3.4.5 A tag with attributes
Our HelloWorldTag is predictable; in fact, it always does exactly the same thing.
In the dynamic world of web development, that is seldom the case, so let's look at
a tag that behaves realistically, based on some user-specified attributes.
A web page might, for instance, need to display the value stored in a cookie such
as a user name. Rather than forcing the page author to learn Java to access that value,
we'll build a simple tag that does this for him. The tag should be flexible enough
to be used in retrieving the value of any accessible cookie, so we'll create a tag
attribute called cookieName to allow this. The first step in supporting this new
attribute is to modify our tag handler class to receive and make use of this new
attribute( listing 3. 6):
Listing 3. 6 Source code for the CookieValueTag handler class
The field that will get set by the attribute.
2. Prints the value of the cookie to the response.
Returns SKIP_ BODY to tell the JSP runtime to skip the body if one exists.
Invokes the set method when the JSP runtime encounters this attribute.
All we needed to do was add a set method called setCookieName() and assign a
variable within it. The value of that variable is examined within our tag handler's doStartTag()
to decide which cookie value to return. Now we need to inform the JSP
runtime of this new tag and its attribute. Recall that the TLD is where we spec-ify
this kind of information, so we need to modify our previous TLD to support CookieValueTag.
The tag declaration in our TLD file (listing 3.7) now looks like
the following:
Listing 3. 7 The new TLD file with our CookieValueTag
<? xml version=" 1.0" encoding=" ISO-8859-1" ?>
<! DOCTYPE taglib
PUBLIC "-// Sun Microsystems, Inc.// DTD JSP Tag Library 1. 1// EN"
"http:// java. sun. com/ j2ee/ dtds/ web-jsptaglibrary_ 1_ 1.dtd">
<taglib>
<tlibversion> 1. 0</ tlibversion>
<jspversion> 1.1</ jspversion>
<shortname> simp</ shortname>
<uri> http:// www. manning. com/ jsptagsbook/ simple-taglib </ uri>
<info>
A simple sample tag library
</ info>
<tag>
<name> hello</ name>
<tagclass> book. simpletasks. HelloWorldTag</ tagclass>
<bodycontent> empty</ bodycontent>
<info>
Say hello.
</ info>
</ tag>
<tag>
<name> cookievalue</ name>
<tagclass> book. simpletasks. CookieValueTag</ tagclass>
<bodycontent> empty</ bodycontent>
<info>
Get a cookie's value.
</ info>
<attribute>
<name> cookiename</ name>
<required> true</ required>
</ attribute>
</ tag>
</ taglib>
This tag will have an attribute called cookiename.
Specifies that this attribute is always required for this tag.
The tag definition itself should look familiar, since it is very similar to our Hello-WorldTag.
The important difference is, of course, the attribute we've included.
Note that the name of an attribute, in our case cookiename, is used by the JSP run-time
to find setCookieName() to use in the tag handler; therefore, these need to
match exactly for the tag to function.
To use this attribute within a JSP, syntax such as in listing 3. 8 works well:
Listing 3. 8 A JSP file to drive HelloWorldTag
Declares that the JSP file uses the library referenced by the URI and that the library's
tags are referenced by jspx.
Uses the cookeivalue tag to retrieve a cookie called "username".
Assuming we've used this tag in a case
where a cookie named "username" will be
accessible, we'll see a message like that
shown in figure 3.3.
Adding attributes to your tags makes
them much more flexible and useful to the
web pages where they are used. We
explore the use of tag attributes in further
detail in chapters 4 and 6.
3.4.6 Packaging tags for shipment
Once the tags have been tested to your
satisfaction, it's time to package them in a
standard deployable manner. Packaging
tags means putting the implementation classes along with the library descriptor in a
.jar file following a convention that further instructs you to:
- Put your tag class files inside the .jar archive while maintaining their package structure. .
- Put your TLD in the .jar file in a directory called META-INF.
For example, packaging our lone HelloWorldTag will require the following .jar
file structure:
/book/ simpletasks/ HelloWorldTag. class
/META-INF/ simpletags. tld
This .jar packaging need not be complicated; all that's required is to create the
desired directory structure on your file system and use the jar command (bundled
with the JDK) to archive this structure into the .jar file. The command to place our
class and TLD in a jar called hello. jar looks like this:
jar cf hello. jar META-INF book
Now you can distribute your tag.
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