XML - Quick Guide
XML - Overview
XML stands for Extensible Markup Language. It is a text-based markup language derived from Standard Generalized Markup Language (SGML).
XML tags identify the data and are used to store and organize the data, rather than specifying how to display it like HTML tags, which are used to display the data. XML is not going to replace HTML in the near future, but it introduces new possibilities by adopting many successful features of HTML.
There are three important characteristics of XML that make it useful in a variety of systems and solutions −
XML is extensible − XML allows you to create your own self-descriptive tags, or language, that suits your application.
XML carries the data, does not present it − XML allows you to store the data irrespective of how it will be presented.
XML is a public standard − XML was developed by an organization called the World Wide Web Consortium (W3C) and is available as an open standard.
XML Usage
A short list of XML usage says it all −
XML can work behind the scene to simplify the creation of HTML documents for large web sites.
XML can be used to exchange the information between organizations and systems.
XML can be used for offloading and reloading of databases.
XML can be used to store and arrange the data, which can customize your data handling needs.
XML can easily be merged with style sheets to create almost any desired output.
Virtually, any type of data can be expressed as an XML document.
What is Markup?
XML is a markup language that defines set of rules for encoding documents in a format that is both human-readable and machine-readable. So what exactly is a markup language? Markup is information added to a document that enhances its meaning in certain ways, in that it identifies the parts and how they relate to each other. More specifically, a markup language is a set of symbols that can be placed in the text of a document to demarcate and label the parts of that document.
Following example shows how XML markup looks, when embedded in a piece of text −
<message>
<text>Hello, world!</text>
</message>
This snippet includes the markup symbols, or the tags such as <message>...</message> and <text>... </text>. The tags <message> and </message> mark the start and the end of the XML code fragment. The tags <text> and </text> surround the text Hello, world!.
Is XML a Programming Language?
A programming language consists of grammar rules and its own vocabulary which is used
to create computer programs. These programs instruct the computer to perform specific tasks. XML does not qualify to be a programming language as it does not perform any computation or algorithms. It is usually stored in a simple text file and is processed by special software that is capable of interpreting XML.
XML - Syntax
In this chapter, we will discuss the simple syntax rules to write an XML document. Following is a complete XML document −
<?xml version = "1.0"?>
<contact-info>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</contact-info>
You can notice there are two kinds of information in the above example −
Markup, like <contact-info>
The text, or the character data, Howcodex and (040) 123-4567.
The following diagram depicts the syntax rules to write different types of markup and text in an XML document.
Let us see each component of the above diagram in detail.
XML Declaration
The XML document can optionally have an XML declaration. It is written as follows −
<?xml version = "1.0" encoding = "UTF-8"?>
Where version is the XML version and encoding specifies the character encoding used in the document.
Syntax Rules for XML Declaration
The XML declaration is case sensitive and must begin with "<?xml>" where "xml" is written in lower-case.
If document contains XML declaration, then it strictly needs to be the first statement of the XML document.
The XML declaration strictly needs be the first statement in the XML document.
An HTTP protocol can override the value of encoding that you put in the XML declaration.
Tags and Elements
An XML file is structured by several XML-elements, also called XML-nodes or XML-tags.
The names of XML-elements are enclosed in triangular brackets < > as shown below −
<element>
Syntax Rules for Tags and Elements
Element Syntax − Each XML-element needs to be closed either with start or with end elements as shown below −
<element>....</element>
or in simple-cases, just this way −
<element/>
Nesting of Elements − An XML-element can contain multiple XML-elements as its children, but the children elements must not overlap. i.e., an end tag of an element must have the same name as that of the most recent unmatched start tag.
The Following example shows incorrect nested tags −
<?xml version = "1.0"?>
<contact-info>
<company>Howcodex
</contact-info>
</company>
The Following example shows correct nested tags −
<?xml version = "1.0"?>
<contact-info>
<company>Howcodex</company>
<contact-info>
Root Element − An XML document can have only one root element. For example, following is not a correct XML document, because both the x and y elements occur at the top level without a root element −
<x>...</x>
<y>...</y>
The Following example shows a correctly formed XML document −
<root>
<x>...</x>
<y>...</y>
</root>
Case Sensitivity − The names of XML-elements are case-sensitive. That means the name of the start and the end elements need to be exactly in the same case.
For example, <contact-info> is different from <Contact-Info>
XML Attributes
An attribute specifies a single property for the element, using a name/value pair. An XML-element can have one or more attributes. For example −
<a href = "http://www.howcodex.com/">Howcodex!</a>
Here href is the attribute name and http://www.howcodex.com/ is attribute value.
Syntax Rules for XML Attributes
Attribute names in XML (unlike HTML) are case sensitive. That is, HREF and href are considered two different XML attributes.
Same attribute cannot have two values in a syntax. The following example shows incorrect syntax because the attribute b is specified twice
−
<a b = "x" c = "y" b = "z">....</a>
<a b = x>....</a>
In the above syntax, the attribute value is not defined in quotation marks.
XML References
References usually allow you to add or include additional text or markup in an XML document. References always begin with the symbol "&" which is a reserved character and end with the symbol ";". XML has two types of references −
Entity References − An entity reference contains a name between the start and the end delimiters. For example & where amp is name. The name refers to a predefined string of text and/or markup.
Character References − These contain references, such as A, contains a hash mark (“#”) followed by a number. The number always refers to the Unicode code of a character. In this case, 65 refers to alphabet "A".
XML Text
The names of XML-elements and XML-attributes are case-sensitive, which means the name of start and end elements need to be written in the same case. To avoid character encoding problems, all XML files should be saved as Unicode UTF-8 or UTF-16 files.
Whitespace characters like blanks, tabs and line-breaks between XML-elements and between the XML-attributes will be ignored.
Some characters are reserved by the XML syntax itself. Hence, they cannot be used directly. To use them, some replacement-entities are used, which are listed below −
| Not Allowed Character |
Replacement Entity |
Character Description |
| < |
< |
less than |
| > |
> |
greater than |
| & |
& |
ampersand |
| ' |
' |
apostrophe |
| " |
" |
quotation mark |
XML - Documents
An XML document is a basic unit of XML information composed of elements and other markup in an orderly package. An XML document can contains wide variety of data. For example, database of numbers, numbers representing molecular structure or a mathematical equation.
XML Document Example
A simple document is shown in the following example −
<?xml version = "1.0"?>
<contact-info>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</contact-info>
The following image depicts the parts of XML document.
Document Prolog Section
Document Prolog comes at the top of the document, before the root element. This section contains −
- XML declaration
- Document type declaration
You can learn more about XML declaration in this chapter − XML Declaration
Document Elements Section
Document Elements are the building blocks of XML. These divide the document into a hierarchy of sections, each serving a specific purpose. You can separate a document into multiple sections so that they can be rendered differently, or used by a search engine. The elements can be containers, with a combination of text and other elements.
You can learn more about XML elements in this chapter − XML Elements
XML - Declaration
This chapter covers XML declaration in detail. XML declaration contains details that prepare an XML processor to parse the XML document. It is optional, but when used, it must appear in the first line of the XML document.
Syntax
Following syntax shows XML declaration −
<?xml
version = "version_number"
encoding = "encoding_declaration"
standalone = "standalone_status"
?>
Each parameter consists of a parameter name, an equals sign (=), and parameter value inside a quote. Following table shows the above syntax in detail −
| Parameter |
Parameter_value |
Parameter_description |
| Version |
1.0 |
Specifies the version of the XML standard used. |
| Encoding |
UTF-8, UTF-16, ISO-10646-UCS-2, ISO-10646-UCS-4, ISO-8859-1 to ISO-8859-9, ISO-2022-JP, Shift_JIS, EUC-JP |
It defines the character encoding used in the document. UTF-8 is the default encoding used. |
| Standalone |
yes or no |
It informs the parser whether the document relies on the information from an external source, such as external document type definition (DTD), for its content. The default value is set to no. Setting it to yes tells the processor there are no external declarations required for parsing the document. |
Rules
An XML declaration should abide with the following rules −
If the XML declaration is present in the XML, it must be placed as the first line in the XML document.
If the XML declaration is included, it must contain version number attribute.
The Parameter names and values are case-sensitive.
The names are always in lower case.
The order of placing the parameters is important. The correct order is: version, encoding and standalone.
Either single or double quotes may be used.
The XML declaration has no closing tag i.e. </?xml>
XML Declaration Examples
Following are few examples of XML declarations −
XML declaration with no parameters −
<?xml >
XML declaration with version definition −
<?xml version = "1.0">
XML declaration with all parameters defined −
<?xml version = "1.0" encoding = "UTF-8" standalone = "no" ?>
XML declaration with all parameters defined in single quotes −
<?xml version = '1.0' encoding = 'iso-8859-1' standalone = 'no' ?>
XML - Tags
Let us learn about one of the most important part of XML, the XML tags. XML tags form the foundation of XML. They define the scope of an element in XML. They can also be used to insert comments, declare settings required for parsing the environment, and to insert special instructions.
We can broadly categorize XML tags as follows −
Start Tag
The beginning of every non-empty XML element is marked by a start-tag. Following is an
example of start-tag −
<address>
End Tag
Every element that has a start tag should end with an end-tag. Following is an example of end-tag −
</address>
Note, that the end tags include a solidus ("/") before the name of an element.
Empty Tag
The text that appears between start-tag and end-tag is called content. An element which has no content is termed as empty. An empty element can be represented in two ways as follows −
A start-tag immediately followed by an end-tag as shown below −
<hr></hr>
A complete empty-element tag is as shown below −
<hr />
Empty-element tags may be used for any element which has no content.
XML Tags Rules
Following are the rules that need to be followed to use XML tags −
Rule 1
XML tags are case-sensitive. Following line of code is an example of wrong syntax </Address>, because of the case difference in two tags, which is treated as erroneous syntax in XML.
<address>This is wrong syntax</Address>
Following code shows a correct way, where we use the same case to name the start and the end tag.
<address>This is correct syntax</address>
Rule 2
XML tags must be closed in an appropriate order, i.e., an XML tag opened inside another element must be closed before the outer element is closed. For example −
<outer_element>
<internal_element>
This tag is closed before the outer_element
</internal_element>
</outer_element>
XML - Elements
XML elements can be defined as building blocks of an XML. Elements can behave as containers to hold text, elements, attributes, media objects or all of these.
Each XML document contains one or more elements, the scope of which are either delimited by start and end tags, or for empty elements, by an empty-element tag.
Syntax
Following is the syntax to write an XML element −
<element-name attribute1 attribute2>
....content
</element-name>
where,
element-name is the name of the element. The name its case in the start and end tags must match.
attribute1, attribute2 are attributes of the element separated by white spaces. An attribute defines a property of the element. It associates a name with a value, which is a string of characters. An attribute is written as −
name = "value"
name is followed by an = sign and a string value inside double(" ") or single(' ') quotes.
Empty Element
An empty element (element with no content) has following syntax −
<name attribute1 attribute2.../>
Following is an example of an XML document using various XML element −
<?xml version = "1.0"?>
<contact-info>
<address category = "residence">
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</address>
</contact-info>
XML Elements Rules
Following rules are required to be followed for XML elements −
An element name can contain any alphanumeric characters. The only punctuation mark allowed in names are the hyphen (-), under-score (_) and period (.).
Names are case sensitive. For example, Address, address, and ADDRESS are different names.
Start and end tags of an element must be identical.
An element, which is a container, can contain text or elements as seen in the above example.
XML - Attributes
This chapter describes the XML attributes. Attributes are part of XML elements. An element can have multiple unique attributes. Attribute gives more information about XML elements. To be more precise, they define properties of elements. An XML attribute is always a name-value pair.
Syntax
An XML attribute has the following syntax −
<element-name attribute1 attribute2 >
....content..
< /element-name>
where attribute1 and attribute2 has the following form −
name = "value"
value has to be in double (" ") or single (' ') quotes. Here, attribute1 and attribute2 are unique attribute labels.
Attributes are used to add a unique label to an element, place the label in a category, add a Boolean flag, or otherwise associate it with some string of data. Following example demonstrates the use of attributes −
<?xml version = "1.0" encoding = "UTF-8"?>
<!DOCTYPE garden [
<!ELEMENT garden (plants)*>
<!ELEMENT plants (#PCDATA)>
<!ATTLIST plants category CDATA #REQUIRED>
]>
<garden>
<plants category = "flowers" />
<plants category = "shrubs">
</plants>
</garden>
Attributes are used to distinguish among elements of the same name, when you do not want to create a new element for every situation. Hence, the use of an attribute can add a little more detail in differentiating two or more similar elements.
In the above example, we have categorized the plants by including attribute category and assigning different values to each of the elements. Hence, we have two categories of plants, one flowers and other shrubs. Thus, we have two plant elements with different attributes.
You can also observe that we have declared this attribute at the beginning of XML.
Attribute Types
Following table lists the type of attributes −
| Attribute Type |
Description |
| StringType |
It takes any literal string as a value. CDATA is a StringType. CDATA is character data. This means, any string of non-markup characters is a legal part of the attribute. |
| TokenizedType |
This is a more constrained type. The validity constraints noted in the grammar are applied after the attribute value is normalized. The TokenizedType attributes are given as −
ID − It is used to specify the element as unique. IDREF − It is used to reference an ID that has been named for another element. IDREFS − It is used to reference all IDs of an element. ENTITY − It indicates that the attribute will represent an external entity in the document. ENTITIES − It indicates that the attribute will represent external entities in the document. NMTOKEN − It is similar to CDATA with restrictions on what data can be part of the attribute. NMTOKENS − It is similar to CDATA with restrictions on what data can be part of the attribute.
|
| EnumeratedType |
This has a list of predefined values in its declaration. out of which, it must assign one value. There are two types of enumerated attribute −
NotationType − It declares that an element will be referenced to a NOTATION declared somewhere else in the XML document. Enumeration − Enumeration allows you to define a specific list of values that the attribute value must match.
|
Element Attribute Rules
Following are the rules that need to be followed for attributes −
An attribute name must not appear more than once in the same start-tag or empty-element tag.
An attribute must be declared in the Document Type Definition (DTD) using an Attribute-List Declaration.
Attribute values must not contain direct or indirect entity references to external entities.
The replacement text of any entity referred to directly or indirectly in an attribute value must not contain a less than sign (<)
XML - Comments
This chapter explains how comments work in XML documents. XML comments are similar to HTML comments. The comments are added as notes or lines for understanding the purpose of an XML code.
Comments can be used to include related links, information, and terms. They are visible only in the source code; not in the XML code. Comments may appear anywhere in XML code.
Syntax
XML comment has the following syntax −
<!--Your comment-->
A comment starts with <!-- and ends with -->. You can add textual notes as comments between the characters. You must not nest one comment inside the other.
Example
Following example demonstrates the use of comments in XML document −
<?xml version = "1.0" encoding = "UTF-8" ?>
<!--Students grades are uploaded by months-->
<class_list>
<student>
<name>Tanmay</name>
<grade>A</grade>
</student>
</class_list>
Any text between <!-- and --> characters is considered as a comment.
XML Comments Rules
Following rules should be followed for XML comments −
- Comments cannot appear before XML declaration.
- Comments may appear anywhere in a document.
- Comments must not appear within attribute values.
- Comments cannot be nested inside the other comments.
XML - Character Entities
This chapter describes the XML Character Entities. Before we understand the Character Entities, let us first understand what an XML entity is.
As put by W3 Consortium the definition of an entity is as follows −
"The document entity serves as the root of the entity tree and a starting-point for an XML processor".
This means, entities are the placeholders in XML. These can be declared in the document prolog or in a DTD. There are different types of entities and in this chapter we will discuss Character Entity.
Both, HTML and XML, have some symbols reserved for their use, which cannot be used as content in XML code. For example, < and > signs are used for opening and closing XML tags. To display these special characters, the character entities are used.
There are few special characters or symbols which are not available to be typed directly from the keyboard. Character Entities can also be used to display those symbols/special characters.
Types of Character Entities
There are three types of character entities −
- Predefined Character Entities
- Numbered Character Entities
- Named Character Entities
Predefined Character Entities
They are introduced to avoid the ambiguity while using some symbols. For example, an ambiguity is observed when less than ( < ) or greater than ( > ) symbol is used with the angle tag (<>). Character entities are basically used to delimit tags in XML. Following is a list of pre-defined character entities from XML specification. These can be used to express characters without ambiguity.
Ampersand − &
Single quote − '
Greater than − >
Less than − <
Double quote − "
Numeric Character Entities
The numeric reference is used to refer to a character entity. Numeric reference can either be in decimal or hexadecimal format. As there are thousands of numeric references available, these are a bit hard to remember. Numeric reference refers to the character by its number in the Unicode character set.
General syntax for decimal numeric reference is −
decimal number ;
General syntax for hexadecimal numeric reference is −
Hexadecimal number ;
The following table lists some predefined character entities with their numeric values −
| Entity name |
Character |
Decimal reference |
Hexadecimal reference |
| quot |
" |
" |
" |
| amp |
& |
& |
& |
| apos |
' |
' |
' |
| lt |
< |
< |
< |
| gt |
> |
> |
> |
Named Character Entity
As it is hard to remember the numeric characters, the most preferred type of character
entity is the named character entity. Here, each entity is identified with a name.
For example −
XML - CDATA Sections
In this chapter, we will discuss XML CDATA section. The term CDATA means, Character Data. CDATA is defined as blocks of text that are not parsed by the parser, but are otherwise recognized as markup.
The predefined entities such as &lt;, &gt;, and &amp; require typing and are generally difficult to read in the markup. In such cases, CDATA section can be used. By using CDATA section, you are commanding the parser that the particular section of the document contains no markup and should be treated as regular text.
Syntax
Following is the syntax for CDATA section −
<![CDATA[
characters with markup
]]>
The above syntax is composed of three sections −
CDATA Start section − CDATA begins with the nine-character delimiter <![CDATA[
CDATA End section − CDATA section ends with ]]> delimiter.
CData section − Characters between these two enclosures are interpreted as characters, and not as markup. This section may contain markup characters (<, >, and &), but they are ignored by the XML processor.
Example
The following markup code shows an example of CDATA. Here, each character written inside the CDATA section is ignored by the parser.
<script>
<![CDATA[
<message> Welcome to Howcodex </message>
]] >
</script >
In the above syntax, everything between <message> and </message> is treated as character data and not as markup.
CDATA Rules
The given rules are required to be followed for XML CDATA −
- CDATA cannot contain the string "]]>" anywhere in the XML document.
- Nesting is not allowed in CDATA section.
XML - WhiteSpaces
In this chapter, we will discuss whitespace handling in XML documents. Whitespace is a collection of spaces, tabs, and newlines. They are generally used to make a document more readable.
XML document contains two types of whitespaces - Significant Whitespace and Insignificant Whitespace. Both are explained below with examples.
Significant Whitespace
A significant Whitespace occurs within the element which contains text and markup present together. For example −
<name>TanmayPatil</name>
and
<name>Tanmay Patil</name>
The above two elements are different because of the space between Tanmay and Patil. Any program reading this element in an XML file is obliged to maintain the distinction.
Insignificant Whitespace
Insignificant whitespace means the space where only element content is allowed. For example −
<address.category = "residence">
or
<address....category = "..residence">
The above examples are same. Here, the space is represented by dots (.). In the above
example, the space between address and category is insignificant.
A special attribute named xml:space may be attached to an element. This indicates that whitespace should not be removed for that element by the application. You can set this attribute to default or preserve as shown in the following example −
<!ATTLIST address xml:space (default|preserve) 'preserve'>
Where,
XML - Processing
This chapter describes the Processing Instructions (PIs). As defined by the XML 1.0 Recommendation,
"Processing instructions (PIs) allow documents to contain instructions for applications. PIs are not part of the character data of the document, but MUST be passed through to the application.
Processing instructions (PIs) can be used to pass information to applications. PIs can appear anywhere in the document outside the markup. They can appear in the prolog, including the document type definition (DTD), in textual content, or after the document.
Syntax
Following is the syntax of PI −
<?target instructions?>
Where
A PI starts with a special tag <? and ends with ?>. Processing of the contents ends immediately after the string ?> is encountered.
Example
PIs are rarely used. They are mostly used to link XML document to a style sheet. Following is an example −
<?xml-stylesheet href = "howcodexstyle.css" type = "text/css"?>
Here, the target is xml-stylesheet. href="howcodexstyle.css" and type="text/css" are data or instructions the target application will use at the time of processing the given XML document.
In this case, a browser recognizes the target by indicating that the XML should be transformed before being shown; the first attribute states that the type of the transform is XSL and the second attribute points to its location.
Processing Instructions Rules
A PI can contain any data except the combination ?>, which is interpreted as the closing delimiter. Here are two examples of valid PIs −
<?welcome to pg = 10 of tutorials point?>
<?welcome?>
XML - Encoding
Encoding is the process of converting unicode characters into their equivalent binary representation. When the XML processor reads an XML document, it encodes the document depending on the type of encoding. Hence, we need to specify the type of encoding in the XML declaration.
Encoding Types
There are mainly two types of encoding −
UTF stands for UCS Transformation Format, and UCS itself means Universal Character Set. The number 8 or 16 refers to the number of bits used to represent a character. They are either 8(1 to 4 bytes) or 16(2 or 4 bytes). For the documents without encoding information, UTF-8 is set by default.
Syntax
Encoding type is included in the prolog section of the XML document. The syntax for UTF-8 encoding is as follows −
<?xml version = "1.0" encoding = "UTF-8" standalone = "no" ?>
The syntax for UTF-16 encoding is as follows −
<?xml version = "1.0" encoding = "UTF-16" standalone = "no" ?>
Example
Following example shows the declaration of encoding −
<?xml version = "1.0" encoding = "UTF-8" standalone = "no" ?>
<contact-info>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</contact-info>
In the above example encoding="UTF-8", specifies that 8-bits are used to represent the characters. To represent 16-bit characters, UTF-16 encoding can be used.
The XML files encoded with UTF-8 tend to be smaller in size than those encoded with UTF-16 format.
XML - Validation
Validation is a process by which an XML document is validated. An XML document is said to be valid if its contents match with the elements, attributes and associated document type declaration(DTD), and if the document complies with the constraints expressed in it. Validation is dealt in two ways by the XML parser. They are −
- Well-formed XML document
- Valid XML document
Well-formed XML Document
An XML document is said to be well-formed if it adheres to the following rules −
Non DTD XML files must use the predefined character entities for amp(&), apos(single quote), gt(>), lt(, quot(double quote).
It must follow the ordering of the tag. i.e., the inner tag must be closed before closing the outer tag.
Each of its opening tags must have a closing tag or it must be a self ending tag.(<title>....</title> or <title/>).
It must have only one attribute in a start tag, which needs to be quoted.
amp(&), apos(single quote), gt(>), lt(, quot(double quote) entities other than these must be declared.
Example
Following is an example of a well-formed XML document −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>
<!DOCTYPE address
[
<!ELEMENT address (name,company,phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone (#PCDATA)>
]>
<address>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</address>
The above example is said to be well-formed as −
It defines the type of document. Here, the document type is element type.
It includes a root element named as address.
Each of the child elements among name, company and phone is enclosed in its self explanatory tag.
Order of the tags is maintained.
Valid XML Document
If an XML document is well-formed and has an associated Document Type Declaration (DTD), then it is said to be a valid XML document. We will study more about DTD in the chapter XML - DTDs.
XML - DTDs
The XML Document Type Declaration, commonly known as DTD, is a way to describe XML language precisely. DTDs check vocabulary and validity of the structure of XML documents against grammatical rules of appropriate XML language.
An XML DTD can be either specified inside the document, or it can be kept in a separate document and then liked separately.
Syntax
Basic syntax of a DTD is as follows −
<!DOCTYPE element DTD identifier
[
declaration1
declaration2
........
]>
In the above syntax,
The DTD starts with <!DOCTYPE delimiter.
An element tells the parser to parse the document from the specified root element.
DTD identifier is an identifier for the document type definition, which may be the path to a file on the system or URL to a file on the internet. If the DTD is pointing to external path, it is called External Subset.
The square brackets [ ] enclose an optional list of entity declarations called Internal Subset.
Internal DTD
A DTD is referred to as an internal DTD if elements are declared within the XML files. To refer it as internal DTD, standalone attribute in XML declaration must be set to yes. This means, the declaration works independent of an external source.
Syntax
Following is the syntax of internal DTD −
<!DOCTYPE root-element [element-declarations]>
where root-element is the name of root element and element-declarations is where you declare the elements.
Example
Following is a simple example of internal DTD −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>
<!DOCTYPE address [
<!ELEMENT address (name,company,phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone (#PCDATA)>
]>
<address>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</address>
Let us go through the above code −
Start Declaration − Begin the XML declaration with the following statement.
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>
DTD − Immediately after the XML header, the document type declaration follows, commonly referred to as the DOCTYPE −
<!DOCTYPE address [
The DOCTYPE declaration has an exclamation mark (!) at the start of the element name. The DOCTYPE informs the parser that a DTD is associated with this XML document.
DTD Body − The DOCTYPE declaration is followed by body of the DTD, where you declare elements, attributes, entities, and notations.
<!ELEMENT address (name,company,phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone_no (#PCDATA)>
Several elements are declared here that make up the vocabulary of the <name> document. <!ELEMENT name (#PCDATA)> defines the element name to be of type "#PCDATA". Here #PCDATA means parse-able text data.
End Declaration − Finally, the declaration section of the DTD is closed using a closing bracket and a closing angle bracket (]>). This effectively ends the definition, and thereafter, the XML document follows immediately.
Rules
The document type declaration must appear at the start of the document (preceded only by the XML header) − it is not permitted anywhere else within the document.
Similar to the DOCTYPE declaration, the element declarations must start with an exclamation mark.
The Name in the document type declaration must match the element type of the root element.
External DTD
In external DTD elements are declared outside the XML file. They are accessed by specifying the system attributes which may be either the legal .dtd file or a valid URL. To refer it as external DTD, standalone attribute in the XML declaration must be set as no. This means, declaration includes information from the external source.
Syntax
Following is the syntax for external DTD −
<!DOCTYPE root-element SYSTEM "file-name">
where file-name is the file with .dtd extension.
Example
The following example shows external DTD usage −
<?xml version = "1.0" encoding = "UTF-8" standalone = "no" ?>
<!DOCTYPE address SYSTEM "address.dtd">
<address>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</address>
The content of the DTD file address.dtd is as shown −
<!ELEMENT address (name,company,phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone (#PCDATA)>
Types
You can refer to an external DTD by using either system identifiers or public identifiers.
System Identifiers
A system identifier enables you to specify the location of an external file containing DTD declarations. Syntax is as follows −
<!DOCTYPE name SYSTEM "address.dtd" [...]>
As you can see, it contains keyword SYSTEM and a URI reference pointing to the location of the document.
Public Identifiers
Public identifiers provide a mechanism to locate DTD resources and is written as follows −
<!DOCTYPE name PUBLIC "-//Beginning XML//DTD Address Example//EN">
As you can see, it begins with keyword PUBLIC, followed by a specialized identifier. Public identifiers are used to identify an entry in a catalog. Public identifiers can follow any format, however, a commonly used format is called Formal Public Identifiers, or FPIs.
XML - Schemas
XML Schema is commonly known as XML Schema Definition (XSD). It is used to describe and validate the structure and the content of XML data. XML schema defines the elements, attributes and data types. Schema element supports Namespaces. It is similar to a database schema that describes the data in a database.
Syntax
You need to declare a schema in your XML document as follows −
Example
The following example shows how to use schema −
<?xml version = "1.0" encoding = "UTF-8"?>
<xs:schema xmlns:xs = "http://www.w3.org/2001/XMLSchema">
<xs:element name = "contact">
<xs:complexType>
<xs:sequence>
<xs:element name = "name" type = "xs:string" />
<xs:element name = "company" type = "xs:string" />
<xs:element name = "phone" type = "xs:int" />
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>
The basic idea behind XML Schemas is that they describe the legitimate format that an XML document can take.
Elements
As we saw in the XML - Elements chapter, elements are the building blocks of XML document. An element can be defined within an XSD as follows −
<xs:element name = "x" type = "y"/>
Definition Types
You can define XML schema elements in the following ways −
Simple Type
Simple type element is used only in the context of the text. Some of the predefined simple types are: xs:integer, xs:boolean, xs:string, xs:date. For example −
<xs:element name = "phone_number" type = "xs:int" />
Complex Type
A complex type is a container for other element definitions. This allows you to specify which child elements an element can contain and to provide some structure within your XML documents. For example −
<xs:element name = "Address">
<xs:complexType>
<xs:sequence>
<xs:element name = "name" type = "xs:string" />
<xs:element name = "company" type = "xs:string" />
<xs:element name = "phone" type = "xs:int" />
</xs:sequence>
</xs:complexType>
</xs:element>
In the above example, Address element consists of child elements. This is a container for other <xs:element> definitions, that allows to build a simple hierarchy of elements in the XML document.
Global Types
With the global type, you can define a single type in your document, which can be used by all other references. For example, suppose you want to generalize the person and company for different addresses of the company. In such case, you can define a general type as follows −
<xs:element name = "AddressType">
<xs:complexType>
<xs:sequence>
<xs:element name = "name" type = "xs:string" />
<xs:element name = "company" type = "xs:string" />
</xs:sequence>
</xs:complexType>
</xs:element>
Now let us use this type in our example as follows −
<xs:element name = "Address1">
<xs:complexType>
<xs:sequence>
<xs:element name = "address" type = "AddressType" />
<xs:element name = "phone1" type = "xs:int" />
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name = "Address2">
<xs:complexType>
<xs:sequence>
<xs:element name = "address" type = "AddressType" />
<xs:element name = "phone2" type = "xs:int" />
</xs:sequence>
</xs:complexType>
</xs:element>
Instead of having to define the name and the company twice (once for Address1 and once for Address2), we now have a single definition. This makes maintenance simpler, i.e., if you decide to add "Postcode" elements to the address, you need to add them at just one place.
Attributes
Attributes in XSD provide extra information within an element. Attributes have name and type property as shown below −
<xs:attribute name = "x" type = "y"/>
XML - Tree Structure
An XML document is always descriptive. The tree structure is often referred to as XML Tree and plays an important role to describe any XML document easily.
The tree structure contains root (parent) elements, child elements and so on. By using tree structure, you can get to know all succeeding branches and sub-branches starting from the root. The parsing starts at the root, then moves down the first branch to an element, take the first branch from there, and so on to the leaf nodes.
Example
Following example demonstrates simple XML tree structure −
<?xml version = "1.0"?>
<Company>
<Employee>
<FirstName>Tanmay</FirstName>
<LastName>Patil</LastName>
<ContactNo>1234567890</ContactNo>
<Email>tanmaypatil@xyz.com</Email>
<Address>
<City>Bangalore</City>
<State>Karnataka</State>
<Zip>560212</Zip>
</Address>
</Employee>
</Company>
Following tree structure represents the above XML document −
In the above diagram, there is a root element named as <company>. Inside that, there is one more element <Employee>. Inside the employee element, there are five branches named <FirstName>, <LastName>, <ContactNo>, <Email>, and <Address>. Inside the <Address> element, there are three sub-branches, named <City> <State> and <Zip>.
XML - DOM
The Document Object Model (DOM) is the foundation of XML. XML documents have a hierarchy of informational units called nodes; DOM is a way of describing those nodes and the relationships between them.
A DOM document is a collection of nodes or pieces of information organized in a hierarchy. This hierarchy allows a developer to navigate through the tree looking for specific information. Because it is based on a hierarchy of information, the DOM is said to be tree based.
The XML DOM, on the other hand, also provides an API that allows a developer to add, edit, move, or remove nodes in the tree at any point in order to create an application.
Example
The following example (sample.htm) parses an XML document ("address.xml") into an XML DOM object and then extracts some information from it with JavaScript −
<!DOCTYPE html>
<html>
<body>
<h1>Howcodex DOM example </h1>
<div>
<b>Name:</b> <span id = "name"></span><br>
<b>Company:</b> <span id = "company"></span><br>
<b>Phone:</b> <span id = "phone"></span>
</div>
<script>
if (window.XMLHttpRequest)
{// code for IE7+, Firefox, Chrome, Opera, Safari
xmlhttp = new XMLHttpRequest();
}
else
{// code for IE6, IE5
xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
}
xmlhttp.open("GET","/xml/address.xml",false);
xmlhttp.send();
xmlDoc = xmlhttp.responseXML;
document.getElementById("name").innerHTML=
xmlDoc.getElementsByTagName("name")[0].childNodes[0].nodeValue;
document.getElementById("company").innerHTML=
xmlDoc.getElementsByTagName("company")[0].childNodes[0].nodeValue;
document.getElementById("phone").innerHTML=
xmlDoc.getElementsByTagName("phone")[0].childNodes[0].nodeValue;
</script>
</body>
</html>
Contents of address.xml are as follows −
<?xml version = "1.0"?>
<contact-info>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</contact-info>
Now let us keep these two files sample.htm and address.xml in the same directory /xml and execute the sample.htm file by opening it in any browser. This should produce the following output.
Here, you can see how each of the child nodes is extracted to display their values.
XML - Namespaces
A Namespace is a set of unique names. Namespace is a mechanisms by which element and attribute name can be assigned to a group. The Namespace is identified by URI(Uniform Resource Identifiers).
Namespace Declaration
A Namespace is declared using reserved attributes. Such an attribute name must either be xmlns or begin with xmlns: shown as below −
<element xmlns:name = "URL">
Syntax
The Namespace starts with the keyword xmlns.
The word name is the Namespace prefix.
The URL is the Namespace identifier.
Example
Namespace affects only a limited area in the document. An element containing the declaration and all of its descendants are in the scope of the Namespace. Following is a simple example of XML Namespace −
<?xml version = "1.0" encoding = "UTF-8"?>
<cont:contact xmlns:cont = "www.howcodex.com/profile">
<cont:name>Tanmay Patil</cont:name>
<cont:company>Howcodex</cont:company>
<cont:phone>(011) 123-4567</cont:phone>
</cont:contact>
Here, the Namespace prefix is cont, and the Namespace identifier (URI) as www.howcodex.com/profile. This means, the element names and attribute names with the cont prefix (including the contact element), all belong to the www.howcodex.com/profile namespace.
XML - Databases
XML Database is used to store huge amount of information in the XML format. As the use of XML is increasing in every field, it is required to have a secured place to store the XML documents. The data stored in the database can be queried using XQuery, serialized, and exported into a desired format.
XML Database Types
There are two major types of XML databases −
- XML- enabled
- Native XML (NXD)
XML - Enabled Database
XML enabled database is nothing but the extension provided for the conversion of XML document. This is a relational database, where data is stored in tables consisting of rows and columns. The tables contain set of records, which in turn consist of fields.
Native XML Database
Native XML database is based on the container rather than table format. It can store large amount of XML document and data. Native XML database is queried by the XPath-expressions.
Native XML database has an advantage over the XML-enabled database. It is highly capable to store, query and maintain the XML document than XML-enabled database.
Example
Following example demonstrates XML database −
<?xml version = "1.0"?>
<contact-info>
<contact1>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</contact1>
<contact2>
<name>Manisha Patil</name>
<company>Howcodex</company>
<phone>(011) 789-4567</phone>
</contact2>
</contact-info>
Here, a table of contacts is created that holds the records of contacts (contact1 and contact2), which in turn consists of three entities − name, company and phone.
XML - Viewers
This chapter describes THE various methods to view an XML document. An XML document can be viewed using a simple text editor or any browser. Most of the major browsers supports XML. XML files can be opened in the browser by just double-clicking the XML document (if it is a local file) or by typing the URL path in the address bar (if the file is located on the server), in the same way as we open other files in the browser. XML files are saved with a ".xml" extension.
Let us explore various methods by which we can view an XML file. Following example (sample.xml) is used to view all the sections of this chapter.
<?xml version = "1.0"?>
<contact-info>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</contact-info>
Text Editors
Any simple text editor such as Notepad, TextPad, or TextEdit can be used to create or view an XML document as shown below −
Firefox Browser
Open the above XML code in Chrome by double-clicking the file. The XML code displays
coding with color, which makes the code readable. It shows plus(+) or minus (-) sign at
the left side in the XML element. When we click the minus sign (-), the code hides. When
we click the plus (+) sign, the code lines get expanded. The output in Firefox is as shown below −
Chrome Browser
Open the above XML code in Chrome browser. The code gets displayed as shown below −
Errors in XML Document
If your XML code has some tags missing, then a message is displayed in the browser. Let us try to open the following XML file in Chrome −
<?xml version = "1.0"?>
<contact-info>
<name>Tanmay Patil</name>
<company>Howcodex</company>
<phone>(011) 123-4567</phone>
</contact-info>
In the above code, the start and end tags are not matching (refer the contact_info tag), hence an error message is displayed by the browser as shown below −
XML - Editors
XML Editor is a markup language editor. The XML documents can be edited or created using existing editors such as Notepad, WordPad, or any similar text editor. You can also find a professional XML editor online or for downloading, which has more powerful editing features such as −
- It automatically closes the tags that are left open.
- It strictly checks syntax.
- It highlights XML syntax with colour for increased readability.
- It helps you write a valid XML code.
- It provides automatic verification of XML documents against DTDs and Schemas.
Open Source XML Editors
Following are some open source XML editors −
Online XML Editor − This is a light weight XML editor which you can use online.
Xerlin − Xerlin is an open source XML editor for Java 2 platform released under an Apache license. It is a Java based XML modelling application, for creating and editing XML files easily.
CAM - Content Assembly Mechanism − CAM XML Editor tool comes with XML+JSON+SQL Open-XDX sponsored by Oracle.
XML - Parsers
XML parser is a software library or a package that provides interface for client applications to work with XML documents. It checks for proper format of the XML document and may also validate the XML documents. Modern day browsers have built-in XML parsers.
Following diagram shows how XML parser interacts with XML document −
The goal of a parser is to transform XML into a readable code.
To ease the process of parsing, some commercial products are available that facilitate the breakdown of XML document and yield more reliable results.
Some commonly used parsers are listed below −
MSXML (Microsoft Core XML Services) − This is a standard set of XML tools from Microsoft that includes a parser.
System.Xml.XmlDocument − This class is part of .NET library, which contains a number of different classes related to working with XML.
Java built-in parser − The Java library has its own parser. The library is designed such that you can replace the built-in parser with an external implementation such as Xerces from Apache or Saxon.
Saxon − Saxon offers tools for parsing, transforming, and querying XML.
Xerces − Xerces is implemented in Java and is developed by the famous open source Apache Software Foundation.
XML - Processors
When a software program reads an XML document and takes actions accordingly, this is called processing the XML. Any program that can read and process XML documents is known as an XML processor. An XML processor reads the XML file and turns it into in-memory structures that the rest of the program can access.
The most fundamental XML processor reads an XML document and converts it into an internal representation for other programs or subroutines to use. This is called a parser, and it is an important component of every XML processing program.
Processor involves processing the instructions, that can be studied in the chapter Processing Instruction.
Types
XML processors are classified as validating or non-validating types, depending on whether or not they check XML documents for validity. A processor that discovers a validity error must be able to report it, but may continue with normal processing.
A few validating parsers are − xml4c (IBM, in C++), xml4j (IBM, in Java), MSXML (Microsoft, in Java), TclXML (TCL), xmlproc (Python), XML::Parser (Perl), Java Project X (Sun, in Java).
A few non-validating parsers are − OpenXML (Java), Lark (Java), xp (Java), AElfred (Java), expat (C), XParse (JavaScript), xmllib (Python).
character with acute accent.
with grave accent.