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Java Standard Edition (SE)

Taglet Overview


Taglet Overview

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Taglet API

Taglets are programs written in the Java™ programming language that implement the Taglet API. Taglets support both block tags, such as @todo, and inline tags, such as {@underline}. (See block and inline tags.) The Taglet API consists of one interface and two abstract classes:
  • Taglet API
       Taglet Interface
    Implement when writing a block taglet that does not need to support inline tags, or when writing a general inline tag
    BaseExecutableMemberTaglet class
    Extend when writing a block taglet that supports inline tags
    BaseInlineTaglet class
    Extends when writing an inline tag that needs access to the HtmlStandardWriter and/or the doc object
A taglet can modify and format the text argument of a custom tag, and it can redirect the text to a file or other stream. A taglet can override a standard tag.

Taglets are a the foundation beneath the -tag option. When you use the -tag option, it uses a built-in taglet to generate default HTML formatting that is similar to that generated for @return.

Writing a Taglet

Choose which type of taglet you want to write, and implement the appropriate interface or extend the appropriate abstract class:
  • Block taglets
    • that support inline tags - extend BaseExecutableMemberTaglet
    • that do not support inline tags - implement Taglet
  • Inline taglets
    • general inline tags - implement Taglet
    • that need access to the HtmlStandardWriter or Doc object - extend BaseInlineTag
The BaseExecutableMemberTaglet class extends Taglet to provide the additional functionality to support inline tags. It supports both standard inline tags, such as {@link} and {@docroot}, and custom inline tags you might write.

If you write an inline tag that needs access to the HtmlStandardWriter or the Doc object, it should extend BaseInlineTag. For example, the {@value} inline tag needs the Doc object to retrieve the constant value from the field. Most tags only need the HtmlStandardWriter to report errors using the doclet error reporting methods.

Here are the basic steps you need to follow to create and use your own taglet:

  1. Write the Java program that constitutes your taglet. Include import statements appropriate to the taglet you're writing. For example, if extending BaseExecutableMemberTaglet, your program should import the following:
    import com.sun.tools.doclets.Taglet;                                        // Taglet API
    import com.sun.tools.doclets.standard.tags.BaseExecutableMemberTaglet;      // For inline tags
    import com.sun.tools.doclets.standard.HtmlStandardWriter;                   // For error reporting
    import com.sun.javadoc.*;                                                   // Doclet API
    import java.util.Map;                                                       // Used in register(Map)
    
    The Tag interface used in toString(Tag) is part of the Doclet API.
  2. In addition to the requirements of the interface you are implementing or abstract class you are extending, your program must implement the following static method. Then enables the taglet to be loaded at runtime. See the example below.
        public static void register(Map tagletMap)  
    
    
    Your class must also implement the requirements of the interface it extends or class it implements. The toString method is where all of the custom work is implemented to modify, format or redirect the text argument passed in to your custom tag.
        toString(Doc, HtmlStandardWriter)
    
  3. Compile your doclet. Use javac compiler version 1.4.0 (or later) in the Java 2 SDK. The required class files are in the lib\tools.jar file in the SDK. For the first example below, assuming the SDK is installed at C:\Program Files\j2sdk1.4.0.
       javac -classpath "C:\Program Files\j2sdk1.4.0\lib\tools.jar"  ToDoTaglet.java
    
  4. Run the javadoc tool using the -taglet and -tagletpath options. For example, if your taglet class file is defined to be in package com.sun and is stored in C:\taglets\com\sun\Taglet.class, then you should set tagletpath to C:\taglets. This example calls javadoc on package com.package1, including ToDoTaglet tags:
       javadoc -taglet ToDoTaglet -tagletpath C:\taglets com.package1
    

The following are examples of block and inline taglets.

Example - A Simple Block Taglet

The source code for an example of a block taglet implementing @todo is included at:

The corresponding class file ToDoTaglet.class is already compiled and saved in the same directory as this source file.

To run the doclet, include the -tagletpath and -taglet options. Assuming the ToDoTaglet.class is located in /home/user/taglet, and the @todo tag is located in the Test.java file:

% javadoc -d html -tagletpath /home/user/taglet -taglet ToDoTaglet ./Test.java
If the file Test.java contains the following doc comment:
    /**
     * @todo Fix this!
     */
then the ToDo taglet formats the HTML output as follows:
To Do:
Fix this!

Implementation Description

Let's look at the source code. To name the tag and define the header text, define two private fields:
    private static final String NAME = "todo";
    private static final String HEADER = "To Do:";
To make this a block tag rather than an inline tag, you set isInlineTag to return false:
    public boolean isInlineTag() {
        return false;
    }
There are other methods inField, inMethod, inType, inPackage and inOverview that you specify true or false to indicate in which doc comments in the source code this tag can be used.

The toString(Tag) method determines how the text is inserted into the output when a single {@todo} tag is encountered. This code creates a bold heading followed by a table with a yellow background that contains the text (specified by tag.text()).

    public String toString(Tag tag) {`
        return "<DT><B>" + HEADER + "</B><DD>"
               + "<table cellpadding=2 cellspacing=0><tr><td bgcolor=\"yellow\">"
               + tag.text() 
               + "</td></tr></table></DD>\n";
    }
Similarly, the toString(Tag[]) method (which takes an array of tags) determines how the text is inserted into the output when multiple {@todo} tags are encountered.

Example - A Block Taglet That Handles Inline Tags

The source code for an example of a block taglet implementing @todo is included at:

The corresponding class file ToDoTaglet.class is already compiled and saved in the same directory as this source file.

To run the doclet, include the -tagletpath and -taglet options. Assuming the ToDoTaglet.class is located in /home/user/taglet, and the @todo tag is located in the Test.java file:

% javadoc -d html -tagletpath /home/user/taglet -taglet ToDoTaglet ./Test.java
If the file Test.java contains the following doc comment:
    /**
     * @todo Fix this!
     */
then the ToDo taglet formats the HTML output as follows:
To Do:
Fix this!

Implementation Description

Let's look at the source code. To name the tag and define the header text, define two private fields:
    private static final String NAME = "todo";
    private static final String HEADER = "To Do:";
To make this a block tag rather than an inline tag, you set isInlineTag to return false:
    public boolean isInlineTag() {
        return false;
    }
There are other methods inField, inMethod, inType, inPackage and inOverview that you specify true or false to indicate in which doc comments in the source code this tag can be used.

The toString(Tag) method determines how the text is inserted into the output when a single {@todo} tag is encountered. This code creates a bold heading followed by a table with a yellow background that contains the text (specified by tag.text()).

    public String toString(Tag tag) {`
        return "<DT><B>" + HEADER + "</B><DD>"
               + "<table cellpadding=2 cellspacing=0><tr><td bgcolor=\"yellow\">"
               + tag.text() 
               + "</td></tr></table></DD>\n";
    }
Similarly, the toString(Tag[]) method (which takes an array of tags) determines how the text is inserted into the output when multiple {@todo} tags are encountered.

Example - An Inline Taglet

Unline block tags, a custom inline tag can only be implemented using a taglet (rather than using the -tag option). This is because there is no default behavior for inline tags.

The source code for an example of an inline taglet implementing {@underline} is included at:

The corresponding class file UnderlineTaglet.class is already compiled and saved in the same directory as this source file.

This taglet formats the output for the {@underline} tag. A doc comment containing the following tag:

    /**
     * Be sure to insert the value at the {@underline start} of the array.
     */
would be output in HTML as follows:

Be sure to insert the value at the start of the array.

Implementation Description

Let's look at how this source code differs from the previous example. Of course the tag name is different (and inline tags have no heading, so none is defined):
    private String NAME = "underline";
To define this as an inline tag rather than a block tag, you set isInlineTag to return true:
    public boolean isInlineTag() {
        return true;
    }

The methods inField, inMethod, inConstructor, inType, inPackage and inOverview apply only to block tags and must all be set to false for inline tags.

The toString(Tag) method determines how the text is inserted into the output when an {@underline} tag is encountered. This code creates surrounds the text with the HTML underline tags <ul> and </ul>.

    public String toString(Tag tag) {
        return "<u>" + tag.text() + "</u>";
    }

It is not possible to have an array of inline tags to process, like it is with block tags. Therefore, the toString(Tag[]) method (which takes an array of tags) is ignored for inline tags.

Handling Errors and Warnings

Errors - A taglet can report an error and stop the Javadoc tool by simply printing the error message and then calling System.exit().

Warnings - A taglet can report a warning using a MessageRetriever given to it by the Doclet instance. The MessageRetriever is obtained from the Configuration object. The Configuration object is obtained from the doclet. For example, if the Taglet is designed to be used with the Standard doclet, the Configuration can be retrieved using Standard.configuration(), a static method. As an example, this is how the SimpleTaglet prints warnings, which is the default taglet for the -tag option.



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