Professional Java Server Programming J2EE Edition
Chapter 12
<jsp:forward page="next.jsp" />
We can also to add additional parameters to the request before
forwarding with an extended usage of <jsp:forward>, which
nests one or more <jsp:param> tags within the
<jsp:forward> tag:
<:RE>
<jsp:forward page="next.jsp" >
<jsp:param name="image" value="house.gif" />
</jsp:forward>
Tag extensions allow a vast range of new functionality to be added to
the JSP language and they can be invoked in a similarly intuitive way.
For example, I could create a tag named <wrox:forward>, specify
what attributes and subtags, if any, it requires, and implement it to
perform a custom action before forwarding. Not only can this be added
simply into the web page, it enforces separation of code and presentation,
decouples the call from the class that implements the functionality
associated with the tag, and can be simply incorporated into a design
tool.
The key concepts in tag extensions are:
- Tag name:
- A JSP tag is uniquely identifed by a combination of prefix (in this case jsp), and suffix (in this
case forward), separated by a colon.
- Attributes:
- Tags may have attributes, which use the XML syntax for attributes. The <jsp:forward> tag
above has one attribute (page), while the <jsp:param> attribute has two (name and value).
Attributes may be required or optional.
- Nesting:
- Note how the <jsp:param> subtag is used in the second example above. Tag extensions can
detect nested tags at runtime and cooperate. A tag directly enclosing another tag is called the
parent of the tag it encloses: in the example above, the <jsp:forward> tag is the parent of
the <jsp:param> tag.
- Body content:
- This is anything between the start and end elements in a JSP tag, excluding subtags. A tag
extension can access and manipulate its body content. Neither the <jsp:forward> nor
<jsp:param> tags require body content. We will see later an example of a tag that can
reverse its body content. It will be invoked like this (the body content is shown in bold):
<examples:reverse>
Able was I ere I saw Elba
The functionality associated with a tag is implemented by one or more Java classes. The
tag handler (the class implementing the tag itself) is a JavaBean, with properties matching
the tag's XML attributes. A Tag Library Descriptor (TLD) file is an XML document that
describes a tag library, which contains one or more tag extensions. The JSP taglib
directive must be used to import the tag library's tags in each JSP that wishes to use any of
them.
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