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This article compares Document Type Definition (DTD) and XML Schema elements. Java 2 Enterprise Edition (J2EE) developers use DTDs and schemas in J2EE/XML applications. When a DTD for an XML document is provided and validation with an XML Schema is required, the DTD-to-XML Schema conversion creates an XML Schema document corresponding to the DTD document.

Overview
A DTD defines the structure of an XML document and defines a document's element types, subelement types, and the order and number of each element type. It also declares the attributes, entities, notations, processing instructions, and comments. An XML Schema is an XML-based representation of the structure of a XML document. Among the advantages of XML Schema is support for data types and namespaces. XML Schema, being XML based, is an extensible, complex representation of an XML document structure.

In this tutorial the different DTD declarations and their equivalent declarations in an XML Schema, and the equivalent of the XML document DOCTYPE declaration, are discussed, including:

• DOCTYPE
• Comment
• Processing instructions
• Element
• Attribute
• Entity
• Notation

DOCTYPE Declaration
The DOCTYPE declaration in an XML document refers to a public URI and the system URI of an external DTD. If the XML document is validated, the document elements and attributes are validated with the element and attribute definitions specified in the DOCTYPE DTD. A DTD specified with a system and public identifiers is an external subset DTD. A DTD may also be specified as an internal subset. An internal subset DTD is specified in the DOCTYPE declaration, which may consist of a combination of internal and external DTDs.

The DOCTYPE declaration in an XML document is specified with the <!DOCTYPE> declaration.

<!DOCTYPE root_element ((SYSTEM "system_uri")|(PUBLIC "public_uri"
"system_uri")>

The system URI of a schema document is specified in an XML document with the xsi:schemaLocation and xsi:noNamespaceSchemaLocation attributes in the root element. The xsi:schemaLocation is used to specify a namespace schema. The xsi:noNamespaceSchemaLocation is used to specify a schema without a namespace.

<root_element xmlns:xsi=http://www.w3.org/2001/
XMLSchema-instance
xsi:noNamespaceSchemaLocation=
"system_uri"
xsi:schemaLocation=
"namespace_uri system_uri ">

Comment Declaration
Comments in an XML document or a DTD are not displayed in the output document. They may be placed only after the XML declaration. They may not be placed within a tag, but may be placed within a DOCTYPE declaration.

In a DTD comments are specified with a comment declaration.

<!-- DTD Comment-->

In a schema comments are declared in the <xs:documentation/> element.

<xs:annotation>
<xs:documentation>Schema Comment
</xs:documentation>
</xs:annotation>

Processing Instructions Declaration
Processing instructions are used to include information in a DTD to be processed by applications. The XML declaration is an example of a processing instruction. Processing instructions in a DTD are specified with a processing instruction declaration.

<? PITarget Instruction?>

The PITarget "xml" or "XML" may only be used in an XML declaration. In a schema the <xs:appInfo/> element specifies the instruction for an application.

<xs:annotation>
<xs:appinfo>
<fn:application
Instruction>
Instruction for Application
</fn:application
Instruction>
</xs:appinfo>
</xs:annotation>

Element Declaration
Element declarations in a DTD specify the type and number of elements that may be included in an XML document. The element declaration also specifies the subelements and the order of the subelements.

Elements in a DTD are represented with <!ELEMENT> declarations.

<!ELEMENT A (B, C)>

Element A has subelements B and C. Element declarations in an XML Schema are represented with the <xsd:element/>.

<xsd:element name="element_name"
type="element_type" />

An <xs:element/> declaration in a schema may be in a <xs:schema/>, <xs:sequence/>, <xs:choice/>, or a<xs:all/> element. The following sections discuss different types of element declarations.

Text element
A text element may consist only of text. A text element is declared in a DTD by specifying the element as a parsed character data (PCDATA) element.

<!ELEMENT A (#PCDATA)>

In an XML Schema a text element is declared by specifying the type attribute of the element as xs:string.

<xs:element name="A" type="xs:string"/>

A PCDATA element may also be declared in a schema with a <xs:complexType/>.

<xsd:element name="A">
<xsd:complexType mixed="true">
<xsd:complexContent>
<xsd:restriction
base="xsd:anyType">
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
</xsd:element>

Empty element An empty element does not consist of any subelements or text. An empty element is declared in a DTD by specifying the element as an EMPTY element.

<!ELEMENT A EMPTY>

In a schema an empty element is represented by setting attribute "mixed" of element <xs:complexType> to "false."

<xsd:element name="A">
<xsd:complexType mixed="false">
<xsd:complexContent>
<xsd:restriction
base="xsd:anyType">
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
</xsd:element>

Any element
An element may be specified with variable text or subelements within the element. A variable element is declared in a DTD by specifying the element as an ANY element.

<!ELEMENT A ANY>

Element A may consist of variable text or subelements. The schema representation of a variable element is specified with the <xs:any/> element.

<xs:element name="A">
<xs:complexType>
<xs:sequence>
<xs:any minOccurs="0"
maxOccurs="unbounded" />
</xs:sequence>
</xs:complexType>
</xs:element>

Element sequence
An element in an XML document may consist of a sequence of subelements. An element sequence is specified in a DTD with a list of subelements.

<!ELEMENT A(B, C, D)>

Elements B, C, and D are a sequence of elements in Element A. In a schema a sequence of elements is specified with <xs:sequence/> .

<xs:element name="A" >
<xs:complexType>
<xs:sequence> <xs:element name="B"
type="xs:string"/>
<xs:element name="C"
type="xs:string"/>
<xs:element name="D"
type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>

Element choice
An element in an XML document may consist of a choice of elements. An element choice is specified in a DTD with a choice (|) of subelements.

<!ELEMENT A (B|C|D)>

Elements B, C, and D are a choice of elements in Element A. In a schema a choice of elements is specified with <xs:choice/>.

<xs:element name="A" >
<xs:complexType>
<xs:choice>
<xs:element name="B"
type="xs:string"/>
<xs:element name="C"
type="xs:string"/>
<xs:element name="D"
type="xs:string"/>
</xs:choice>
</xs:complexType>
</xs:element>

Element with sequence and choice
An element in an XML document may consist of a sequence and a choice of subelements.

An element sequence and choice is specified in a DTD with a combination of a list and choice (|) declarations.

<!ELEMENT A ((B,C)|D>

In a schema an element consisting of a sequence and a choice is declared with a combination of <xs:sequence/> and <xs:choice/> elements.

<xs:element name="A" >
<xs:complexType>
<xs:choice>
<xs:sequence>
<xs:element name="B"
type="xs:string"/>
<xs:element name="C"
type="xs:string"/>
</xs:sequence>
<xs:element name="D"
type="xs:string"/>
</xs:choice>
</xs:complexType>
</xs:element>

Element with cardinality
The cardinality of an element is the number of occurences of that element. In a DTD a cardinality of 0 or 1 is represented with "?"; a cardinality of 0 or more is represented with "*"; and a cardinality of 1 or more is represented with "+". An element or an element group may have a cardinality. An element with subelements with a cardinality may be specified in a DTD with the cardinality notation (?, *, +).

<!ELEMENT A (B?, (C*, D+)*)>

In a schema, cardinality is represented with the minOccurs and maxOccurs attributes (see Listing 1).

The minOccurs and maxOccurs attributes may be declared in the <xs:element/>, <xs:all/>, <xs:choice/>, <xs:sequence/>, and <xs:any/> elements.

Element with PCDATA and subelements
An element in an XML document may consist of both text and subelements. An element with text and subelements is declared in a DTD by specifying the element as a list of subelements consisting of a PCDATA subelement.

<!ELEMENT A(#PCDATA, B, C)>

An element with text and subelements in a schema is specified with the <xs:complexType/> attribute "mixed" set to "true".

<xs:element name="A" >
<xs:complexType mixed="true">
<xs:sequence>
<xs:element name="B"
type="xs:string"/>
<xs:element name="C"
type="xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>

Attribute Declaration
Attributes are additional information associated with XML document elements. An attribute in a DTD is defined with an <!ATTLIST> declaration.

<!ATTLIST Element_Name (Attribute_
Name attribute_type default_value)*>

An ATTLIST may define multiple attributes. The attribute_type may be "CDATA" enumerations of values "NOTATION", "ID", "IDREF", "IDREFS", "ENTITY", "ENTITIES", "NMTOKEN", or "NMTOKENS". The default_type may be "#REQUIRED", "#IMPLIED", or "#FIXED AttValue".

In a schema an attribute is declared with the <xs:attribute/> element.

<xs:attribute default ="default_
value" fixed ="fixed_value" name
="attribute_name" use = (optional |
required) : optional/>

The <xs:attribute/> declaration may be in an <xs:schema/>, <xs:complexType/>, <xs:restriction/>, <xs:extension/>, or <xs:attributeGroup/> element.

CDATA attributes
Character data (CDATA) attributes are text attributes. Text attributes in a DTD are declared by specifying an attribute as a CDATA attribute.

<!ATTLIST A
a CDATA #REQUIRED
b CDATA #IMPLIED
c CDATA #FIXED "AttValue">

Element A has attributes a, b, and c. In a schema a CDATA attribute is specified with the attribute type xs:string.

<xs:element name="A" >
<xs:complexType>
<xs:attribute name="a"
type="xs:string"
use="required"/>
<xs:attribute name="b"
type="xs:string"
use="optional"/>
<xs:attribute name="c"
type="xs:string" fixed="AttValue"/>
</xs:complexType>
</xs:element>

The attributes may be included in an element as a <xs:attributeGroup/>. The <xs:attributeGroup/> element declares a group of attributes (see Listing 2).

Enumerated Attributes
Enumerated attributes specify a choice of attribute values. In a DTD an enumerated attribute is declared with a choice(|) of attribute values.

<!ATTLIST A a(x|y|z)>

In a schema an enumerated attribute is declared with the <xs:enumeration/> element (see Listing 3).

Data types attributes
An attribute in a DTD with data type "NOTATION", "ID", "IDREF", "IDREFS", "ENTITY," "ENTITIES", "NMTOKEN", or "NMTOKENS" is specified as in the following DTD declaration.

<!ATTLIST A
a NOTATION
b ID
c IDREF
d IDREFS
e ENTITY
f ENTITIES
g NMTOKEN
h NMTOKENS>

In a schema an element with "NOTATION", "ID", "IDREF", "IDREFS", "ENTITY", "ENTITIES", "NMTOKEN", or "NMTOKENS" data type attributes is specified by setting the "type" attribute of the <xs:attribute/> element (see Listing 4).

Entity Declaration
Entities reference data that is abbreviated or that may be found at an external location. In a DTD an entity may be a general entity or a parameter entity. A general entity is represented in a DTD with an <!ENTITY> declaration.

<!ENTITY Entity_name Entity_value|((SYSTEM System_uri | PUBLIC Public_uri
System_uri) NDATA Notation_name)>

A parameter entity is represented with a <!ENTITY> declaration.

<!ENTITY Entity_name Entity_value|(SYSTEM System_uri | PUBLIC Public_uri
System_uri)>

System_uri is entity's system identifier and Public_uri is entity's public identifier.

XML Schema does not have an equivalent of an ENTITY declaration. In a schema an entity is specified with the <xs:simpleType/> element. An entity is represented in a DTD by the following ENTITY declaration:

<!ENTITY % complexDerivationSet "CDATA">

which is represented in a schema with the following <xs;simpleType/> declaration:

<xs:simpleType name="complexDerivationSet">
<xs:restriction base="xs:string"/>
</xs:simpleType>

Notation Declaration
Notations are used to identify the format of unparsed entities, elements with notation attributes, or processing instructions. In a DTD a notation is specified with a <!NOTATION> declaration.

<!NOTATION notation_name (ExternalID | PublicID)>

In a schema a notation is specified with the <xs:notation/> element.

<xs:notation name ="notation_name" public ="PublicID" system ="ExternalID"/>

A notation is declared in a DTD with the following declaration:

<!NOTATION gif PUBLIC "image/gif">

and represented in a schema with the following declaration:

<xs:notation name="gif" public="image/gif">

Conclusion
Based on the DTD declarations and the equivalent XML Schema declarations an XML/J2EE developer should be able to convert a DTD to an XML Schema, and an XML Schema to a DTD document.