WebSphere Application Server - IBM Redbooks

In practice, it is often best to provide the JSP author with choice. By
implementing the logic of the tag within a bean and providing the custom tag as a
wrapper to the bean’s interface, then you may be able to provide the author with
the choice of scripting or using custom tags.
9.1.3 Common custom tag usages
9.1.4 How they work
When custom tags are used, they often perform a function that falls into one of
the following categories:
► Markup generation: Generation of markup language output, for example, to
create an HTML form.
► Scripting generation: Generation of client-side scripting code such as
JavaScript, for example, to perform client-side data validation. This is
illustrated in the Online Catalog sample.
► Environment access: Providing access to and modification of implicit objects.
► API access: Simplifying access to APIs such as JDBC, JNDI, and JavaMail.
This is illustrated in the Online Catalog sample.
► Content manipulation: Manipulating content before output, and caching of
generated content.
In order for the Web container to understand and execute a custom tag, the code
implementing the tag and information on how to translate the tag and find and
invoke the code must be available to the container. Tags for a particular function
are normally aggregated into a tag library.
Tag libraries, or taglibs, are normally packaged as JAR files. The taglib is made
up of the following elements:
► Tag library definition (TLD): An XML file containing elements that describe
each tag in the library. The TLD will be used by the Web container during
translation time to interpret each tag, and at request time to find and invoke
the tag code.
► Tag handler: A Java class that provides the Web container with access to the
functionality of the tag. However, this does not necessarily mean that the tag
handler implements the tag functionality. As we hinted, this may be contained
within other classes/JavaBeans. The taglib will contain a tag handler class for
each custom tag.
► Supplemental classes: The implementation of a custom tag may include
some optional files that, if used, need to be included within the taglib.
Chapter 9. Using tag libraries 279