Substitution groups are an
XML Schema feature that provides a way of substituting one element for another
in an XML message.
A substitution group is a list of global elements that can appear in place
of another global element, called the head element.
A substitution group is defined by setting the substitution
group property on one global element (the member element)
to point at another global element (the head element). This adds
the member element to the substitution group of the head element.
Tip: If your messages are never rendered as XML, or if you have
a simple message model, use a complex type or a group with Composition set
to Choice, instead of using
substitution groups.
Elements
- Head elements
- A head element is simply an element that can be substituted. When a message
is parsed, one of its member elements can appear in place of the head element
without causing a validation error.
- Abstract elements
- An abstract element is a head element which must be substituted. The 'abstract'
attribute on the element indicates this. Typically, abstract elements have
other elements in their substitution group - otherwise they are of little
use. Wherever an abstract element appears in a message definition, a member
of its substitution group must appear instead.
Attributes
- The block attribute on elements
- The block attribute on an element
limits the set of global elements that can substitute for the element. The block attribute can take any subset of the
values restriction, extension, substitution,
or all.
- If the block attribute contains restriction, an element that is based
on a restriction of the element's type cannot substitute for the element.
- If the block attribute contains extension, an element that is based on
an extension of the element's type cannot substitute for the element.
- If the block attribute contains substitution, an element that is a member
of the element's substitution group cannot substitute for the element.
- If the block attribute contains all, all of the above limits apply.
- The final attribute on elements
- The final attribute on an element
limits the set of global elements that can be a member of the element's substitution
group. The final attribute can take
any subset of the values restriction, extension, or all.
- If the final attribute contains restriction, an element that is based
on a restriction of the element's type cannot be in the substitution group
of the element.
- If the final attribute contains extension, an element that is based on
an extension of the element's type cannot be in the substitution group of
the element.
- If the final attribute contains all, both of the above limits apply.
- The block attribute on complex
types
- The block attribute on a complex
type limits the set of other types that can substitute for that type. The block attribute can take values restriction, extension,
or all. The meanings for these
values are the same as those shown for the block attribute on an element above.
An element that is a member of a substitution group can only substitute for
the head element if its type is compatible with the block attribute on the
type of the head element.
- Default block and final attributes
- A default for the block and final attributes can be set at the message
definition file level. If a default for one or both of these attributes has
been set and the relevant block or final attribute has not been set at the object
level, the default setting is used for that object. You can override the default
setting at the object level.