Professional Java Server Programming J2EE Edition
Chapter 12
TagSupport and BodyTagSupport are concrete, not abstract classes, so they provide complete
implementations of the corresponding interfaces, which do nothing except return the appropriate values to cause
the JSP engine to continue rendering the page. So developers can safely omit methods that they are not interested
in. Developers don't usually concern themselves with handling the parent property and setPageContext().
The release() method can also be omitted if it is not necessary to free resources or return the tag to its
default state. The methods that a developer will normally want to override are doStartTag() and
doEndTag() for all tags, and doInitBody() and doAfterBody() for BodyTags specifically.
TagSupport also makes an important convenience variable available to subclasses: pageContext (the
saved PageContext which was set by the JSP engine when the tag was first used in a page).
BodyTagSupport provides a getBodyContent() method, necessary to obtain a tag's BodyContent
before manipulating it.
Like me, you might find the names TagSupport and BodyTagSupport confusing. These classes
are standard implementations of the Tag and BodyTag interfaces. It should be pointed out that this
naming is inconsistent with Sun's usual practice and Java convention; I think of them as TagImpl and
BodyTagImpl.
The javax.servlet.jsp.tagext.TagExtraInfo Class
Metadata classes extending the TagExtraInfo abstract class may be associated with tag handlers to provide
extra information to a JSP engine. This optional association is specified in the tag library descriptor.
We will look at the implementation of a TagExtraInfo class in more detail later in this chapter, but the two
methods you are most likely to override are:
VariableInfo[] getVariableInfo(TagData td)
This method is used to return information about scripting variables that the tag makes available to JSPs using
it. It returns an array of VariableInfo objects, which contain information about the name of each scripting
variable and its fully qualified class name. It describes whether or not the variable should be declared or
whether the tag will merely overwrite the value of an existing variable, and the scope of the variable.
boolean isValid(TagData data)
This is sometimes used to validate the attributes passed to a tag at translation time. As an example, consider a
tag with four attributes. Three may be optional, but if one of the optional attributes is specified the whole
three must be present. There is no way to specify this behavior in a tag library descriptor. However, the
isValid() method of the appropriate TagExtraInfo subclass could be implemented to return false if
the parameters supplied at translation time are invalid. The default implementation of isValid() in
TagExtraInfo always returns true. Note that runtime attribute validation is entirely different, and is the
responsibility of the tag handler concerned.
Objects Available to Tag Handlers
All tag handlers have access to more context information than do most beans. This is available through the
PageContext object they are passed on initialization. As you'll recall, javax.servlet.jsp.PageContext
is a convenient holder for information about the runtime of a JSP page, including the request and
response objects, and references to objects such as beans associated with the JSP.
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