JavaServer Pages, 3rd Edition

Dynamic Attribute Values and Types

Throughout this book, you’ve seen how action element attributes can be given dynamic values, evaluated at runtime. A dynamic attribute value can be assigned by an EL expression, a Java expression (as shown in Chapter 16), or by a <jsp:attribute> element.

Not all attributes accept dynamic values, though. To tell the container that a custom action attribute accepts a dynamic value, or a request-time attribute value as it’s also called, you have to declare this fact in the TLD:

<tag>
  <name>geekContestEntry</name>
  <tag-class>com.xmp.GeekContextEntry</tag-class>
  <description>
    Saves the submitted data in the Geek Contest database.
  </description>
  
  <attribute>
    <name>yearsSinceLastVacation</name>
    <rtexprvalue>true</rtexprvalue>
  </attribute>
  <attribute>
    <name>hoursWithoutSleep</name>
    <rtexprvalue>true</rtexprvalue>
  </attribute>
  <attribute>
    <name>employersInAMonth</name>
    <rtexprvalue>true</rtexprvalue>
  </attribute>
</tag>

An <rtexprexprvalue> element with the value true enables this feature. You can then assign dynamic values to the attributes in a page like this:

<xmp:geekContestEntry
  yearsSinceLastVacation="${param.noVacation}"
  hoursWithoutSleep='<%= request.getParameter("noSleep") %>'>
  <jsp:attribute name="employersInAMonth">
     <xmp:getAvgEmployers id="${param.geekId}" />
  </jsp:attribute>
</xmp:geekContestEntry>

An EL expression assigns the value of the noVacation request parameter to the first attribute, a Java expression assigns the noSleep parameter value to ...