Use Entity References
An entity
reference (also called an entity
declaration in some circles) is one of those topics in XML
that seems a little obscure. However, just think of an entity
reference as a variable in XML. That variable has a declared value,
and every time the variable occurs, the parser substitutes that value
in the XML output. In that regard, an entity reference is like a
static final variable in Java in that it cannot
alter its value from an initial value defined in a Document Type
Definition (DTD).
An entity reference often refers to an online resource (you’ll see examples of this later in the Section 5.2), but it can also have a value defined in a DTD, such as the following:
<!ENTITY phoneNumber "800-775-7731">
Instead of typing the phone number for O’Reilly several times in your XML document, and possibly introducing typographical errors, you can just refer to the value through its reference:
<content>O'Reilly's phone number is &phoneNumber;.</content>Of course, this seems pretty trivial, so let’s look at a more realistic example. Example 5-1 shows a simple XML document fragment intended for display on a web page.
<page> <title>O'Reilly Java Enterprise Best Practices</title> <content type="html"> <center><h1>O'Reilly Java Enterprise Best Practices</h1></center> <p> Welcome to the website for <i>O'Reilly Java Enterprise Best Practices</i>. This book was written by O'Reilly's Java authors for Java Enterprise professionals. ...