Cooperating Tags

The first style requires that a shared object be named and stored in the page context (one of the implicit objects accessible to both JSP pages and tag handlers). To access objects created and named by another tag, a tag handler uses the pageContext.getAttribute(name, scope) method.

In the second style of object sharing, an object created by the enclosing tag handler of a group of nested tags is available to all inner tag handlers. This form of object sharing has the advantage that it uses a private namespace for the objects, thus reducing the potential for naming conflicts.

To access an object created by an enclosing tag, a tag handler must first obtain its enclosing tag with the static method TagSupport.findAncestorWithClass(from, class) or the TagSupport.getParent() method. The former method should be used when a specific nesting of tag handlers cannot be guaranteed. Once the ancestor has been retrieved, a tag handler can access any statically or dynamically created objects. Statically created objects are members of the parent. Private objects can also be created dynamically created. Such objects can be stored in a tag handler with the setValue method and retrieved with the getValue method.

The following example illustrates a tag handler that supports both the named and private object approaches to sharing objects. In the example, the handler for a query tag checks whether an attribute named connection has been set in the doStartTag method. If the connection attribute has been set, the handler retrieves the connection object from the page context. Otherwise, the tag handler first retrieves the tag handler for the enclosing tag, and then retrieves the connection object from that handler.

public class QueryTag extends BodyTagSupport {
   private String connectionId;
   public int doStartTag() throws JspException {
      String cid = getConnection();
      if (cid != null) {
      // there is a connection id, use it
         connection =(Connection)pageContext.
            getAttribute(cid);
      } else {
         ConnectionTag ancestorTag =
            (ConnectionTag)findAncestorWithClass(this,
               ConnectionTag.class);
         if (ancestorTag == null) {
            throw new JspTagException("A query without
               a connection attribute must be nested
               within a connection tag.");
         }
         connection = ancestorTag.getConnection();
      }
   }
}

<tlt:connection id="con01" ....> ... </tlt:connection>
<tlt:query id="balances" connection="con01"> 
   SELECT account, balance FROM acct_table 
      where customer_number = <%= request.getCustno()%> 
</tlt:query>

<tlt:connection ...>
   <x:query id="balances"> 
      SELECT account, balance FROM acct_table 
         where customer_number = <%= request.getCustno()%> 
   </x:query>
</tlt:connection>

<tag>
   <attribute>
   <name>connection</name>
   <required>false</required>
</tag>