Sams Teach Yourself XML in 21 Days

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Using XML Schema Tools

Today and tomorrow you're going to see how to write XML schemas, but before you dig into the details (and there are plenty of them), it's worth noting that more and more software tools are appearing that can generate XML schemas for you. Although XML authors should know how to write XML schemas, tools that do the work for you can be very handy, so you'll start by introducing them.

Creating Schemas by Using XML Schema-Creation Tools

A growing number of XML schema-creation tools are becoming available; here's a sampling of the ones that are out there as of this writing:

• HiT Software( http://www.hitsw.com/xml_utilites/ )— This is an online automatic XML schema generator and DTD to XML schema converter. You just let it upload a document, and it creates an XML schema for free.
• xmlArchitect ( http://www.sysonyx.com/products/xmlArchitect )— This is an XML editor for creating schemas.
• XMLspy ( http://www.xmlspy.com )— XMLspy is a product family of tools that aid in the creation of XML schemas.
• XRay ( http://architag.com/xray )— This tool provides support for XML schemas and has an integrated online XML tutorial system.
• Microsoft Visual Studio .NET ( http://www.microsoft.com )— Visual Studio .NET can also generate XML schemas for you automatically.

As an example of the schema-creation process, let's take a look at the XML schema generator in the Microsoft Visual Studio .NET development tool. Take a look at this XML document:

<?xml version="1.0"?>
<document xmlns="http://xmlpowercorp">
    <text>
        Welcome to XML Schemas!
    </text>
</document>

What if you want to automatically generate an XML schema for this document? You start by opening the XML document in Visual Studio .NET (that is, in Visual Studio .NET, you create a new project and then select Project, Add Existing Item to open the Add Existing Item dialog box, where you can browse to the XML document for which you want to create a schema). Figure 6.1 shows the short sample XML document opened in Visual Studio .NET.

Figure 6.1 An XML document in Visual Studio .NET.

Next, you select XML, Create Schema to create an XML schema for the document. The XML that is generated is shown in Figure 6.2.

Figure 6.2 Creating an XML schema in Visual Studio .NET.

Here's what the generated XML schema looks like (note that it's about four times as long as the original XML document) :

<?xml version="1.0"?>
<xs:schema id="NewDataSet"
targetNamespace="http://xmlpowercorp"
xmlns:mstns="http://xmlpowercorp" xmlns="http://xmlpowercorp"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:msdata="urn:schemas-microsoft-com:xml-msdata"
attributeFormDefault="qualified" elementFormDefault="qualified">
  <xs:element name="document" msdata:Prefix="ch06">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="text" msdata:Prefix="ch06"
            type="xs:string" minOccurs="1" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>
  <xs:element name="NewDataSet" msdata:IsDataSet="true"
    msdata:Prefix="ch06" msdata:EnforceConstraints="False">
    <xs:complexType>
      <xs:choice maxOccurs="unbounded">
        <xs:element ref="document" />
      </xs:choice>
    </xs:complexType>
  </xs:element>
</xs:schema>

This is a valid XML schema, although it uses a namespace for Microsoft-specific data types that you're not going to use, and it adds more elements than you'll need.

As mentioned earlier today, you can also use free online XML schema generators such as the one at http://www.hitsw.com/xml_utilities/ to upload XML documents and create XML schemas. Figure 6.3 shows the XML schema that this generator creates for the sample XML document.

Figure 6.3 Using an online XML schema generator.

Validating XML Documents by Using XML Schemas

When you want to validate an XML document by using an XML schema, you can choose from the many XML validators that are available. Here's a starter list:

• Visual Studio .NET ( http://www.microsoft.com )— You just select XML, Validate XML Data to begin validating an XML document.
• Topologi Schematron Validator ( http://www.topologi.com )— This is a free Windows-based tool that validates XML schemas.
• XML Schema Quality Checker ( http://www.alphaworks.ibm.com/tech/xmlsqc )— This is the IBM AlphaWorks XML schema validator.
• Xerces ( http://xml.apache.org/xerces2-j/index.html )— The Apache Project's Xerces 2 XML processor includes XML schema validation.
• XSD Schema Validator ( http://apps.gotdotnet.com/xmltools/xsdvalidator )— This tool is a .NET-based XML schema validator.
• XSV ( http://www.w3.org/2001/03/webdata/xsv )— This is the online W3C XML schema validator.
• Xerces J ( http://tools.decisionsoft.com/schemaValidate.html )— This is a DecisionSoft XML validator.
• Internet Explorer ( http://www.microsoft.com/windows/ie/default.asp )— You can use Microsoft Internet Explorer to validate schemas.

The most widely used of these XML schema validation tools is Internet Explorer. The XML support in Internet Explorer is built into the MSXML package (which was called the Microsoft XML Parser until MSXML version 4.0, when it was named the Microsoft XML Core Services). MSXML version 4.0 is the version that supports XML Schema Definition Language (XSD) schemas. (Note that before version 4.0, MSXML only supported a smaller and different version of XML schemas, which Microsoft called XML-Data Reduced [XDR] schemas.) Table 6.1 lists support for XML schemas, by MSXML version.

Table 6.1. XML Schema Support by MSXML Version

Let's put MSXML 4.0 and Internet Explorer 6 to work now. If you want Internet Explorer to validate an XML document by using an XML schema, you have to do a little extra work by using JScript, Internet Explorer's version of JavaScript. You'll take a more detailed look at JavaScript on Day 15, "Using JavaScript and XML," and you don't have to write any JavaScript until then—all the work is already done for you in the HTML document ch06_01.html, which is shown in Listing 6.1. You can load this HTML document in an XML document, as stored in a file named ch06_02.xml, and validate it.

Example 6.1. An HTML Document That Can Validate an XML Document (ch06_01.xml)

<HTML>
    <HEAD>
        <TITLE>
            Validating With XML Schemas
        </TITLE>
        <SCRIPT LANGUAGE="JavaScript">
            document.write("<H1>Validating With XML Schemas</H1>");
            var parser = new ActiveXObject("MSXML2.DOMDocument.4.0");
            parser.validateOnParse = true;

            if (parser.load("ch06_02.xml")) {
                document.write("The document is valid!");
            } else {
                if (parser.parseError.errorCode != 0) {
                    document.write(parser.parseError.reason);
                }
            }
        </SCRIPT>
    </HEAD>

    <BODY></BODY>
</HTML>

As an example, you can validate the following sample XML document, which you have already seen today:

<?xml version="1.0"?>
<document xmlns="http://xmlpowercorp">
    <text>
        Welcome to XML Schemas!
    </text>
</document>

How do you connect an XML schema to an XML document? Different XML processors do it in different ways, but, slowly, a standard is emerging. If you define a namespace, usually named xsi, for the URI www.w3.org/2001/XMLSchema-instance, you can then use an attribute named xsi:schemaLocation in the document element to specify the URI of the document's XML schema.

Not many XML processors support this attribute yet, but Internet Explorer does. To use this attribute so that Internet Explorer will understand it, you assign it a text string, giving the namespace you're using in our XML document, which is http://xmlpowercorp here, and the URI of the XML schema, which is ch0603.xsd in this case (assuming that ch0603.xsd is in the same directory as the XML document), like this: xsi:schemaLocation="http://xmlpowercorp ch06_03.xsd". (If you're not using a namespace in our XML document, you can use the xsi:noNamespaceSchemaLocation attribute.) Listing 6.2 shows how this works in the XML document ch06_02.xml.

Example 6.2. A Sample XML Document to Verify (ch06_02.xml)

<?xml version="1.0"?>
<document xmlns="http://xmlpowercorp"

       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

       xsi:schemaLocation="http://xmlpowercorp ch06_03.xsd">
    <text>
        Welcome to XML Schemas!
    </text>
</document>

Listing 6.3 shows an XML schema (ch06_03.xsd) you can use for this example.

Example 6.3. The XML Schema for the First Example (ch06_03.xsd)

<?xml version="1.0"?>
<xsd:schema targetNamespace="http://xmlpowercorp"
xmlns="http://xmlpowercorp"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
attributeFormDefault="qualified" elementFormDefault="qualified">
  <xsd:element name="document">
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element name="text" type="xsd:string" minOccurs="1" />
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>
</xsd:schema>

Just by looking at the XML schema in Listing 6.3, you can get an idea of what's going on: You declare an element named <document> and another one named <text> to match what's in the XML document. Then when you open ch06_01.html, Internet Explorer loads in the XML document, ch06_02.xml, and then the XML schema, ch06_03.xsd, and it validates the XML document, as shown in Figure 6.4. (Note that to run this example, ch06_01.html, ch06_02.xml, and ch06_03.xsd should all be in the same directory. )

Figure 6.4 Validating with an XML schema in Internet Explorer.

On the other hand, what if the XML processor you're using objects to the xsi:schemaLocation attribute, which you use to connect the XML schema to an XML document, as nonstandard? It turns out that it's actually not necessary to embed an xsi:schemaLocation attribute in an XML document to validate it by using Internet Explorer—you can use JavaScript to tell Internet Explorer where to find the XML schema. Listing 6.4 shows an HTML document (ch06_04.html) that does that.

Example 6.4. An HTML Document That Verifies an XML Document (ch06_04.html)

<HTML>
    <HEAD>
        <TITLE>
            Validating With XML Schemas
        </TITLE>
        <SCRIPT LANGUAGE="JScript">
            document.write("<H1>Validating With XML Schemas</H1>");
            var schemaHandler = new ActiveXObject("MSXML2.XMLSchemaCache.4.0");
            schemaHandler.add("http://xmlpowercorp", "ch06_03.xsd");

            var parser = new ActiveXObject("MSXML2.DOMDocument.4.0");
            parser.schemas = schemaHandler;
            parser.validateOnParse = true;

            if (parser.load("ch06_05.xml")) {
                document.write("The document is valid!");
            } else {
                if (parser.parseError.errorCode != 0) {
                    document.write(parser.parseError.reason);
                }
            }
        </SCRIPT>
    </HEAD>

    <BODY></BODY>
</HTML>

By using ch06_04.html, you don't have to use the xsi:schemaLocation attribute, as you can see in the new version of the sample XML document, ch06_05.xml, which is shown in Listing 6.5.

Example 6.5. The XML Document to Be Verified (ch06_05.xml)

<?xml version="1.0"?>
<document xmlns="http://xmlpowercorp">
    <text>
        Welcome to XML Schemas!
    </text>
</document>

When you open this HTML document, ch06_04.html, in Internet Explorer, you get the same results shown in Figure 6.4 as it validates the new version of the XML document. (Note that to run this example, ch06_04.html, ch06_05.xml, and ch06_03.xsd should all be in the same directory.)

Now that you have some experience using software both in generating XML schemas and using them to validate XML, it's time to get into the meat of today's discussion: creating our own XML schemas.