JavaServer Pages, 3rd Edition

Developing a Basic Action

The main difference between the simple and classic tag handler APIs is that while the SimpleTag interface has only one method for asking the tag handler to complete its processing, the Tag interface has two: doStartTag( ) and doEndTag( ) (the Tag subinterfaces for more specialized classic tag handlers add even more methods). The container calls these methods when the start tag and end tag are encountered, as shown in Figure 21-4.

Figure 21-4. Tag interface methods and property setter methods

Attribute values are set using bean setter methods, just as for the simple tag handler. The doStartTag( ) and doEndTag( ) method return values that controls what happens next, for instance how to deal with the custom action body. This is another significant difference compared to the simple tag handler; a classic tag handler needs to ask the container (through the return value) to evaluate the body rather than doing it itself.

A tag handler that implements just the Tag interface can add dynamic content to the response body and set response headers, add or remove variables in one of the JSP scopes, and tell the container to either include the action element’s body in the response or ignore it.

Here are the most important methods of the Tag interface:

public void setPageContext(PageContext pageContext); public int doStartTag( ) throws JspException; public int doEndTag( ) ...