JavaServer Pages, 3rd Edition

Using Undeclared Attributes

Occasionally, declaring all attributes for a tag file can be a hassle. Say you want to develop a tag file that generates an HTML table, and you want the page author to be able to specify all standard attributes that an HTML table element supports. That’s a lot of attributes and the tag file would need to test for the existence of each one. A better approach for this scenario is to use the tag directive’s dynamic-attributes attribute. This attribute declares that the tag file accepts any custom action element attribute. The attribute value is the name of a local page scope variable that holds a collection (a Map) with all undeclared attribute names and values. Example 11-3 shows an example of a tag file that uses this approach to generate a table with all request header values.

Example 11-3. Using undeclared attributes in a tag file (headers.tag)

<%@ tag body-content="empty" dynamic-attributes="dynattrs" %>
<%@ attribute name="caption" required="true" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<table
  <c:forEach items="${dynattrs}" var="a">
    ${a.key}="${a.value}"
  </c:forEach>
>
  <caption>${caption}</caption>
  <tr>
    <th>Name</th>
    <th>Value</th>
  </tr>
  <c:forEach items="${header}" var="h">
    <tr>
      <td>${h.key}</td>
      <td>${h.value}</td>
    </tr>
  </c:forEach>
</table>

The dynamic-attributes attribute declares a variable named dynattrs to hold the undeclared attributes, and a JSTL <c:forEach> action loops through the collection and adds the name and ...