What is an XML?

XML (eXtensible Markup Language) is a markup language designed to store and transport data in a structured and hierarchical format. It is both human-readable and machine-readable and has been widely used for data exchange between systems, especially in web services and configurations.

Key Features of XML

  1. Structured and Hierarchical:
    • XML organizes data in a tree-like structure, where elements can contain sub-elements (children), allowing complex data structures to be represented.
  2. Self-descriptive:
    • XML uses tags to define elements, similar to HTML, but unlike HTML, the tags are not predefined. You can define your own tags based on the data you’re representing.
    • The structure of the data and the data itself are both stored together, making it easier to understand the context of the information.
  3. Platform-independent:
    • XML is a platform-independent and language-independent format, making it widely used in data exchange between different systems and applications.
  4. Extensible:
    • There are no fixed tags in XML. You can create custom tags based on the needs of your application or data model.

XML Structure

An XML document consists of:

  • Elements: Represented by tags, which are the building blocks of XML. They start with a start tag (<tag>), may contain data or other elements, and end with an end tag (</tag>).
  • Attributes: Provide additional information about elements. Attributes are specified within the start tag.
  • Prolog: Optional and typically contains information like the XML version and character encoding.
  • Root Element: Every XML document must have a single root element that contains all other elements.

Example of an XML Document 

<?xml version="1.0" encoding="UTF-8"?>
<bookstore>
    <book id="1">
        <title>Harry Potter and the Philosopher's Stone</title>
        <author>J.K. Rowling</author>
        <price>29.99</price>
        <published>1997</published>
    </book>
    <book id="2">
        <title>The Lord of the Rings</title>
        <author>J.R.R. Tolkien</author>
        <price>39.99</price>
        <published>1954</published>
    </book>
</bookstore>

Explanation

  1. Prolog:
    • The first line <?xml version="1.0" encoding="UTF-8"?> is the XML declaration, specifying the XML version and the character encoding.
  2. Root Element:
    • <bookstore> is the root element of this XML document, and it contains all other elements.
  3. Elements and Tags:
    • Each <book> element represents a book and contains child elements like <title>, <author>, <price>, and <published>.
  4. Attributes:
    • The id attribute (e.g., id="1") provides additional information about each <book> element.

XML vs. HTML

  • Purpose:
    • XML is designed for transporting and storing data, while HTML is designed for displaying data in web browsers.
  • Tags:
    • In XML, tags are custom and self-defined, while in HTML, the tags are predefined and have specific meanings (e.g., <div>, <a>, <p>).
  • Strictness:
    • XML is strict about syntax (e.g., every opening tag must have a corresponding closing tag), while HTML is more lenient.

Common Uses of XML

  1. Data Exchange:
    • XML is widely used for exchanging data between different systems, applications, or services (e.g., REST or SOAP web services).
  2. Configuration Files:
    • Many applications use XML for storing configuration settings (e.g., .xml files in Java applications or Android’s AndroidManifest.xml og pom.xml).
  3. Document Formats:
    • XML is used as the base format for many document standards (e.g., Microsoft Office files like .docx and .xlsx are internally XML-based).
  4. APIs and Web Services:
    • XML is often used in SOAP web services, where data is exchanged between clients and servers in an XML format.

XML Schemas and Validation

  1. DTD (Document Type Definition):
    • DTD is used to define the structure and allowed elements/attributes in an XML document.
  2. XSD (XML Schema Definition):
    • XSD is a more powerful and flexible way to define the structure, data types, and constraints for XML documents.

Example of XML with XSD Schema 

<person>
    <name>John Doe</name>
    <age>30</age>
</person>

This XML document could be validated against an XSD schema to ensure it follows the correct structure (e.g., that age must be a number).

Pros and Cons of XML

Pros

  • Self-descriptive: XML data includes metadata that describes itself, making it easier to understand without external documentation.
  • Extensible: You can define your own tags based on your specific use case.
  • Widely Supported: XML is supported by a wide range of technologies and platforms.
  • Structured: XML’s hierarchical structure allows complex data to be easily modeled.

Cons

  • Verbose: XML tends to be more verbose compared to other formats like JSON, as both opening and closing tags are required for each element.
  • Performance: Parsing XML can be slower than more lightweight formats like JSON.
  • Human Readability: While XML is human-readable, its verbosity can make it harder to read compared to more concise formats like JSON.

Conclusion

XML is a flexible and widely adopted format for representing structured data. Its ability to define custom tags and its platform independence have made it a popular choice for data interchange, configuration files, and web services. However, its verbosity and slower parsing speed have led to alternatives like JSON being preferred in certain contexts (e.g., modern web APIs).

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3. semester efterår 2024