Declaring Tag Attributes for Tag Handlers (The Java EE 5 Tutorial)

The Java EE 5 Tutorial

Declaring Tag Attributes for Tag Handlers

For each tag attribute, you must specify whether the attribute is required, whether the value can be determined by an expression, the type of the attribute in an attribute element (optional), and whether the attribute is a fragment. If the rtexprvalue element is true or yes, then the type element defines the return type expected from any expression specified as the value of the attribute. For static values, the type is always java.lang.String. An attribute is specified in a TLD in an attribute element. Table 8–10 lists the subelements of the attribute element.

Table 8–10 attribute Subelements


Element	Description
`description`	(optional) A description of the attribute.
`name`	The unique name of the attribute being declared. A translation error results if more than one `attribute` element appears in the same tag with the same `name`.
`required`	(optional) Whether the attribute is required. The default is `false`.
`rtexprvalue`	(optional) Whether the attribute’s value can be dynamically calculated at runtime by an EL expression. The default is `false`. When this element is set to `true` and the attribute definition also includes either a `deferred-value` or `deferred-method` element then the attribute accepts both dynamic and deferred expressions.
`type`	(optional) The runtime type of the attribute’s value. Defaults to `java.lang.String` if not specified.
`fragment`	(optional) Whether this attribute is a fragment to be evaluated by the tag handler (`true`) or a normal attribute to be evaluated by the container before being passed to the tag handler. If this attribute is `true`: You do not specify the `rtexprvalue` attribute. The container fixes the `rtexprvalue` attribute at `true`. You do not specify the `type` attribute. The container fixes the `type` attribute at `javax.servlet.jsp.tagext.JspFragment`. Defaults to `false`.
`deferred-value`	(optional) Indicates that the tag attribute accepts deferred value expressions. This element includes an optional `type` child element, which indicates the type of object to which the expression resolves. If no `type` element is included, the type is `java.lang.Object`. Either the `deferred-value` or `deferred-method` element (but not both) can be defined for the same attribute.
`deferred-method`	(optional) Indicates that the tag attribute accepts deferred method expressions. This element includes an optional `method-signature` child element, which indicates the signature of the method that the expression invokes. If no method signature is defined, the method signature default is `void` `methodName()`. Either the `deferred-value` or `deferred-method` element (but not both) can be defined for the same attribute.

If a tag attribute is not required, a tag handler should provide a default value.

The tag element for a tag that outputs its body if a test evaluates to true declares that the test attribute is required and that its value can be set by a runtime expression.

<tag>
    <name>present</name>
         <tag-class>condpkg.IfSimpleTag</tag-class>
    <body-content>scriptless</body-content>
    ...
    <attribute>
        <name>test</name>
        <required>true</required>
        <rtexprvalue>true</rtexprvalue>
    </attribute>
    ...
</tag>