Defining a Tag Attribute Type

As explained in the previous section, all kinds of expressions can be used in tag attributes. Which kind of expression and how it is evaluated, whether immediately or deferred, are determined by the type attribute of the tag’s definition in the View Description Language (VDL) that defines the tag.

If you plan to create custom tags, for each tag in the VDL, you need to specify what kind of expression to accept. Table 6-1 shows the kinds of tag attributes that accept EL expressions, gives examples of expressions they accept, and provides the type definitions of the attributes that must be added to the VDL. You cannot use #{} syntax for a dynamic attribute, meaning an attribute that accepts dynamically calculated values at runtime. Similarly, you also cannot use the ${} syntax for a deferred attribute.

Table 6-1 Definitions of Tag Attributes That Accept EL Expressions

<rtexprvalue>true</rtexprvalue>

<rtexprvalue>true</rtexprvalue>

<deferred-value>
   <type>java.lang.String</type>
</deferred-value>

<deferred-value>
   <type>int</type>
</deferred-value>

<deferred-method>
   <method-signature>
      java.lang.String submit()
   </method-signature>
<deferred-method>

<deferred-method>
   <method-signature>
      double calcTotal(int, double)
   </method-signature>
</deferred-method>

In addition to the tag attribute types shown in Table 6-1, you can define an attribute to accept both dynamic and deferred expressions. In this case, the tag attribute definition contains both an rtexprvalue definition set to true and either a deferred-value or deferred-method definition.