Java Enterprise Best Practices

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.

Example 5-1. Sample document without entity references

<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. ...