XML is not a replacement for HTML.
HTML is about displaying information, while XML is about carrying information.
Upgrading to new systems (hardware or software platforms), is always time consuming. Large
XML data is stored in text format. This makes it easier to expand or upgrade to new operating
systems, new applications, or new browsers, without losing data.
Different applications can access your data, not only in HTML pages, but also from XML data
sources.
With XML, your data can be available to all kinds of "reading machines" (Handheld computers,
voice machines, news feeds, etc.), and make it more available for blind people, or people with other disabilities.
Several Internet languages are written in XML. Here are some examples:
The first line is the XML declaration. It defines the XML version (1.0).
The next line describes the root element of the document (like saying: "this document is a
note"):
The next 4 lines describe 4 child elements of the root (to, from, heading, and body):
You can assume, from this example, that the XML document contains a note to Tove from Jani.
The elements in an XML document form a document tree. The tree starts at the root and
branches to the lowest level of the tree.
The terms parent, child, and sibling are used to describe the relationships between elements. Parent elements have children. Children on the same level are called siblings (brothers or sisters).
All elements can have text content and attributes (just like in HTML).
<author>J K. Rowling</author>
<author>Erik T. Ray</author>
The root element in the example is <bookstore>. All <book> elements in the document are
contained within <bookstore>.
An XML element is everything from (including) the element's start tag to (including) the
element's end tag.
An element can contain:
- other elements
- text
- attributes
- or a mix of all of the above...
<bookstore>
<book category="CHILDREN">
<title>Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
<book category="WEB">
<title>Learning XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>
</bookstore>
In the example above, <bookstore> and <book> have element contents, because they contain
other elements. <book> also has an attribute (category="CHILDREN")<title>, <author>, <year>, and <price> have text content because they contain text.
Empty XML Elements
-
An element with no content is said to be empty.
In XML, you can indicate an empty element like this:
<element></element>
or you can use an empty tag, like this (this sort of element syntax is called self-closing):
<element />
The two forms above produce identical results in an XML parser.
Note: Empty elements do not have any content, but they can have attributes!
XML Naming Rules
XML elements must follow these naming rules:
- Element names are case-sensitive
- Element names must start with a letter or underscore
- Element names cannot start with the letters xml (or XML, or Xml, etc)
- Element names can contain letters, digits, hyphens, underscores, and periods
- Element names cannot contain spaces
Any name can be used, no words are reserved (except xml).
Best Naming Practices
-
Create descriptive names, like this: <person>,<firstname>, <lastname>.
Create short and simple names, like this: <book_title> not like this: <the_title_of_the_book>.
Avoid "-". If you name something "first-name", some software may think you want to subtract
"name" from "first".
Avoid ".". If you name something "first.name", some software may think that "name" is a
property of the object "first".
Avoid ":". Colons are reserved for namespaces (more later).
Non-English letters like éòá are perfectly legal in XML, but watch out for problems if your
software doesn't support them.
Naming Styles
There are no naming styles defined for XML elements. But here are some commonly used:
If you choose a naming style, it is good to be consistent!
XML documents often have a corresponding database. A good practice is to use the naming rules of your database for the elements in the XML documents.
XML Elements are Extensible
XML elements can be extended to carry more information.
Look at the following XML example:
<note>
<to>Tove</to>
<from>Jani</from>
<body>Don't forget me this weekend!</body>
</note>
Let's imagine that we created an application that extracted the <to>, <from>, and <body>
elements from the XML document to produce this output:
MESSAGE
To: Tove
From: Jani
Don't forget me this weekend!
Imagine that the author of the XML document added some extra information to it:
<note>
<date>2008-01-10</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
Should the application break or crash?
No. The application should still be able to find the <to>, <from>, and <body> elements in the
XML document and produce the same output.
One of the beauties of XML, is that it can be extended without breaking applications.
XML Attributes:-
XML elements can have attributes, just like HTML.
Attributes provide additional information about an element.
XML Attributes
In HTML, attributes provide additional information about elements:
<img src="computer.gif">
<a href="demo.asp">
Attributes often provide information that is not a part of the data. In the example below, the file
type is irrelevant to the data, but can be important to the software that wants to manipulate the element:
<file type="gif">computer.gif</file>
XML Attributes Must be Quoted
-
Attribute values must always be quoted. Either single or double quotes can be used. For a
person's gender, the person element can be written like this:
<person gender="female">
or like this:
<person gender='female'>
If the attribute value itself contains double quotes you can use single quotes, like in this example:
<gangster name='George "Shotgun" Ziegler'>
or you can use character entities:
<gangster name="George "Shotgun" Ziegler">
XML Elements vs. Attributes
-
Take a look at these examples:
<person gender="female">
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
<person>
<gender>female</gender>
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>
In the first example gender is an attribute. In the last, gender is an element. Both examples
provide the same information.
There are no rules about when to use attributes or when to use elements. Attributes are handy in
HTML. In XML my advice is to avoid them. Use elements instead.
My Favorite Way
The following three XML documents contain exactly the same information:
A date attribute is used in the first example:
<note date="2008-01-10">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
A date element is used in the second example:
<note>
<date>2008-01-10</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
An expanded date element is used in the third: (THIS IS MY FAVORITE):
<note>
<date>
<year>2008</year>
<month>01</month>
<day>10</day>
</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
Avoid XML Attributes?
Some of the problems with using attributes are:
- attributes cannot contain multiple values (elements can)
- attributes cannot contain tree structures (elements can)
- attributes are not easily expandable (for future changes)
Attributes are difficult to read and maintain. Use elements for data. Use attributes for information that is not relevant to the data.
Don't end up like this:
<note day="10" month="01" year="2008"
to="Tove" from="Jani" heading="Reminder"
body="Don't forget me this weekend!">
</note>
XML Attributes for Metadata
Sometimes ID references are assigned to elements. These IDs can be used to identify XML elements in much the same way as the id attribute in HTML. This example demonstrates this:
<messages>
<note id="501">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
<note id="502">
<to>Jani</to>
<from>Tove</from>
<heading>Re: Reminder</heading>
<body>I will not</body>
</note>
</messages>
The I'd attributes above are for identifying the different notes. It is not a part of the note itself.
What I'm trying to say here is that metadata (data about data) should be stored as attributes, and the date itself should be stored as elements.
XML Namespace:-
XML Namespaces provide a method to avoid element name conflicts.
Name Conflicts
In XML, element names are defined by the developer. This often results in a conflict when trying to mix XML documents from different XML applications.
This XML carries HTML table information:
<table>
<tr>
<td>Apples</td>
<td>Bananas</td>
</tr>
</table>
This XML carries information about a table (a piece of furniture):
<table>
<name>African Coffee Table</name>
<width>80</width>
<length>120</length>
</table>
If these XML fragments were added together, there would be a name conflict. Both contain a <table> element, but the elements have different content and meaning.
A user or an XML application will not know how to handle these differences.
Solving the Name Conflict Using a Prefix-
Name conflicts in XML can easily be avoided using a name prefix.
This XML carries information about an HTML table, and a piece of furniture:
<h:table>
<h:tr>
<h:td>Apples</h:td>
<h:td>Bananas</h:td>
</h:tr>
</h:table>
<f:table>
<f:name>African Coffee Table</f:name>
<f:width>80</f:width>
<f:length>120</f:length>
</f:table>
In the example above, there will be no conflict because the two <table> elements have different
names.
XML Namespaces - The xmlns Attribute-
When using prefixes in XML, a so-called namespace for the prefix must be defined.
The namespace is defined by the xmlns attribute in the start tag of an element.
The namespace declaration has the following syntax. xmlns:prefix="URI".
<root>
<h:table xmlns:h="http://www.w3.org/TR/html4/">
<h:tr>
<h:td>Apples</h:td>
<h:td>Bananas</h:td>
</h:tr>
</h:table>
<f:table xmlns:f="http://www.w3schools.com/furniture">
<f:name>African Coffee Table</f:name>
<f:width>80</f:width>
<f:length>120</f:length>
</f:table>
</root>
In the example above, the xmlns attribute in the <table> tag give the h: and f: prefixes a qualified namespace.
When a namespace is defined for an element, all child elements with the same prefix are
associated with the same namespace.
Namespaces can be declared in the elements where they are used or in the XML root element:
<root xmlns:h="http://www.w3.org/TR/html4/"
xmlns:f="http://www.w3schools.com/furniture">
<h:table>
<h:tr>
<h:td>Apples</h:td>
<h:td>Bananas</h:td>
</h:tr>
</h:table>
<f:table>
<f:name>African Coffee Table</f:name>
<f:width>80</f:width>
<f:length>120</f:length>
</f:table>
</root>
Note: The namespace URI is not used by the parser to look up information.
The purpose is to give the namespace a unique name. However, often companies use the namespace as a pointer to a web page containing namespace information.
Uniform Resource Identifier (URI):-
A Uniform Resource Identifier (URI) is a string of characters which identifies an Internet
Resource.
The most common URI is the Uniform Resource Locator (URL) which identifies an Internet domain address. Another, not so common type of URI is the Universal Resource Name (URN).
In our examples we will only use URLs.
Default Namespaces
Defining a default namespace for an element saves us from using prefixes in all the child
elements. It has the following syntax:
xmlns="namespaceURI"
This XML carries HTML table information:
<table xmlns="http://www.w3.org/TR/html4/">
<tr>
<td>Apples</td>
<td>Bananas</td>
</tr>
</table>
This XML carries information about a piece of furniture:
<table xmlns="http://www.w3schools.com/furniture">
<name>African Coffee Table</name>
<width>80</width>
<length>120</length>
</table>
Namespaces in Real Use
-
XSLT is an XML language that can be used to transform XML documents into other formats, like HTML.
In the XSLT document below, you can see that most of the tags are HTML tags.
The tags that are not HTML tags have the prefix xsl, identified by the namespace xmlns:xsl="http://www.w3.org/1999/XSL/Transform":
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
<html>
<body>
<h2>My CD Collection</h2>
<table border="1">
<tr>
<th style="text-align:left">Title</th>
<th style="text-align:left">Artist</th>
</tr>
<xsl:for-each select="catalog/cd">
<tr>
<td><xsl:value-of select="title"/></td>
<td><xsl:value-of select="artist"/></td>
</tr>
</xsl:for-each>
</table>
</body>
</html>
</xsl:template>
</xsl:stylesheet>
XML Encoding:-
XML documents can contain international characters, like Norwegian æøå, or French êèé.
To avoid errors, you should specify the encoding used, or save your XML files as UTF-8.
Character Encoding
Character encoding defines a unique binary code for each different character used in a document.
In computer terms, character encoding are also called character set, character map, code set, and
code page.
The Unicode Consortium
The Unicode Consortium develops the Unicode Standard. Their goal is to replace the existing
character sets with its standard Unicode Transformation Format (UTF).
The Unicode Standard has become a success and is implemented in HTML, XML, Java,
JavaScript, E-mail, ASP, PHP, etc. The Unicode standard is also supported in many operating systems and all modern browsers.
The Unicode Consortium cooperates with the leading standards development organizations, like
ISO, W3C, and ECMA.
The Unicode Character Sets
Unicode can be implemented by different character sets. The most commonly used encodings are UTF-8 and UTF-16.
UTF-8 uses 1 byte (8-bits) to represent basic Latin characters, and two, three, or four bytes for
the rest.
UTF-16 uses 2 bytes (16 bits) for most characters, and four bytes for the rest.
UTF-8 = The Web Standard
UTF-8 is the standard character encoding on the web.
UTF-8 is the default character encoding for HTML5, CSS, JavaScript, PHP, SQL, and XML.
XML Encoding
The first line in an XML document is called the prolog:
<?xml version="1.0"?>
The prolog is optional. Normally it contains the XML version number.
It can also contain information about the encoding used in the document. This prolog specifies
UTF-8 encoding:
<?xml version="1.0" encoding="UTF-8"?>
The XML standard states that all XML software must understand both UTF-8 and UTF-16.
UTF-8 is the default for documents without encoding information.
In addition, most XML software systems understand encodings like ISO-8859-1, Windows-
1252, and ASCII.
XML Errors
Most often, XML documents are created on one computer, uploaded to a server on a second computer, and displayed by a browser on a third computer.
If the encoding is not correctly interpreted by all the three computers, the browser might display meaningless text, or you might get an error message.
For high quality XML documents, UTF-8 encoding is the best to use. UTF-8 covers international characters, and it is also the default, if no encoding is declared.
Conclusion
When you write an XML document:
- Use an XML editor that supports encoding
- Make sure you know what encoding the editor uses
- Describe the encoding in the encoding attribute
- UTF-8 is the safest encoding to use
- UTF-8 is the web standard
Displaying XML:-
Raw XML files can be viewed in all major browsers.
Don't expect XML files to be displayed as HTML pages.
Viewing XML Files
<?xml version="1.0" encoding="UTF-8"?>
- <note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
Look at the XML file above in your browser: note.xml
Notice that an XML document will be displayed with color-coded root and child elements. A plus (+) or minus sign (-) to the left of the elements can be clicked to expand or collapse the element structure. To view the raw XML source (without the + and - signs), select "View Page
Source" or "View Source" from the browser menu.
Note: In Safari, only the element text will be displayed. To view the raw XML, you must right click the page and select "View Source".
Viewing an Invalid XML File
If an erroneous XML file is opened, some browsers report the error, and some only display it incorrectly.
Try to open the following XML file in Chrome, IE, Firefox, Opera, and Safari:
note_error.xml.
Other XML Examples
Viewing some XML documents will help you get the XML feeling:
This is a CD collection, stored as XML.
This is a plant catalog from a plant shop, stored as XML.
This is a breakfast food menu from a restaurant, stored as XML.
Why Does XML Display Like This?
XML documents do not carry information about how to display the data.
Since XML tags are "invented" by the author of the XML document, browsers do not know if a tag like <table> describes an HTML table or a dining table.
Without any information about how to display the data, most browsers will just display the XML document as it is.
Displaying XML Files with CSS?
Below is an example of how to use CSS to format an XML document.
We can use an XML file like cd_catalog.xml and a style sheet like cd_catalog.css
RESULT: The CD catalog formatted with the CSS file
Below is a fraction of the XML file. The second line links the XML file to the CSS file:
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/css" href="cd_catalog.css"?>
<CATALOG>
<CD>
<TITLE>Empire Burlesque</TITLE>
<ARTIST>Bob Dylan</ARTIST>
<COUNTRY>USA</COUNTRY>
<COMPANY>Columbia</COMPANY>
<PRICE>10.90</PRICE>
<YEAR>1985</YEAR>
</CD>
<CD>
<TITLE>Hide your heart</TITLE>
<ARTIST>Bonnie Tyler</ARTIST>
<COUNTRY>UK</COUNTRY>
<COMPANY>CBS Records</COMPANY>
<PRICE>9.90</PRICE>
<YEAR>1988</YEAR>
</CD>
.
.
.
</CATALOG>
Formatting XML with CSS is not recommended. Use JavaScript or XSLT instead.
XML Document Types
An XML document with correct syntax is called "Well Formed".
A "Valid" XML document must also conform to a document type definition.
Well Formed XML Documents
An XML document with correct syntax is "Well Formed".
The syntax rules were described in the previous chapters:
XML documents must have a root element
XML elements must have a closing tag
XML tags are case sensitive
XML elements must be properly nested
XML attribute values must be quoted
<?xml version="1.0" encoding="UTF-8"?>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
An XML Validator
To help you check the syntax of your XML files, we have created an XML validator to syntax-check your XML.
Valid XML Documents
A "valid" XML document is not the same as a "well formed" XML document.
A "valid" XML document must be well formed. In addition it must conform to a document type definition.
Rules that defines the legal elements and attributes for XML documents are called Document Type Definitions (DTD) or XML Schemas.
There are two different document type definitions that can be used with XML:
DTD - The original Document Type Definition
XML Schema - An XML-based alternative to DTD
When to Use a DTD/Schema?
With a DTD, independent groups of people can agree to use a standard DTD for interchanging
data.
Your application can use a standard DTD to verify that the data you receive from the outside world is valid.
You can also use a DTD to verify your own data.
When to NOT to Use a DTD/Schema?
XML does not require a DTD/Schema.
When you are experimenting with XML, or when you are working with small XML files, creating DTDs may be a waste of time.
If you develop applications, wait until the specification is stable before you add a document definition. Otherwise, your software might stop working because of validation errors.
XML Validator
Use our XML validator to syntax-check your XML.
XML Errors Will Stop You
Errors in XML documents will stop your XML applications.
The W3C XML specification states that a program should stop processing an XML document if it finds an error. The reason is that XML software should be small, fast, and compatible.
HTML browsers will display HTML documents with errors (like missing end tags).
With XML, errors are not allowed.
Syntax-Check Your XML
To help you syntax-check your XML, we have created an XML validator.
Paste your XML into the text area below, and syntax-check it by clicking the "Validate" button.
Photo
Syntax-Check an XML File
You can syntax-check an XML file by typing the URL of the file into the input field below, and then click the "Validate" button:
Filename:
Photo
If you get "Access Denied" or "Network Error", it is because your browser does not allow file access across domains.
The file "note_error.xml" demonstrates your browsers error handling. If you want to see an error-free message, substitute the "note_error.xml" with "cd_catalog.xml".
A General XML Validator
To help you check your xml files, you can syntax-check any XML file here.
Parse Errors
You can read more about the parse errors in our XML DOM tutorial.
XML Schema
An XML Schema describes the structure of an XML document, just like a DTD.
An XML document with correct syntax is called "Well Formed".
An XML document validated against an XML Schema is both "Well Formed" and "Valid".
XML Schema
XML Schema is an XML-based alternative to DTD:
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
The Schema above is interpreted like this:
<xs:element name="note"> defines the element called "note"
<xs:complexType> the "note" element is a complex type
<xs:sequence> the complex type is a sequence of elements
<xs:element name="to" type="xs:string"> the element "to" is of type string (text)
<xs:element name="from" type="xs:string"> the element "from" is of type string
<xs:element name="heading" type="xs:string"> the element "heading" is of type string
<xs:element name="body" type="xs:string"> the element "body" is of type string
Everything is wrapped in "Well Formed" XML.
XML Schemas are More Powerful than DTD
XML Schemas are written in XML
XML Schemas are extensible to additions
XML Schemas support data types
XML Schemas support namespaces
Why Use an XML Schema?
With XML Schema, your XML files can carry a description of its own format.
With XML Schema, independent groups of people can agree on a standard for interchanging data.
With XML Schema, you can verify data.
XML Schemas Support Data Types
One of the greatest strength of XML Schemas is the support for data types:
It is easier to describe document content
It is easier to define restrictions on data
It is easier to validate the correctness of data
It is easier to convert data between different data types
XML Schemas use XML Syntax
Another great strength about XML Schemas is that they are written in XML:
You don't have to learn a new language
You can use your XML editor to edit your Schema files
You can use your XML parser to parse your Schema files
You can manipulate your Schemas with the XML DOM
You can transform your Schemas with XSLT
If you want to study XML Schema, please read our XML Schema Tutorial.
Thanking you for read my blog 😁😁
