Home > Articles > Web Services > XML

This chapter is from the book

This chapter is from the book

Sams Teach Yourself XML in 21 Days, 3rd EditionSams Teach Yourself XML in 21 Days, 3rd Edition

Learn More Buy

This chapter is from the book

This chapter is from the book

Sams Teach Yourself XML in 21 Days, 3rd EditionSams Teach Yourself XML in 21 Days, 3rd Edition

Learn More Buy

Using XML Namespaces

There's a lot of freedom in XML, because you get to create your own markup. As time went on, however, XML authors started noticing a problem that the original creators of XML hadn't really anticipated—conflicting tag names.

For example, you've already seen that two popular XML applications are XHTML, which is the derivation of HTML in XML, and MathML, which lets you format and display math equations. Suppose that you want to display an equation in an XHTML Web page. That could be a problem, because because the tag set in XHTML and MathML overlap—in particular, each XML application defines a <var> and <select> element.

The way to solve this problem is to use namespaces. Namespaces give you a way to make sure that one set of tags will not conflict with another. You prefix a name to tag and attribute names. Changing the resulting names won't conflict with others that have a different prefix.

XML namespaces are one of those XML companion recommendations that keep being added to the XML specification. You can find the specification for namespaces at http://www.w3.org/TR/REC-xml-names/. There's still a lot of debate about this one (mostly because namespaces can make writing DTDs difficult), but it's an official W3C recommendation now.

Creating Namespaces

An example will make namespaces and why they're important clearer. For example, suppose you're the boss of one of the employees in our sample document, ch03_01.xml:

<employee>
  <name>
    <lastname>Kelly</lastname>
    <firstname>Grace</firstname>
  </name>
  <hiredate>October 15, 2005</hiredate>
  <projects>
    <project>
      <product>Printer</product>
      <id>111</id>
      <price>$111.00</price>
    </project>
    <project>
      <product>Laptop</product>
      <id>222</id>
      <price>$989.00</price>
    </project>
  </projects>
</employee>

Now suppose that you want to add your own comments to this employee's data in a <comment> element. The problem with that is that the XML data on this employee comes from the Human Resources department, and they haven't created an element named <comment>. You can indeed create your own <comment> element, but first you should confine the human resource's department's XML data to its own namespace to indicate that your comments are not part of the Human Resource Department's set of XML tags.

To define a new namespace, use the xmlns:prefix attribute, where prefix is the prefix you want to use for the namespace. In this case, you'll define a new namespace called hr for the Human Resources department:

<employee>

xmlns:hr="http://www.superduperbigco.com/human_resources">

  <name>
    <lastname>Kelly</lastname>
    <firstname>Grace</firstname>
  </name>
  <hiredate>October 15, 2005</hiredate>
  <projects>
    <project>
      <product>Printer</product>
      <id>111</id>
      <price>$111.00</price>
    </project>
    <project>
      <product>Laptop</product>
      <id>222</id>
      <price>$989.00</price>
    </project>
  </projects>
</employee>

To define a namespace, you assign the xmlns:prefix attribute to a unique identifier, which in XML is usually a URI that might direct the XML processor to a DTD for the namespace (but doesn't have to). So what's a URI?

Defining Namespaces with URIs

The XML specification expands the idea of standard URLs (Uniform Resource Locators) into URIs (Uniform Resource Identifiers). In HTML and on the Web, you use URLs; in XML, you use URIs. URIs are supposed to be more general than URLs, as we'll see when we discuss XLinks and XPointers in Day 14, "Handling XLinks, XPointers, and XForms."

For example, in theory, a URI can point not just to a single resource, but to a cluster of resources, or to arcs of resources along a path. The truth is that the whole idea of URIs as the next step after URLs is still being developed, and in practice, URLs are almost invariably used in XML—but you still call them URIs. Some software accepts more general forms of URIs, letting you, for example, access only a specific section of an XML document, but such usage and the associated syntax is far from standardized yet.

TIP

You might want to look up the current formal definition of URIs, which you can find in its entirety at http://www.ics.uci.edu/pub/ietf/uri/rfc2396.txt.

When you define a namespace with the xmlns:prefix attribute, you usually assign a URI to that attribute (in practice, this URI is always a URL today). The document that URI points to can describe more about the namespace you're creating; an example of this is the XHTML namespace, which uses the namespace http://www.w3.org/1999/xhtml

<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/tr/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns:xhtml="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">

    .
    .
    .

A namespace's URI can also hold a DTD or XML schema that defines the syntax for the XML elements you can use in that namespace (then it's up to the XML processor to use that DTD or XML schema, if it's been written to be smart enough to interpret namespaces in this way—most aren't). All that's really necessary, however, is that you assign a unique identifier, which can be any text, to the xmlns:prefix attribute.

After defining the hr namespace in our example, you can preface every tag and attribute name in this namespace with hr: like this:

<hr:employee

xmlns:hr="http://www.superduperbigco.com/human_resources">

  <hr:name>
    <hr:lastname>Kelly</hr:lastname>
    <hr:firstname>Grace</hr:firstname>
  </hr:name>
  <hr:hiredate>October 15, 2005</hr:hiredate>
  <hr:projects>
    <hr:project>
      <hr:product>Printer</hr:product>
      <hr:id>111</hr:id>
      <hr:price>$111.00</hr:price>
    </hr:project>
    <hr:project>
      <hr:product>Laptop</hr:product>
      <hr:id>222</hr:id>
      <hr:price>$989.00</hr:price>
    </hr:project>
  </hr:projects>
</hr:employee>

Now you've made it clear that all these tags come from the Human Resources department. Note how this works—the actual tag names themselves have been changed, because a colon is a legal character to use in tag names. (Now you know why you shouldn't use colons in tag names, although they're legal—they can make it look like you're using namespaces when you're not.) For example, the <product> tag has now become the <hr:product> tag. In other words, using namespaces keeps elements separate by actually changing tag and attribute names. This was a clever solution to the problem of tag and attribute name conflicts, because this way, even XML processors that have never heard of namespaces can still "support" them.

At this point, all tag and attribute names from the hr namespace are in their own namespace, so you can add your own namespace to the document, allowing you to use your own elements without fear of conflict. Since you're the boss, you might start by defining a new namespace named boss:

<hr:employee
  xmlns:hr="http://www.superduperbigco.com/human_resources"

xmlns:boss="http://www.superduperbigco.com/big_boss">

  <hr:name>
    <hr:lastname>Kelly</hr:lastname>
    <hr:firstname>Grace</hr:firstname>
  </hr:name>
  <hr:hiredate>October 15, 2005</hr:hiredate>
  <hr:projects>
    <hr:project>
      <hr:product>Printer</hr:product>
      <hr:id>111</hr:id>
      <hr:price>$111.00</hr:price>
    </hr:project>
    <hr:project>
      <hr:product>Laptop</hr:product>
      <hr:id>222</hr:id>
      <hr:price>$989.00</hr:price>
    </hr:project>
  </hr:projects>
</hr:employee>

Now you can use the new boss namespace to add your own markup to the document, as you see in Listing 3.2.

Listing 3.2 XML Document with Namespaces (ch03_02.xml)

<hr:employee
  xmlns:hr="http://www.superduperbigco.com/human_resources"
  xmlns:boss="http://www.superduperbigco.com/big_boss">
  <hr:name>
    <hr:lastname>Kelly</hr:lastname>
    <hr:firstname>Grace</hr:firstname>
  </hr:name>
  <hr:hiredate>October 15, 2005</hr:hiredate>

<boss:comment>Needs much supervision.</boss:comment>

  <hr:projects>
    <hr:project>
      <hr:product>Printer</hr:product>
      <hr:id>111</hr:id>
      <hr:price>$111.00</hr:price>
    </hr:project>
    <hr:project>
      <hr:product>Laptop</hr:product>
      <hr:id>222</hr:id>
      <hr:price>$989.00</hr:price>
    </hr:project>
  </hr:projects>
</hr:employee>

You can also add your own attributes in the boss namespace as long as you prefix them with boss: this way:

<hr:employee>
  xmlns:hr="http://www.superduperbigco.com/human_resources"
  xmlns:boss="http://www.superduperbigco.com/big_boss">
  <hr:name>
    <hr:lastname>Kelly</hr:lastname>
    <hr:firstname>Grace</hr:firstname>
  </hr:name>
  <hr:hiredate>October 15, 2005</hr:hiredate>

<boss:comment boss:date="10/15/2006">

Needs much supervision.

</boss:comment>

  <hr:projects>
    <hr:project>
      <hr:product>Printer</hr:product>
      <hr:id>111</hr:id>
      <hr:price>$111.00</hr:price>
    </hr:project>
    <hr:project>
      <hr:product>Laptop</hr:product>
      <hr:id>222</hr:id>
      <hr:price>$989.00</hr:price>
    </hr:project>
  </hr:projects>
</hr:employee>

And that's how namespaces work—you can use them to separate tags, even tags with the same name, so there's no conflict. As you can see, using multiple namespaces in the same document is no problem at all—just use the xmlns:prefix attribute in the enclosing element to define the appropriate namespace. In fact, you can use this attribute attribute in child elements to redefine an enclosing namespace, if you want to.

Namespace prefixes are really just text prefixed to (prepended is the offical term) tag and attribute names. They follow the same rules for naming tags and attributes. For example, in XML 1.0, a namespace name can start with a letter or an underscore. The following characters can include underscores, letters, digits, hyphens, and periods. Note also that although colons are legal in tag names, you can't use a colon in a namespace name, for obvious reasons. Also, there are two namespace names that are reserved: xml and xmlns.

Creating Local Namespaces

The xmlns:prefix attribute can be used in any element, not just the document element. Just bear in mind that this attribute defines a namespace for the current element and any enclosed element, which means you shouldn't use the namespace prefix until you've defined the namespace with an attribute like xmlns:prefix.

For example, you can create the boss: namespace prefix and use it in the same element, as you see in Listing 3.3.

Listing 3.3 XML Document with a Local Namespaces (ch03_03.xml)

<hr:employee
  xmlns:hr="http://www.superduperbigco.com/human_resources">
  <hr:name>
    <hr:lastname>Kelly</hr:lastname>
    <hr:firstname>Grace</hr:firstname>
  </hr:name>
  <hr:hiredate>October 15, 2005</hr:hiredate>

<boss:comment

xmlns:boss="http://www.superduperbigco.com/big_boss"

boss:date="10/15/2006">

Needs much supervision.

</boss:comment>

  <hr:projects>
    <hr:project>
      <hr:product>Printer</hr:product>
      <hr:id>111</hr:id>
      <hr:price>$111.00</hr:price>
    </hr:project>
    <hr:project>
      <hr:product>Laptop</hr:product>
      <hr:id>222</hr:id>
      <hr:price>$989.00</hr:price>
    </hr:project>
  </hr:projects>
</hr:employee>

You can see ch03_03.xml in the Internet Explorer, complete with namespaces, in Figure 3.1.

Figure 3.1Figure 3.1 Viewing an XML document with local namespaces.

Creating Default Namespaces

You can use the xmlns:prefix attribute to define a namespace, or you can use the xmlns attribute by itself to define a default namespace. When you define a default namespace, elements and attributes without a namespace prefix are in that default namespace.

To see how this works, we'll come full circle and put to work the example that introduced our discussion of namespaces in the first place—mixing XHTML with MathML. We'll start with some XHTML (all the details on XHTML are coming up in Day 11, "Extending HTML with XHTML," and Day 12, "Putting XHTML to Work"), like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/tr/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
  <head>
    <title>
      Using XHTML and MathML Together
    </title>
  </head>

  <body>
    <center>
      <h1>
        Using XHTML and MathML Together
      </h1>
    </center>
    <br/>
    Consider the equation
    .
    .
    .
  </body>
</html>

You'll see what you need to create XHTML documents like this, such as the <!DOCTYPE> element, in Day 11. Note in particular here that in the <html> element, the xmlns attribute defines a default namespace for the <html> and all enclosed elements. (This namespace is the XHTML namespace, which W3C defines as "http://www.w3.org/1999/xhtml".) When you use the xmlns attribute alone this way, without specifying any prefix, you are defining a default namespace. The current element and all child elements are assumed to belong to that namespace. Making use of a default namespace in this way, you can use the standard XHTML tag names without any prefix, as you see here.

However, we also want to use MathML markup in this document, and to do that, we add a new namespace, named m to this document, using the namespace W3C has specified for MathML, "http://www.w3.org/1998/Math/MathML":

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/tr/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"

xmlns:m="http://www.w3.org/1998/Math/MathML">

  <head>
    <title>
      Using XHTML and MathML Together
    </title>
  </head>

  <body>
    <center>
      <h1>
        Using XHTML and MathML Together
      </h1>
    </center>
    <br/>
    Consider the equation
    .
    .
    .
  </body>
</html>

Now you can use MathML as you like, as long as you prefix it with the m namespace. You can see this at work in ch03_04.html (XHTML documents use the extension .html), shown in Listing 3.4, where we're using the MathML we developed in Day 1 to display an equation.

Listing 3.4 An XML Document Combining XHTML and MathML (ch03_04.html)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/tr/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"
  xmlns:m="http://www.w3.org/1998/Math/MathML">
  <head>
    <title>
      Using XHTML and MathML Together
    </title>
  </head>

  <body>
    <center>
      <h1>
        Using XHTML and MathML Together
      </h1>
    </center>
    <br/>
    Consider the equation

<m:math>

<m:mrow>

<m:mrow>

<m:mn>4</m:mn>

<m:mo>&InvisibleTimes;</m:mo>

<m:msup>

<m:mi>x</m:mi>

<m:mn>2</m:mn>

</m:msup>

<m:mo>-</m:mo>

<m:mrow>

<m:mn>5</m:mn>

<m:mo>&InvisibleTimes;</m:mo>

<m:mi>x</m:mi>

</m:mrow>

<m:mo>+</m:mo>

<m:mn>6</m:mn>

</m:mrow>

<m:mo>=</m:mo>

<m:mn>0.</m:mn>

</m:mrow>

</m:math>

    <br/>
    What, you may ask, are this equation's roots?
  </body>
</html>

Thanks to namespaces, this XHTML/MathML document works just as it should, as you can see in the W3C Amaya browser in Figure 3.2.

Figure 3.2Figure 3.2 Viewing an XML document with local namespaces.

You'll be seeing XML namespaces throughout this book, especially when we use the popular XML applications available, such as XHTML.

That finishes the main topics for today's discussion—well-formed documents and namespaces. Before getting into validation in tomorrow's work, however, we'll round off our discussion of XML documents by taking a look at XML infosets and canonical XML. These two topics are worth discussing before we start talking about validation, because they're terms you'll run across as you work with XML, but we're going to consider them optional topics—if you want to skip them and get directly to DTDs, just turn to Day 4.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020