XML Entities

These are placeholders for code and data that can be expanded within a XML document. There are several types of entities

• External

  • Used inside a XML file but the contents are referenced from an external file or URL.

    • <!DOCTYPE note [
      <!ENTITY ext SYSTEM "http://example.com/external.dtd">
      ]>
      <note>
              <info>&ext;</info>
      </note>

• Internal

  • Used within a XML document to define content that can be used multiple times.

    • <!DOCTYPE note [
      <!ENTITY inf "This is a test.">
      ]>
      <note>
              <info>&inf;</info>
      </note>

• Parameter

  • special types of entities used within DTDs to define reusable structures or to include external DTD subsets. Useful to maintain large scale XML applications

    • <!DOCTYPE note [
      <!ENTITY % common "CDATA">
      <!ELEMENT name (%common;)>
      ]>
      <note>
              <name>John Doe</name>
      </note>

• General

  • can be declared either internally or externally. They are used to define substitutions that can be used within the body of the XML document.

    • <!DOCTYPE note [
      <!ENTITY author "John Doe">
      ]>
      <note>
              <writer>&author;</writer>
      </note>

• Character

  • Used for special or reserved characters that cannot be used directly in XML documents.

    • <note>
              <text>Use &lt; to represent a less-than symbol.</text>
      </note>

    • Example:

      • < for the less-than symbol (<)

      • > for the greater-than symbol (>)

      • & for the ampersand (&)