What is Javadoc comment

Javadoc comment is multiline comment /* */ that starts with * character and placed above class definition, interface definition, enum definition, method definition or field definition.

For example, here is java file:

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

}
      
Javadoc content is:
 * My <b>class</b>.
 * @see AbstractClass
      
Attention that java comment starts with /*, following with Identificator of comment type. Javadoc Identificator is *. All symbols after Javadoc Identificator till */ are part of javadoc comment.

Please not that javadoc-like comment inside a method is not a javadoc comment and skiped by Sun/Oracle javadoc tool and by our parser.

In internet you can find different types of documentation generation tools similar to javadoc. Such tools rely on specific Identificator: "!", "#", "$". Comments looks like "/*! some comment */" , "/*# some comment */" , "/*$ some comment */". Such multiline comments are not a javadoc.

Limitations

Javadoc by specification could contain any HTML tags that let user generate content he needs. All tags are copied as is to result javadoc html pages by Sun/Oracle javadoc tool. All bad formatting is responsibility of user and web-browser. To validate Chekcstyle to parse input to predictable structure - Abstract Syntax Tree(AST). It is very difficult to parse free style format, so input text need to follow some format, so limitation appears.

The comment should be written in Tight-HTML to build nested AST Tree 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(ommitable) by HTML specification (example is TR), so, Checkstyle won't show error about missing end tag, however, it leads to broken Tight-HTML structure and as a result leads to not-nested content of the HTML tags in Abstract Syntax Tree of the Javadoc comment.
In other words, if HTML tags are not closed Javadoc grammar cannot determine content of these tags, so structure of the parse tree will not be nested like it is while using Tight-HTML code. It is done just to not fail on every Javadoc comment, because there are tons of using unclosed tags, etc.

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(ommitable) 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

Principle of writing Javadoc Checks is similar to writing regular Checks. You just extend another abstract class and use another token types.

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

Difference between Java Grammar and Javadoc comments Grammar

Java grammar parses java file base on Java language specifications. So, there are singleline comments and multiline/block comments in it. Java compiler doesn't know about Javadoc because it is just a multiline comment. To parse multiline comment as a Javadoc comment, checkstyle has special Parser that is based on ANTLR Javadoc grammar. So, it's supposed to proccess block comments that start with Javadoc Identificator and parse them to Abstract Syntax Tree (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.

Main Java grammar skips any whitespaces and newlines, so in Java Abstract Syntax Tree there are no whitespace/newline nodes. In Javadoc comment every whitespace matters, and Javadoc Checks need all those whitespaces and newline nodes to verify format and content of the Javadoc comment. Because of that Javadoc grammar includes all whitespaces, newlines to the parse tree (WS, NEWLINE).

Tool to print Javadoc tree structure

Checkstyle can print Abstract Syntax Tree for Java and Javadoc trees. You need to run checkstyle jar file with -J argument, providing 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 -> \n * My <b>class</b>.\n * @see AbstractClass\n <EOF> [1:0]
|   |   |       |--NEWLINE -> \n [1:0]
|   |   |       |--LEADING_ASTERISK ->  * [2:0]
|   |   |       |--TEXT ->  My  [2:2]
|   |   |       |--HTML_ELEMENT -> <b>class</b> [2:6]
|   |   |       |   `--HTML_TAG -> <b>class</b> [2:6]
|   |   |       |       |--HTML_ELEMENT_OPEN -> <b> [2:6]
|   |   |       |       |   |--OPEN -> < [2:6]
|   |   |       |       |   |--HTML_TAG_NAME -> b [2:7]
|   |   |       |       |   `--CLOSE -> > [2:8]
|   |   |       |       |--TEXT -> class [2:9]
|   |   |       |       `--HTML_ELEMENT_CLOSE -> </b> [2:14]
|   |   |       |           |--OPEN -> < [2:14]
|   |   |       |           |--SLASH -> / [2:15]
|   |   |       |           |--HTML_TAG_NAME -> b [2:16]
|   |   |       |           `--CLOSE -> > [2:17]
|   |   |       |--TEXT -> . [2:18]
|   |   |       |--NEWLINE -> \n [2:19]
|   |   |       |--LEADING_ASTERISK ->  * [3:0]
|   |   |       |--WS ->   [3:2]
|   |   |       |--JAVADOC_TAG -> @see AbstractClass\n  [3:3]
|   |   |       |   |--SEE_LITERAL -> @see [3:3]
|   |   |       |   |--WS ->   [3:7]
|   |   |       |   |--REFERENCE -> AbstractClass [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]
      

As you 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: classes, methods, comments, etc.

In most cases while developing Javadoc Check you need only parse tree of the exact Javadoc comment. To do that just copy Javadoc comment to separate file and remove /** at the begining 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 ->  * My <b>class</b>.\r\n * @see AbstractClass<EOF> [0:0]
|--LEADING_ASTERISK ->  * [0:0]
|--TEXT ->  My  [0:2]
|--HTML_ELEMENT -> <b>class</b> [0:6]
|   `--HTML_TAG -> <b>class</b> [0:6]
|       |--HTML_ELEMENT_OPEN -> <b> [0:6]
|       |   |--OPEN -> < [0:6]
|       |   |--HTML_TAG_NAME -> b [0:7]
|       |   `--CLOSE -> > [0:8]
|       |--TEXT -> class [0:9]
|       `--HTML_ELEMENT_CLOSE -> </b> [0:14]
|           |--OPEN -> < [0:14]
|           |--SLASH -> / [0:15]
|           |--HTML_TAG_NAME -> b [0:16]
|           `--CLOSE -> > [0:17]
|--TEXT -> . [0:18]
|--NEWLINE -> \r\n [0:19]
|--LEADING_ASTERISK ->  * [1:0]
|--WS ->   [1:2]
|--JAVADOC_TAG -> @see AbstractClass [1:3]
|   |--SEE_LITERAL -> @see [1:3]
|   |--WS ->   [1:7]
|   `--REFERENCE -> AbstractClass [1:8]
|       `--CLASS -> AbstractClass [1:8]
`--EOF -> <EOF> [1:21]
      

Access Java AST from Javadoc Check

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

For example, to write a JavadocCheck that verifies @param tags in Javadoc comment of a method definition, you also need all method's parameter names. To get method definition AST you should access java DetailAST tree from javadoc Check. For this purpose use getBlockCommentAst() method that returns 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 picked just to have a list of elements whose end tag is optional(ommitable) and a list of void elements (also known as empty html tags, for example BR tag).

HTML4 elements whose end tag is optional(ommitable): <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>.

To make Checkstyle support HTML5 tags whose end tag is optional(ommitable) and HTML5 void elements we should update Javadoc Parser bacause each element that breaks Tight-HTML rules have to be defined in Javadoc grammar. In future we should update Javadoc grammar if those tag lists extend (new tags, new HTML standard, etc.). (We already have an issue on updating Javadoc grammar to HTML5)

If Checkstyle meets unknown tag (for example HTML5 tag) it doesn't fail and parses this tag as HTML_TAG Javadoc token type. Just follow Tight-HTML rules to make Checkstyle javadoc parser make nested AST, even though tags are unknown.

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

Here is what you get if unknown tag doesn't have matching end tag (for example, HTML5 tag <audio>):
Input:

<audio>test
Output:
[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 Javadoc parser prints error and doesn't build AST if unknown HTML tag doesn't have matching end tag. If that a case please create an issue against Checkstyle to upgrade parser.

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>
      

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

More examples:

1) Unclosed paragraph HTML tag. As you see in the tree, content of the paragraph tag is not nested to this tag. That is because HTML tags are not closed by pair tag </p>, and Checkstyle requires Tight-HTML code to predictably parse Javadoc comments. 2) Here is correct version with open and closed HTML tags.
<p> First
<p> Second
          
<p> First </p>
<p> Second </p>
          
JAVADOC -> <p> First\r\n<p> Second<EOF> [0:0]
|--HTML_ELEMENT -> <p> [0:0]
|   `--P_TAG_OPEN -> <p> [0:0]
|       |--OPEN -> < [0:0]
|       |--P_HTML_TAG_NAME -> p [0:1]
|       `--CLOSE -> > [0:2]
|--TEXT ->  First [0:3]
|--NEWLINE -> \r\n [0:9]
|--HTML_ELEMENT -> <p> [1:0]
|   `--P_TAG_OPEN -> <p> [1:0]
|       |--OPEN -> < [1:0]
|       |--P_HTML_TAG_NAME -> p [1:1]
|       `--CLOSE -> > [1:2]
|--TEXT ->  Second [1:3]
`--EOF -> <EOF> [1:10]
          
JAVADOC -> <p> First </p>\r\n<p> Second </p><EOF> [0:0]
|--HTML_ELEMENT -> <p> First </p> [0:0]
|   `--PARAGRAPH -> <p> First </p> [0:0]
|       |--P_TAG_OPEN -> <p> [0:0]
|       |   |--OPEN -> < [0:0]
|       |   |--P_HTML_TAG_NAME -> p [0:1]
|       |   `--CLOSE -> > [0:2]
|       |--TEXT ->  First  [0:3]
|       `--P_TAG_CLOSE -> </p> [0:10]
|           |--OPEN -> < [0:10]
|           |--SLASH -> / [0:11]
|           |--P_HTML_TAG_NAME -> p [0:12]
|           `--CLOSE -> > [0:13]
|--NEWLINE -> \r\n [0:14]
|--HTML_ELEMENT -> <p> Second </p> [1:0]
|   `--PARAGRAPH -> <p> Second </p> [1:0]
|       |--P_TAG_OPEN -> <p> [1:0]
|       |   |--OPEN -> < [1:0]
|       |   |--P_HTML_TAG_NAME -> p [1:1]
|       |   `--CLOSE -> > [1:2]
|       |--TEXT ->  Second  [1:3]
|       `--P_TAG_CLOSE -> </p> [1:11]
|           |--OPEN -> < [1:11]
|           |--SLASH -> / [1:12]
|           |--P_HTML_TAG_NAME -> p [1:13]
|           `--CLOSE -> > [1:14]
`--EOF -> <EOF> [1:15]
          

Boolean flag indicating the presence of unclosed HTML tags

Not implemented yet, will be named "hasUnclosedTag". See Github Issue #3311. For expected behavior please read Javadoc parser behavior section.

Checkstyle SDK GUI

Checkstyle GUI allows to showing javadoc tree in java files. To run in use

java -cp checkstyle-7.4-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 parsed javadoc tree as child of comment block.

screenshot

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

There are four methods in 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 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 about how to write Javadoc Checks could be taken from existing Checks.

Javadoc parser behavior for current HTML version and new HTML version

This section shows how parser should/will behave during parsing of current HTML versoin and any new HTML version. Current version is HTML4, new version that need 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(ommitable) end tag:

Input Current standard (HTML4) Current standard with hasUnclosedTag flag After parser update for new standard (HTML5) After parser update for new standard with hasUnclosedTag flag
<p>text</p> No errors, Nested tree, SpecialToken No errors, Nested tree, SpecialToken, hasUnclosedTag=false No errors, Nested tree, SpecialToken No errors, Nested tree, SpecialToken, hasUnclosedTag=false
<p>text No errors, Non-nested tree, SpecialToken No errors, Non-nested tree, SpecialToken, hasUnclosedTag=true No errors, Non-nested tree, SpecialToken No errors, Non-nested tree, SpecialToken, hasUnclosedTag=true
<rb>text</rb>
New HTML5 tag with optional(ommitable) end tag
No errors, Nested tree, GeneralToken No errors, Nested tree, GeneralToken, hasUnclosedTag=false No errors, Nested tree, SpecialToken No errors, Nested tree, SpecialToken, hasUnclosedTag=false
<rb>text
New HTML5 tag with optional(ommitable) end tag
Parse error Parse error No errors, Non-nested tree, SpecialToken No errors, Non-nested tree, SpecialToken, hasUnclosedTag=true
<qwerty>text</qwerty>
Unknown HTML tag
No errors, Nested tree, GeneralToken No errors, Nested tree, GeneralToken, hasUnclosedTag=false No errors, Nested tree, GeneralToken No errors, Nested tree, GeneralToken, hasUnclosedTag=false
<qwerty>text
Unknown HTML tag
Parse error Parse error Parse error Parse error

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.
Input Current standard (HTML4) After parser update for new standard (HTML5)
<br/> No errors, SpecialToken No errors, SpecialToken
<br> No errors, SpecialToken No errors, SpecialToken
<embed/>
New HTML5 tag
No errors, GeneralToken No errors, SpecialToken
<embed>
New HTML5 tag
Parse Error No errors, SpecialToken
<basefont/>
Supported in HTML4. Not supported tag in HTML5
No errors, SpecialToken No errors, SpecialToken
<basefont>
Supported in HTML4. Not supported tag in HTML5
No errors, SpecialToken No errors, SpecialToken
<qwerty/>
Unknown HTML tag
No errors, GeneralToken No errors, GeneralToken
<qwerty>
Unknown HTML tag
Parse Error Parse Error