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The Java EE 5 Tutorial

Tags That Define Variables

The mechanisms for defining variables in classic tags are similar to those described in Chapter 8, Custom Tags in JSP Pages. You must declare the variable in a variable element of the TLD or in a tag extra info class. Use PageContext().setAttribute(name,value) or PageContext.setAttribute(name,value,scope) methods in the tag handler to create or update an association between a name that is accessible in the page context and the object that is the value of the variable. For classic tag handlers, Table 9–3 illustrates how the availability of a variable affects when you may want to set or update the variable’s value.

Table 9–3 Variable Availability

Value 

Availability 

In Methods 

NESTED

Between the start tag and the end tag 

doStartTag, doInitBody, and doAfterBody

AT_BEGIN

From the start tag until the end of the page 

doStartTag, doInitBody, doAfterBody, and doEndTag

AT_END

After the end tag until the end of the page 

doEndTag

A variable defined by a custom tag can also be accessed in a scripting expression. For example, the web service described in the preceding section can be encapsulated in a custom tag that returns the response in a variable named by the var attribute, and then var can be accessed in a scripting expression as follows:

<ws:hello var="response"
         name="<%=request.getParameter("username")%>" />
<h2><font color="black"><%= response %>!</font></h2>

Remember that in situations where scripting is not allowed (in a tag body where the body-content is declared as scriptless and in a page where scripting is specified to be invalid), you wouldn’t be able to access the variable in a scriptlet or an expression. Instead, you would have to use the JSP expression language to access the variable.