Taglet Overview

Taglet API
Writing a Taglet
Example - A Block Taglet
Example - An Inline Taglet
Handling Errors and Warnings

Taglet API

Taglets are programs written in the Java^TM programming language that implement the Taglet API. Taglets can be written as either block tags, such as @todo, or inline tags, such as {@underline}. Block taglets do not currently support inline tags in their text. (See block and inline tags.) The Taglet API consists of one interface:

Taglet interface

A taglet can modify and format the text argument of a custom tag, and do other things, such as redirect the text to a file or other stream. A taglet can override a standard tag.

Taglets are 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

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

Write the Java program that constitutes your taglet. Your program should import the following:

   import com.sun.tools.doclets.Taglet;    // Taglet API
   import com.sun.javadoc.*;               // Doclet API
   import java.util.Map;                   // Used in register(Map)

The com.sun.javadoc classes must be imported for the Tag interface used in toString(Tag).

Your class must implement the requirements of the interface it extends. 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.
```
    public String toString(Tag tag)
```
In addition to the requirements of the interface you are implementing, your program must implement the following static method. Then enables the taglet to be loaded at runtime. See the source code in the example below.
```
    public static void register(Map tagletMap)  
```
Compile your doclet. Use javac compiler version 1.4.0 (or later) in the JDK. The required class files are in the lib\tools.jar file in the JDK. For the first example below, assuming the JDK is installed at C:\Program Files\j2sdk1.4.1.
```
   javac -classpath "C:\Program Files\j2sdk1.4.1\lib\tools.jar"  ToDoTaglet.java
```
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 examples are of block and inline taglets.

Example - A Block Taglet

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

ToDoTaglet.java

The corresponding class file ToDoTaglet.class is already compiled and saved in the same directory as this source file.
This taglet formats the output for the @todo tag. A doc comment containing the following tag:
    /**
     * @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.
The register method
    /**
     * Register this Taglet.
     * @param tagletMap  the map to register this tag to.
     */
    public static void register(Map tagletMap) {
       ToDoTaglet tag = new ToDoTaglet();
       Taglet t = (Taglet) tagletMap.get(tag.getName());
       if (t != null) {
           tagletMap.remove(tag.getName());
       }
       tagletMap.put(tag.getName(), tag);
    }

Example - An Inline Taglet

Unlike 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:

UnderlineTaglet.java

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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