Professional Java Server Programming J2EE Edition
Chapter 12
Tag Nesting
One might expect that specifying tag nesting of custom tags would be done in the TLD. However, TLDs don't
allow for this, and nesting must be implemented by cooperating tag handler classes. Fortunately, the API helps
us in this respect by providing methods on tag handlers we can use to obtain information about their parents
and other ancestors. Although programmers of tag handlers must ensure that they enforce their desired tag
nesting, the dynamic discovery of tag nesting at runtime allows for greater flexibility than would be possible if
nesting were mandated in a static file. However, the absence of a formal grammar like a DTD or XML schema
does place a responsibility on developers to ensure that any cooperation between tags is well documented.
Why might we use tag nesting? A common reason is to handle iteration (nested tags can simulate nested
loops). Another is to let nested tags benefit from the context of the enclosing tag or tags.
Suppose we have additional information we want to display for the people named in our list, but that this
information is expensive to retrieve from a database or legacy system. So we don't want the hellos tag to
retrieve these additional fields with every iteration of the loop. (This need to retrieve regardless of usage is a
disadvantage of using scripting variables.)
One solution is to use a descendant tag that draws its context from the enclosing tag and performs the
additional lookups only when required: that is, only when the descendant tag is used. Let's implement a
NameTag that requires no attributes, but retrieves additional information for the user its parent is currently
processing. This information, nationality and city, will be exposed through scripting variables. Note that the
child tag, like any body content, will be evaluated each time the parent iterates over its body content.
The JSP code invoking this functionality might look like this (using a scriptlet to define and display the extra
information only when desired):
<examples:hellos names="<%=names%>" >
Hello <%=name%>. You're entry <%=index%> in my list.
<% if (condition) { %>
<examples:nameInfo>
<b>Nationality:</b> <%=nationality%> <b>City:</b> <%=city%>
</examples:nameInfo>
<% } %>
<br/>
</examples:hellos>
To implement this, we'll first need to add a method to VarHelloBodyTag that exposes the necessary
context, String getName().
While we're at it, we'll create an interface NameContext that contains this new method. This way, we
could make VarHelloTag, which already defines a getName() method, implement the interface and
provide the necessary context for our new subtags.
With tag extensions, as always, remember to program to interfaces rather than concrete
classes.
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