Reviews : Java Books :
Professional Java Server Programming J2EE Edition : Chapter 12

Title: Professional Java Server Programming J2EE Edition ISBN: 1861004656 US Price: $ 64.99 Canadian Price: C$ 97.95 UK Price: £ 46.99 Publication Date: September 2000 Pages: 1633 © Wrox Press Limited, US and UK.

 
RowOpenTag.class
RowsTag.class
RowsTagExtraInfo.class
RowTagsExtraInfo.class
StyledXMLTag.class
TableTag.class

Note that although I wrote this particular example using Windows NT, the jar tool (which is written in Java) uses identical syntax when running on other operating systems.

Warning: Tomcat 3.1 can produce translation errors if .java source files are included in tag library JARs.

Combination of WAR and JAR

A combination of the second and third delivery methods is often useful. For example, consider a web application that uses a tag library that may also be of value in other web applications. The best approach is to package the tag library as a JAR, place this JAR in the /META-INF/lib directory of the web application's WAR, and create a mapping in the WAR's web.xml file to the tag library's TLD. This approach works well; the only issue is that the TLD must be extracted from the tag library JAR for it to be picked up by the web.xml file or JSPs in the WAR. Unfortunately the <taglib> element's mapping does not work directly to a JARed tag library. The tag library JAR can be directly imported into JSPs if it is publicly accessible, under the web application's root, but this is somewhat less elegant. It's seldom a good idea to publish more information than is strictly necessary.

This combination approach will be used later in the next chapter, as we'll need to reuse some of the tags to support the application developed in Chapter 14.

Writing Tag Extensions

Once the initial concepts are grasped, implementing tag extensions is surprisingly easy. Processing Attributes Our simple example is all very well, but it doesn't take advantage of the dynamic potential of custom tags. We could interrogate the PageContext to implement context-specific behavior, but there are far better alternatives.

The easiest way to parameterize tags is to pass in XML attributes.

How do we make our tags handle attributes? The answer, not surprisingly, is that attributes in a TLD tag element map onto bean properties of the corresponding tag handlers. The mapping of attributes onto tag handler properties is, as we might expect, handled by the JSP engine using reflection and not only does it work with primitive types, we can pass any type to a tag handler. (Draft versions of the JSP 1.1 specification included a type subelement of the attribute TLD element; this has now been removed.)

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