The tag extension mechanism described in this chapter addresses the following
goals. It is designed to be:
- Portable - An action described in a tag library must be usable in any JSP con-tainer.
- Simple - Unsophisticated users must be able to understand and use this mech-anism.
Vendors of JSP functionality must find it easy to make it available to
users as actions.
- Expressive - The mechanism must support a wide range of actions, including
nested actions, scripting elements inside action bodies, and creation, use and
updating of scripting variables.
- Usable from different scripting languages - Although the JSP specification
currently only defines the semantics for scripts in the Java programming lan-guage,
we want to leave open the possibility of other scripting languages.
- Built upon existing concepts and machinery- We do not want to reinvent what
exists elsewhere. Also, we want to avoid future conflicts whenever we can
In clear, they allow you to define your own set of tags (i.e. your own tag library) as an extension of the existing set of basic JSP tags.
For example: you've defined a database connection in a custom tag, opening, calling db, iterating records and such types of things can be once defined in your tag and be used in multiple applications and you don't have to bother to write it again for other app.
Hope it helps a bit and clarifies your querry.