Java Web Services: Up and Running, 2nd Edition

The WSDL Service Contract in Detail

The WSDL document, which is XML, is structured as follows:

• The document or root element is named definitions. This is appropriate because the WSDL defines the web service thoroughly enough that utilities such as wsimport can use the WSDL to generate code, typically but not exclusively client-side support code.
• The first child element of definitions, named types, is technically optional but almost always present in a modern WSDL. This element contains (or links to) an XML Schema or the equivalent—a grammar that specifies the data types for the messages involved in the service. In a modern SOAP-based web service, the arguments passed to web service operations are typed—but the SOAP messages themselves are also typed. For this reason, the receiver of a SOAP message can check, typically at the library level, whether the received message satisfies the constraints that the message’s type impose.
• Next come one or more message elements, which list the messages whose data types are given in the types section immediately above. Every message has a corresponding complexType entry in the schema from the types section, assuming that the types section is nonempty.
• The portType section comes next. There is always exactly one portType element. The portType is essentially the service interface: a specification of the service’s operations and the message patterns that the operations exemplify. For example, in the request/response pattern, the client begins the ...