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14.2 Defining a Basic Tag

315

The Tag Handler Class

Tags that either have no body or that merely include the body verbatim

should extend the 

TagSupport

 class. This is a built in class in the

javax.servlet.jsp.tagext

 package that implements the 

Tag

 interface

and contains much of the standard functionality basic tags need. Because of

other classes you will use, your tag should normally import classes in the

javax.servlet.jsp

 and 

j

ava.io

 packages as well. So, most tag implementa 

tions contain the following 

import

 statements after the package declaration:

import javax.servlet.jsp.*;

import javax.servlet.jsp.tagext.*;

import java.io.*;

I recommend that you download an example from 

http://www.coreserv 

lets.com/

 and use it as the starting point for your own implementations.

For a tag without attributes or body, all you need to do is override the

doStartTag

 method, which defines code that gets called at request time

where the element's start tag is found. To generate output, the method

should obtain the 

JspWriter

 (the specialized 

PrintWriter

 available in JSP

pages through use of the predefined 

out

 variable) from the 

pageContext

field by means of 

getOut

. In addition to the 

getOut

 method, the 

pageCon 

text

 field (of type 

PageContext

) has methods for obtaining other data struc 

tures associated with the request. The most important ones are 

getRequest

,

getResponse

, 

getServletContext

, and 

getSession

.

Since the 

print

 method of 

JspWriter

 throws 

IOException

, the 

print

statements should be inside a 

try

/

catch

 block. To report other types of

errors to the client, you can declare that your 

doStartTag

 method throws a

JspException

 and then throw one when the error occurs.

If your tag does not have a body, your 

doStartTag

 should return the

SKIP_BODY

 constant. This instructs the system to ignore any content between

the tag's start and end tags. As we will see in Section 14.5 (Optionally Includ 

ing the Tag Body), 

SKIP_BODY

 is sometimes useful even when there is a tag

body, but the simple tag we're developing here will be used as a stand alone

tag (

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