|
|
JSP Tags
Another important syntax element of JSP are tags. JSP tags do not use
<%, but just the < character. A JSP tag is somewhat like
an HTML tag. JSP tags can have a "start tag", a "tag body" and an "end
tag". The start and end tag both use the tag name, enclosed in < and >
characters. The end starts with a / character after the <
character. The tag names have an embedded colon character : in them, the
part before the colon describes the type of the tag. For instance:
<some:tag>
body
</some:tag>
If the tag does not require a body, the start and end can be conveniently merged
together, as
<some:tag/>
Here by closing the start tag with a /> instead of > character, we are
ending the tag immediately, and without a body. (This syntax convention is
the the same as XML.)
Tags can be of two types: loaded from an external tag library, or predefined
tags. Predefined tags start with jsp: characters. For
instance, jsp:include is a predefined tag that is used to include other pages.
We have already seen the include directive. jsp:include is similar.
But instead of loading the text of the included file in the original file, it
actually calls the included target at run-time (the way a browser would call
the included target. In practice, this is actually a simulated request
rather than a full round-trip between the browser and the server).
Following is an example of jsp:include usage
<HTML>
<BODY>
Going to include hello.jsp...<BR>
<jsp:include page="hello.jsp"/>
</BODY>
</HTML>
Try it and see what you get. Now change the "jsp:include" to "jsp:forward" and see what is
the difference. These two predefined tags are frequently very useful.
Exercise: Write a JSP to do either a forward or an include, depending upon
a boolean variable (hint: The concepts of mixing HTML and scriptlets work
with JSP tags also!) |