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JavaWorld.com > Java Enterprise Edition >

Use XML data binding to do your laundry

Explore JAXB and Castor from the ground up

By Sam Brodkin, JavaWorld.com, 12/28/01

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Page 3 of 6

The answer to these deficiencies: XML Schema. XML Schemas are in XML format; they allow data typing, user-defined types, and range value constraints. XML Schema's popularity and software support is growing because it is now a final W3C Recommendation. Here you'll find an example of constraining the same document type using XML Schema:

Listing 3. socks.xsd

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <xsd:element name="socks">
      <xsd:complexType>
        <xsd:sequence>
          <xsd:element name="sock" minOccurs="0" maxOccurs="unbounded">
            <xsd:complexType>
              <xsd:sequence>
                <xsd:element name="name" type="xsd:string"/>
                <xsd:element name="image" type="imageType"/>
                <xsd:element name="color" type="colorType"/>
                <xsd:element name="price" type="money"/>
                <xsd:element name="smell" type="smellType"/>
                </xsd:sequence>
                <xsd:attribute name="number" type="xsd:string" use="required"/>
            </xsd:complexType>
          </xsd:element>
        </xsd:sequence>
      </xsd:complexType>
    </xsd:element>
    <xsd:simpleType name="imageType">
      <xsd:restriction base="xsd:string">
        <xsd:pattern value="(.)+\.(gif|jpg|jpeg|bmp)"/>
      </xsd:restriction>
    </xsd:simpleType>
    <xsd:simpleType name="colorType">
      <xsd:restriction base="xsd:string">
        <xsd:enumeration value="black"/>
        <xsd:enumeration value="white"/>
      </xsd:restriction>
    </xsd:simpleType>
    <xsd:simpleType name="money">
      <xsd:restriction base="xsd:decimal">
        <xsd:fractionDigits value="2"/>
      </xsd:restriction>
    </xsd:simpleType>
    <xsd:simpleType name="smellType">
      <xsd:annotation>
        <xsd:documentation>0=clean and 10=smells terrible</xsd:documentation>
      </xsd:annotation>
      <xsd:restriction base="xsd:nonNegativeInteger">
        <xsd:minInclusive value="0"/>
         <xsd:maxInclusive value="10"/>
      </xsd:restriction>
    </xsd:simpleType>
</xsd:schema>


That looks a little more complicated, but believe me, the complexity is worth it. In addition to the constraints we specified in the DTD, XML Schema lets us add the following:

  • The contents of the <image> tag must end with an image type extension (like .gif).
  • The <price> must be a number with two fractional digits (like 5.34).
  • The <smell> must be an int between 0 and 10. There is also a documentation comment stating 0=clean and 10=smells terrible.


For more on XML Schemas, see the schema tutorial in Resources.

To associate socks.xml with our XML Schema, we'll change only some attributes in the root (socks) tag:

Listing 4. socks.xml for use with the XML Schema

<?xml version="1.0"?>
<socks xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="socks.xsd">
  <sock number="1">
        <name>black socks</name>
...


How do we check to see if an XML document is valid against a DTD or XML Schema? We use an XML parser like Apache Xerces to verify that an XML document conforms to the socks.dtd or socks.xsd constraints. I've provided a Windows batch file in my sample code called validate.bat to do that:

validate.bat <path_to_socks>\socks.xml

This invokes the Xerces parser in validation mode, which asks it to please check socks.xml against its stated document type. Xerces now supports both DTD and XML Schema validation.

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