The idea behind tag libraries in JSP looks very good on paper. Tag libraries allow you
to associate Java code to a series of XML-like tags that are packaged as a library. The
user of tag libraries (mainly the JSP developer) will use these tags to develop an
application. The more complete the tag libraries are, the need to write Java code directly
inside the JSP page or via JavaBeans diminishes. Taglibs provide a reusable framework for
J2EE application development, but their key to success is availability. The same way we go
to repositories to retrieve a particular DTD or XML Schema, we need a repository of
taglibs.
The Jakarta taglib project (www.apache.org) is
one such repository of custom tag libraries contributed by various developers. The spectrum
of libraries is very broad including taglibs for JNDI, JDBC, and XSL processing. Every
library is placed in its own directory. The Java class files making up the library are
packaged as a jar file. The Tag Library Descriptor (TLD) file contains the necessary
information for deployment and using the taglib. Reviewing the TLD file is a good way to
get familiarizes with the library, its components and its purpose. Documentation and
example applications using the taglib are packaged as WAR files ready to be deployed to a
servlet/JSP container such as Tomcat.
In the latest distribution, there are 19 libraries. Some of them like request, response,
and session, cover the basic aspects of servlet/JSP API by wrapping these objects and
making them available via the taglib interface. Yet, others provide unique and interesting
functionality. For example, the scrape library can be used to extract portions of other web
pages. The extracted pieces can then be used inside the current page. Many portal engines
are based on this idea. Using tags like <scrape> and <result>
and by modifying their attributes, one can create a JSP page that contains content from
other pages. Here is a code fragment from the sample application extracting weather
information:
<scrp:pageurl="http://weather.noaa.gov/cgi-bin/fmtbltn.pl?
file=forecasts/city/mo/columbia.txt" time="11">
<scrp:scrape id="weather1" begin="<PRE>"
end="</PRE>" anchors="true"/>
<scrp:scrape id="weather2" begin="<PRE>"
end="</PRE>" anchors="false"/>
<scrp:scrape id="weather3" begin="<PRE>"
end="</PRE>" anchors="false" strip="true"/>
</scrp:page>
The result can be inserted in the page with a single line like this:
<scrp:result scrape="weather1"/>
Another interesting library is regexp bringing Perl regular expressions to JSP.
<regexp> identifies the regular expression and <text> identifies the text
string in use. Tags like <existmatch>, <substitute>, <split> and
<match> can be used to apply the regular expression to the text string.
The jdbc and jndi libraries provide a gateway to JDBC and JNDI APIs. They encapsulate
the most common aspects of these APIs like opening a connection, performing a query/lookup
and retrieval of the results. It is conceivable that taglibs for EJB and JMS interactions
can be written in a similar manner. To ease integration of XML documents inside a JSP page,
you can use the xsl library. It allows you to transform XML documents via XSL to be used by
the current application or others.
Taglibs represent distinct software components that can easily be reused. The interface
to these reusable components is not JavaBeans or a Java class file, but XML-like tags and
attributes. So while under the hood, there are Java classes and code that make up the
taglib, the user of the taglib will never see them. The user will merely be putting
together tags to make up the application. Of course this model only works when libraries
are available for the application function the user is interested in and repositories such
as Jakarta taglib are a step in the right direction to host and share the various
libraries.
About the Author
Piroz Mohseni is president of Bita
Technologies, focusing on business improvement through the effective use of
technology. His areas of interest include enterprise Java, XML, and e-commerce
applications.
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