You can insert external text and even documents into XML documents by using external entities.
XML comes with a native mechanism for including text from both internal and external sources. The mechanism is called entities (http://www.w3.org/TR/REC-xml.html/#sec-physical-struct). This feature allows you to make XML documents modular. Entities can be declared and stored internally in a document, in an external file, and even across a network. Entities are declared in DTDs and can contain just small bits of non-XML text, XML markup, or even large amounts of text.
XML has a concept of a document entity , which is a starting point for an XML processor. A document entity, from one standpoint, may exist in a file with an associated name. However, from the standpoint of the XML spec, a document entity does not have a name and might be an input stream that has no means of identification at all.
The rather minimal XML document entity.xml declares one internal entity (line 3) and two external entities (lines 4 and 5), as shown in Example 2-17.
Example 2-17. entity.xml
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE time [ <!ENTITY tm "59"> <!ENTITY tme SYSTEM "tm.ent"> <!ENTITY rmt SYSTEM "http://www.wyeast.net/rmt.ent"> ]> <!-- a time instant --> <time timezone="PST"> <hour>11</hour> <minute>&tm;</minute> <second>&tme;</second> <meridiem>p.m.</meridiem> &rmt; </time>
This kind of DTD is called the internal subset because it is internal to the XML ...