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Week 2 - XML Schema and DTD ​

Table of content ​

  • Review Week 1 Tutorial.
  • Review XML structure.
  • Review XML rules.
  • Explain what an XML schema is.
  • Explain what a document definition is.
  • Create a DTD document definition.
  • Understand the main elements of DTD.

XML Schema? ​

  • The schema describes the structure.
    • What data is allowed.
    • What data is required.
    • How data is organized.
  • Example: maged#zy.cdut

Well Formed and Valid XML ​

XML document is well-formed:

  • structurally and syntactically correct.
  • Adheres to the general XML rules.
  • Not necessarily valid. XML document is valid:
  • Satisfies the schema document.
  • Validity can vary between schema documents.
  • XML documents that fail validation are considered invalid.

XML Schema Types ​

The XML schema has two types:

  • DTD: Document Type Definition.
  • XSD: XML Schema Definition.

DTDs ​

Document Type Definition (DTD) ​

DTD is a structure (standard)

  • A DTD is a set of markup declarations that define the document structure.
  • Uses a list of validated elements and attributes.
  • What elements can/must appear.
  • What order they must be in.

DTD basics ​

DTD determines:

  • What the root element is.
  • Number of instances of elements. DTD does not determine:
  • Data types.
  • Meaning of an element.

Is DTD also an XML file? ​

No, DTDs are NOT xml

DTD location ​

Internal: declared inside the XML document.

  • Appears after the XML declaration, and before the document body. External: declared outside the XML document (as an external reference).

DOCTYPE ​

  • internal
xml
<!DOCTYPE Name [
    Document definition
]>
  • external
xml
<!DOCTYPE events SYSTEM "xxx.dtd">

The contents of a DTD ​

DTD Elements:

  • Elements are the allowed/required tags.
  • Created with an Element declaration.
  • ElementName = XML Tag.
  • Type = Tag Type

Internal DTD Example ​

xml
<?xml version="1.0" standalone="yes"?>
<!DOCTYPE Foods [
    <!ELEMENT Foods (Food)*>
    <!ELEMENT Food (Fruits,Vegetables,Meat,Fish)>
    <!-- 上面的元素定义了食品的结构（Food内的标签及其顺序，即Fruits,Vegetables,Meat,Fish） -->
    <!ELEMENT Fruits (#PCDATA)>
    <!ELEMENT Vegetables (#PCDATA)>
    <!ELEMENT Meat (#PCDATA)>
    <!ELEMENT Fish (#PCDATA)>
]>
<!-- 在浏览器中查看XML文件时，DOCTYPE部分并不会显示 -->
<Foods>
    <Food>
        <Fruits>Apple</Fruits>
        <Vegetables>Carrot</Vegetables>
        <Meat>Lamb</Meat>
        <Fish>Salmon</Fish>
    </Food>
    <Food>
        <Fruits>Orange</Fruits>
        <Vegetables>Cucumber</Vegetables>
        <Meat>Mutton</Meat>
        <Fish>Cod</Fish>
    </Food>
</Foods>

External DTD Example ​

Foods.xml

xml
<?xml version="1.0" standalone="no"?>
<!DOCTYPE Foods SYSTEM “FoodsDTD.dtd">
<Foods>
    <Food>
        <Fruits>Apple</Fruits>
        <Vegetables>Carrot</Vegetables>
        <Meat>Lamb</Meat>
        <Fish>Salmon</Fish>
    </Food>
    <Food>
        <Fruits>Orange</Fruits>
        <Vegetables>Cucumber</Vegetables>
        <Meat>Mutton</Meat>
        <Fish>Cod</Fish>
    </Food>
</Foods>

FoodsDTD.dtd

xml
<!ELEMENT Foods (Food)*>
<!ELEMENT Food (Fruits,Vegetables,Meat,Fish)>
<!ELEMENT Fruits (#PCDATA)>
<!ELEMENT Vegetables (#PCDATA)>
<!ELEMENT Meat (#PCDATA)>
<!ELEMENT Fish (#PCDATA)>
  • Private external DTD: <!DOCTYPE name SYSTEM “pathToFile”>
  • External DTD on a website: <!DOCTYPE name SYSTEM “URL”>

Element type declarations ​

Mixed content需要把#PCDATA写在最前面

  • (#PCDATA): The content can be only character data.
    • e.g.,
      • DTD: <!ELEMENT first (#PCDATA)>
      • XML: <first> John </first>
  • EMPTY: The element is declared to be an empty one - no content allowed.
    • e.g.,
      • DTD: <!ELEMENT empty_element EMPTY>
      • XML: <empty_element></empty_element> or <empty_element />
  • ANY: The element can have any element or character data.
    • e.g.,
      • DTD: <!ELEMENT any_element ANY>
      • XML:
xml
<any_element>
    This is a line of characters
    <other_element> ... </other_element>
</any_element>
  • Child elements: The content can only contain the child elements (no character data). The sequence, alternative, and cardinality can be expressed using commas(,) Ors (|).
    • e.g. DTD: <!ELEMENT a (x,y,z)> Element a must have an element x, followed by y, followed by z.
    • e.g. DTD: <!ELEMENT b (x|y|z)> Element b must have an element x, or y, or z
  • Mixed content
    • Must use OR | to separate elements
    • #PCDATA must come first
    • * Must appear at the end if there are child elements
    • e.g., DTD: <!ELEMENT mix_element (#PCDATA | first | last)*>
xml
<mix_element>
    My first name is
    <first>John</first>
    and my last name is
    <last>smith </last>
</mix_element>

DTD Element Options (modifiers) ​

  • <!ELEMENT Foods (Food)*>
    • *: zero or more times.
    • +: one or more times.
    • ?: zero or one time.
    • No symbol means the element must appear only once.
    • e.g. DTD: <!ELEMENT c (x*, y+, z?) >

DTD Element Options ​

Elements with children

xml
<!ELEMENT person (child1,child2)>
<!ELEMENT person (child1+,child2)>
<!ELEMENT person (child1*,child2)>
<!ELEMENT person (child1?,child2)>
<!ELEMENT person (child1|(child2,child3))*>

DTD Attributes ​

<!ATTLIST ElementName AttributeName AttType DefaultValue>

  • Some common attribute data types:
    • CDATA: Character data (text values).
    • ID: unique key.
    • Enumerator: list of values (value1 | value2 | value3). e.g. DTD: <!ATTLIST event cancelled (y|n) #REQUIRED>
  • Some common default values:
    • #REQUIRED: <!ATTLIST Food orderNo CDATA #REQUIRED>
    • #IMPLIED: the attribute is optional
    • “default character data” <!ATTLIST person sex CDATA “male">
    • #FIXED: <!ATTLIST Form language CDATA #FIXED “EN”>

DTD Attributes Example ​

xml
<?xml version="1.0" standalone=“yes"?>
<!DOCTYPE Foods [
    <!ELEMENT Foods (Food)*>
    <!ELEMENT Food (Fruits,Vegetables,Meat,Fish)>
    <!ATTLIST Food orderNo CDATA "1">
    <!ELEMENT Fruits (#PCDATA)>
    <!ELEMENT Vegetables (#PCDATA)>
    <!ELEMENT Meat (#PCDATA)>
    <!ELEMENT Fish (#PCDATA)>
]>
<Foods>
    <Food orderNo ="1">
        <Fruits>Apple</Fruits>
        <Vegetables>Carrot</Vegetables>
        <Meat>Lamb</Meat>
        <Fish>Salmon</Fish>
    </Food>
    <Food orderNo ="2">
        <Fruits>Orange</Fruits>
        <Vegetables>Cucumber</Vegetables>
        <Meat>Mutton</Meat>
        <Fish>Cod</Fish>
    </Food>
</Foods>

Structure: CDATA ​

CDATA / PCDATA sections can be declared in both DTD and XML files

  • Attributes can be defined in DTD using CDATA
  • Elements can be defined in DTD using #PCDATA
  • You can also declare a CDATA section directly in an XML file with the following processing instruction : <![CDATA[<tags>content</tags>]]>

Entity declarations ​

To declare in DTD: <!ENTITY entityName "Long string or other character">
To display: &entityName;
Predefined entities:

xml
<?xml version="1.0" standalone="yes"?>
<!DOCTYPE Universities [
    <!ELEMENT Universities (University)*>
    <!ELEMENT University (#PCDATA)>
    <!ENTITY CDUT "Chengdu University of Technology">
    <!ENTITY OBU "Oxford Brookes University">
]>
<Universities>
    <University>&CDUT;</University>
    <University>&OBU;</University>
</Universities>

Result:

xml
<Universities>
    <University>Chengdu University of Technology </University>
    <University>Oxford Brookes University </University>
</Universities>

Comments ​

html
<!-- comments are written in the same way as XML and HTML comments-->

Issues with Document definitions ​

  • If your document contains a link to a definition and the definition changes your document may break.
  • If you link to a document definition and the link breaks your XML will not work.

Remember ​

XML Validation ​

Check the validity of your XML

  • Is it well formed?
  • Is it valid?

Useful Information ​

Once NP++ and XML tools are installed, go to: Plugins > XML Tools > Options
Set "Prohibit DTD" to FALSE. This allows the use of internal DTDs in your XML files. By default, these are disabled.