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  1. 1. XML Presented by Prof. Venkat Krishnan
  2. 2. Prof Venkat Krishnan (SCJP, SCWCD, SCJD, MCSD, Rational Certified)
  3. 3. <ul><li>Visiting Faculty at: </li></ul><ul><ul><li>S.P.Jain Inst of Management </li></ul></ul><ul><ul><li>Somaiya Institute of Management </li></ul></ul><ul><ul><li>CMIT </li></ul></ul><ul><li>Corporate Trainings offered at: </li></ul><ul><ul><li>Air India </li></ul></ul><ul><ul><li>ABN Amro </li></ul></ul><ul><ul><li>Kale Consultants </li></ul></ul><ul><ul><li>Supreme Technologies </li></ul></ul><ul><ul><li>IDBI </li></ul></ul><ul><ul><li>Associated with: </li></ul></ul><ul><ul><li>Champlain College (UK) </li></ul></ul><ul><ul><li>Southern New Hampshire (UK) </li></ul></ul><ul><ul><li>Malboro College (US) </li></ul></ul><ul><ul><li>Charles Strut University (Aus) </li></ul></ul><ul><ul><li>Northwood University (US) </li></ul></ul>Prof.Venkat Krishnan
  4. 4. Coming up… <ul><li>Basics about XML (1 to 65) </li></ul><ul><li>DTD (66 to 96) </li></ul><ul><li>Schema (97 to 180) </li></ul><ul><li>W3C DOM & Data Binding (181 to 218) </li></ul><ul><li>XSL and XSLT (219 to 293) </li></ul><ul><li>Java & XML </li></ul>
  5. 5. What Is XML?
  6. 6. <ul><li>XML stands for Extensible Markup Language </li></ul><ul><li>XML is a set of rules for defining semantic tags that break a document into parts and identify the different parts of the document. </li></ul><ul><li>It is a meta-markup language that defines a syntax in which other field-specific markup languages can be written. </li></ul><ul><li>XML, however, is a meta-markup language. It’s a language in which you make up the tags you need as you go along. These tags must be organized according to certain general principles, but they’re quite flexible in their meaning. </li></ul><ul><li>The tags you create can be documented in a Document Type Definition (DTD). </li></ul>
  7. 7. <ul><li>XML defines the meta syntax that field-specific markup languages such as MusicML, MathML, and CML must follow. It specifies the rules for the low-level syntax, saying how markup is distinguished from content, how attributes are attached to elements, and so forth without saying what these tags, elements, and attributes are or what they mean. </li></ul><ul><li>A browser does not need to know in advance each and every tag that might be used by thousands of different markup languages. Instead, it discovers the tags used by any given document as it reads thedocument or its DTD. </li></ul>
  8. 8. <ul><li>XML is the Extensible M arkup L anguage </li></ul><ul><ul><li>Similar to HTML in format </li></ul></ul><ul><ul><ul><li>HTML specifies presentation </li></ul></ul></ul><ul><ul><ul><li>XML specifies data structure </li></ul></ul></ul><ul><ul><li>Allows definition of your own tags and attributes (extensible) </li></ul></ul><ul><li>How XML and SGML Are Related </li></ul><ul><li>SGML provides the specification for writing markup languages </li></ul><ul><ul><li>HTML would be an implementation of SGML </li></ul></ul><ul><ul><li>XML is a subset of SGML </li></ul></ul><ul><li>XML and SGML do not define the meaning of the language elements </li></ul><ul><ul><li>This is the role of the Document Type Definition (DTD) or Extensible Style sheet Language (XSL) file </li></ul></ul><ul><li>SGML is very powerful </li></ul><ul><ul><li>With that power comes complexity, making the language difficult to use </li></ul></ul><ul><li>XML was designed with the goal of capturing all that was good in SGML, while keeping it as simple as possible </li></ul>
  9. 9. A Simple XML Example: an Address Book <ul><li><?xml version=&quot;1.0&quot;?> </li></ul><ul><li><addressBook> </li></ul><ul><li><entry> </li></ul><ul><li><name> </li></ul><ul><li> <first>Venkat</first> </li></ul><ul><li><last>Krishnan</last> </li></ul><ul><li> </name> </li></ul><ul><li><address> </li></ul><ul><li><street1>I-101 Pranay Nagar Borivali</street1> <street2/> </li></ul><ul><li><city>Mumbai</city> </li></ul><ul><li><zipcode>400092</zipcode> </li></ul><ul><li> </address> </li></ul><ul><li></entry> </li></ul><ul><li></addressBook> </li></ul>
  10. 10. XML describes structure and semantics, not formatting <ul><li>The thing to understand about XML is that XML markup describes a document’s structure and meaning. It does not describe the formatting of the elements on the page. Formatting can be added to a document with a style sheet. The document itself only contains tags that say what is in the document, not what the document looks like. </li></ul><ul><li>By contrast, HTML encompasses formatting, structural, and semantic markup. <B> is a formatting tag that makes its content bold. </li></ul><ul><li>Open Standard </li></ul><ul><ul><li>World Wide Web Consortium (W3C) </li></ul></ul><ul><ul><li>Derived from the Standard Generalized Markup Language (SGML) </li></ul></ul><ul><ul><li>Passed as W3C Standard February 1998 (although many aspects of XML are still being defined) </li></ul></ul><ul><li>Text Based </li></ul><ul><ul><li>International support via the Unicode character set </li></ul></ul>
  11. 11. <ul><li>Extensible Markup Language ( XML ) is a buzzword you will see everywhere on the Internet, but it's also a rapidly maturing technology with powerful real-world applications, particularly for the management, display and organization of data. </li></ul><ul><li>XML is a technology concerned with the description and structuring of data , so before we can really delve into the concepts behind XML, we need to understand how data is stored and accessed by computers. For our purposes, there are two kinds of data files that are understood by computers: text files and binary files . </li></ul><ul><li>A binary file , at its simplest, is just a stream of bits (1's and 0's). It's up to the application that created a binary file to understand what all of the bits mean. That's why binary files can only be read and produced by certain computer programs, which have been specifically written to understand them. </li></ul><ul><li>Like binary files, text files are also streams of bits. However, in a text file these bits are grouped together in standardized ways, so that they always form numbers. These numbers are then further mapped to characters. </li></ul>More on XML
  12. 12. A Brief History of Markup <ul><li>We can see that there are advantages to binary file formats (easy to understand by a computer, compact), as well as advantages to text files (universally interchangeable). Wouldn't it be ideal if there were a format that combined the universality of text files with the efficiency and rich information storage capabilities of binary files? </li></ul><ul><li>An early attempt to combine a universally interchangeable data format with rich information storage capabilities was SGML ( Standard Generalized Markup Language ). This is a text-based language that can be used to mark up data–that is, add meta data–in a way which is self-describing . </li></ul><ul><li>A very well-known language, based on the SGML work, is the HyperText Markup Language , or HTML . HTML uses many of SGML's concepts to provide a universal markup language for the display of information, and the linking of different pieces of information. </li></ul><ul><li>Unfortunately, SGML is such a complicated language that it's not well suited for data interchange over the web. And, although HTML has been incredibly successful, it's also limited in its scope: it is only intended for displaying documents in a browser. </li></ul>
  13. 13. <ul><li>The tags it makes available do not provide any information about the content they encompass, only instructions on how to display that content. </li></ul><ul><li>XML is a subset of SGML, with the same goals (mark up of any type of data), but with as much of the complexity eliminated as possible. XML was designed to be fully compatible with SGML, which means that any document which follows XML's syntax rules is by definition also following SGML's syntax rules, and can therefore be read by existing SGML tools. It doesn't go both ways though, so an SGML document is not necessarily an XML document.might create an XML file like this: </li></ul><ul><li><name> </li></ul><ul><li><first>Venkat</first> </li></ul><ul><li><last>Krishnan</last></name> </li></ul><ul><li>Even from this simple example you can see why markup languages like SGML and XML are called &quot;self-describing&quot;. Looking at the data, you can easily tell that this is information about a <name>, and you can see that there is data called <first> and more data called <last>. I could have given the tags any names I liked, however, if you're going to use XML, you might as well use it right, and give things meaningful names.Now open the file inside the IE Browser </li></ul>
  14. 14. How It Works <ul><li>Although our XML file has no information concerning display, IE formats it nicely for us, with our information in bold, and our markup displayed in different colors. Also, <name> is collapsible, like your file folders in Windows Explorer; try clicking on the - sign next to <name> in the browser window. For large XML documents, where you only need to concentrate on a smaller subset of the data, this can be quite handy. </li></ul><ul><li>This is one reason why IE can be so helpful when authoring XML: it has a default stylesheet built in, which applies this default formatting to any XML document. </li></ul><ul><li>XML styling is accomplished through another document dedicated to the task, called a stylesheet. In a stylesheet the designer specifies rules that determine the presentation of the data. The same stylesheet can then be used with multiple documents to create a similar appearance among them. There are a variety of languages that can be used to create stylesheets. </li></ul>
  15. 15. XML Parsers <ul><li>If we just follow the rules specified by XML, we can be sure that it will be easy to get at our information. This is because there are programs written, called parsers , which are able to read XML syntax and get the information out for us. We can use these parsers within our own programs, meaning our applications never have to even look at the XML directly; a large part of the workload will have been done for us. </li></ul>
  16. 16. Why &quot;Extensible&quot;? <ul><li>Since we have full control over the creation of our XML document, we can shape the data in any way we wish, so that it makes sense to our particular application. If we want to create data in a way that only one particular computer program will ever use, we can do so. And if we feel that we want to share our data with other programs, or even other companies across the Internet, XML gives us the flexibility to do that as well. We are free to structure the same data in different ways that suit the requirements of an application or category of applications. </li></ul><ul><li>There is, of course, a fundamental difference between HTML and XML: </li></ul><ul><li>HTML is designed for a specific application; to convey information to humans (usually visually, through a web browser) </li></ul><ul><li>XML has no specific application; it is designed for whatever use you need it for </li></ul>
  17. 17. Hierarchies in HTML <ul><li>For example, when working with Dynamic HTML ( DHTML ) there is an object model available for working with HTML documents, called the Document Object Model or DOM . This allows us to write code in an HTML document, like the following JavaScript: </li></ul><ul><li>alert(document.title); </li></ul><ul><li>Here we are using the alert() function to pop up a message box telling us the title of an HTML document. That's done by accessing an object called document, which contains all of the information needed about the HTML document. The document object includes a property called title, which returns the title of the current HTML document. </li></ul><ul><li>Hierarchies in XML </li></ul><ul><li>XML also groups information in hierarchies. The items in our documents relate to each other in parent/child and sibling/sibling relationships. These &quot;items&quot; are called elements . </li></ul>
  18. 18. What Is the World Wide Web Consortium? <ul><li>One of the reasons that HTML and XML are so successful is that they're standards . That means that anyone can follow these standards, and the solutions they develop will be able to interoperate. So who creates these standards? </li></ul><ul><li>The World Wide Web Consortium ( W3C ) was started in 1994, according to their web site ( http://www.w3.org/ ), &quot;to lead the World Wide Web to its full potential by developing common protocols that promote its evolution and ensure its interoperability&quot;. Recognizing this need for standards, the W3C produces Recommendations that describe the basic building blocks of the Web. They call them recommendations, instead of standards, because it is up to others to follow the recommendations to provide the interoperability. </li></ul><ul><li>Their most famous contribution to the Web is, of course, the HTML Recommendation; when a web browser producer claims that their product follows version 3.2 or 4.01 of the HTML Recommendation, they're talking about the Recommendation developed under the authority of the W3C. </li></ul>
  19. 19. <ul><li>Namespaces provide a means to distinguish one XML vocabulary from another, which allows us to create richer documents by combining multiple vocabularies into one document type. </li></ul><ul><li>XPath describes a querying language for addressing parts of an XML document. This allows applications to ask for a specific piece of an XML document, instead of having to always deal with one large &quot;chunk&quot; of information. For example, XPath could be used to get &quot;all the last names&quot; from a document </li></ul><ul><li>As we discussed earlier, in some cases we may want to display our XML documents. For simpler cases, we can use Cascading Style Sheets ( CSS ) to define the presentation of our documents. And, for more complex cases, we can use Extensible Stylesheet Language ( XSL ), that consists of XSLT , which can transform our documents from one type to another. </li></ul><ul><li>XLink and XPointer are languages that are used to link XML documents to each other, in a similar manner to HTML hyperlinks. </li></ul><ul><li>To provide a means for more traditional applications to interface with XML documents, there is a document object model–the DOM. An alternative way for programmers to interface with XML documents from their code is to use the Simple API for XML ( SAX ). </li></ul>
  20. 20. Why Are Developers Excited About XML? <ul><li>Design of field-specific markup languages: </li></ul><ul><li>XML enables individual professions (e.g., music, chemistry, human resources) to develop their own field-specific markup languages. These languages make it possible for practitioners in the field to trade notes, data, and information without worrying about whether or not the person on the receiving end has the particular proprietary payware that was used to create the data. They can even send documents to people outside the profession with reasonable confidence that the people who receive them will at least be able to view the documents. </li></ul><ul><li>Self-describing data: </li></ul><ul><li>At a higher level, XML is self-describing. The tags are self explanatory </li></ul><ul><li>Interchange of data among applications: </li></ul><ul><li>Because XML is nonproprietary and easy to read and write, it’s an excellent formatfor the interchange of data among different applications. </li></ul><ul><li>Structured and integrated data: </li></ul><ul><li>XML is ideal for large and complex documents because the data is structured. It not only lets you specify a vocabulary that defines the elements in the document; it also lets you specify the relations between elements. For example, if you’re putting together a Web page of sales contacts, you can require that every contact have a phone number and an e-mail address. If you’re inputting data for a database, you can make sure that no fields are missing. You can even provide default values to be used when no data is entered. </li></ul>
  21. 21. The Myths: <ul><li>XML is a replacement for HTML </li></ul><ul><ul><li>HTML is a markup language </li></ul></ul><ul><ul><li>XML is a markup language and a specification language </li></ul></ul><ul><li>XML will solve all your problems </li></ul><ul><ul><li>XML provides an infrastructure for a solution, but it is not the solution </li></ul></ul><ul><ul><li>XML is only part of the application architecture; alone it does nothing </li></ul></ul><ul><li>XML is very simple: there's not much too it </li></ul><ul><ul><li>Learning the syntax of XML is easy, but it is not enough </li></ul></ul><ul><ul><li>There is more to XML than &quot;tags&quot; </li></ul></ul>
  22. 22. XML Document Life Cycle
  23. 23. Comparing HTML to XML <ul><li>HTML Strengths and Weaknesses </li></ul><ul><ul><li>HTML Strengths </li></ul></ul><ul><ul><ul><li>Portable </li></ul></ul></ul><ul><ul><ul><li>Easy to understand and use </li></ul></ul></ul><ul><ul><ul><li>Widely used </li></ul></ul></ul><ul><ul><ul><li>Free tools </li></ul></ul></ul><ul><ul><li>HTML Weaknesses </li></ul></ul><ul><ul><ul><li>Data is mixed with presentation </li></ul></ul></ul><ul><ul><ul><li>Does not allow precise searching of content </li></ul></ul></ul><ul><ul><ul><li>Inflexible - fixed number of tags </li></ul></ul></ul><ul><ul><ul><li>Lacks data structure </li></ul></ul></ul>
  24. 24. Solving the Tags Dilemma <ul><li>Split Data from Presentation </li></ul><ul><ul><li>XML deals with data </li></ul></ul><ul><ul><li>Style sheets deal with presentation </li></ul></ul><ul><li>No Predefined Tags </li></ul><ul><ul><li>The author of the XML document makes up the tags </li></ul></ul><ul><ul><li>Definition of the tags are held in a dictionary called the Document Type Definition (DTD) </li></ul></ul><ul><li>Strict Rules </li></ul><ul><ul><li>Precise rules are needed to define the structure of the document </li></ul></ul><ul><ul><li>Processing is made easier with stricter rules </li></ul></ul><ul><ul><ul><li>Browsing would be much faster if HTML was stricter </li></ul></ul></ul>
  25. 25. URLs and URIs <ul><li>XML documents can live on the Web, just like HTML and other documents. When they do, they are referred to by Uniform Resource Locators (URLs), just like HTML files. For example, at the URL http://www.hypermedic.com/style/xml/ tempest.xml you’ll find the complete text of Shakespeare’s Tempest marked up in XML. </li></ul><ul><li>Although URLs are well understood and well supported, the XML specification uses Uniform Resource Identifiers (URIs) instead. URIs are a superset of URLs. A URI is a more general means of locating a resource; URIs focus a little more on the resource and a little less on the location. Furthermore, they aren’t necessarily limited to resources on the Internet. For instance, the URI for any Imaginary XML book would be uri:isbn:123456. This doesn’t refer to the specific copy you might be having but it refers to the almost all the books of that particular edition </li></ul>
  26. 26. URNs <ul><li>URNs are not as commonly seen as URLs. In fact, most people, even those who have been on the Internet for years, have never seen a URN. They exist to provide a persistent, location-independent name for a resource. </li></ul><ul><li>For example, a person's name is similar to a URN, because the person has the same name, no matter where they are. Even after a person dies, the name still refers to the person who used to have it when they were alive. A name is different from a URN, though, because more than one person can have the same name, whereas URNs are designed to be unique across time and space. </li></ul><ul><li>A URN looks something like this: </li></ul><ul><li>urn:foo:a123,456 </li></ul><ul><li>First comes the string urn, upper-or lowercase, and a colon. After the first colon comes the Namespace Identifier, or NID, (foo in this case) followed by another colon. And finally comes the Namespace Specific String, or NSS (a123,456 for example). As you can see from the terminology, URNs were designed with namespaces already in mind. </li></ul><ul><li>The NID portion of the URN declares what type of URN this is. For example, to create URNs for Canadian citizens, we might declare an NID of Canadian-Citizen. </li></ul><ul><li>The NSS portion of the URN is the part that must be unique, and persistent. In Canada, all citizens are assigned unique Social Insurance Numbers. So, a URN for a Canadian citizen with a Social Insurance Number of 000-000-000 might look like this: </li></ul><ul><li>urn:Canadian-Citizen:000-000-000 </li></ul>
  27. 27. XLinks and XPointers <ul><li>As long as XML documents are posted on the Internet, people will want to link them to each other. Standard HTML link tags can be used in XML documents, and HTML documents can link to XML documents. For example, this HTML link points to the aforementioned copy of the Tempest in XML: </li></ul><ul><li><A HREF=”http://www.hypermedic.com/style/xml/tempest.xml”> The Tempest by Shakespeare </A> </li></ul><ul><li>However, XML lets you go further with XLinks for linking to documents and XPointers for addressing individual parts of a document. </li></ul><ul><li>XLinks enable any element to become a link, not just an A element. For example, in XML, the above link might be written like this: </li></ul><ul><li><PLAY xlink:type=”simple” xmlns:xlink=”http://www.w3.org/1999/xlink” </li></ul><ul><li>xlink:href=”http://www.hypermedic.com/style/xml/tempest.xml”> </li></ul><ul><li><TITLE>The Tempest</TITLE> by <AUTHOR>Shakespeare</AUTHOR> </li></ul><ul><li></PLAY> </li></ul>
  28. 28. <ul><li>Furthermore, XLinks can be bidirectional, multidirectional, or even point to multiple mirror sites from which the nearest is selected. XLinks use normal URLs to identify the site they’re linking to. As new URI schemes become available, XLinks will be able to use those, too. </li></ul><ul><li>XPointers allow URLs to point not just to a particular document at a particular location, but to a particular part of a particular document. An XPointer can refer to a particular element of a document, to the first, the second, or the seventeenth such element, to the first element that’s a child of a given element, and so on. XPointers provide extremely powerful connections between documents that do not require the targeted document to contain additional markup just so its individual pieces can be linked to from other documents. </li></ul>
  29. 29. XML Applications <ul><li>XML is a meta-markup language for designing domain-specific markup languages. Each specific XML-based markup language is called an XML application . This is not an application that uses XML, such as the Mozilla Web browser, the Gnumeric spreadsheet, or the XML Spy editor; rather, it is an application of XML to a specific field such as the Chemical Markup Language (CML) for chemistry. </li></ul><ul><li>Each XML application has its own syntax and vocabulary. This syntax and vocabulary adheres to the fundamental rules of XML. This is much like human languages, each of which has its own vocabulary and grammar, while adhering to certain fundamental rules. </li></ul>
  30. 30. Real-World XML Examples <ul><li>Exchanging data between different systems </li></ul><ul><li>Using Internet search Engines </li></ul><ul><li>Capturing data from the web </li></ul><ul><li>Electronic Data Interchange (EDI) </li></ul>
  31. 31. XML Applications <ul><li>Channel Definition Format (CDF) </li></ul><ul><ul><li>Used in push technology applications </li></ul></ul><ul><li>Open Financial Exchange (OFX) - now International Financial Exchange (IFX) </li></ul><ul><ul><li>Standardizes information exchange in the financial sector </li></ul></ul><ul><li>Mathematical Markup Language (MathML) </li></ul><ul><ul><li>XML applications for mathematical equations </li></ul></ul><ul><li>Chemical Markup Language (CML) </li></ul><ul><ul><li>Organizes complex chemical objects </li></ul></ul><ul><li>Synchronized Multimedia Integration Language (SMIL) </li></ul><ul><ul><li>W3C recommendation for writing TV-like multimedia presentations for the Web </li></ul></ul>
  32. 32. Your First XML Document <ul><li><?xml version=”1.0”?> </li></ul><ul><li><FOO> </li></ul><ul><li>Hello XML! </li></ul><ul><li></FOO> </li></ul><ul><li>The above document is a well-formed XML document. (XML has special terms for documents that it considers “good” depending on exactly which set of rules they satisfy.) </li></ul><ul><li>The first line of the simple XML document is the XML declaration. </li></ul><ul><li>The XML declaration has a version attribute. An attribute is a name-value pair separated by an equals sign. The name is on the left side of the equals sign, and the value is on the right side between double quote marks. Every XML document should begin with an XML declaration that specifies the version of XML in use. (Some XML documents omit this for reasons of backward compatibility, but you should include a version declaration unless you have a specific reason to leave it out.) In the previous example, the version attribute says that this document conforms to the XML 1.0 specification. There isn’t any version of XML except 1.0. This attribute just exists to allow the possibility of future revisions. </li></ul><ul><li><FOO> </li></ul><ul><li>Hello XML! </li></ul><ul><li></FOO> </li></ul><ul><li>Collectively these three lines form a FOO element . Separately, <FOO> is a start tag ; </FOO> is an end tag ; and Hello XML! is the content of the FOO element. Divided another way, the start tag, end tag, and XML declaration are all markup . The text Hello XML! is character data . </li></ul>
  33. 33. Assigning Meaning to XML Tags <ul><li>Markup can indicate three kinds of meaning: structural, semantic, or stylistic. Structure specifies the relations between the different elements in the document. Semantics relates the individual elements to the real world outside of the document itself. Style specifies how an element is displayed. </li></ul><ul><li>Structure merely expresses the form of the document, without regard for differences between individual tags and elements. </li></ul><ul><li>Semantic meaning exists outside the document, in the mind of the author or reader, or in some computer program that generates or reads these files. For instance, a Web browser that understands HTML, but not XML, would assign the meaning “paragraph” to the tags <P> and </P> but not to the tags <GREETING> and </GREETING>, <FOO> and </FOO>, or <DOCUMENT> and </DOCUMENT>. </li></ul><ul><li>The third kind of meaning that can be associated with a tag is stylistic. Style says how the content of a tag is to be presented on a computer screen or other output device. Style says whether a particular element is bold, italic, green, two inches high, or what have you. </li></ul>
  34. 34. Writing a Style Sheet for an XML Document <ul><li>XML allows you to create any tags that you need. Of course, since you have almost complete freedom in creating tags, there’s no way for a generic browser to anticipate your tags and provide rules for displaying them. Therefore, you also need to write a style sheet for the XML document that tells browsers how to display particular tags. Like tag sets, style sheets can be shared between different documents and different people, and the style sheets you create can be integrated with style sheets others have written. </li></ul><ul><li><?xml version=”1.0”?> </li></ul><ul><li><GREETING> </li></ul><ul><li>Hello XML! </li></ul><ul><li></GREETING> </li></ul><ul><li>The above greeting.xml contains only one tag, <GREETING>, so all you need to do is define the style for the GREETING element. </li></ul>
  35. 35. <ul><li>The following is a very simple style sheet that specifies that the contents of the GREETING element should be rendered as a block-level element in 24-point bold type. </li></ul><ul><li>GREETING {display: block; font-size: 24pt; font-weight: bold} </li></ul><ul><li>The above code should be typed in a text editor and saved in a new file called greeting.css The .css extension stands for Cascading Style Sheet. Again, the .css extension is important, although the exact filename is not important. </li></ul>
  36. 36. Attaching a Style Sheet to an XML Document <ul><li>Once you’ve written an XML document and a Cascading Style Sheet for that document, you need to tell the browser to apply the style sheet to the document. In the long-term, there are likely to be a number of different ways to do this, including browser-server negotiation via HTTP headers, naming conventions, and browserside defaults. However, right now the only way that works is to include an </li></ul><ul><li><?xml-stylesheet?> processing instruction in the XML document to specify the style sheet to be used. </li></ul><ul><li>The <?xml-stylesheet?> processing instruction has two required attributes: type and href. The type attribute specifies the style sheet language used, and the href attribute specifies a URL, possibly relative, where the style sheet can be found. </li></ul><ul><li><?xml version=”1.0”?> </li></ul><ul><li><?xml-stylesheet type=”text/css” href=”greeting.css”?> </li></ul><ul><li><GREETING> </li></ul><ul><li>Hello XML! </li></ul><ul><li></GREETING> </li></ul>
  37. 37. Organization of the XML data <ul><li>XML is based on a containment model. Each XML element can contain text or other XML elements called children. A few XML elements may contain both text and child elements. This is called mixed content . However, in data heavy documents it should be avoided. </li></ul><ul><li>Complete Example of Baseball League </li></ul><ul><li>Starting the document: XML declaration and root element </li></ul><ul><li><?xml version=”1.0”?> </li></ul><ul><li><SEASON> </li></ul><ul><li></SEASON> </li></ul><ul><li>The XML declaration is not an element or a tag. Therefore, it does not need to be contained inside the root element SEASON. But every element that you put in this document will go between the <SEASON> start tag and the </SEASON> end tag. </li></ul><ul><li>This choice of root element means that you will not be able to store multiple seasons in a single file. If you want to do that, however, you can define a new root element that contains seasons. </li></ul>
  38. 38. <ul><li>Major league baseball in the United States is divided into two leagues, the American League and the National League. Each league has a name. The two names can be encoded like this: </li></ul><ul><li><?xml version=”1.0”?> </li></ul><ul><li><SEASON> </li></ul><ul><li><YEAR>1998</YEAR> </li></ul><ul><li><LEAGUE> </li></ul><ul><li><LEAGUE_NAME>National League</LEAGUE_NAME> </li></ul><ul><li></LEAGUE> </li></ul><ul><li><LEAGUE> </li></ul><ul><li><LEAGUE_NAME>American League</LEAGUE_NAME> </li></ul><ul><li></LEAGUE> </li></ul><ul><li></SEASON> </li></ul><ul><li>I’ve chosen to define the name of a league with a LEAGUE_NAME element, rather than simply a NAME element because NAME is too generic and likely to be used in other contexts. For instance, divisions, teams, and players also have names. </li></ul><ul><li>Each league can be divided into East, West, and Central divisions. </li></ul>
  39. 39. XMLizing player data <ul><li>Each team is composed of players. Each player has a first name and a last name. It’s important to separate the first and last names so that you can sort by either one. The data for the starting pitchers in the 1998 Yankees lineup can be encoded as follows: </li></ul><ul><li><TEAM> </li></ul><ul><li><TEAM_CITY>New York</TEAM_CITY> </li></ul><ul><li><TEAM_NAME>Yankees</TEAM_NAME> </li></ul><ul><li><PLAYER> </li></ul><ul><li><GIVEN_NAME>Orlando</GIVEN_NAME> </li></ul><ul><li><SURNAME>Hernandez</SURNAME> </li></ul><ul><li></PLAYER> </li></ul><ul><li><PLAYER> </li></ul><ul><li><GIVEN_NAME>David</GIVEN_NAME> </li></ul><ul><li><SURNAME>Cone</SURNAME> </li></ul><ul><li></PLAYER> </li></ul><ul><li></TEAM> </li></ul><ul><li>The tags <GIVEN_NAME> and <SURNAME> are preferable to the more obvious <FIRST_NAME> and <LAST_NAME> or <FIRST_NAME> and <FAMILY_NAME>. </li></ul><ul><li>Whether the family name or the given name comes first or last varies from culture to culture. Furthermore, surnames aren’t necessarily family names in all cultures. </li></ul>
  40. 40. XMLizing player statistics <ul><li>The next step is to provide statistics for each player. Statistics look a little different for pitchers and batters, especially in the American League where few pitchers bat. Below is Joe Girardi’s 1998 statistics. He’s a catcher so he has batting statistics: </li></ul><ul><li><PLAYER> </li></ul><ul><li><GIVEN_NAME>Joe</GIVEN_NAME> </li></ul><ul><li><SURNAME>Girardi</SURNAME> </li></ul><ul><li><POSITION>Catcher</POSITION> </li></ul><ul><li><GAMES>78</GAMES> </li></ul><ul><li><GAMES_STARTED>76</GAMES_STARTED> </li></ul><ul><li><AT_BATS>254</AT_BATS> </li></ul><ul><li><HITS>70</HITS> </li></ul><ul><li><DOUBLES>11</DOUBLES> </li></ul><ul><li><TRIPLES>4</TRIPLES> </li></ul><ul><li></PLAYER> </li></ul>
  41. 41. <ul><li>There are several benefits of XML and Among them the most important are :- </li></ul><ul><li>The data is self-describing. </li></ul><ul><li>The data can be manipulated with standard tools. </li></ul><ul><li>The data can be viewed with standard tools. </li></ul><ul><li>Different views of the same data are easy to create with style sheets. </li></ul><ul><li>The first major benefit of the XML format is that the data is self-describing. The meaning of each number is clearly and unmistakably associated with the number itself. When reading the document, you know that the 70 in <HITS>70</HITS> refers to hits and not runs batted in or strikeouts. If the person typing in the document inadvertently leaves out a statistic, that doesn’t mean that every number after it is misinterpreted. HITS is still HITS even if the preceding RUNS element is missing. </li></ul>
  42. 42. Attributes <ul><li>In the last chapter, all information was provided either in the form of a tag name or as the text content of an element. This is a straightforward and easy-to-understand approach, but it’s not the only one possible. As in HTML, XML elements may have attributes. An attribute is a name-value pair associated with an element. The name and the value are each strings. You’re already familiar with attribute syntax from HTML. For example, consider this <IMG> tag: </li></ul><ul><li><IMG SRC=cup.gif WIDTH=89 HEIGHT=67 ALT=”Cup of coffee”> </li></ul><ul><li>Attribute syntax fits the baseball example quite nicely. One advantage is that it makes the markup somewhat more concise. For example, instead of containing a YEAR child element, the SEASON element only needs a YEAR attribute: </li></ul><ul><li><SEASON YEAR=”1998”> </SEASON> </li></ul><ul><li>On the other hand, LEAGUE should be a child of the SEASON element rather than an attribute. For one thing, there are two leagues in a season. Anytime there’s likely to be more than one of something, child elements are called for. Attribute names must be unique within an element. Thus, you cannot, for example, write a SEASON element like this: </li></ul>
  43. 43. <ul><li><SEASON YEAR=”1998” LEAGUE=”National” League=”American”> </li></ul><ul><li></SEASON> </li></ul><ul><li>The second reason LEAGUE is naturally a child element rather than an attribute is that it has substructure; that is, it, itself, is subdivided into DIVISION elements. Attribute values are flat text. XML elements can conveniently encode structure. Attribute values cannot. </li></ul><ul><li>However, the name of a league is unstructured, flat text; and there’s only one name per league, so LEAGUE elements can easily have a NAME attribute instead of a LEAGUE_NAME child element: </li></ul><ul><li><LEAGUE NAME=”National League”> </li></ul><ul><li></LEAGUE> </li></ul><ul><li>Because an attribute is more closely tied to its element than a child element is, you don’t run into problems by using NAME instead of LEAGUE_NAME for the name of the attribute. Divisions and teams can also have NAME attributes without any fear of confusion with the name of a league. Because an element can have more than one attribute (as long as the attributes have different names). </li></ul>
  44. 44. Attributes versus Elements <ul><li>The last code’s no-attribute approach was an extreme position. It’s also possible to swing to the other extreme—storing all the information in the attributes and none in the content. In general, I don’t recommend this approach. Storing all the information in element content is much easier to work with in practice. </li></ul><ul><li>There are no hard and fast rules about when to use child elements and when to use attributes. Generally, you’ll use whichever suits your application. With experience, you’ll gain a feel for when attributes are easier than child elements and vice versa. Until then, one good rule of thumb is that the data itself should be stored in elements. Information about the data (metadata) should be stored in attributes. And when in doubt, put the information in the elements. </li></ul><ul><li>To differentiate between data and metadata, ask yourself whether someone reading the document would want to see a particular piece of information. If the answer is yes, then the information probably belongs in a child element. If the answer is no, then the information probably belongs in an attribute. If all tags were stripped from the document along with all the attributes, the basic information should still be present. </li></ul>
  45. 45. <ul><li>Attributes are good places to put ID numbers, URLs, references, and other information not directly or immediately relevant to the reader. However, there are many exceptions to the basic principle of storing metadata as attributes. Reasons for making an exception include: </li></ul><ul><li>Attributes can’t hold structure well. </li></ul><ul><li>Elements allow you to include meta-metadata (information about the information about the information). </li></ul><ul><li>Not everyone always agrees on what is and isn’t metadata. </li></ul><ul><li>Elements are more extensible in the face of future changes. </li></ul><ul><li>IMP: </li></ul><ul><li>CSS styles only apply to element content, not to attributes. If you use CSS, any data that you want to display to the reader should be part of an element’s content rather than one of its attributes. </li></ul>
  46. 46. Where Is XML Used, and Where Can it Be Used? <ul><li>Well, that's quite a question. XML is platform and language independent, which means it doesn't matter that one computer may be using, for example, Visual Basic on a Microsoft operating system, and the other is a UNIX machine running Java code. Really, any time one computer program needs to communicate with another program, XML is a potential fit for the exchange format. </li></ul><ul><li>Web Site Content </li></ul><ul><li>The W3C uses XML to write its specifications. These XML documents can then be transformed to HTML for display (by XSLT), or transformed to a number of other presentation formats. </li></ul><ul><li>Some web sites also use XML entirely for their content, where traditionally HTML would have been used. This XML can then be transformed to HTML via XSLT, or displayed directly in browsers via CSS. In fact, the web servers can even determine dynamically what kind of browser is retrieving the information, and then decide what to do. For example, transform the XML to HTML for older browsers, and just send the XML straight to the client for newer browsers, reducing the load on the server. </li></ul>
  47. 47. <ul><li>Remote Procedure Calls </li></ul><ul><li>XML is also used as a protocol for Remote Procedure Calls ( RPC ). RPC is a protocol that allows objects on one computer to call objects on another computer to do work, allowing distributed computing </li></ul><ul><li>Using XML and HTTP for these RPC calls, using a technology called the Simple Object Access Protocol (SOAP) , allows this to occur even through a firewall, which would normally block such calls, providing greater opportunities for distributed computing. </li></ul><ul><li>E-Commerce </li></ul><ul><li>E-commerce is one of those buzzwords that you hear all over the place. Companies are discovering that by communicating via the Internet, instead of by more traditional methods (such as faxing, human-to-human communication, etc.), they can streamline their processes, decreasing costs and increasing response times. Whenever one company needs to send data to another, XML is the perfect fit for the exchange format. </li></ul><ul><li>When the companies involved have some kind of on-going relationship this is known as business-to-business ( B2B ) e-commerce. There are also business to consumer (B2C) transactions–you may even have used this to buy this book if you bought it on the Internet. Both types have their potential uses for XML. </li></ul><ul><li>Web Server Configurations </li></ul>
  48. 48. <ul><li>The text starting with a &quot;<&quot; character, and ending with a &quot;>&quot; character, is an XML tag. The information in our document (our data) is contained within the various tags that constitute the markup of the document. This makes it easy to distinguish the information in the document from the markup . </li></ul><ul><li>As you can see, the tags are paired together, so that any opening tag also has a closing tag. In XML parlance, these are called start-tags and end-tags . The end-tags are the same as the start-tags, except that they have a &quot;/&quot; right after the opening < character. </li></ul><ul><li>In this regard, XML tags work the same as start-tags and end-tags do in HTML. For example, you would mark a section of HTML bold like this: </li></ul><ul><li><B>This is bold.</B> As you can see, there is a <B> start-tag, and a </B> end-tag, just like we use for XML. </li></ul><ul><li>All of the information from the start of a start-tag to the end of an end-tag, and including everything in between, is called an element . So: </li></ul><ul><li><first> is a start-tag </li></ul><ul><li></first> is an end-tag </li></ul><ul><li><first>John</first> is an element </li></ul>Tags and Text and Elements
  49. 49. What are the Pieces that Make Up XML? <ul><li>Structuring information&quot; is a pretty broad topic, and as such it would be futile to try and define a specification to cover it fully. For this reason there are a number of inter-related specifications that all work together to form the XML family of technologies, with each specification covering different aspects of communicating information. Here are some of the more important ones: </li></ul><ul><li>XML 1.0 is the base specification upon which the XML family is built. It describes the syntax that XML documents have to follow, the rules that XML parsers have to follow, and anything else you need to know to read or write an XML document. It also defines DTDs, although they sometimes get treated as a separate technology. See the next bullet. </li></ul><ul><li>Because we can make up our own structures and element names for our documents, DTDs and Schemas provide ways to define our document types. We can check to make sure other documents adhere to these templates, and other developers can produce compatible documents </li></ul>
  50. 50. <ul><li>The text between the start-tag and end-tag of an element is called the element content . The content between our tags will often just be data (as opposed to other elements). In this case, the element content is referred to as Parsed Character DATA , which is almost always referred to using its acronym, PCDATA , or with a more general term such as &quot;text content&quot; or even &quot;text node&quot;. </li></ul><ul><li>Rules for Elements </li></ul><ul><li>Every start-tag must have a matching end-tag, or be a self-closing tag </li></ul><ul><li>Tags can't overlap </li></ul><ul><li>XML documents can have only one root element </li></ul><ul><li>Element names must obey XML naming conventions </li></ul><ul><li>XML is case-sensitive </li></ul><ul><li>XML will keep whitespace in your text </li></ul><ul><li>Element Names </li></ul><ul><li>If we're going to be creating elements we're going to have to give them names, and XML is very generous in the names we're allowed to use. For example, there aren't any reserved words to avoid in XML, as there are in most programming languages, so we have a lot flexibility in this regard. </li></ul>
  51. 51. <ul><li>However, there are some rules that we must follow: </li></ul><ul><li>Names can start with letters (including non-Latin characters) or the &quot;_&quot; character, but not numbers or other punctuation characters. </li></ul><ul><li>After the first character, numbers are allowed, as are the characters &quot;-&quot; and &quot;.&quot;. </li></ul><ul><li>Names can't contain spaces. </li></ul><ul><li>Names can't contain the &quot;:&quot; character. Strictly speaking, this character is allowed, but the XML specification says that it's &quot;reserved&quot;. You should avoid using it in your documents, unless you are working with namespaces. </li></ul><ul><li>Names can't start with the letters &quot;xml&quot;, in uppercase, lowercase, or mixed–you can't start a name with &quot;xml&quot;, &quot;XML&quot;, &quot;XmL&quot;, or any other combination. </li></ul><ul><li>There can't be a space after the opening &quot;<&quot; character; the name of the element must come immediately after it. However, there can be space before the closing &quot;>&quot;character, if desired. </li></ul><ul><li>Another important point to keep in mind is that the tags in XML are case-sensitive. </li></ul><ul><li>Warning! Because XML is case-sensitive, you could legally create an XML document which has both <first> and <First> elements, which have different meanings. This is a bad idea, and will cause nothing but confusion! You should always try to give your elements distinct names, for your sanity, and for the sanity of those to come after you. </li></ul>
  52. 52. Whitespace in PCDATA <ul><li>There is a special category of characters, called whitespace . This includes things like the space character, new lines (what you get when you hit the Enter key), and tabs. Whitespace is used to separate words, as well as to make text more readable. </li></ul><ul><li>Attributes </li></ul><ul><li>In addition to tags and elements, XML documents can also include attributes . </li></ul><ul><li>Why Use Attributes? </li></ul><ul><li>There have been many debates in the XML community about whether attributes are really necessary, and if so, where they should be used. Here are some of the main points in that debate: </li></ul>
  53. 53. <ul><li>Attributes Can Provide Meta Data that May Not be Relevant to Most Applications Dealing with Our XML </li></ul><ul><li>For example, if we know that some applications may care about a CD's serial number, but most won't, it may make sense to make it an attribute. This logically separates the data most applications will need from the data that most applications won't need. </li></ul><ul><li>In reality, there is no such thing as &quot;pure meta data&quot;–all information is &quot;data&quot; to some application. Think about HTML; you could break the information in HTML into two types of data: the data to be shown to a human, and the data to be used by the web browser to format the human-readable data. From one standpoint, the data used to format the data would be meta data, but to the browser or the person writing the HTML, the meta data is the data. Therefore, attributes can make sense when we're separating one type of information from another. </li></ul>
  54. 54. <ul><li>What Do Attributes Buy Me that Elements Don't? </li></ul><ul><li>Can't elements do anything attributes can do? </li></ul><ul><li>In other words, on the face of it there's really no difference between: </li></ul><ul><li><name nickname='Shiny John'></name> </li></ul><ul><li>And </li></ul><ul><li><name> </li></ul><ul><li><nickname>Shiny John</nickname> </li></ul><ul><li></name> </li></ul><ul><li>So why bother to pollute the language with two ways of doing the same thing? </li></ul><ul><li>The main reason that XML was invented was that SGML could do some great things, but it was too massively difficult to use without a fully-fledged SGML expert on hand. So one concept behind XML is a simpler, kinder, gentler SGML. For this reason, many people don't like attributes, because they add a complexity to the language that they feel isn't needed. </li></ul><ul><li>On the other hand, some people find attributes easier to use–for example, they don't require nesting and you don't have to worry about crossed tags. </li></ul>
  55. 55. <ul><li>Attributes are Un-Ordered </li></ul><ul><li>The order of attributes is considered irrelevant. Hence, sometimes you may need to use elements, rather than attributes, for information that must come in the document in a certain order. </li></ul><ul><li>Why Use Attributes When Elements Look So Much Better? I Mean, Why Use Elements When Attributes Look So Much Better? </li></ul><ul><li>Many people have different opinions as to whether attributes or child elements &quot;look better&quot;. In this case, it comes down to a matter of personal preference and style. </li></ul><ul><li>In fact, much of the attributes versus elements debate comes from personal preference. Many, but not all, of the arguments boil down to &quot;I like the one better than the other&quot;. But since XML has both elements and attributes, and neither one is going to go away, you're free to use both. Choose whichever works best for your application, whichever looks better to you, or whichever you're most comfortable with. </li></ul><ul><li>Comments </li></ul><ul><li>Comments provide a way to insert into an XML document text that isn't really part of the document, but rather is intended for people who are reading the XML source itself. </li></ul>
  56. 56. <ul><li>Anyone who has used a programming language will be familiar with the idea of comments: you want to be able to annotate your code (or your XML), so that those coming after you will be able to figure out what you were doing. (And remember: the one who comes after you may be you! Code you wrote six months ago might be as foreign to you as code someone else wrote.) </li></ul><ul><li>Of course, comments may not be as relevant to XML as they are to programming languages; after all, this is just data, and it's self-describing to boot. But you never know when they're going to come in handy, and there are cases where comments can be very useful, even in data. </li></ul><ul><li>Comments start with the string <!-- and end with the string -->, as shown here: </li></ul><ul><li><name nickname='Venky K'> </li></ul><ul><li><first>Venkat</first> </li></ul><ul><li><!—This is a Comment--> </li></ul><ul><li><middle></middle> </li></ul><ul><li><last>Krishnan</last> </li></ul><ul><li></name> </li></ul><ul><li>There are a couple of points that we need to note about comments. First, you can't have a comment inside a tag. </li></ul><ul><li>The XML specification states that an XML parser doesn't need to pass these comments on to the application, meaning that you should never count on being able to use the information inside a comment from your application. Comments are only there for the benefit of someone reading your XML markup. </li></ul>
  57. 57. <ul><li>Empty Elements </li></ul><ul><li>Sometimes an element has no PCDATA. In this case, you also have the option of writing this element using the special empty element syntax : </li></ul><ul><li><middle/> </li></ul><ul><li>This is the one case where a start-tag doesn't need a separate end-tag, because they are both combined together into this one tag. In all other cases, they do. </li></ul><ul><li>Recall from our discussion of element names that the only place we can have a space within the tag is before the closing &quot;>&quot;. This rule is slightly different when it comes to empty elements. The &quot;/&quot; and &quot;>&quot; characters always have to be together, so you can create an empty element like this: </li></ul><ul><li><middle /> </li></ul><ul><li>but not like these: </li></ul><ul><li><middle/ > </li></ul><ul><li><middle / > </li></ul>
  58. 58. <ul><li>XML Declaration </li></ul><ul><li>It is often very handy to be able to identify a document as being of a certain type. XML provides the XML declaration for us to label documents as being XML, along with giving the parsers a few other pieces of information. You don't need to have an XML declaration, but it's considered good practice to include it. </li></ul><ul><li>A typical XML declaration looks like this: </li></ul><ul><li><?xml version='1.0' encoding='UTF-16' standalone='yes'?> </li></ul><ul><li>Some things to note about the XML declaration: </li></ul><ul><li>The XML declaration starts with the characters <?xml, and ends with the characters ?>. </li></ul><ul><li>If you include it, you must include the version, but the encoding and standalone attributes are optional. </li></ul><ul><li>The version, encoding, and standalone attributes must be in that order. </li></ul>
  59. 59. <ul><li>Currently, the version should be 1.0. If you use a number other than 1.0, XML parsers that were written for the version 1.0 specification can reject the document. (If there is ever a new version of the XML Recommendation, the version number in the XML declaration will be used to signal which version of the specification your document claims to support.) </li></ul><ul><li>The XML declaration must be right at the beginning of the file. That is, the first character in the file should be that &quot;<&quot;; no line breaks or spaces. Some parsers are more forgiving about this than others. </li></ul><ul><li>So an XML declaration can be as full as the previous one, or as simple as: </li></ul><ul><li><?xml version='1.0'?> </li></ul><ul><li>A character code is a one-to-one mapping between a set of characters and the corresponding numbers to represent those characters. A character encoding is the method used to represent the numbers in a character code digitally (in other words how many bytes should be used for each number, etc.) </li></ul><ul><li>One character code that you might have come across is the American Standard Code for Information Interchange ( ASCII ). For example, in ASCII the character &quot;a&quot; is represented by the number 97, and the character &quot;A&quot; is represented by the number 65. </li></ul>
  60. 60. Illegal PCDATA Characters <ul><li>There are some reserved characters that you can't include in your PCDATA because they are used in XML syntax, the &quot;<&quot; and &quot;&&quot; characters: </li></ul><ul><li><!--This is not well-formed XML!--><comparison>6 is < 7 & 7 > 6</comparison> </li></ul><ul><li>This means that the XML parser comes across the &quot;<&quot; character, and expects a tag name, instead of a space. (Even if it had got past this, the same error would have occurred at the &quot;&&quot; character.) </li></ul><ul><li>There are two ways you can get around this: escaping characters , or enclosing text in a CDATA section . </li></ul><ul><li>Escaping Characters </li></ul><ul><li>To escape these two characters, you simply replace any &quot;<&quot; characters with &quot;&lt;&quot; and any &quot;&&quot; characters with &quot;&amp;&quot;. (In addition, you can also escape the &quot;>&quot; character with &quot;&gt;&quot;. It isn't necessary, but it does make things more consistent, since you need to escape all of the &quot;<&quot; characters.) The above XML could be made well-formed by doing the following: </li></ul><ul><li><comparison>6 is &lt; 7 &amp; 7 &gt; 6 </comparison> lt; and &amp; are known as entity references . The following entities are defined in XML: </li></ul><ul><li>&amp;-the & character </li></ul><ul><li>&lt;-the < character </li></ul><ul><li>&gt;-the > character </li></ul><ul><li>&apos;-the ' character </li></ul><ul><li>&quot;-the &quot; character </li></ul>
  61. 61. <ul><li>CDATA Sections </li></ul><ul><li>If you have a lot of &quot;<&quot; and &quot;&&quot; characters that need escaping, you may find that your document quickly becomes very ugly and unreadable. Luckily, there are also CDATA sections. </li></ul><ul><li>Using CDATA sections, we can tell the XML parser not to parse the text, but to let it all go by until it gets to the end of the section. CDATA sections look like this: </li></ul><ul><li><comparison><![CDATA[6 is < 7 & 7 > 6]]></comparison> </li></ul><ul><li>Everything starting after the <![CDATA[ and ending at the ]]> is ignored by the parser, and passed through to the application as is. The only character sequence that can't occur within a CDATA section is &quot;]]>&quot;, since the XML parser would think that you were closing the CDATA section. If you needed to include this sequence, you would be better off keeping it out of the CDATA section, like this: </li></ul><ul><li><![CDATA[This text contains the sequence ']]>]]><![CDATA[' in it.]]> </li></ul>
  62. 62. <ul><li>Parsing XML </li></ul><ul><li>The main reason for creating all of these rules about writing well-formed XML documents is so that we can create a computer program to read in the data, and easily tell markup from information. </li></ul><ul><li>Apache Xerces </li></ul><ul><li>The Apache Software Foundation's Xerces sub-project of the Apache XML Project ( http:// xml.apache.org / ) has resulted in XML parsers in Java and C++, plus a Perl wrapper for the C++ parser. These tools are free, and the distribution of the code is controlled by the GNU Public License. </li></ul><ul><li>James Clark's Expat </li></ul><ul><li>Expat is an XML 1.0 parser toolkit written in C. More information can be found at http:// www.jclark.com/xml/expat.html and Expat can be downloaded from ftp:// ftp.jclark.com/pub/xml/expat.zip . It is free for both private and commercial use. </li></ul><ul><li>Xml4j </li></ul><ul><li>IBM's AlphaWorks site ( http:// www.alphaworks.ibm.com ) offers a number of XML tools and applications, including the xml4j parser. This is another parser written in Java, available for free, though there are some licensing restrictions regarding its use. </li></ul>
  63. 63. <ul><li>Errors in XML </li></ul><ul><li>As well as specifying how a parser should get the information out of an XML document, it is also specified how a parser should deal with errors in XML. There are two types of error in the XML specification: errors and fatal errors . </li></ul><ul><li>An error is simply a violation of the rules in the specification, where the results are undefined; the XML processor is allowed to recover from the error and continue processing. </li></ul><ul><li>Fatal errors are more serious: according to the specification a parser is not allowed to continue as normal when it encounters a fatal error. (It may, however, keep processing the XML document to search for further errors.) This is called draconian error handling . Any error which causes an XML document to cease being well-formed is a fatal error. </li></ul><ul><li>The reason for this drastic handling of non-well-formed XML is simple: it would be extremely hard for parser writers to try and handle &quot;well-formedness&quot; errors, and it is extremely simple to make XML well-formed. (Web browsers don't force documents to be as strict as XML does, but this is one of the reasons why web browsers are so incompatible; they must deal with all of the errors they may encounter, and try to figure out what the person who wrote the document was really trying to code.) </li></ul>
  64. 64. <ul><li>But draconian error handling doesn't just benefit the parser writers; it also benefits us when we're creating XML documents. If I write an XML document that doesn't properly follow XML's syntax, I can find out right away and fix my mistake. On the other hand, if the XML parser tries to recover from these errors, it may misinterpret what I was trying to do, but I wouldn't know about it because no error would be raised. In this case, bugs in my software would be much harder to track down, instead of being caught right at the beginning when I was creating my data. Even worse, if I sent my XML document to someone else, their parser might interpret the mistake differently. </li></ul>
  65. 65. Namespaces <ul><li>A URI (Uniform Resource Identifier) is a string of characters that identifies a resource. It can come in one of two flavors: URL (Uniform Resource Locator), or URN (Universal Resource Name). </li></ul><ul><li>How XML Namespaces Work </li></ul><ul><li>To use XML Namespaces in your documents, elements are given qualified names . (In most W3C specifications &quot;qualified name&quot; is abbreviated to QName .) These qualified names consist of two parts: the local part , which is the same as the names we have been giving elements all along, and the namespace prefix , which specifies to which namespace this name belongs. </li></ul><ul><li>For example, to declare a namespace called http://sernaferna.com/pers, and associate a <person> element with that namespace, we would do something like the following: </li></ul><ul><li><pers:person xmlns:pers=&quot;http://sernaferna.com/pers&quot;/> </li></ul><ul><li>The key is the xmlns:pers attribute (&quot;xmlns&quot; stands for XML NameSpace). Here we are declaring the pers namespace prefix, and the URI of the namespace which it represents (http://sernaferna.com/pers). We can then use the namespace prefix with our elements, as in pers:person. </li></ul>
  66. 66. Document Type Definitions <ul><li>The need to validate documents against a vocabulary is common in markup languages. In fact, it is so common that the creators of XML included a method for checking validity in the XML Recommendation. A document is valid if its XML content complies with a definition of allowable elements, attributes, and other document pieces. By utilizing special Document Type Definition syntaxes, or DTDs, you can check the content of a document type with special parsers. </li></ul><ul><li>More specifically, DTDs outline the following features within the DTD: </li></ul><ul><li>The element types that will be allowed within the XML document </li></ul><ul><li>The various characteristics of each element type, along with the attributes used with each element and the content each element is capable of having </li></ul><ul><li>Any notations that can be encountered within the document </li></ul><ul><li>The entities that can be used within the document </li></ul><ul><li>The XML Recommendation separates parsers into two categories-validating and nonvalidating. Validating parsers, according to the Recommendation, must implement validity checking using DTDs. Therefore, if we have a validating parser, we can remove the content checking code from our application and depend on the external processor. </li></ul><ul><li><?xml version=&quot;1.0&quot;?> </li></ul><ul><li><!DOCTYPE name [ </li></ul><ul><li><!ELEMENT name (first, middle, last)> </li></ul><ul><li><!ELEMENT first (#PCDATA)> </li></ul><ul><li><!ELEMENT middle (#PCDATA)> </li></ul><ul><li><!ELEMENT last (#PCDATA)> </li></ul>
  67. 67. <ul><li>]> </li></ul><ul><li><name> </li></ul><ul><li><first>John</first> </li></ul><ul><li><middle>Fitzgerald</middle> </li></ul><ul><li><last>Doe</last> </li></ul><ul><li></name> </li></ul><ul><li>Save the file as name1.xml. </li></ul><ul><li><!DOCTYPE name [ </li></ul><ul><li>Immediately following the XML header is the Document Type Declaration , commonly referred to as the DOCTYPE. This informs the parser that there is a DTD associated with this XML Document. </li></ul><ul><li>If the document contains a DTD 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. The DOCTYPE declaration has an exclamation mark (!) at the start of the element name. </li></ul><ul><li>Following the name of the root element, you have several options for specifying the rest of the Document Type Declaration. In our <name> example, our element declarations appeared within our XML Document, between the [ and ] of our Document Type Declaration. Any declarations within the XML document, as in our sample, are the internal subset . The XML declaration also allows you to refer to an external DTD. Declarations that appear in an external DTD are the external subset . You can refer to an external DTD in one of two ways </li></ul><ul><li>System identifiers </li></ul><ul><li>Public identifiers </li></ul>
  68. 68. System Identifiers <ul><li>A system identifier allows you to specify the physical location of a file on your system. It is comprised of two parts: the keyword SYSTEM followed by a URI reference to a document with a physical location. </li></ul><ul><li><!DOCTYPE name SYSTEM &quot;name.dtd&quot; [...]> </li></ul><ul><li>This can be a file on your local hard-drive, for example, or, perhaps, a file on your intranet or the Internet. To indicate that you are using a system identifier, you must type the word SYSTEM after the name of the root element in your declaration. Immediately following the SYSTEM keyword is the URI reference to the location of the file in quotation marks. Some valid system identifiers include: </li></ul><ul><li><!DOCTYPE name SYSTEM &quot;file:///c:/name.dtd&quot; [...]> </li></ul><ul><li><!DOCTYPE name SYSTEM &quot;http://sernaferna.com/hr/name.dtd&quot; [...]> </li></ul><ul><li><!DOCTYPE name SYSTEM &quot;name.dtd&quot;> </li></ul><ul><li>Notice that in the last example we have omitted the [ and ]. This is perfectly normal. Specifying an internal subset is optional. You can use an external and internal subset at the same time. </li></ul>
  69. 69. Public Identifiers <ul><li>Public identifiers provide a second mechanism to locate DTD resources. </li></ul><ul><li><!DOCTYPE name PUBLIC &quot;-//Wrox Press//DTD Name example from Chapter 1//EN&quot;> </li></ul><ul><li>Much like the system identifier, the public identifier begins with a keyword, PUBLIC, followed by a specialized identifier. However, instead of a reference to a file, public identifiers are used to identify an entry in a catalog. According to the XML specification, public identifiers can follow any format, however, there is a commonly used format called Formal Public Identifiers , or FPIs . </li></ul><ul><li>The syntax for FPIs follows the basic structure: </li></ul><ul><li>-//Owner//Class Description//Language//Version </li></ul>
  70. 70. Try It Out-The External DTD <ul><li>We have alluded to the idea that external DTDs can be very useful. By utilizing an external DTD, we can easily share our vocabulary with others in our industry, or even our own company. By this same logic, we can utilize vocabularies that others have already developed, by referring to external files they have created. Therefore, let's reconfigure our <name> example so that our DTD is defined external to our XML document. </li></ul><ul><li>EXTERNAL DTD.doc </li></ul>
  71. 71. How It Works <ul><li>In this Try It Out, we used an external DTD to check our XML content. As you may have guessed, the syntax for the DTD changed very little. The main difference between the internal DTD and external DTD was that there was no DOCTYPE declaration within the external DTD. The DOCTYPE declaration is always located within the XML document. In addition, within our name4.xml document there was no internal subset. Instead, we used a public identifier to indicate what DTD our validation program should use. </li></ul><ul><li>Of course, our validation program has no way to resolve public identifiers. The processor instead used the optional URI reference that we included to find the correct DTD for validation. In this case, our XML parser must find the file identified as &quot;name4.dtd&quot;. Because this is a relative URL reference (it does not contain a web site address or drive letter), our parser will begin looking in the current directory-where the XML document it is parsing is located. The XML Recommendation does not specify how parsers should handle relative URL references; however, most XML parsers will treat the path of the XML document as the base path, just as ours has done. </li></ul>
  72. 72. Element Declarations <ul><li>Throughout the beginning of this chapter, we have seen element declarations in use, but have not yet looked at the element declaration in detail. When using a DTD to define the content of an XML document, you must declare each element that appears within the document. As we will soon see, DTDs may also include declarations for optional elements, elements that may or may not appear in the XML document. </li></ul><ul><li><!ELEMENT name (first, middle, last)> </li></ul><ul><li>Element declarations can be broken down into three basic parts: </li></ul><ul><li>The ELEMENT declaration </li></ul><ul><li>The element name </li></ul><ul><li>The element content model </li></ul><ul><li>As we have seen with the DOCTYPE declaration, the ELEMENT declaration is used to indicate to the parser that we are about to define an element. Much like the DOCTYPE declaration, the ELEMENT declaration begins with an exclamation mark. The declaration can appear only within the context of the DTD. </li></ul>
  73. 73. <ul><li>Immediately following the ELEMENT keyword is the name of the element that you are defining. Just as we saw in the DOCTYPE, the element name must appear exactly as it will within the XML document, including any namespace prefix. </li></ul><ul><li>The content model of the element appears after the element name. An element's content model defines the allowable content within the element. An element may be empty, or its content may consist of text, element children, or a combination of children and text. This is essentially the crux of the DTD, where the entire document's structure is defined. As far as the XML Recommendation is concerned, there are four kinds of content model: </li></ul><ul><ul><li>Empty </li></ul></ul><ul><ul><li>Element </li></ul></ul><ul><ul><li>Mixed </li></ul></ul><ul><ul><li>Any </li></ul></ul><ul><li>To define an element with an empty content model you simply include the word EMPTY following the element name in the declaration: </li></ul><ul><li><!ELEMENT br EMPTY> (<br> in XHTML) </li></ul><ul><li>There are two fundamental ways of specifying that an element will contain more than one child: </li></ul><ul><li>Sequences (If the elements should be in distinct order) </li></ul><ul><li>Choices </li></ul>
  74. 74. <ul><li>Suppose we needed to allow one element or another, but not both. Obviously, we would need a choice mechanism of some sort. For example, let's say we were designing a DTD that could be used to catalog our music library. Within our catalog, we had several <item> elements. Within each <item> element we wanted to allow the four choices <CD>, <cassette>, <record>, and <MP3>. We could declare our <item> element as follows: </li></ul><ul><li><!ELEMENT item (CD | cassette | record | MP3)> </li></ul><ul><li>This declaration would allow our <item> element to contain one <CD> or one <cassette> or one <record> or one <MP3> element. If our <item> element were empty, or if it contained more than one of these elements the parser would raise an error. </li></ul><ul><li>The choice content model is very similar to the sequence content model. Instead of separating the elements by commas, however, you must use the vertical bar (|) character. The vertical bar functions as an exclusive OR. An exclusive OR allows one and only one element out of the possible choices. </li></ul><ul><li>When creating our <name> declaration we would need to specify that the content may include either a <fullname> element or the <first>, <middle>, and <last> sequence of elements, but not both. The XML Recommendation allows us to mix sequences and choices. Because of this, we can declare our model as follows: </li></ul><ul><li><!ELEMENT name (fullname | (first, middle, last))> </li></ul>
  75. 75. Mixed Content <ul><li>The XML Recommendation specifies that any element with text in its content is a mixed content model element. Within mixed content models, text may appear by itself or it may be interspersed between elements. </li></ul><ul><li>We have already seen some examples of the simplest mixed content model-text only: </li></ul><ul><li><!ELEMENT artist (#PCDATA)> </li></ul><ul><li>There is only one way to declare a mixed content model within DTDs. In the mixed content model, we must use the choice mechanism when adding elements. This means that each element within our content model must be separated by the vertical bar (|) character: </li></ul><ul><li><!ELEMENT p (#PCDATA | b | i)*> </li></ul><ul><li>In the sample above, we have attempted to describe the <p> element (actually, this definition is horribly lacking-there are many more allowable children of a real <p> element). Notice that we have used the choice mechanism to describe our content model; a vertical bar separates each element. We cannot use commas to separate the choices. </li></ul>
  76. 76. <ul><li>When including elements in the mixed content model, the #PCDATA keyword must always appear first in the list of choices. This allows validating parsers to immediately recognize that it is processing a mixed content model, rather than an element content model. Unlike element-only content models, you are not allowed to have inner content models in a mixed declaration. </li></ul><ul><li>You should also notice that we have included the '*' outside of the parentheses of our content model. When you are including elements within your mixed content model you are required to include the '*' at the end of your content model. The '*' character is known as a cardinality indicator . </li></ul><ul><li>To simplify, every time you declare elements within a mixed content model, they must follow four rules: </li></ul><ul><li>They must use the choice mechanism to separate elements. </li></ul><ul><li>The #PCDATA keyword must appear first in the list of elements. </li></ul><ul><li>There must be no inner content models. </li></ul><ul><li>The '*' cardinality indicator must appear at the end of the model. </li></ul>
  77. 77. Any Content <ul><li>Finally, we can declare our element utilizing the ANY keyword: </li></ul><ul><li><!ELEMENT catalog ANY> </li></ul><ul><li>In the above, the ANY keyword indicates that any elements declared within the DTD may be used within the content of the <catalog> element, and that they may be used in any order any number of times. The ANY keyword does not allow you to include elements in your XML document that are not declared within the DTD. In addition, any character data may appear within the <catalog> element. Because DTDs are used to restrict content, the ANY keyword is not very popular as it does very little to restrict the allowed content of the element you are declaring. </li></ul><ul><li>Cardinality </li></ul><ul><li>An element's cardinality defines how many times it will appear within a content model. Each element within a content model may have an indicator of how many times it will appear following the element name. DTDs allow four indicators for cardinality: </li></ul>
  78. 78. Indicates that the element may appear zero or more times. * Indicates that the element may appear one or more times. + Indicates that the element may appear either once or not at all. ? As we have seen in all of our content models thus far, when no cardinality indicator is used it indicates that the element must appear once and only once. This is the default behavior for elements used in content models. [none] Description Indicator
  79. 79. <ul><li>Let's look at these indicators in action: </li></ul><ul><li>< !ELEMENT parody (title?, artist?)> </li></ul><ul><li>Suppose we used cardinality indicators when declaring the content for our <parody> element. By including a '?' when specifying our title and artist elements, we inform the processor that these elements may appear once or not at all within our content model. If we were to validate the document again, the parser would not raise validity errors if the <title> element and <artist> elements were missing. With our new declaration, the only allowable <parody> elements include: </li></ul><ul><li><parody> </li></ul><ul><li><title>Like A Virgin</title> </li></ul><ul><li><artist>Madonna</artist> </li></ul><ul><li></parody> </li></ul><ul><li><parody> </li></ul><ul><li><title>Like A Virgin</title> </li></ul><ul><li></parody> </li></ul><ul><li><parody> </li></ul><ul><li><artist>Madonna</artist> </li></ul><ul><li></parody><parody> </li></ul><ul><li></parody> </li></ul><ul><li>In each of the above cases, we can see that the <title> element may or may not appear. In addition, the <artist> element may or may not appear. In the final example, we see that the <parody> element can be completely empty, where neither element appears. An important point to remember is that, because we used the '?' cardinality indicator, both the <title> and <artist> elements may appear within our <parody> element at most one time. It is also important to notice that when both elements do appear, they must be in the order that we defined within our sequence. </li></ul>
  80. 80. Attribute Declarations <ul><li>Attribute declarations are in many ways similar to element declarations. Instead of declaring content models for allowable elements, however, DTDs allow you to declare a list of allowable attributes for each element. These lists are called ATTLIST declarations. </li></ul><ul><li><!ELEMENT catalog (CD | cassette | record | MP3)*> </li></ul><ul><li><!ATTLIST catalog name CDATA #IMPLIED> </li></ul><ul><li>Above, we have an element declaration for our <catalog> element from our catalog example. Following the element declaration, we have an ATTLIST declaration that declares the allowable attributes of our <catalog> element. This particular ATTLIST declares only one attribute, name, for the <catalog> element. </li></ul><ul><li>An ATTLIST declaration can be broken down into three basic parts: </li></ul><ul><li>The ATTLIST keyword </li></ul><ul><li>The associated element's name </li></ul><ul><li>The list of declared attributes </li></ul>
  81. 81. <ul><li>Just as we have seen in all of our other declarations, the ATTLIST declaration begins with an exclamation mark to indicate that it is part of the DTD. Immediately following the ATTLIST keyword is the name of the associated element. In our example above, the name of our associated element is catalog. We indicated that we were building a list of attributes for a <catalog> element by including the word catalog immediately following the ATTLIST keyword. </li></ul><ul><li>Following the ATTLIST name, we declared each attribute in the list. An ATTLIST declaration may include any number of attributes. Each attribute in the list can be broken down into three parts: </li></ul><ul><li>The attribute name </li></ul><ul><li>The attribute type </li></ul><ul><li>The attribute value declaration </li></ul><ul><li>Let's look at each section of name attribute declaration: </li></ul><ul><li>name CDATA #IMPLIED </li></ul><ul><li>In the above declaration, the name of the attribute is, ironically, name. We have declared that our name attribute may contain character data by using the CDATA keyword-this is our attribute's type. Finally, we have declared that our attribute has no default value, and further, that this attribute does not need to appear within our element using the #IMPLIED keyword. </li></ul>
  82. 82. <ul><li>Let's look at an example document: </li></ul><ul><li><?xml version=&quot;1.0&quot;?> </li></ul><ul><li><!DOCTYPE names [ </li></ul><ul><li> <!ELEMENT names (name)*> </li></ul><ul><li> <!ELEMENT name (first, middle?, last)> </li></ul><ul><li> <!ATTLIST name social ID #REQUIRED> </li></ul><ul><li> <!ELEMENT first (#PCDATA)> </li></ul><ul><li> <!ELEMENT middle (#PCDATA)> </li></ul><ul><li> <!ELEMENT last (#PCDATA)> </li></ul><ul><li>]> </li></ul><ul><li><names> </li></ul><ul><li><name social=&quot;_111-22-3333&quot;> </li></ul><ul><li><first>John</first> </li></ul><ul><li><middle>Fitzgerald</middle> </li></ul><ul><li><last>Doe</last> </li></ul><ul><li></name> </li></ul><ul><li><name social=&quot;_777-88-9999&quot;> </li></ul><ul><li><first>Jane</first> </li></ul><ul><li><middle>Mary</middle> </li></ul><ul><li><last>Doe</last> </li></ul><ul><li></name> </li></ul><ul><li></names> </li></ul>
  83. 83. Attribute Value Declarations <ul><li>Within each attribute declaration, you must also specify how the value will appear in the document. Often we will want to provide a default value for our attribute declaration. At times we may simply want to require that the attribute be specified in the document, still other times we may need to require that the value of the attribute be fixed at a given value. Each attribute can be declared with these properties. </li></ul><ul><li>The XML Recommendation allows us to specify that the attribute: </li></ul><ul><li>Has a default value </li></ul><ul><li>Has a fixed value </li></ul><ul><li>Is required </li></ul><ul><li>Is implied </li></ul><ul><li>Sometimes we need to provide a value for an attribute even if it hasn't been included in the XML document. By specifying a default value for our attributes, we can be sure that it will be included in the final output. As the document is being processed, a validating parser will automatically insert the attribute with the default value if the attribute has been omitted. If the attribute has been included in the document, the parser will use the included attribute. </li></ul>
  84. 84. <ul><li>There are some circumstances where an attribute's value must always be fixed. When an attribute's value can never change we use the #FIXED keyword followed by the fixed value . Fixed values operate much like default values. As the parser is validating the file, if the fixed attribute is encountered then the parser will check that the fixed value and attribute value match. If they do not match, the parser will raise a validity error. If the parser does not encounter the attribute within the element, it will insert the attribute with the fixed value. </li></ul><ul><li>When you specify that an attribute is required , it must be included within the XML document. Often a document must have the attribute to function properly, at other times it is simply a matter of exercising control over the document content. Imagine you are developing a program that can display images. Within your XML document, you have <image> elements that refer to external image files. In order for your application to display the images, it needs to know what kind of image is being referenced; and without this information, the file cannot be displayed. </li></ul>
  85. 85. Entities <ul><li>An entity is a powerful XML tool that allows DTD and document developers to associate a text alias with non-ASCII text or binary resources and large or frequently used blocks of text and markup. Document Type Definitions (DTDs) that take advantage of the entity mechanism are well organized, easier to read and work with, and able to support literally thousands of nonstandard characters. Without entities, binary resources, such as image and audio files, cannot be included in Extensible Markup Language (XML) documents. </li></ul>
  86. 86. Placing A Character Entity In A Document <ul><li>To use special reserved characters within a document, you need to use a character entity that has already been defined within the XML standard. For example, if you want to use the ampersand sign (&) in the name of a company, you need to specify the XML character entity that represents the ampersand, because the ampersand is used to specify built-in character entities. </li></ul><ul><li>Entities that represent reserved characters always start with an ampersand, are followed by the entity name, and are closed by adding a semicolon to the end of the entity name. No additional white space is permitted around or inside the entity. To specify a built-in entity, place the following code within the content of your XML document: </li></ul><ul><li>&amp; </li></ul><ul><li>This code can be used anywhere within the content of an XML document, for example: </li></ul><ul><li>Welcome to the Diggins &amp; Kirk Consulting Firm! </li></ul><ul><li>The content, once processed, would display as follows: </li></ul><ul><li>Welcome to the Diggins & Kirk Consulting Firm! </li></ul>
  87. 87. Creating Your Own Entity <ul><li>To create your own entities to specify groups of character data, you must first declare them in your DTD or within the beginning of your XML document as you do with elements and attributes. The name of the entity can be made up of letters, digits, periods, dashes, underscores, or a colon and must begin with a letter or an underscore. The value of the entity, called the entity definition, must be enclosed in quotes and can contain any valid markup. The syntax of an entity declaration is the entity element !ENTITY , the name of the entity, and then the entity declaration. To define an entity that specifies the city and state, Anchorage, Alaska, include the following code in your XML document or within your DTD: </li></ul><ul><li><!ENTITY AK &quot;Anchorage, Alaska&quot;> </li></ul>
  88. 88. Using The Entity You Have Declared <ul><li>Once you have declared an entity, you can place it anywhere within the content of the XML document. The entity reference must start with an ampersand (&), include the entity name, and end with a semicolon (;). Case is important, so if you’ve defined an entity in uppercase, you must also use uppercase when you place the entity notation in the document. To place the previously defined entity in your document, use the following code where you want the entity value to display within the content: </li></ul><ul><li>Kirk Consulting is located in beautiful downtown &AK; </li></ul><ul><li>This would be processed through the XML processor and displayed within the XML application accordingly: </li></ul><ul><li>Kirk Consulting is located in beautiful downtown Anchorage, Alaska. </li></ul>
  89. 89. DTD Limitations <ul><li>They allow us to validate our content without application specific code, allow us to supply default values for attributes, and even create modular XML documents. Throughout your XML career, you will use existing DTDs and often design your own. Because of XML's strong SGML foundation, much of the early XML development focused on the markup of technical documents. Since that time, XML has been used in areas no one ever expected. While this was a great achievement for the XML community, it began to reveal some limitations of DTDs. </li></ul><ul><li>Some limitations of DTDs include: </li></ul><ul><li>DTD syntax is different from XML syntax </li></ul><ul><li>Poor support for XML namespaces </li></ul><ul><li>Poor data typing </li></ul><ul><li>Limited content model descriptions </li></ul>
  90. 90. DTD Syntax <ul><li>The DTD syntax is different from the generic XML syntax we learned in the first few chapters. Why is the syntax so different? Early on, we learned that XML is based on SGML. Because many of the developers turning to XML used SGML, the creators of XML chose to adopt the DTD syntax that was originally developed for SGML. </li></ul><ul><li>This proved to be both a benefit and a limitation within XML. Initially this made migration from SGML to XML a snap. Many users had already developed DTDs for their SGML documents. Instead of having to completely redesign their vocabularies, they could reuse what they had already done with minimal changes. As support for XML grew, new XML tools and standards were developed that allowed users to manipulate their XML data. Unfortunately, these tools were meant for generic XML, not for DTDs. </li></ul>
  91. 91. XML Namespaces <ul><li>Whenever element or attribute names are declared the namespace prefix and colon must be included in the declaration. In addition, DTDs must treat namespace declarations as attributes. This is because the completion of the XML Recommendation occurred before the syntax for XML namespaces was finalized. Forcing users to declare namespace prefixes in advance defeats the purpose of namespaces altogether. Merging documents from multiple namespaces when the prefixes are predefined can be hazardous as well. </li></ul>
  92. 92. Data Typing <ul><li>As XML developers began using DTDs to model more complex data (such as databases and programming objects) the need for stronger data types emerged. Within DTDs the only available data types are limited to use in attribute declarations, and even then provide only a fraction of the needed functionality. There is no method for constraining the data within a text-only element to a specific type. For example, if you were modeling a database and needed to specify that data within a specific element needed to be numeric, you couldn't use DTDs. </li></ul>
  93. 93. Limited Content Model Descriptions <ul><li>In addition to needing more advanced data types, limitations in content model descriptions soon became apparent. Developers wanted the capability to mimic object inheritance in their XML content models. Developers also found the cardinality operators limiting. DTDs lack strict control on the number of times an element occurs. </li></ul>
  94. 94. <ul><li><!ATTLIST image format NOTATION (jpeg | gif | bmp) #REQUIRED> In the example above, we have specified that the format attribute must appear within the <image> element in the document. If our parser encounters an <image> element without a format attribute as it is processing the document it will raise an error. </li></ul><ul><li>In most cases the attribute you are declaring won't be required, and often won't even have a default or fixed value. In these circumstances, the attribute may or may not occur within the element. These attributes are called implied attributes , as there is sometimes no explicit value available. When the attributes do occur within the element, a validating parser will simply check that the value specified within the XML document follows the rules for the declared attribute type </li></ul>
  95. 95. INCLUDE and IGNORE <ul><li>The CDATA marked section is a special case of the more general marked section construct provided by SGML </li></ul><ul><li>Within the body of a DTD (but not within the body of a document), two other kinds of marked section are possible: an IGNORE marked section, and an INCLUDE marked section. As the names suggest, material within an IGNORE marked section is ignored during processing, while material within an INCLUDE marked section is included. These can be used to include and exclude portions of a DTD selectively, so as to adjust it to relevant circumstances. </li></ul>
  96. 96. <ul><li>Suppose, for example, that we want to allow for poems which contain either only stanzas, or only couplets. A content model to enforce this rule is easy to define, but it does require us to to declare both possibilities — we must provide declarations for both <stanza> and <line> elements, even though in a given document we know that only one will appear. An alternative approach might be to provide two different declarations for <poem> , as follows: </li></ul><ul><li><![INCLUDE[ <!ELEMENT poem (stanza+)> <!ELEMENT stanza (line+)> ]]> </li></ul><ul><li><![IGNORE[ <!ELEMENT poem (couplet+)> <!ELEMENT couplet (line,line)> ]]> </li></ul><ul><li>The first declaration here will be the one used, since it is within an INCLUDE marked section. The second one will be ignored. </li></ul>
  97. 97. XML Schemas <ul><li>In 1999, the W3C began to develop XML Schemas in response to the growing need for a more advanced format for describing XML documents. Already, work had begun on several efforts that were intended to better model the types of document that were being created by XML developers. The W3C's effort took the best of these early technologies and then added additional features that could be used to fully describe any well-formed XML document </li></ul><ul><li>So, what is a schema? A schema is any type of model document that defines the structure of something. In this case, our &quot;something&quot; is an XML document. Many of you may have dealt with database schemas–the documents that are used to define the structure of the database tables and fields. In fact, DTDs are forms of schemas. Throughout this book, we have been using the term &quot;vocabulary&quot; where we could have used the word &quot;schema&quot;. So, what is an XML Schema? This is where it gets confusing. The term XML Schema is used to refer to the specific W3C XML Schema technology. W3C XML Schemas, much like DTDs allow you to describe the structure for an XML document. Properly, when referring to this technology, you should capitalize the &quot;S&quot; in &quot;Schema&quot;. XML Schema definitions are also commonly referred to as XSD. </li></ul>
  98. 98. Benefits of XML Schemas <ul><li>At this point, you already have invested time in learning DTDs. You know the syntax and can create complex, even modular, definitions for your vocabulary. Although XML Schemas are the next great thing, it may be helpful for you to understand some of the benefits of XML Schemas before jumping in. </li></ul><ul><li>XML Schemas are created using XML, not an alternative SGML syntax </li></ul><ul><li>XML Schemas fully support the Namespace Recommendation </li></ul><ul><li>XML Schemas allow you to validate text element content based on built-in and user-defined data types. </li></ul><ul><li>XML Schemas allow you to more easily create complex and reusable content models </li></ul><ul><li>XML Schemas allow you to model concepts such as object inheritance and type substitution </li></ul><ul><li>Let's look at some of these in more detail. </li></ul>
  99. 99. <ul><li>XML Schemas Use XML Syntax </li></ul><ul><li>The syntax, as we learned, doesn't follow the basic rules for XML well-formedness. Instead, you must declare your DTDs using a separate syntax. When defining an XML Schema, the syntax is entirely in XML; although you still have to learn the rules for which elements and attributes are required in given declarations, you can use XML tools that have no understanding of the rules to process XML Schema documents. As we learn new XML technologies through this book, we will see how to apply them to any XML document. Unfortunately, powerful tools, such as XSLT, often cannot be used on DTDs. </li></ul><ul><li>XML Schema Namespace Support </li></ul><ul><li>Because XML Schemas were finalized after the Namespace Recommendation, the concept of namespaces was available to be included in the design. Unlike DTDs, which do not support the full functionality of namespaces, XML Schemas enable you to define vocabularies that utilize namespace declarations. More importantly, XML Schemas allow you to mix namespaces in XML documents with less rigidity. For example, when designing an XML Schema, it is not necessary to specify namespace prefixes, as you must in DTDs. Instead, the XML Schema leaves that decision to the end-user. </li></ul>
  100. 100. <ul><li>XML Schema Datatypes </li></ul><ul><li>When we were developing our DTDs, we could specify that an element had mixed element, or empty content. Unfortunately, when our elements contained only text we could not exercise any constraints on the type of data that was allowed. Attribute declarations gave us more control, but even then, our types were very limited. </li></ul><ul><li>Important : XML Schema divides data types into two broad categories: simple and complex. Elements that may contain attributes or other elements are complexTypes . Attributes values and elements that contain only text content are simpleTypes . </li></ul><ul><li>XML Schemas allow you to specify the type of textual data allowed within attributes and elements using simpleType declarations. By utilizing these types, for example, you could specify that an element may contain only dates, or only positive numbers, or numbers within a certain range. Many commonly used simpleTypes are built into XML Schemas. This is, perhaps, the single most important feature within XML Schemas. By allowing you to specify allowable type of data within an element, you can more rigidly control documents that are intended to represent databases, programming languages, and objects within programming languages. </li></ul>
  101. 101. <ul><li>XML Schema Content Models </li></ul><ul><li>In order to reuse a content model within a DTD we had to utilize parameter entities. As we saw, this can lead to situations that make it difficult to reuse parts of the DTD. XML Schemas provide several mechanisms for reusing content models. In addition to the simple models we created in DTDs, XML Schemas can model complex programming concepts like type inheritance and element substitution. The advanced features of XML Schemas allow you to build content models upon content models, modifying the definition in each step </li></ul><ul><li>Basic XML Schemas </li></ul><ul><li>As we progress through this chapter, you should begin to see more benefits of XML Schemas, which provide many features that were never possible using DTDs </li></ul><ul><li>The XML Schema Document </li></ul><ul><li>Although most XML Schemas are stored within a separate XML document, the XML Schema Recommendation takes great pains to ensure that the XML Schema can be defined using alternate methods as well. In this respect, XML Schemas function very similarly to external DTDs, an XML document contains a reference to the XML Schema that defines its vocabulary. An XML document that adheres to a particular XML Schema is an XML Schema instance document. </li></ul>
  102. 102. Try It Out–What's in a Name? <ul><li>In this example, we will create an XML Schema that defines our name vocabulary. We will also see how to refer to the XML Schema from the instance document. At the end of this example, we will break down each step to see how it works. </li></ul><ul><li>WHATS N A NAME.doc </li></ul>
  103. 103. How It Works <ul><li>In this Try It Out example, we created an XML Schema for our name vocabulary. We used the XML Schema to check if our instance document was schema valid. In order to connect the two documents we included a reference to the XML Schema within our instance document. The internal process by which schema validators compare the document structure against the vocabulary varies greatly. At the most basic level