Professional Java Server Programming J2EE Edition
Chapter 12
The javax.servlet.jsp.tagext.Tag Interface
The Tag interface defines a simple interaction between the JSP engine and the tag handler, sufficient for tags
that don't need to manipulate their body content. Its core methods are the calls implementing classes will
receive when the JSP engine encounters the tag's opening and closing tags, doStartTag() and
doEndTag(). Before we look at the method contracts in more detail, a sequence diagram helps to visualize
the calls made to the tag handler by the compiled servlet. Assume that the container already has a tag handler
instance available, and in the default state:
Let's look at the messages in more detail:
- The container initializes the tag handler by setting the tag handler's pageContext property,
which the tag handler can use to access information available to the JSP currently using it.
-
The container sets the tag handler's parent property. (Parent may be set to null, if the tag is
not enclosed in another tag.)
-
Any tag attributes defined by the developer will be set. This is a mapping from the XML
attributes of the tag to the corresponding properties of the tag handler bean. For example, in
the case of a tag invoked like this: <mytags:test name="John" age="43" />, the
container will attempt to call the setName() and setAge() methods on the tag handler. The
container will attempt to convert each attribute to the type of the corresponding bean
property: for example, the String "43" will be converted to an int in this case. If the type
conversion fails, an exception will be thrown and must be handled by the calling JSP page.
(From a JSP's point of view, there is no difference between an exception thrown by a tag
handler and one thrown by an expression of scriptlet in the page.)
New on the Java Boutique:
New Review:
Time Management Made Easy with the Quartz Enterprise Job Scheduler
Why not just use the Java timer API? This open source scheduling
API boasts simplicity, ease-of-integration, a well-rounded feature
set, and it's free!
New Applet:
Reverse Complement
Reverse Complement is a simple applet that converts DNA or RNA
sequences into three useful formats.
Elsewhere on internet.com:
WebDeveloper Java
Lots of Java information on webdeveloper.com
WDVL Java
Thorough Java resource at the Web Developer's Virtual Library.
ScriptSearch Java
Hundreds of free Java code files to download.
jGuru: Your View of the Java Universe
Customizable portal with online training, FAQs, regular news updates, and tutorials.
