The Java EE 5 Tutorial (PDF) - Oracle Software Downloads

Tag Handlers forTags with Attributes
This section describes how to define attributes for a tag handler and how to validate attribute
values.
Defining Attributes in aTag Handler
For each tag attribute, you must define a set method in the tag handler that conforms to the
JavaBeans architecture conventions. For example, consider the tag handler for the JSTL c:if
tag:
This tag handler contains the following method:
public void setTest(boolean test) {
this.test = test;
}
As shown by the preceding example, the name of the attribute must match the name of the set
method.
AttributeValidation
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” on page 260).
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 has the following TLD attribute element:
attr1
true
true
This declaration indicates that the value of attr1 can be determined at runtime.
Programming SimpleTag Handlers
Chapter 8 • CustomTags in JSP Pages 255