Java Web Services in a Nutshell

Describing and Discovering Web Services

One of the clauses in the W3C definition of a web service states that its “definition can be discovered by other software systems.” In order to make this possible, the web service standards include:

• A language used to define the interfaces provided by a web service, in a manner that is not dependent on the platform on which it is running or the programming language used to implement it

• A provision for a registry within which these definitions can be placed

Since all access to web services uses XML messaging, it is appropriate that the language used to describe a web service should itself be XML-based. The Web Service Description Language (WSDL) was originally defined by Microsoft, IBM, and Ariba, and is now subject to standardization by the W3C consortium. W3C recently published a draft of its official variant of WSDL (Version 1.2), which can be downloaded from its web site at http://www.w3c.org. At the time of this writing, however, most existing WSDL-aware software (including that provided by Sun Microsystems) is based on WSDL Version 1.1.

As you’ll see in Chapter 5, WSDL describes a web service by defining the messages that it accepts and the reply messages that it returns. These messages are actually defined first in abstract terms and then bound to one or more message and transport protocols. Today, of course, web services use SOAP as the messaging protocol, and therefore almost all WSDL files will define a binding of the service to SOAP ...