Tags That Define Scripting Variables

Tag Handlers

If the value of the scripting variable is dependent on an object present in the tag handler's context it can retrieve the object using the pageContext.getAttribute(name, scope) method.

The usual procedure is that the tag handler retrieves a scripting variable value object, performs some processing on the object, and then sets the scripting variable's value using the pageContext.setAttribute(name, object) method.

The scope that an object can have is summarized in Table 3. The scope constrains the accessibility and lifetime of the object.

Table 3 Scope of Objects

Name	Accessible From	Lifetime
page	Current page	Until the response has been sent back to the user or the request is passed to a new page
request	Current page and any included or forwarded pages	Until the response has been sent back to the user
session	Current request and any subsequent request from the same browser (subject to session lifetime).	The life of the user's session
application	Current and any future request from the same Web application	The life of the application

In addition to setting the value of the variable within the tag handler, you must define a class derived from TagExtraInfo that provides information to the JSP container about the nature of the variable. A TagExtraInfo must implement the method getVariableInfo to return an array of VariableInfo objects containing the following information:

• Variable name
  
  
• Variable class
  
  
• Whether the variable refers to a new or existing object value.
  
  
• The availability of the variable
  
  

Table 4 Scripting Variable Availability

Value	Availability	Methods
NESTED	Between the start tag and the end tag.	In doInitBody and doAfterBody for a tag handler implementing BodyTag; otherwise in doStartTag.
AT_BEGIN	From the start tag until the end of the page.	In doInitBody, doAfterBody, and doEndTag for a tag handler implementing BodyTag; otherwise in doStartTag and doEndTag.
AT_END	After the end tag until the end of the page.	In doEndTag.

The JSP container passes a parameter called data to the getVariableInfo method that contains an attribute-value tuples for each of the tag's attributes. These attributes can be used to provide the VariableInfo object with a scripting variable's name and class.

Recall the scripting variable example described in the first section:

<tlt:lookup id="tx" type="UserTransaction" 
   name="java:comp/UserTransaction" />
<% tx.begin(); %>

public LookupTag extends TagSupport {
   private String type;
   private String name;
   public int doStartTag() {
      return SKIP_BODY;
   }
   public int doEndTag() throws JspException {
   try {
      InitialContext context = new InitialContext();
      Object obj = (Object)context.lookup(name);
      pageContext.setAttribute(getId(), obj);
      } catch(javax.naming.NamingException e) {
         throw new JspException("Unable to look up " + name 
            + " due to " + e.getMessage()); 
      }
      return EVAL_PAGE;
   }
}

public class LookupTagTEI extends TagExtraInfo {
   public VariableInfo[] getVariableInfo(TagData data) {
      VariableInfo info1
         = new VariableInfo(
            data.getAttributeString("id"),
            data.getAttributeString("type"),
            true,
            VariableInfo.AT_END);
      VariableInfo[] info = { info1 } ;
      return info;
   }
}

TLD teiclass Element

<tag>
   ...
   <teiclass>LookupTagTEI<teiclass>
   </tag>