An attribute value of the form

 <%= 

scriptlet_expr

 %>  or `<%= 

scriptlet_expr

 %>' denotes 

a 

request time attribute value

. The value denoted is that of the scriptlet expression involved. 

Request time attribute values can only be used in actions. If there are more than one such 

attribute in a tag, the expressions are evaluated left to right.

Only attribute values can be denoted this way (e.g. the name of the attribute is always an 

explicit name), and the expression must appear by itself (e.g. multiple expressions, and 

mixing of expressions and string constants are not permitted; instead perform these 

operations within the expression).

The resulting value of the expression depends upon the expected type of the attribute's value. 

The type of an action element indicates the valid Java programming languag type for each 

attribute value; the default is 

java.lang.String

.

By default, all attributes have page translation time semantics. Attempting to specify a 

scriptlet expression as a value for an attribute that has page translation time semantics is 

illegal, and will result in a fatal translation error. The type of an action element indicates 

whether a given attribute will accept request time attribute values.

Most attributes in the actions defined in the JSP 1.1 specification have page translation time 

semantics.

The following attributes accept request time attribute expressions:

The 

value

 and 

beanName

 attributes of 

jsp:setProperty

 (2.13.2).

The 

page

 attribute of 

jsp:include

 (2.13.4).

The 

page

 attribute of 

jsp:forward

 (2.13.5).

The 

value

 attribute of 

jsp:param 

(

2.13.6)

.

2.12.2

The 

id

 Attribute

The 

id= name 

 attribute/value tuple in an element has special meaning to a JSP container, 

both at page translation time, and at client request processing time; in particular:

the 

name

 must be unique within the translation unit, and identifies the particular element 

in which it appears to the JSP container and page.

Duplicate 

id

's found in the same translation unit shall result in a fatal translation error.

In addition, if the action type creates one or more object instance at client request 

processing time, one of these objects will usually be associated by the JSP container with 

the named value and can be accessed via that name in various contexts through the 

pagecontext 

object described later in this specification. 

Furthermore, the 

name

 is also used to expose a variable (name) in the page's scripting 

language environment. The scope of this scripting language dependent variable is 

dependent upon the scoping rules and capabilities of the actual scripting language used in 

the page. Note that this implies that the 

name

 value syntax shall comply with the variable 

naming syntax rules of the scripting language used in the page.

Chapter 2

Standard Syntax and Semantics

58