Tag Libraries Tutorial

How Is a Tag Handler Invoked?

The Tag interface defines the basic protocol between a tag handler and JSP page implementation class. It defines the life cycle and the methods to be invoked when the start and end tag of an action are encountered.

The JSP page implementation class invokes the setPageContext, setParent, and attribute setting methods before calling doStartTag. The JSP page implementation class also guarantees that release will be invoked on the tag handler before the end of the page.

Here is a typical tag handler method invocation sequence:

ATag t = new ATag();
t.setPageContext(...);
t.setParent(...);
t.setAttribute1(value1);
t.setAttribute2(value2);
t.doStartTag();
t.doEndTag();
t.release();

The BodyTag interface extends Tag by defining additional methods that let a tag handler access its body. The interface provides three new methods:

setBodyContent - creates body content and adds to tag handler

doInitBody - called before evaluation of tag body

doAfterBody - called after evaluation of tag body

A typical invocation sequence is:

t.doStartTag();
out = pageContext.pushBody();
t.setBodyContent(out);
// perform any initialization needed after body content is set
t.doInitBody();
t.doAfterBody();
// while doAfterBody returns EVAL_BODY_TAG we 
// iterate body evaluation
...
t.doAfterBody();
t.doEndTag();
t.pageContext.popBody();
t.release();

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