Attribute Validation

The documentation for a tag library should describe valid values for tag attributes. When a JSP page is translated, a web container will enforce any constraints contained in the TLD element for each attribute.

The attributes passed to a tag can also be validated at translation time using the validate method of a class derived from TagExtraInfo. This class is also used to provide information about variables defined by the tag (see TagExtraInfo Class).

The validate method is passed the attribute information in a TagData object, which contains attribute-value tuples for each of the tag’s attributes. Because the validation occurs at translation time, the value of an attribute that is computed at request time will be set to TagData.REQUEST_TIME_VALUE.

The tag <tt:twa attr1="value1"/> has the following TLD attribute element:

<attribute>
    <name>attr1</name>
    <required>true</required>
    <rtexprvalue>true</rtexprvalue>
</attribute>

This declaration indicates that the value of attr1 can be determined at runtime.

The following validate method checks whether the value of attr1 is a valid Boolean value. Note that because the value of attr1 can be computed at runtime, validate must check whether the tag user has chosen to provide a runtime value.

public class TwaTEI extends TagExtraInfo {
    public ValidationMessage[] validate(TagData data) {
        Object o = data.getAttribute("attr1");
        if (o != null && o != TagData.REQUEST_TIME_VALUE) {
            if (((String)o).toLowerCase().equals("true") ||
                 ((String)o).toLowerCase().equals("false") )
                 return null;
            else
                return new ValidationMessage(data.getId(),
                    "Invalid boolean value.");
        }
        else
            return null;
    }
}