

Attention that java comment starts with /*, following with Identifier of comment type. Javadoc Identifier is *. All symbols after Javadoc Identifier till */ are part of javadoc comment.

Please note that javadoc-like (multiline comment with javadoc identifier) comment inside a method is not a javadoc comment and skipped by Sun/Oracle javadoc tool and by our javadoc comment matcher, but such comment will be in AST.

You can find different types of documentation generation tools similar to javadoc on the Internet. Such tools rely on specific Identifier: "!", "#", "$". Comments look like "/*! some comment */" , "/*# some comment */" , "/*$ some comment */". Such multiline comments are not a javadoc.

Limitations

Since Oracle itself does not provide a comprehensive language specification for Javadocs, we have interpreted the existing behavior of the Javadoc tool and accepted standards to form our Javadoc grammar and Abstract Syntax Tree (AST). For this reason, we must impose the following limitations on Javadoc parsing and checks.

The comment should be written in Tight-HTML to build ASTs that most Checks expect.

For more details about parsing of HTML into AST read HTML Code In Javadoc Comments and Javadoc parser behavior section.

Tight-HTML rules

Every HTML tag should have matching end HTML tag or it is a void element.

The only exceptions are HTML 4 tags whose end tag is optional (omittable) by HTML specification (example is TR), so, Checkstyle won't show an error about missing end tags, however, it leads to broken Tight-HTML structure and as a result leads to non-nested content of the HTML tags in the Abstract Syntax Tree of the Javadoc comment.
In other words, if HTML tags are not closed in Javadoc, our parser cannot determine the content of these tags, so structure of the parse tree will not be nested like it is while using Tight-HTML.

Other rules:

• Document Structure elements (DOCTYPE, <html>, <body>, etc) are not mandatory.
• Elements must always be closed, except HTML4 elements whose end tag is optional (omittable) and HTML4 void elements. See HTML Code In Javadoc Comments section
• HTML elements can be either in lowercase or in uppercase
• Attribute names can be either in lowercase or in uppercase
• Attribute values can be either quoted or not be quoted

How to create Javadoc Check

Writing Javadoc Checks is similar to writing Java Checks, however you should extend AbstractJavadocCheck<link> and use JavadoctokenTypes<link>.

To start implementing new Check create a new class and extend AbstractJavadocCheck. It has two abstract methods you should implement:

• getDefaultJavadocTokens() - return int array of javadoc token types your Check is going to process. The array should contain int constants from JavadocTokenTypes class. (There is also TokenTypes class in Checkstyle. Make sure you use JavadocTokenTypes class in your Check, because the TokenTypes is used to describe standard Java DetailAST token type.)
• visitJavadocToken(DetailNode) - it's a place you should put tree nodes processing. The argument is Javadoc tree node of type you described before in getDefaultJavadocTokens() method.

Difference between Java Grammar and Javadoc comments Grammar

Our Java parser parses all multiline comments (C-style comments and Javadoc comments) as block comments, whether they are Javadocs or not. Our Javadoc parser then transforms these block comment ASTs into our Javadoc AST.

The difference is that Java grammar uses ANTLR v2, while Javadoc grammar uses ANTLR v4. Because of that, these two grammars and their trees are not compatible. Java AST consists of DetailAST objects, while Javadoc AST consists of DetailNode objects.

Our Java grammar ignores whitespace in most cases, since Java itself does not care about whitespace; however, whitespace is critical in Javadocs. For this reason, our Javadoc AST retains most whitespace that is present in the Javadoc. (WS, NEWLINE).

Tool to print Javadoc tree structure

Checkstyle can print a combined Java and Javadoc Abstract Syntax Tree. You need to run the checkstyle jar file with the -J argument, providing a java file.

For example, here is MyClass.java file:

/**
 * My <b>class</b>.
 * @see AbstractClass
 */
public class MyClass {

}
      

Command:

          java -jar checkstyle-X.XX-all.jar -J MyClass.java
        

Output:

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]
|   |   |   `--JAVADOC -> JAVADOC [1:3]
|   |   |       |--NEWLINE -> \n [1:3]
|   |   |       |--LEADING_ASTERISK ->  * [2:0]
|   |   |       |--TEXT ->  My  [2:2]
|   |   |       |--HTML_ELEMENT -> HTML_ELEMENT [2:6]
|   |   |       |   `--HTML_TAG -> HTML_TAG [2:6]
|   |   |       |       |--HTML_ELEMENT_START -> HTML_ELEMENT_START [2:6]
|   |   |       |       |   |--START -> < [2:6]
|   |   |       |       |   |--HTML_TAG_NAME -> b [2:7]
|   |   |       |       |   `--END -> > [2:8]
|   |   |       |       |--TEXT -> class [2:9]
|   |   |       |       `--HTML_ELEMENT_END -> HTML_ELEMENT_END [2:14]
|   |   |       |           |--START -> < [2:14]
|   |   |       |           |--SLASH -> / [2:15]
|   |   |       |           |--HTML_TAG_NAME -> b [2:16]
|   |   |       |           `--END -> > [2:17]
|   |   |       |--TEXT -> . [2:18]
|   |   |       |--NEWLINE -> \n [2:19]
|   |   |       |--LEADING_ASTERISK ->  * [3:0]
|   |   |       |--WS ->   [3:2]
|   |   |       |--JAVADOC_TAG -> JAVADOC_TAG [3:3]
|   |   |       |   |--SEE_LITERAL -> @see [3:3]
|   |   |       |   |--WS ->   [3:7]
|   |   |       |   |--REFERENCE -> REFERENCE [3:8]
|   |   |       |   |   `--CLASS -> AbstractClass [3:8]
|   |   |       |   |--NEWLINE -> \n [3:21]
|   |   |       |   `--WS ->   [4:0]
|   |   |       `--EOF -> <EOF> [4:1]
|   |   `--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]
      

In most cases while developing Javadoc Checks, you need to only parse the tree of the exact Javadoc comment. To do that just copy Javadoc comment to a separate file and remove /** at the beginning and */ at the end. After that, run checkstyle with -j argument.

MyJavadocComment.javadoc file:

 * My <b>class</b>.
 * @see AbstractClass
      

Command:

          java -jar checkstyle-X.XX-all.jar \
              -j MyJavadocComment.javadoc
        

Output:

JAVADOC -> JAVADOC [0:0]
|--LEADING_ASTERISK ->  * [0:0]
|--TEXT ->  My  [0:2]
|--HTML_ELEMENT -> HTML_ELEMENT [0:6]
|   `--HTML_TAG -> HTML_TAG [0:6]
|       |--HTML_ELEMENT_START -> HTML_ELEMENT_START [0:6]
|       |   |--START -> < [0:6]
|       |   |--HTML_TAG_NAME -> b [0:7]
|       |   `--END -> > [0:8]
|       |--TEXT -> class [0:9]
|       `--HTML_ELEMENT_END -> HTML_ELEMENT_END [0:14]
|           |--START -> < [0:14]
|           |--SLASH -> / [0:15]
|           |--HTML_TAG_NAME -> b [0:16]
|           `--END -> > [0:17]
|--TEXT -> . [0:18]
|--NEWLINE -> \n [0:19]
|--LEADING_ASTERISK ->  * [1:0]
|--WS ->   [1:2]
|--JAVADOC_TAG -> JAVADOC_TAG [1:3]
|   |--SEE_LITERAL -> @see [1:3]
|   |--WS ->   [1:7]
|   `--REFERENCE -> REFERENCE [1:8]
|       `--CLASS -> AbstractClass [1:8]
`--EOF -> <EOF> [1:21]
      

Access Java AST from Javadoc Check

As you already know the Javadoc AST is a result of parsing a block comment. There is a method to get the original block comment from a Javadoc Check. You may need this block comment to check its position in the DetailAST tree.

For example, to write a JavadocCheck that verifies @param tags in the Javadoc comment of a method definition, you also need all of the method's parameter names. To get a method definition AST you should access the DetailAST tree from a javadoc Check. For this purpose use the getBlockCommentAst() method that returns a DetailAST node.

Example:

class MyCheck extends AbstractJavadocCheck {

    @Override
    public int[] getDefaultJavadocTokens() {
        return new int[]{JavadocTokenTypes.PARAMETER_NAME};
    }

    @Override
    public void visitJavadocToken(DetailNode paramNameNode) {
        String javadocParamName = paramNameNode.getText();
        DetailAST blockCommentAst = getBlockCommentAst();

        if (BlockCommentPosition.isOnMethod(blockCommentAst)) {

            DetailAST methodDef = blockCommentAst.getParent();
            DetailAST methodParam = findMethodParameter(methodDef);
            String methodParamName = methodParam.getText();

            if (!javadocParamName.equals(methodParamName)) {
                log(methodParam, "params.dont.match");
            }

        }
    }
}
      

HTML Code In Javadoc Comments

Checkstyle supports HTML4 tags in Javadoc comments: all HTML4 elements .

HTML4 is designed to have a list of elements whose end tag is optional(omittable) and a list of void elements (also known as empty html tags, for example BR tag).

HTML4 elements whose end tag is optional (omittable): <P>, <LI>, <TR>, <TD>, <TH>, <BODY>, <COLGROUP>, <DD>, <DT>, <HEAD>, <HTML>, <OPTION>, <TBODY>, <THEAD>, <TFOOT>.

Void HTML4 elements: <AREA>, <BASE>, <BASEFONT>, <BR>, <COL>, <FRAME>, <HR>, <IMG>, <INPUT>, <ISINDEX>, <LINK>, <META>, <PARAM>.

Checkstyle does not support HTML5 tags with optional end tags. See issue for details.

If the parser consumes an unknown tag (for example HTML5 tag) it doesn't fail, but parses this tag as a HTML_TAG Javadoc token type. Always follow Tight-HTML rules to make the Checkstyle javadoc parser create nested ASTs, even though tags are unknown.

<audio><source src="horse.ogg" type="audio/ogg"/></audio>
        

JAVADOC -> JAVADOC [0:0]
|--HTML_ELEMENT -> HTML_ELEMENT [0:0]
|   `--HTML_TAG -> HTML_TAG [0:0]
|       |--HTML_ELEMENT_START -> HTML_ELEMENT_START [0:0]
|       |   |--START -> < [0:0]
|       |   |--HTML_TAG_NAME -> audio [0:1]
|       |   `--END -> > [0:6]
|       |--HTML_ELEMENT -> HTML_ELEMENT [0:7]
|       |   `--SINGLETON_ELEMENT -> SINGLETON_ELEMENT [0:7]
|       |       `--EMPTY_TAG -> EMPTY_TAG [0:7]
|       |           |--START -> < [0:7]
|       |           |--HTML_TAG_NAME -> source [0:8]
|       |           |--WS ->   [0:14]
|       |           |--ATTRIBUTE -> ATTRIBUTE [0:15]
|       |           |   |--HTML_TAG_NAME -> src [0:15]
|       |           |   |--EQUALS -> = [0:18]
|       |           |   `--ATTR_VALUE -> "horse.ogg" [0:19]
|       |           |--WS ->   [0:31]
|       |           |--ATTRIBUTE -> ATTRIBUTE [0:32]
|       |           |   |--HTML_TAG_NAME -> type [0:32]
|       |           |   |--EQUALS -> = [0:36]
|       |           |   `--ATTR_VALUE -> "audio/ogg" [0:37]
|       |           `--SLASH_END -> /> [0:49]
|       `--HTML_ELEMENT_END -> HTML_ELEMENT_END [0:51]
|           |--START -> < [0:51]
|           |--SLASH -> / [0:52]
|           |--HTML_TAG_NAME -> audio [0:53]
|           `--END -> > [0:58]
`--EOF -> <EOF> [0:59]
        

This is an example of parsing an unknown tag that doesn't have a matching end tag (for example, HTML5 tag <audio>):
Input:

<audio>test

          [ERROR:0] Javadoc comment at column 1 has parse error. Missed HTML close tag 'audio'.
          Sometimes it means that close tag missed for one of previous tags.
        

As you see, the Javadoc parser prints an error and doesn't build the AST if an unknown HTML tag doesn't have a matching end tag. If that's the case, please create an issue for Checkstyle to consider adding support for this tag.

There are also HTML tags that are marked as "Not supported in HTML5" (HTML Element Reference). Checkstyle Javadoc parser can parse those tags too if they are written in Tight-HTML.
Example:
Input:

<acronym title="as soon as possible">ASAP</acronym>

        

JAVADOC -> JAVADOC [0:0]
|--HTML_ELEMENT -> HTML_ELEMENT [0:0]
|   `--HTML_TAG -> HTML_TAG [0:0]
|       |--HTML_ELEMENT_START -> HTML_ELEMENT_START [0:0]
|       |   |--START -> < [0:0]
|       |   |--HTML_TAG_NAME -> acronym [0:1]
|       |   |--WS ->   [0:8]
|       |   |--ATTRIBUTE -> ATTRIBUTE [0:9]
|       |   |   |--HTML_TAG_NAME -> title [0:9]
|       |   |   |--EQUALS -> = [0:14]
|       |   |   `--ATTR_VALUE -> "as soon as possible" [0:15]
|       |   `--END -> > [0:37]
|       |--TEXT -> ASAP [0:38]
|       `--HTML_ELEMENT_END -> HTML_ELEMENT_END [0:42]
|           |--START -> < [0:42]
|           |--SLASH -> / [0:43]
|           |--HTML_TAG_NAME -> acronym [0:44]
|           `--END -> > [0:51]
`--EOF -> <EOF> [0:52]

        

More examples:

<p> First
<p> Second

              

<p> First </p>
<p> Second </p>

              

JAVADOC -> JAVADOC [0:0]
|--HTML_ELEMENT -> HTML_ELEMENT [0:0]
|   `--P_TAG_START -> P_TAG_START [0:0]
|       |--START -> < [0:0]
|       |--P_HTML_TAG_NAME -> p [0:1]
|       `--END -> > [0:2]
|--TEXT ->  First [0:3]
|--NEWLINE -> \n [0:9]
|--HTML_ELEMENT -> HTML_ELEMENT [1:0]
|   `--P_TAG_START -> P_TAG_START [1:0]
|       |--START -> < [1:0]
|       |--P_HTML_TAG_NAME -> p [1:1]
|       `--END -> > [1:2]
|--TEXT ->  Second [1:3]
`--EOF -> <EOF> [1:10]

              

JAVADOC -> JAVADOC [0:0]
|--HTML_ELEMENT -> HTML_ELEMENT [0:0]
|   `--PARAGRAPH -> PARAGRAPH [0:0]
|       |--P_TAG_START -> P_TAG_START [0:0]
|       |   |--START -> < [0:0]
|       |   |--P_HTML_TAG_NAME -> p [0:1]
|       |   `--END -> > [0:2]
|       |--TEXT ->  First  [0:3]
|       `--P_TAG_END -> P_TAG_END [0:10]
|           |--START -> < [0:10]
|           |--SLASH -> / [0:11]
|           |--P_HTML_TAG_NAME -> p [0:12]
|           `--END -> > [0:13]
|--NEWLINE -> \n [0:14]
|--HTML_ELEMENT -> HTML_ELEMENT [1:0]
|   `--PARAGRAPH -> PARAGRAPH [1:0]
|       |--P_TAG_START -> P_TAG_START [1:0]
|       |   |--START -> < [1:0]
|       |   |--P_HTML_TAG_NAME -> p [1:1]
|       |   `--END -> > [1:2]
|       |--TEXT ->  Second  [1:3]
|       `--P_TAG_END -> P_TAG_END [1:11]
|           |--START -> < [1:11]
|           |--SLASH -> / [1:12]
|           |--P_HTML_TAG_NAME -> p [1:13]
|           `--END -> > [1:14]
`--EOF -> <EOF> [1:15]

              

Checks can also be configured to log violations upon encountering non-tight HTML tags. The violateExecutionOnNonTightHtml property can be used for this purpose in the checks that support it. A custom check needs to extend AbstractJavadocCheck to have this functionality readily available. Do note that a check which has this property set to true, will log violations only for the first not-tight HTML tag found. To allow a check to skip processing of javadocs with non-tight HTML, the acceptJavadocWithNonTightHtml method in class AbstractJavadocCheck can be overridden in the check. The following example illustrates how to use this property.

Input:

/**
  * <body>
  * <p> This class is only meant for testing. </p>
  * <p> This p tag is not closed. It is non-tight. Will lead to violations if
  * <tt>violateExecutionOnNonTightHtml</tt> is set to true for the check.
  * <li>tight li tag <p>non-tight p tag, but only the 1st non-tight tag is logged in violation</li>
  * </body>
  */
 public class Test {
     /**
      <p><p>
      paraception. Will result in a violation from the <tt>JavadocParagraph</tt> check due to
      redundant tags.
      </p></p>
      */
     private int field1;

     /**<tr> Zero </p> nesting despite `tr` is closed </tr>*/
     private int field2;

     /**
      * <p> this paragraph is closed and would be nested in javadoc tree </p>
      * <li> this list has an <p> unclosed para, but still the list would get nested </li>
      */
     private int field3;

     /**
      * <li> Complete <p> nesting </p> </li>
      */
     private int field4;
}

      

Output with violateExecutionOnNonTightHtml set to false:

<!DOCTYPE module PUBLIC
          "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
          "https://checkstyle.org/dtds/configuration_1_3.dtd">
<module name="Checker">
  <module name="TreeWalker">
    <module name="JavadocParagraph">
      <property name="violateExecutionOnNonTightHtml" value="false"/>
    </module>
  </module>
</module>
              

Starting audit...
[ERROR] Test.java:11: Redundant <p> tag. [JavadocParagraph]
Audit done.
Checkstyle ends with 1 errors.
              

Output with violateExecutionOnNonTightHtml set to true:

<!DOCTYPE module PUBLIC
          "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
          "https://checkstyle.org/dtds/configuration_1_3.dtd">
<module name="Checker">
  <module name="TreeWalker">
    <module name="JavadocParagraph">
      <property name="violateExecutionOnNonTightHtml" value="true"/>
    </module>
  </module>
</module>
              

Starting audit...
[ERROR] Test.java:4: Unclosed HTML tag found: p [JavadocParagraph]
[ERROR] Test.java:11: Redundant <p> tag. [JavadocParagraph]
[ERROR] Test.java:11: Unclosed HTML tag found: p [JavadocParagraph]
[ERROR] Test.java:18: Unclosed HTML tag found: tr [JavadocParagraph]
[ERROR] Test.java:23: Unclosed HTML tag found: p [JavadocParagraph]
Audit done.
Checkstyle ends with 5 errors.
              

Checkstyle SDK GUI

The Checkstyle GUI provides us with a way to show javadoc trees in java files. To run it, use

          java -cp checkstyle-10.21.0-all.jar com.puppycrawl.tools.checkstyle.gui.Main
        

and choose "JAVA WITH JAVADOC MODE" in dropdown list in bottom of frame.

Now you can see a parsed javadoc tree as a child of comment block.

Notice that only files with ".java" extension can be opened.

For detail reference you can see Checkstyle GUI documentation .

Customize token types in Javadoc Checks

Java checks are controlled by methods setTokens(), getDefaultTokens(), getAccessibleTokens(), getRequiredTokens(). JavaDoc checks use the same model plus 4 additional methods for Javadoc tokens. As Java AST and Javadoc AST are not bound, it is highly recommended for Javadoc checks to not use customization of java tokens and except to be executed only on javadoc tokens.

There are four methods in the AbstractJavadocCheck class to control the processed JavadocTokenTypes - one setter setJavadocTokens(), which is used to define a custom set (which is different from the default one) of the processed JavadocTokenTypes via config file and three getters, which have to be overridden: getDefaultJavadocTokens(), getAcceptableJavadocTokens(), getRequiredJavadocTokens().

• setJavadocTokens() - method then define actual set of tokens to run on.
• getDefaultJavadocTokens() - returns a set of JavadocTokenTypes which are processed in visitToken() method by default.
• getRequiredJavadocTokens() - returns a set of JavadocTokenTypes which Check must be subscribed to for a valid execution. If the user wants to specify a custom set of JavadocTokenTypes then this set must contain all the JavadocTokenTypes from RequiredJavadocTokens.
• getAcceptableJavadocTokens() - returns a set, which contains all the JavadocTokenTypes that can be processed by the check. Both DefaultJavadocTokens and RequiredJavadocTokens and any custom set of JavadocTokenTypes are subsets of AcceptableJavadocTokens.

Integrating new Javadoc Check

Javadoc Checks as well as regular Checks extend AbstractCheck class. So integrating a new Javadoc Check is similar to integrating other Checks.

Declare check's external resource locations

See Declare check's external resource locations.

Examples of Javadoc Checks

The best source knowledge on how to write Javadoc Checks can be taken from existing Checks .

Javadoc parser behavior for current HTML version and new HTML version

This section shows how the parser should/will behave during parsing of the current HTML version and any new HTML version. The current version is HTML4, a new version that needs to be supported is HTML5. GeneralToken - mean that after parsing there will be general AstToken - HTML_TAG. SpecialToken - mean that after parsing there will be special AstToken - PARAGRAPH, .... .

Tags with optional (omittable) end tag:

Void tags:

Note: "Nested"/"Non-Nested" is not applicable for this type of tags - all of them are looks like Non-Nested. Flas "hasUnclosedTag" is "false" for all cases.