XML easy learning general section

Foreword

XML is becoming more and more popular, and basic tutorials on XML can be found everywhere on the Internet. However, a lot of concepts and terms are often daunting. Many friends asked me: What is the use of XML? Do we need to learn it? I want to write a more comprehensive introduction article based on my personal learning process and experience. First of all, there are two points that need to be affirmed:

First: XML is definitely the future development trend. Whether you are a web designer or a network programmer, you should learn and understand it in time. Waiting will only make you lose opportunities;

Second Two: There will definitely be many new concepts in new knowledge. Only by trying to understand and accept them can you improve. Don't be afraid and run away, after all we are still young.

Outline

This article is divided into five parts. They are XML quick introduction, XML concepts, XML terminology, XML implementation, and XML example analysis. The final appendix introduces XML-related resources. The author stands from the perspective of ordinary web designers and uses plain and vivid language to tell you all aspects of XML, helping you to lift the mystery of XML and quickly enter the new field of XML.

Chapter 1: Quick Start with XML

1. What is XML?

2. Is XML a new concept?

3. What are the benefits of using XML?

4. Is XML difficult to learn?

5. The difference between XML and HTML

6. The strict format of XML

7. More about XML

1. What is XML?


This is often the first question, and you often don’t understand it on the first question, because most textbooks answer this:

XML is the abbreviation of Extensible Markup Language, an extensible markup language. . This is the standard definition. So what is a markup language and why is it called extensibility? It's already a bit confusing. I think it would be better to understand it this way:

You are already very familiar with HTML. It is a markup language. Do you remember its full name: "Hypertext Markup Language". Understood? At the same time, there are many tags in HTML, such as, etc., which are standardized and defined in HTML
4.0, and XML allows you to create such tags yourself, so it is called extensibility.

Here are a few confusing concepts to remind everyone:

1.XML is not a markup language. It is just a metalanguage used to create markup languages ​​(such as HTML). God, I'm confused again! It doesn't matter, you just need to know this: XML and HTML are different, and its uses are much wider than HTML, which we will introduce in detail later.

2.XML is not a replacement for HTML. XML is not an upgrade of HTML, it is just a supplement to HTML, extending more functions to HTML. We will continue to use HTML for a long time to come. (But it is worth noting that XHTML, the upgraded version of HTML, is indeed moving closer to adapting to XML.)

3. You cannot use XML to write web pages directly. Even if it contains XML data, it still needs to be converted into HTML format before it can be displayed on the browser.

The following is an XML sample document (Example 1), used to represent the information of this article:







ajie



ajie@aolhoo.com



< ;date>20010115






Note:


1. This code is just code to give you a preliminary understanding of XML, and it cannot What specific applications can be achieved;

2. Statements like < title>, < author> are self-created tags. They are different from HTML tags. For example, < title> here means the title of the article. , < title> in HTML is the page title.

2. Is XML a new concept?

No. XML is derived from SGML, a markup language standard that is earlier than HTML.

Let’s take a brief look at SGML. You only need to have a general idea.

The full name of SGML is "Standard Generalized Markup Language". You can tell just by looking at the name: it is a standard for markup languages, which means that all markup languages ​​are developed in accordance with SGML, including HTML, of course. SGML has a wide coverage. All files with a certain format belong to SGML, such as reports, music scores, etc. HTML is the most common file format of SGML on the Internet. Therefore, people jokingly call SGML the "mother" of HTML.


XML is a simplified version of SGML, except that the complex and uncommon parts are omitted. (Oh, I understand! It is the second "mother" of HTML. No wonder it is more powerful than HTML.) Like SGML, XML can also be applied in various fields such as finance and scientific research. What we are talking about here is just the application of XML in the web. Just the application.


At this point, you should have a little understanding: XML is used to create and define a markup language similar to HTML, and then use this markup language to display information. 3. What are the benefits of using XML?

With HTML, why do we need to use XML?

Because network applications are becoming more and more widespread, it is no longer enough to rely on a single file type of HTML to handle ever-changing documents and data. Moreover, the grammar of HTML itself is very loose, which seriously affects the transmission and sharing of network information. (Think about how many designers’ brain cells have been damaged by browser compatibility issues.) People have already begun to explore ways to meet the needs of various applications on the Internet. It is possible to use SGML, but SGML is too large and complicated to program, so I finally chose the "weight-loss" SGML---XML as the data transmission and interaction tool for next-generation web applications.


What are the benefits of using XML? Let’s look at the description from the w3c organization (XML standard setter):

XML makes using the SGML language on the Internet more "simple and direct": It simplifies the process of defining file types, simplifies the process of programming and processing SGML files, and simplifies the process of defining file types. Delivery and sharing on the Web.


1.XML can be widely used anywhere on the web;

2.XML can meet the needs of network applications;

3. Using XML will make programming easier;

4.XML is easy to learn and create;

5.XML code will be clear and easy to read and understand;

It is still a bit abstract. Let us slowly experience the powerful advantages of XML in the following example tutorials!

4. Is XML difficult to learn?

If you are interested in learning XML, you can’t help but ask: Is XML difficult? What kind of foundation is needed to learn XML?

XML is very simple and easy to learn. If you are familiar with HTML, you will find that its documents are very similar to HTML, look at the same sample document (Example 1):

?xml version="1.0"?>



< myfile>







ajie



ajie@aolhoo.com



20010115





The first line is an XML declaration, indicating that the document follows the XML version 1.0 specification.

The second line defines the first element (element) in the document, also called the root element: < myfile>. This is similar to the < HTML> opening tag in HTML. Note that this name is defined arbitrarily by yourself.

Four sub-elements are defined below: title, author, email, and date. Indicate the title, author, email address and date of the article respectively. Of course, you can define these tags in Chinese, which makes them easier to understand:

ajie

ajie@aolhoo.com

20010115

This is an XML document. Any netizen who knows HTML can directly write such a simple XML document.

In addition, to learn XML, you must also master a page scripting language, the most common ones are javascript and VB script. Because XML data uses script to implement calling and interaction in HTML. Let’s look at the simplest example (Example 2):

1. Save the following code as myfile.htm












< body bgcolor="#FFFFFF">

Title:



Author: >



Email:


Date:










2. Save the following code as myfile.xml








ajie

ajie@aolhoo .com

20010115




3. Put them in the same directory, open them with IE5 or above browser, you can see the effect . Learn and master a script, and you will truly understand the extremely powerful functions of XML.

5. The difference between XML and HTML


Both XML and HTML come from SGML. They both contain tags and have similar syntax. The biggest difference between HTML and XML is that HTML is a stereotyped markup language that uses inherent tags. Markup to describe and display the content of the web page. For example, < H1> represents the first line of title and has a fixed size. In contrast, XML has no fixed tags. XML cannot describe the specific appearance and content of a web page. It only describes the data form and structure of the content.


This is a qualitative difference: web pages mix data and display, while XML separates data and display.


Let’s look at the example above. In myfile.htm, we only care about the display mode of the page. We can design different interfaces and layout the page in different ways, but the data is stored in myfile.xml, no need any changes.


(If you are a programmer, you will be surprised to find that this is very similar to the idea of ​​modular object-oriented programming! In fact, isn’t a web page a program?)


It is this difference that makes XML stand out on the Internet Application and information sharing are convenient, efficient and scalable. Therefore, we believe that XML, as an advanced data processing method, will bring the network to a new realm.


6. The strict format of XML


Drawing on the lessons learned from the loose format of HTML, XML has insisted on implementing "good format" from the beginning.

Let’s first look at some statements in HTML, which can be found everywhere in HTML:

1.

sample


2.
3.< td>sample< /TD>


4.< font color=red>samplar< /font>


In the XML document, the syntax of the above statements is wrong. Because:

1. All tags must have a corresponding closing tag;

2. All XML tags must be nested reasonably;

3. All XML tags are case-sensitive;

4. All The attributes of the tag must be enclosed in "";

So the correct way to write the above statement in XML is

1.

sample


2.< b>< i>sample< /i>< /b>


3.< td>sample< /td>


4.< font color="red">samplar< /font>


　 In addition, XML tags must follow the following naming rules :

1. The name can contain letters, numbers and other letters;

2. The name cannot start with numbers or "_" (underscore);

3. The name cannot start with the letters xml (or XML or Xml..) Beginning;

4. The name cannot contain spaces.

Any error in the XML document will result in the same result: the web page cannot be displayed. Browser developers have reached an agreement to implement strict and picky parsing of XML, and any small errors will be reported. You can modify the myfile.xml above, for example, change < email> to < Email>, and then open myfile.xml directly with IE5, you will get an error message page:








ajie

ajie@ aolhoo.com

20010115



7. More about XML

Okay, by now you already know:

1. What It is XML;

2. The relationship and difference between XML, HTML, and SGML;

3. Simple application of XML.

The above is the content of the general chapter on easy learning of XML. For more related content, please pay attention to the PHP Chinese website (www.php.cn)!