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XML Schemas

XML Schemas. Chapter 16. Introduction to Schemas. Schemas are more powerful than DTDs They can describe complex restrictions on elements and attributes. A Schema is a formal description of what comprises a valid document. An XML Schema is written in XML

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XML Schemas

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  1. XML Schemas Chapter 16

  2. Introduction to Schemas • Schemas are more powerful than DTDs • They can describe complex restrictions on elements and attributes. • A Schema is a formal description of what comprises a valid document. • An XML Schema is written in XML • Formal description of what comprises a valid XML document

  3. Introduction to Schemas Pt. 2 • Instance Document – XML document described by schema • If it satisfies schema constraints, document is schema-valid. • xsi:schemaLocation attribute contains list of namespaces within element and URLs of the schemas that validate elements in namespaces. • xsi:noNamespaceSchemaLocation attribute contains URL for schema containing elements not in any namespace.

  4. Schemas Versus DTDs • W3C XML Schema standard includes: • Simple and complex data types • Type derivation and inheritance • Element occurrence constraints • Namespace aware element and attribute declarations. • You can declare your own data types as well • You can inherit from existing data types and reuse types from other schemas. • Better restrict number and sequence of child elements that appear in a given location even when using mixed content. • DTD attributes can have default/fixed values. Schema elements can have default values: <xs: element name=“myName” type=“xs:string” default=“Joe” /> • Unlike default attribute values, default and fixed element values are only added to the document if the element is present. • However, there are no entities in schemas. However the fixed attribute can be used to represent another value.

  5. Namespace Issues • Schemas provide explicit support for namespaces since its recommendation was passed AFTER the namespace recommendation was passed. • Schemas validate against the combination of the namespace URI and local name rather than by the prefixed name. • Namespaces are used within instance documents to include directives to the schema processor. • SchemaLocation and noNamespaceSchemaLocation must be associated with official XML Schema instance namespace URI: http://www.w3.org/2001/XMLSchema-instance or else the schema processor won’t recognize the instruction. • Information preceded with xs: is part of the xml schema instance namespace URI. • Any information without a prefix is part of the structure being defined, unless it is in a user defined namespace.

  6. Document Organization • Every schema document that doesn’t contain namespaces consists of a single root xs:schema element. • xs:schema contains declarations for all elements/attributes that can appear in valid instance document. • XMLelements in a n XML schema must belong to the XML schema namespace at http://www.w3.org/2001/XMLSchema • This namespace is usually associated with the xs prefix. • It is an error to declare two elements with global scope (root elements)

  7. Annotations • Parsers are not obliged to keep comments intact when parsing XML documents. • Use the optional xs:annotation element generally as the first child element. • Include any combination of xs:documentation (human-readable information) and xs:appinfo (machine-readable information) elements as child elements.

  8. xs:documentation Element • See example on 258-259 • Permits xml:lang attribute to identify message’s language • xml:lang can also be added to xs:schema element to set document’s language • xs:documentation can contain any well formed XML

  9. xs:appinfo Element • Little difference between this element and xs:documentation. • Should contain application specific extension information related to a particular schema element. • For example XML text could be added to help an application generate a tool tip. See example on bottom of 259.

  10. Element Declarations • xs:element is widely used schema declaration • <xs:element name=”fullName” type=“xs:string”> indicates that the document will contain an element named name that is of type string and is named “fullName”

  11. Referencing Predefined Elements • Global elements make a schema easier to read. • You can define elements after the annotation. • When the element is going to be used, add the ref attribute to link to the predefined element<xs:element name=“brand” type=“xs:string” />…<xs:complexType> <xs:sequence>-Indicates element order <xs:element ref=“brand” minOccurs=“1” />

  12. Scope • You can define several different elements with the same name as long as they are used in different contexts. • This is another benefit of using Predefined elements. You only have to define the element once and then you can reuse.

  13. Simple Types • Simple content equates with basic data types such as String, integer, etc. • Simple types cannot contain nested content. • See table 16-1 on 260-261 for a list of built in simple schema types. • Attributes can NEVER contain elements so they will always use simple types. • An element that is declared as a simple type CANNOT have any attributes. • The default simple type is “xs:anytype”

  14. Attribute Declarations • Attributes are declared using the xs:attribute element. • May be declared globally or locally. • The code sample on the bottom of 261 shows the work required to define an element that contains an attribute. • the element contains: • <xs:complextype> - indicates the element is now complex. • <xs:simpleContent> - indicates the element’s simple content appears next. • <xs:extension base=“xs:string”>-indicates the data type of the simple content. • <xs:attribute name=“language” type=“xs:language”>-indicates the attribute’s name, and simple type. • use can be optional/required or prohibited, default is similar to #IMPLIED and fixed is similar to #FIXED

  15. Attribute Groups • Similar to DTD’s parameter entities • Names group of xs:attribute declarations or references to other attribute groups. • Can be referenced from complex type definitions. • Must be declared as a global element with a unique name attribute. • Referenced within a complex type by including another xs:attributeGroup element with a ref attribute that matches the group name. • The example on 262 shows how attribute groups can be reused.

  16. Element Groups • You can define a group of elements using <xs:group name=“thegroup”> <xs:sequence> …….. • <xs:group ref=“thegroup” • Groups can be used within choices to require one group or the other.

  17. Complex Types • Schemas assign types to each declared element and attribute. • Complex types may contain nested elements and attributes. • Only elements can have complex types. • When building large and complex schemas, data types will need to be shared among different elements. • Named types facilitate reuse. • If you surround the complex type’s elements with the <xs:all>empty element all of the elements are required. • If you surround the complex type’s elements with the <xs:choice> empty element, then only one of the elements can be included. • <xs:sequence> indicates that the elements should appear in the order in which they were defined. • If you set the xs:complexType mixed attribute to “false”, the complex type can only contain elements. setting it to true allows mixed content to be displayed.

  18. Occurrence Constraints • You can explicitly set the minimum and maximum number of times an element may occur at a particular point in a document using minOccurs and maxOccurs attributes of xs:element element. • See example on middle of page 267. • Default value for both attributes is 1 • If you just set minOccurs to 0 and leave out maxOccurs, then the element may appear 0 or 1 times, similar to using the ? . • You can set maxOccurs to “unbounded”. This means the maximum is unlimited. Like * or + • You an set minOccurs and maxOccurs for xs:choice giving you increased power.

  19. Named Types • The simplest way to create a new type is to start with a complex type and give it a name. • You then define an element of the type you just created. At this point you can also set other attributes such as minOccurs or maxOccurs. • <xs:complexType name=“myNamedType”>…..</xs:complexType> • <xs:element name=“item” type=“myNamedType”> • The new type is NOT prefixed with xs!

  20. Restrictions • You can create a new type out of one that already exists. • You accomplish this by adding conditions to one of XML Schema’s built-in types. • <xs:simpleType name=“USCurrency”> <xs:restriction base=“xs:decimal”> <xs:fractionDigits value=“2” /> </xs:restriction></xs:simpleType> • <xs:element name=“lastPrice” type=“USCurrency” />

  21. Restrictions Part Two • The restriction element tells the processor that we’re creating a subset of an existing base type, decimal • The subtype in the example must have two digits after the decimal point. • Simple types can be restricted on the basis of one or more facets (requirements)

  22. Restrictions Part Three • minInclusive – Minimum value • maxInclusive – Maximum value • minExclusive– Number must be one higher than the value • maxExclusive—Number must be one lower than the value • totalDigits—Required number of digits including before and after the decimal point. • fractionDigits – Required number of digits after the decimal point • length – Refers to total characters or total items in a list • minlength/maxlength • pattern -- regular expression • enumeration – Set of potential values • whiteSpace – set to preserve, data is untouched, set to replace tabs line feeds and carriage returns are replaced with spaces, set to collapse, multiple spaces are replaced with single space.

  23. Enumeration • You can utilize facets by creating an enumerated type to restrict choices for an element or attribute. • <xs:attribute name=“demand”> <xs:simpleType> <xs:restriction base=“xs:string”> <xs:enumeration value=“low” /> <xs:enumeration value=“high” />……..

  24. Lists • It’s similar to an enumeration, however it allows multiple values separated by spaces • Using the demand attribute that contains the restricted enumerations: • <xs:simpleType name=“demandList” <xs:list itemType=“demand” /></xs:simpleType> • <xs:simpleType name=“theDemands” <xs:restriction base=“demandList”> <xs:maxLength value=“2” /> </xs:restriction></xs:simpleType> • INSIDE OF XML DOCUMENT:<theDemands> low high</theDemands>

  25. Adding Attributes to Simple Types by Extension • It is difficult to include elements in a schema that have both attributes and text. • You can’t specify the text type in a simple element, because elements with attributes aren’t simple. • If you define element as complex, you can add attributes easily, but the only way to add content is to add a child element. • You get around the problem by extending the simple type and adding attributes to its model.

  26. Extending a Simple Type: Example • <xs:element name=“originalOwner> <xs:complexType><xs:simpleContent><xs:extension base=“xs:string”> <xs:attribute name=“confirmed” default=“no” /> </xs:extension></xs:simpleContent></xs:complexType></xs:element> • Because it has an attribute, you must declare originalOwner as a complex type. However the xs:simpleContent tag indicates that the element should contain text and no child elements • SimpleContent derives from the base type String. In our example it’s extended to include the confirmed attribute.

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