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Brief History of XML
百度 直到2006年，我又重新生起了这个愿望，终于在2008年初正式启动了编纂工作。XML, or Extensible Markup Language, came into existence in the mid-1990s as a universal standard for structured document markup. The World Wide Web Consortium (W3C), led by Jon Bosak, developed XML as a simplification of the Standard Generalized Markup Language (SGML). The XML 1.0 Specification was accepted as a recommendation by the W3C in February 1998, marking its official status as a web standard.
From the outset, XML aimed to solve the limitations of HTML, designed predominantly for document presentation rather than for data storage and interchange. Moreover, the free open nature of the XML standard and the existence of various schema systems and application programming interfaces (APIs) for XML-based languages contributed to its widespread adoption.
What is XML and its Purpose?
Broadly speaking, XML is a markup language designed for data storage and transport. While HTML is concerned with how data should be displayed, XML defines what the data is. It encodes data in a format that is human-readable and machine-readable, offering a reliable, extensible framework for describing complex documents and data structures.
Its primary purpose is to facilitate the sharing of structured data across different systems, especially those connected via the Internet. XML files are both platform-independent and language-independent, which means they can be processed by any modern language on any operating system. For instance, XML is commonly used in web services to format and exchange data, such as user information, product details, and transactions between the server and client applications.
XML Features
As a markup language and a file format, XML offers a multitude of features that make it versatile for encoding, storing, and transporting data:
Human and machine readability: XML data is stored in plain text format, facilitating interpretation by both humans and machines.
Self-descriptive: XML documents are self-descriptive. The tags in XML provide a general idea of the type of data they contain.
Extensible: XML, true to its name, permits users to define their own tags, thereby offering flexibility to represent a wide range of data structures.
Platform and language independence: XML data can be processed by any system that can process XML, making it an excellent choice for data interchange.
In a way, XML itself does not achieve anything. Instead, it serves as a means to structure and describe data. Developers then use this structured data in various applications for different purposes, such as data exchange, configuration files, message passing in distributed systems, and much more.
Basic XML Examples
When learning the ropes of any technology, it's helpful to see it in action to better understand its structure and principles. In XML, you have the freedom to define your tags according to your specific needs. This makes it versatile enough for various applications, yet also maintains a level of standardization through a defined syntax, ensuring interoperability.
An XML Document
The structure of an XML document revolves around elements, with each document beginning with an XML declaration. What we see below is not an element; the line serves to transmit meta-data about the document.
Note: In XML terminology, it's important to differentiate between "tags" and "elements". A "tag" specifically refers to the start tag or end tag of an element. An "element" is composed of a start tag, content (which can include other nested elements or plain text), and an end tag. While in casual usage, these terms are often used interchangeably, understanding the technical distinction is essential when working with more complex XML structures.
<?xml version="1.0" encoding="UTF-8"?>
We can notice the attributes in use here. version specifies the XML version used in the document, and encoding indicates the character encoding used which in this case is UTF-8.
Logically, XML documents have to meet certain conditions:
The document must be "well-formed" and adhere to all XML syntax rules.
The document must conform to a predefined structure, typically defined by an XML schema or a Document Type Definition (DTD). These blueprints, specify the acceptable structure and content for an XML document, thus enabling consistency and data validation.
Consider the following example:
<?xml version="1.0" encoding="UTF-8"?>
<book>
<title>
Learning XML
<!-- missing </title> -->
</book>
The example above isn't appropriate since it's missing a closing </title> tag. A corrected version would look like this:
<?xml version="1.0" encoding="UTF-8"?>
<book>
<title>
Learning XML
</title>
</book>
In our XML example, <book> and <title> are custom tags. While XML allows you to define your custom tags like these for structuring data, these tags must follow certain rules. They can't begin with a number or punctuation character, and they can't contain spaces.
It's also worth noting that some browsers, like Google Chrome, include developer tools that assist in XML development. These debuggers can identify and underline syntax errors in XML, aiding in the development and troubleshooting process.
While we've touched upon the basic structure of XML, there is more that comprises its syntax. You'll also encounter more complex features of XML, such as the use of entities for representing special characters, CDATA sections for encapsulating large text blocks, processing instructions for embedding code, namespaces for avoiding tag name conflicts, and more.
Final Thoughts
XML has significantly contributed to web development and data exchange, supporting functionalities ranging from RSS feeds to SOAP-based web services. While alternative technologies like JSON have become increasingly popular for data exchange in modern web APIs, thanks to their lighter weight and seamless integration with JavaScript, XML still has a place in some domains.
Specifically, XML is often found in configuration files, some legacy systems, certain industry-specific standards, and SOAP-based web services, where its robustness and complexity prove advantageous. While it may not be the first choice for modern projects and other specific scenarios, knowledge of XML can still be beneficial due to its presence in various established systems.
As we progress, we'll focus on its syntax rules, manipulation techniques, and validation processes. We'll also look at its interaction with different technologies and languages, discuss its relevance, and highlight scenarios where usage is still common, pointing out areas where alternatives might be more suitable.