JSP tag handlers can't have an attribute named “class”

Recently I've been implementing some tag handlers that emit HTML. In them, I want to carry through some of the standard HTML attributes such as id and class. And I've discovered that Tomcat is quite happy to accept the following TLD entry, but at runtime complains that it's unable to find a setter method for the attribute:

<attribute>
  <name>class</name>
  <rtexprvalue>true</rtexprvalue>

  <type>java.lang.String</type>
</attribute>

Sun provides a schema definition for the JSP taglib deployment descriptor, and it defines the type tld-attributeType for attribute declarations. Looking at that type definition, we see this:

<xsd:element name="name"
   type="j2ee:java-identifierType"/>

And jumping to the common definitions schema, here's the definition of java-identifierType:

<xsd:complexType name="java-identifierType">
  <xsd:annotation>
    <xsd:documentation>

        The java-identifierType defines a Java identifier.
        The users of this type should further verify that
        the content does not contain Java reserved keywords.

    </xsd:documentation>
  </xsd:annotation>

  <xsd:simpleContent>
    <xsd:restriction base="j2ee:string">
      <xsd:pattern value="($|_|\p{L})(\p{L}|\p{Nd}|_|$)*"/>
    </xsd:restriction>
  </xsd:simpleContent>

</xsd:complexType>

Did you read the comment? That users of the type should perform keyword validation?!? I couldn't believe it, until I turned to the XML Schema docs and discovered that there's no way to exclude values: the enumeration facet enumerates legal values only.

So, two lessons to draw from this: first, XML schema is not only complex but incomplete, and second, you can't use class as a JSP tag attribute. Or any other Java keyword for that matter. At least not in a servlet container developed by people who have read the schema docs, because this restriction is not mentioned in the JSP specification text.

My solution? The ugly but descriptive htmlClass.

(originally posted on my website 19 Nov 08)