Today we will begin to consider a very popular and convenient XML markup language. Because this format Data representation is very flexible and universal, and it can be used almost anywhere, meaning in connection with something. Therefore, a novice programmer will sooner or later have to deal with this language, and it doesn’t matter what exactly you do, be it web programming or database administration, because everyone uses XML, and you will also use it to implement the tasks you need.
We will start, as usual, with theory, let's look at what kind of language it is, why it is good, how to use it and where it is used.
XML Language Definition
XML (eXtensible Markup Language) is a universal and extensible data markup language that is independent of the operating system and processing environment. Xml is used to present certain data in the form of a structure, and you can develop this structure yourself or customize it for a particular program or service. That's why given language They call it expandable, and this is its main advantage, for which it is so valued.
As you know, there are quite a lot of markup languages, for example, the HTML language, but all of them, one way or another, depend on the processor, for example, the same html, the code of which the browser parses, is standardized and not extensible, there are clear tags there, syntax that cannot be violated, and in xml you can create your own tags, i.e. your markup. The main difference between HTML and XML is that html just describes the markup for displaying data, and xml is an abstract data structure that can be processed and displayed as you wish, anywhere, and therefore there is no need to compare these languages, they have completely different purposes.
As noted above, xml is a very common and universal language, through which almost all applications, both web and just for the computer, use it as an exchange of information, since with the help of it you can very easily exchange data between applications or services that are even written in different languages. In this connection, every novice programmer who is involved in absolutely any programming should have an understanding of XML. If you want to become a web master, then you simply must know XML, and we have already discussed how to become a WEB Master and what you need to know for this.
For example, I once had a task to write a certain service that should return data in xml form upon request, i.e. a kind of development of the server part of the application, and I had no idea what the client that would process this data was written in, and that I wrote a service that returned the data in xml form and that’s it, the application worked perfectly. And this is just an example that I had to deal with, but now imagine how many different organizations collaborate and conscientiously develop software and exchange data, and I would not be surprised that this data will be in xml form.
For example, I once had a task to write a certain service that should return data in xml form upon request, i.e. a kind of development of the server part of the application, and I had no idea what the client that would process this data was written in, and that I wrote a service that returned the data in xml form and that’s it, the application worked perfectly. And this is just an example that I had to deal with, but now imagine how many different organizations collaborate and conscientiously develop software and exchange data, and I would not be surprised that this data will be in xml form.
Also, I once had to store xml data in a MS SQL 2008 database in order to better represent this data and exchange it between the server and the client part of the application, we discussed this in the article - Transact-sql - working with xml.
The XML language itself is very simple, and it is simply impossible to get confused in it; all the complexity arises precisely in the processing and interaction of XML with other applications, technologies, i.e. everything that surrounds xml, which is where you can easily get confused.
Today we are talking only about the basics of XML, and we will not focus on technologies for processing and interacting with this language, since this is true, very voluminous material, but I think in the future we will continue to get acquainted with related technologies.
Let's move on to practice. And I will write all the examples that we will consider in Notepad++ only because it is very convenient, but we will not talk about this now, since we have already discussed this in the article - Why Notepad++ is good for a novice developer.
XML tags
XML language uses tags ( tags are case sensitive), but not the same tags as in html, but those that you come up with yourself, but the xml document also has a clear structure, i.e. there is an opening tag and a closing tag, there are nested tags and, of course, there are values that are located in these tags. In other words, everything you need for basic knowledge xml is just to follow these rules. Together, the opening, closing tag and value are called an element, and the entire xml document consists of elements that together form a data structure. An xml document can only have one root element, remember this, because if you write two root elements, it will be an error.
And it’s time to give an example of xml markup, and the first example for now is for syntax:
<Начало элемента>
<Начало вложенного элемента>Nested element value
As you can see, everything is quite simple, and there can be a lot of such elements nested within each other.
Now let's give an example of a real xml document:
As you can see, I just gave an example of a kind of book catalog here, but I did not declare this document, i.e. I didn’t write an XML declaration that tells the application that will process this data that the xml data is located here and in what encoding it is presented. You can also write comments and attributes, so let's give an example of such a document:
Book 1IvanJust book 1Book 2SergeyJust book 2Book 3NovelJust book 3
Where the first line is the declaration that this is an XML document and must be read in UTF-8 encoding.
This data without processing will look, for example, in a browser (Mozilla Firefox) as follows:
I hope you understand that here catalog is the root element, which consists of the book elements, which in turn consists of the name, author and comment elements, and for the example, I also set several attributes for the catalog element and the book element.
For the basics, I think that's enough, because if we dive deeper and deeper into XML, and into all the technologies that are associated with this language, then this article will never end. So that's all for today. Bye!
XML (Extensible Markup Language) was developed by the XML Working Group of the World Wide Web Consortium (W3C). Here's how its creators describe it:
“Extensible Markup Language (XML) is a component of SGML... It is designed to make SGML easier to use on the Web and perform tasks that are currently accomplished using HTML. XML is designed to improve the use and interoperability of SGML and HTML."
This is an excerpt from the XML specification version 1.0, created by the XML Working Group in February 1998. The entire document can be found on the W3C website at http://www.w3.org/TR/REC/-xml.
XML is a markup language designed specifically for placing information on the World Wide Web, similar to hypertext language HTML markup(Hypertext Markup Language), which initially became the standard language for creating Web pages. Since HTML completely satisfies all our needs, the question arises: why was a completely new language needed for the Web? What are its advantages and advantages? How does it interact with HTML? Will it replace HTML, or just improve upon it? Finally, what is SGML, of which XML is a part, and why can't SGML itself be used for Web pages? In this chapter I will try to answer all these questions.
Purpose of XML
The HTML language provides a fixed set of elements that you can use to place components on a typical Web page. Examples of such elements include headings, paragraphs, lists, tables, images, and links. For example, HTML is great for
creating a personal home page. Below is the description of the home page in HTML codes:
Home Page
Michael Young's Home Page
Welcome to my Web site!
Web Site Contents
Please choose one of the following topics:
Writing
Family
Photo Gallery
Other Interesting Web Sites
Click one of the following to explore another Web site:
"1. XML should become the language of direct use on the Internet."
As you might have guessed, XML was designed primarily for storing and distributing information on the Web.
"2. XML will support a large number of applications."
Although its primary purpose is to distribute information on the Web through servers and browser programs, XML is also designed to be used by other programs. For example, XML is used to exchange information between financial programs, to distribute and update software products, and to write voice scripts when delivering information over the phone.
"3.XML will be compatible with SGML."
XML is a specialized branch of SGML. The advantage here is ease of adaptation software SGML for working with HTML.
"4. It will be easier to write programs that process XML documents."
To make practical use of XML, it is necessary that it be fairly easy to write browsers and other programs that process XML documents. In fact, the main reason for separating XML from SGML was the ease of writing programs to process XML documents.
The minimal number of additional functions in XML makes it easy to write programs to process XML documents. The abundance of additional plug-in functions in SGML was the main reason that determined its practical unsuitability for representing Web documents. Additional SGML features require overriding delimiter characters for tags (usually ) and skipping the end tag so that the processor detects the end of the element. When strictly writing an SGML document processing program, it is necessary to take into account the possibility of all additional functions, even if they are rare.
"6. XML documents should be clear and understandable to the user."
XML is intended to become the lingua franca (universal language) for exchanging information among users and programs around the world. According to this concept, users, as well as specialized programs, should be able to create and read XML documents. Accessibility and transparency for the user distinguish XML from most other formats used in the construction of databases and text documents.
The user can easily read the XML document because it is described in plain text and has a logical hierarchical tree structure. You can simplify XML documents by assigning meaningful names to elements, attributes, and objects, and by adding useful comments. (This will be discussed later in this chapter.)
"7. XML development should be completed fairly quickly."
XML will only become a widely accepted standard if programmers and users accept it. This standard must be created before society accepts the alternative standards that are increasingly being created by software companies.
"8. XML should be formal and concise."
The XML specification is written in a formal language used to represent computer languages, with a notation known as Extended Backus-Naur Form (EBNF). This formal language, although quite difficult to understand, is devoid of ambiguity and greatly facilitates the writing of XML documents, and especially programs for processing them.
"9. XML documents will be easier to create."
The practical use of XML as a markup language for Web documents simplifies not only the writing of processing programs, but also the process of creating the XML documents themselves.
"10. The compressed form is not important in XML markup."
In accordance with point 6 (the XML document must be clear and understandable to the user), the XML markup should not be overly compressed so as not to conflict with the specified purpose.
Standard XML Applications
You can use XML for more than just describing a single document. An individual, company, or standards committee can define the required set of XML elements and document structure to be used for a particular class of documents. Such a set of elements and a description of the document structure is called an XML application or an XML dictionary.
For example, an organization might define an XML application to create documents describing molecular structures, human resources, multimedia presentations, or containing vector graphics. At the end of the chapter is a list of some common XML applications that have already been created and applications that are planned to be created.
An XML application is typically defined by the creation of a document type descriptor (DTD), which is a valid component of the XML document. A DTD is built on a database schema: it establishes and defines the names of elements that can be used in a document, the order in which elements can appear, the attributes of elements that can be used, and other features of the document. To actually use an XML application, you typically include its DTD in your XML document; Having a DTD in a document limits the elements and structures you will use to ensure that your document meets the standards of that application. The XML document definitions discussed earlier in this chapter did not include DTDs. You'll learn how to define and use DTDs in Chapter 5.
The benefits of using standard XML applications when developing your documents are that you can share the documents with all other users of the application, and the document can be processed and displayed using software that has already been built for the application.
XML applications that improve the quality of XML documents
In addition to XML applications for describing specific document classes, there are several XML applications that you can use within any type of XML document. These applications make document creation easier and improve its quality. Below are examples of such applications.
Extensible Stylesheet Language (XSL) allows you to create powerful stylesheets using XML syntax.
XML Schema allows you to develop detailed schemas for your XML documents using standard XML syntax, a more powerful alternative to using DTDs.
XML Linking Language (XLink) gives you the ability to link your XML documents. It supports multiple destination links and other useful features, providing greater freedom than HTML's linking mechanism.
XML Pointer Language (XPointer) allows you to define flexible target links. When XPointer and XLink are used together, you can link to anywhere in the target document - not just jumps to specific points.
XLS will be covered in Chapter 10. Other XML applications are not yet mature and are not covered in this book. (XLink and XPointer are not supported in Internet Explorer 5).
As you can see, XML is not only a useful tool for describing documents, but it also serves as the basis for building applications and extensions that may be in demand as the Internet evolves.
Real Use of XML
Although the concept of XML is quite interesting, you may be wondering how to put it into practice. This section provides a list of examples of such applications of XML, both already widely used and those in the future. If there are corresponding XML applications for practical use, they will be given in parentheses. For example, you will find that the MathML XML application will allow you to format mathematical formulas.
Link. A more complete list of current and upcoming XML applications, including detailed descriptions, can be found on the Oasis SGML/XML Web page (http://www.oasis-open.arg/cover/ocml. htmW applications).
Working with databases. Like traditional databases, XML can be used to assign a label to each field of information within each database record. (For example, you can tag each name, address, and phone number within your address list entries.) You can then display the data in various ways and organize search, sorting, filtering and other processing of data.
Structuring documents. The hierarchical structure of XML documents is ideal for marking up the structure of documents such as novels, scientific papers, and plays. For example, you can use XML to mark up a play into acts, scenes, characters, plot lines, scenery, etc. XML markup allows programs to display or print the document in the required format; find, extract, or manipulate information in a document; generate tables of contents, summaries and annotations; process information in other ways.
Multimedia presentations (SMIL - Synchronized Multimedia Integration Language, HTML + TIME - HTML Timed Interactive Multimedia Extensions).
Description of channels. Channels are Web pages that are automatically sent to subscribers. (CDF - Channel Definition Format).
Description of software packages and their relationships. Such descriptions ensure the distribution and updating of software products on the network (OSD - Open Software Description).
Application communication over the Web using XML-co-communications. These messages are independent of operating systems, object models and computer languages (SOAP - Simple Object Access protocol).
Sending electronic business cards via e-mail.
Exchange of financial information. Information is exchanged in an open and understandable format between financial programs (such as Quicken and Microsoft Money) and financial institutions (banks, public funds) (OFX - Open Financial Exchange).
Creation, management and use of complex digital forms for commercial Internet transactions. Such forms may include digitized signatures that make them legally recognized (XFDL - Extensible Forms Description Language).
Exchange of job requests and resumes (HRMML - Human Resource Management Markup Language).
Description of molecular structures (CML - Chemical Markup Language).
Encoding and displaying information about DNA, RNA and chains (BSML - Bioinformatic Sequence Markup Language).
Coding of genealogical data (GeDML - Genealogical Data Markup Language).
Astronomical data exchange (AML - Astronomical Markup Language).
Creation of musical scores (MusicML -Music Markup Language).
Working with voice scripts to deliver information over the phone. Voice scripts can be used, for example, to generate voice messages, product availability certificates and weather forecasts (VoxML).
Processing and delivery of information by courier services. Federal Express, for example, already uses XML for this purpose.
Presentation of advertising in the press in digital format (AdMarkup).
Filling out legal documents and electronic exchange of legal information (XCL - XML Court Interface).
Coding of weather forecasts (OMF - Weather Observation Markup Format).
Exchange of information on real estate transactions (RETS - Real Estate Transaction Standard).
Exchange of insurance information.
Exchange news and information using open Web standards (XMLNews).
Presentation of religious information and markup of liturgical texts (ThML - Theological Markup Language, LitML - Liturgical Markup Language).
When using HTML, the data displayed is not logically related to the tags that are used for markup, so parsing programs cannot use the tags to find the desired fragments of the document. In addition, the limited set of HTML tags makes it difficult to use for displaying specialized information (for example, mathematical formulas).
To replace HTML, the hypertext markup language XML (Extensible Markup Language) was proposed. This language can be used to describe the grammar of other languages and to control the correctness of documents. XML defines how markup tags are created and makes them easy to extend. This makes it possible to define your own tags to identify the data contained in the document. When creating a document structure, connections between elements and the markup necessary to perform the operations of viewing, searching, and analyzing a document are built. XML can be used as a general-purpose query language for information stores. Thus, XML documents can act as a unique way of storing data, including means for parsing information and presenting it.
The XML language has attracted a lot of attention from developers and Internet users. New languages created on the basis of XML are appearing, and Web servers are appearing that use this technology to organize the information stored on them. Full specifications of XML and related languages are available on the official page W3C
- www.w3.org
The birth year of XML can be considered 1998, when the language specification was approved. It all started with the appearance of the SGML language in 1986. Although the concept of hypertext was introduced by W. Bush back in 1945 and, starting from the 60s, applications using hypertext data began to appear, a surge of activity around this technology began when there was a real need for a mechanism for combining multiple information resources, providing the ability to creating, viewing text. Using SGML, you can describe structured data, organize information contained in documents, and present this information in some standardized format. The SGML language is intended for creating other markup languages; it defines a valid set of tags, their attributes and the internal structure of the document. Control over the correct use of descriptors is carried out using a special set of rules called DTD descriptions, which are used by the client program when parsing the document. For each class of documents, its own set of rules is defined that describe the grammar of the corresponding markup language. But due to its complexity, SGML has been used mainly to describe the syntax of other languages, the most famous of which is HTML.
HTML is a simplified version of the SGML markup language. Much simpler and more convenient than SGML, the HTML language allows you to determine the design of document elements and has a certain limited set of instructions - tags, with the help of which the markup process is carried out. HTML instructions are primarily intended to control the process of displaying the contents of a document on the screen of a client program and thereby determine the way the document is presented, but not its structure. As an element of a hypertext database described by HTML, it is used text file, which can be easily transmitted over the network using the HTTP protocol. This feature, as well as the fact that HTML is an open standard and a huge number of users have the opportunity to use the capabilities of this language to design their documents, certainly influenced the growth of the popularity of HTML and made it today the main mechanism for presenting information on the Web
The use of hypertext breakdown of a text document in modern information systems is largely due to the fact that hypertext allows you to create a mechanism for nonlinear viewing of information. In such systems, data is presented not as a continuous stream of textual information, but as a set of interconnected components, which are navigated using hyperlinks.
The most popular hypertext markup language today, HTML, was created specifically for organizing information distributed on the Internet, and is one of the key components of WWW technology. With the use of the hypertext document model, the way of presenting various information resources on the network has become more orderly, and users have received a convenient mechanism for searching and viewing the necessary information.
However, modern applications require not only a language for presenting data on the client screen, but also a mechanism that allows you to determine the structure of the document and describe the elements it contains. HTML has a simple set of commands and successfully copes with the task of describing hypertext information displayed on the screen by a browser. However, the displayed data itself is not associated with the tags that are used for formatting, so parsing programs do not have the ability to use HTML tags to find the document fragments we need. Those. having encountered, for example, such a description
rose,
the viewer will know what color to display the text contained within the tags and, but it is absolutely indifferent to where in the document this tag was found, what other tags the current fragment is enclosed in, whether fragments nested in it exist, whether the relationships between objects are built correctly. This “indifference” to the structure of a document leads to the fact that searching or analyzing information inside it will be no different from working with a continuous text file that is not broken into elements. And this, as you know, is not the most effective way to work with information.
Another significant disadvantage of HTML is the limited set of its tags. DTD rules for HTML define a fixed set of descriptors and therefore the developer does not have the opportunity to enter his own, special tags.
To summarize all that has been said, it can be argued that HTML does not fully satisfy the requirements that developers place on languages of this kind. HTML complements the hypertext markup language XML, a powerful, flexible, and user-friendly language. What are its advantages?
What is XML?
The term XML refers to a special type of hypertext document. What's wrong with regular text documents and what's special about XML documents?
In any text document you can write anything and any way you want, for example, you can put one space between some words, and two spaces between others. You can highlight a paragraph with a red line of three spaces, and sometimes leave a blank line between adjacent paragraphs. You can write the title of the document in the first line, or you can start writing the document with the author’s last name. But if the document needs to be published, then it will have to be redone - paragraphs should be formatted in a uniform way, the title should be highlighted from the general narrative as a blank line, the author’s surname and a brief summary should be indicated between the title and the first paragraph, etc. In a word, there will be a need to prepare the document in some standard way for this publication. And if you need to publish this document in another place, you will need to change and redo something again. It turns out there is a way out - to help will come XML!
In fact, XML is a way of marking up documents, designed to form some kind of structure in documents and define the relationships between the various elements of this structure. To create such markup, special instructions called tags are used. Tags are placed between characters< и >. Thanks to the presence of tags, unified automatic processing and formatting of XML documents becomes possible. Thanks to XML, it is also possible to control the correctness of data stored in documents, as well as establish a uniform standard for the structure of documents that may contain arbitrary data. XML is indispensable when building complex information systems, in which the issue of information exchange between different consumers is very important. Thanks to XML, you can get rid of many problems associated with incompatibility between different components.
Michael Young's Home Page