The JSP specification provides a Tag Extension mechanism (see Chapter 5) that enables the 

addition of new actions, thus allowing the JSP page  language  to be easily extended in a 

portable

 fashion. A typical example would be elements to support embedded database 

queries. Tag libraries can be used by JSP page authoring tools and can be distributed along 

with JSP pages to any JSP container like Web and Application servers.

The Tag Extension mechanism can be used from JSP pages written using any valid scripting 

language, although the mechanism itself only assumes a Java run time environment. Custom 

actions provide access to the attribute values and to their body; they can be nested and their 

bodies can include scripting elements.

1.4.6

Scripting Languages

Scripting elements

 are commonly used to manipulate objects and to perform computation that 

affects the content generated. There are three classes of scripting elements: 

declarations

, 

scriptlets

 and 

expressions. Declarations 

are used to declare scripting language constructs that 

are available to all other scripting elements. 

Scriptlets

 are used to describe actions to be 

performed in response to some request. Scriplets that are program fragments can also be used 

to do things like iterations and conditional execution of other elements in the JSP page. 

Expressions

 are complete expressions in the scripting language that get evaluated at response 

time; commonly the result is converted into a string and then inserted into the output stream.

All JSP containers must support scripting elements based on the Java programming language. 

Additionally, JSP containers may also support other scripting languages. All such scripting 

languages must support:

Manipulation of Java objects.

Invocation of methods on Java objects.

Catching of Java language exceptions.

The precise definition of the semantics for scripting done using elements based on the Java 

programming language is given in Chapter 4.

The semantics for other scripting languages are not precisely defined in this version of the 

specification, which means that portability across implementations cannot be guaranteed. 

Precise definitions may be given for other languages in the future.

1.4.7

Objects and Variables

An object may be made accessible to code in the scripting elements through a scripting 

language variable. An element can define scripting variables in two places: after its start tag 

and after its end tag. The variables will contain at process request time a reference to the 

object defined by the element, although other references exist depending on the 

scope

 of the 

object (see Section 1.4.3,  Objects and Scopes ).

Chapter 1

Overview

26