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14.3 Assigning Attributes to Tags

321

The Tag Library Descriptor File

Tag attributes must be declared inside the 

tag

 element by means of an

attribute

 element. The 

attribute

 element has three nested elements that

can appear between 

 and 

.

1.

name

, a required element that defines the case sensitive 

attribute name. In this case, I use

length

2.

required

, a required element that stipulates whether the 

attribute must always be supplied (

true

) or is optional (

false

). 

In this case, to indicate that 

length

 is optional, I use

false

If you omit the attribute, no call is made to the 

setAttribute 

Name

 method. So, be sure to give default values to the fields that 

the method sets.

3.

rtexprvalue

, an optional attribute that indicates whether the 

attribute value can be a JSP expression like 

<%= expression %>

 (

true

) or whether it must be a fixed 

string (

false

). The default value is 

false

, so this element is 

usually omitted except when you want to allow attributes to 

have values determined at request time.

Listing 14.8 shows the complete 

tag

 element within the tag library

descriptor file. In addition to supplying an 

attribute

 element to describe

the 

length

 attribute, the 

tag

 element also contains the standard 

name

(

prime

), 

tagclass

  (

coreservlets.tags.PrimeTag

), 

info

 (short descrip 

tion), and 

bodycontent

 (

EMPTY

) elements. 

Listing 14.8 csajsp taglib.tld