This chapter discusses what goes into an XML deployment descriptor; it teaches you how to write deployment descriptors for your beans. Keep in mind that you may never need to write a deployment descriptor by hand; most vendors of integrated development tools and EJB servers will provide tools for creating the descriptor automatically. Even if you have such a tool available, however, you should be familiar enough with deployment descriptors to be able to read them on your own.

This chapter does not attempt to teach you how to read or write correct XML. There are many books on the subject: a good quick reference is XML Pocket Reference by Bob Eckstein (O’Reilly); XML in a Nutshell, by Elliotte Rusty Harold and W. Scott Means (O’Reilly), provides a more detailed treatment. Very briefly, XML looks like HTML, but with different tag names and different attributes inside the tags. You won’t see <h1> and <p> inside a deployment descriptor; you’ll see tags like <ejb-jar>. But otherwise, if you expect an XML document to look like HTML, you’re most of the way toward reading it. The tag names and attribute names for an XML document are defined by a special document called a Document Type Definition (DTD). Therefore, for XML deployment descriptors, there is a DTD that defines the tags and attributes that can be used in the document; the DTDs for deployment descriptors in EJB 2.0 and 1.1 are available online at http://java.sun.com/dtd/ejb-jar_2_0.dtd and http://java.sun.com/j2ee/dtds/ejb-jar_1_1.dtd.

There are a few other important differences between XML and HTML. XML is much more strict; many things that are acceptable in HTML are errors in XML. This should not make a difference if you’re just reading a deployment descriptor, but if you’re writing one, you have to be careful. Two differences are particularly important. First, XML is case-sensitive. You cannot mix uppercase and lowercase in your tag names. HTML does not care about the difference between <h1> and <H1>, but XML does. All the tags and attributes used in deployment descriptors are lowercase. Second, XML will not forgive you if you fail to supply closing tags. In HTML you can write <p>...<p>, without ever putting in a </p> to end the first paragraph. XML never allows you to be sloppy. Whenever you have an opening tag, there must always be a closing tag.

That’s about it. These few paragraphs don’t even qualify as a quick introduction to XML, but the basic ideas are very simple, and that’s really all you should need to get going.