Declaring Empty Elements

XSD Tutorials - Herong's Tutorial Examples

∟Declaring Empty Elements

This section describes a tutorial example on how to declare an empty element by defining a special complexType datatype.

What is an empty element? An empty element is an element with no text content, no sub (child) element, and no attribute. There are ways to declare an empty element:

1. An empty element can be declared with a "simleType" datatype derived from "string" with length of "0". Here is a sample schema, empty_simple.xsd, that declares an empty element using "simpleType":

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<!-- empty_simple.xsd
 - Copyright (c) 2002-2013 HerongYang.com. All Rights Reserved.
-->

 <xs:element name="br">
 <xs:simpleType>
  <xs:restriction base="xs:string">
  <xs:length value="0"/>
  </xs:restriction>
 </xs:simpleType>
 </xs:element>

</xs:schema>

2. An empty element can be declared with a "complexType" datatype with "complexContent" containing no "element" and no "attribute". Here is a sample schema, empty_complex.xsd, that declares an empty element using "complexType":

<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<!-- empty_complex.xsd
 - Copyright (c) 2002-2013 HerongYang.com. All Rights Reserved.
-->

 <xs:element name="br">
 <xs:complexType>
  <xs:complexContent>
   <xs:restriction base="xs:anyType"/>
  </xs:complexContent>
 </xs:complexType>
 </xs:element>

</xs:schema>

The second way seems to be better than the first way. It extends the generic built-in datatype "anyType" with no attribute and no sub element.

To test these 2 schema documents, I wrote the following XML document, empty_error.xml.

<?xml version="1.0"?>
<br> </br>

Here are the errors generated from XsdSchemaValidator.java: They confirm that the second schema is better:

herong> java XsdSchemaValidator empty_simple.xsd empty_error.xml

Error:
   Line number: 2
   Column number: 11
   Message: cvc-length-valid: Value ' ' with length = '1' is not
   facet-valid with respect to length '0' for type '#AnonType_br'.

Error:
   Line number: 2
   Column number: 11
   Message: cvc-type.3.1.3: The value ' ' of element 'br' is not
   valid.

Failed with errors: 2


herong> java XsdSchemaValidator empty_complex.xsd empty_error.xml

Error:
   Line number: 2
   Column number: 11
   Message: cvc-complex-type.2.1: Element 'br' must have no character
   or element information item [children], because the type's content
   type is empty.

Failed with errors: 1