Tag Handlers

When the start tag of a custom tag is encountered, the JSP page implementation class calls methods to initialize the appropriate handler and then invokes the handler's doStartTag method. When the custom end tag is encountered, the handler's doEndTag method is invoked. Additional methods are invoked in between when a tag handler needs to interact with the body of the tag. For further information, see "How Is a Tag Handler Invoked?".

In order to provide a tag handler implementation, you must implement the methods that are invoked at various stages of processing the tag. The methods are summarized in Table 2.

Table 2 Tag Handler Methods

Tag Handler Type	Methods
Simple	doStartTag, doEndTag, release
Attributes	doStartTag, doEndTag, set/getAttribute1...N
Body, No Interaction	doStartTag, doEndTag, release
Body, Interaction	doStartTag, doEndTag, release, doInitBody, doAfterBody

A tag handler has access to an API that allows it to communicate with the JSP page. The entry point to the API is the page context object through which a tag handler can access to all the other implicit objects (request, session, and application) accessible from a JSP page. Implicit objects can have attributes associated with them. Such attributes are accessed using the appropriate [set/get]Attribute method.

If the tag is nested, a tag handler also has access to the handler (called the parent) associated with the enclosing tag.

Tag handlers must implement either the Tag or BodyTag interfaces. Interfaces can be used to take an existing Java object and make it a tag handler. For newly created handlers, you can use the TagSupport and BodyTagSupport classes as base classes. You can download documentation that describes these interfaces and classes from the JSP specification download page.