Reusing Components of a XML Schema

One of the key features of schemas is their ability to support a high degree of reusability among other schemas. This can be done by using import elements.

The include element

The include element is used to include or refer to an external schema that is located at a definite address. The syntax for using the include element is as follows:

<include id="ID" schemaLocation="filename"/>

In the preceding syntax, the include element consists of two attributes, id and schemaLocation. The id attribute is used to specify the element ID. The ID must be unique within the XSD (XML Schema Definition) document. The id attribute is optional. The another attribute is schemaLocation attribute. The value of this attribute specifies the physical location of the schema file.

The include element can have multiple occurrences in an XSD document. The schema element is the parent element of the include element. The only restriction placed on the use of include element is, both the containing and contained schema files must belong to the same target namespace.

A target namespace has a reference to the URI to which the schema belongs. You can create different target namespaces for the different schemas. For example, you can create a schema that contains the declarations for the elements and attributes required to store the purchase order details. You can then specify www.encomatcybershoppe.com/purchase as the targetnamespace for the schema.

The include element allows reference to any external schema that is defined in the context of the same target namespace. The target namespace for a schema is declared using the target Namespace attribute of the schema element.

Consider the following:

<schema xm1ns="http://www.w3.org/2001/XMLSchema"
targetNamespace="www.ecomatcybershoppe.com/purchase">
<simpleType name="prstring">
      <restriction base="string">
            <pattern value="[p] {1} \d {3}"/ >
      </restriction>
</simpleType>
</schema>

In the preceding example, a simple data type called prstring is created in an XML schema. This data type has a restriction that specifies that an element or attribute that contains prstring as its type should match a specific pattern. You can use the prstring data type in other XML schemas, as follows:

<schema xmlns="http://www.w3.org/2001/XMLSchema"
targetNamespace="www.ecomtcybershoppe.com/purchase"
xmlns:prd="www.ecomatcybershoppe.com/purchase">
<include schemaLocation="potype.xad"/>
                :
<element name="PRODID" type="prd:prstring"/>
                :
</schema>

In the preceding code snippet, the default namespace is specified as
http://www.w3.org/2001/XMLSchema. When you do not include any prefix with the element name or the data type, it is assumed that the element or data type belong to the default namespace. The target namespace is specified as www.ecomatcybershoppe.com/purchase.

Note that the target namespaces are the same in both the schemas. The prd prefix is used as an alias to refer to the namespaces URI www.ecomatcybershoppe.com/purchase. Now, can refer to the data types declared in potype.xsd, which is the physical location of the schema file in the target namespace, by using the prd prefix before the name of the data type. If you do not use the prefix, prstring will be considerd to belong to the default namespace.

The import Element

The import element performs the same function as the include element. however, the import element allows you to access components from multiple schemas that may belong to different target namespaces.

The syntax for using the import element is as follows:

<import id="ID" namespace="namespace" schemaLocation="filename" />

In preceding syntax, the import element contains three attributes:

• The id attribute takes the ID of the element as its value. This ID must be unique for the XSD document. This attribute is optional.  

• The namespace attribute specifies a namespace URI to which the imported schema belongs. The namespace attribute also specifies the prefix used to associate an element or an attribute also with a particular namespace.

• The schemaLocation attribute is identical to the value used by the include element. The value of this attribute is set to the physical location of the specified schema file.

While importing an XML schema, the importing  schema must contain a namespace reference to the target namespace of the imported schema. This namespace prefix can then be appended to the elements that are declared in the imported document, but used in the importing document.