Java Reference
In-Depth Information
After dragging the
JSTL If
item to our page, NetBeans prompts us for
additional information.
The value of the
Condition
field must be enclosed in
${}
; this denotes this value as a
JSTL expression
. In our particular example, we are looking for a request parameter
named
displayConditionalText
, whose value is
true
. If (and only if) the request
parameter is present and has the expected value, then the text inside the
<c:if>
tag
will be rendered in the generated HTML page from our JSP.
In the above screenshot,
param
is a
JSTL implicit object
to obtain the value of
a request parameter.
param.displayConditionalText
is equivalent to
request.
getParameter("displayConditionalText")
. As we can see, using the implicit
object allows us to save quite a bit of typing, and it makes our expression a lot more
readable. There are a lot of JSTL implicit objects, the most common ones are
param
,
applicationScope
,
sessionScope
,
requestScope
, and
pageScope
. Like we already
saw,
param
allows us to easily retrieve request parameters. The others in the list
allow us to retrieve attributes in the application, session, request, and page scopes,
respectively. They all use the dot notation we saw in the
param
implicit object, with
the key used to store the attribute following the dot, and return the object attached to
the appropriate scope with said key.
To see all implicit JSTL objects, simply invoke code completion
(
Ctrl+Space
) between the two curly braces in a JSTL expression (
${}
).
Back to our example, the
Variable Name
field is optional, if entered. It will be used
to store the value of the conditional expression in a Boolean variable. The
Scope
field,
is also optional. If a value is selected, this will be the scope of the variable entered
in the
Variable Name
field; if no value is selected for the
Scope
field, and a value is
entered for the
Variable Name
field, then the variable will have a default scope
of page.
