An XML schema depicts the structure of an XML document.
XML structures schema define a class of documents by using schema components to constrain and provide meaning to the documents, usage and relationship among their parts, like, data types, elements and attributes in addition with values. It provides the schema for specification for further information of document. Thus formally can be said that,
“XML Schema: Structures can be used to define, describe and catalogue XML vocabularies for classes of XML documents.”
XML Schema is an XML-based alternative to DTD. And it can also be called XML Schema definition (XSD).
Why XML Schema
- Provide more influential and flexible schema language as compare to DTD
- Represent XML document syntax in XML language
- XML tools can be readily used
- Support non-textual data types
- vital role in B2B, e-Commerce
- can easily manage complex syntax
- With DTD, you cannot specify “a value of an element is in the range of 10 to 100”
Valid Vs Schema-Valid
Initially with XML 1.0, XML Schema was not the part. So that the documents that are validated with DTD syntax are call “Valid” and on the contrary, documents validated with XML schema are “Schema-valid”. Additionally, XML document that conforms to a specific XML schema is “instance document” of that schema.
Definition and Declaration
Definition is to create new types and declaration is to use that definition.
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<!– Definition: Creation of a type –>
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<xsd:simpleType name="zipUnion">
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<xsd:union memberTypes="USState listOfMyIntType"/>
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</xsd:simpleType>
<!– Declaration –>
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<element name=zips type=“zipUnion”>
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The <schema> Element, Simple Types & Complex Types
The <schema> Element
The schema element is the root element of every xml schema document.
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<?xml version="1.0"?>
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<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
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targetNamespace="http://www.w3schools.com"
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xmlns="http://www.w3schools.com"
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elementFormDefault="qualified">
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…
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…
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</xs:schema>
Simple Types
A simple type cannot contain child elements or attributes, so it can only contain text. Well, it can be one of the following types as well, Boolean, string, date etc. or some custom type.
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<xs:element name="xxx"/>
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To define a default value use default attribute and for fixed use Fixed attribute as show in following examples.
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<xs:element name="name" default="papu"/>
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<xs:element name="name" fixed="mike"/>
Note: XML Schema specification defines a number of simple types for values, as shown in Table: Predefined simple types of values
Complex Types
Complex types contain other elements and/or attributes. There are four kinds of complex elements,
- empty elements
- elements that contain only other elements
- elements that contain only text
- elements that contain both other elements and text
Defining a complex type,
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<xs:element name="employee">
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<xs:complexType>
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<xs:sequence>
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<xs:element type="xs:string"/>
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<xs:element type="xs:string"/>
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</xs:sequence>
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</xs:complexType>
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</xs:element>
And its declaration should be like this,
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<employee>
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<firstname>John</firstname>
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<lastname>Smith</lastname>
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</employee>
Importing Sample Schema with no schema location
<!– This is the same schema you saw in Example1 –>
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<xsd:schema
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targetNamespace=‘http://www.SampleStore.com/Account’
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xmlns:xsd=‘http://www.w3.org/1999/XMLSchema’
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xmlns:ACC= ‘http://www.SampleStore.com/Account’>
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<xsd:element name=‘InvoiceNo’ type=‘xsd:positive-integer’/>
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<xsd:element name=‘ProductID’ type=‘ACC:ProductCode’/>
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<xsd:simpleType name=‘ProductCode’ base=‘xsd:string’>
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<xsd:pattern value=‘[A-Z]{1}d{6}’/>
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</xsd:simpleType>
Importing a Schema with schemaLocation
The PART namespace needs to be imported using the import declaration element whose schemaLocation attribute specifies the location of the file that contains the schema because
- Is not a well-known namespace
- Is not a targetNamespace
<!–XML Schema fragment in file schema1.xsd–>
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<schema targetNamespace=‘http://www.SampleStore.com/Account’
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xmlns=‘http://www.w3.org/1999/XMLSchema’
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xmlns:ACC= ‘http://www.SampleStore.com/Account’
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xmlns:PART= ‘http://www.PartnerStore.com/PartsCatalog’>
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<import namespace=‘http://www.PartnerStore.com/PartsCatalog’
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schemaLocation=‘http://www.ProductStandards.org/repository/alpha
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.xsd’/>
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<element name=‘InvoiceNo’ type=‘positive-integer’/>
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<element name=‘ProductID’ type=‘ACC:ProductCode’/>
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<simpleType name=‘ProductCode’ base=‘string’>
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<pattern value=‘[A-Z]{1}d{6}’/>
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</simpleType>
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<element name=‘stickyGlue’ type=‘PART:SuperGlueType’/>
Tools for manipulating XML Schema
Several free and commercial tools are available,
- Netbeans 5.5 and higher
- XML Spy
- Apache Xerces
- DTD to XML Schema Conversion Tool
Resource:
http://www.javapassion.com/webservices/#Web_Services_Overview
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