Java Web Services in a Nutshell

The SOAP Protocol

The web service definition cited earlier in this chapter stated that a web service communicates using “XML-based messages conveyed by Internet protocols.” Although it is expected that there will be a choice of web service protocols in the future, at the present time, web services communicate using Simple Object Access Protocol (SOAP). SOAP is an XML-based protocol that can be carried over any transport mechanism capable of delivering a byte stream. In practice, SOAP messages are usually exchanged between clients and services that are resident in web containers and are typically encapsulated inside an HTTP request or response message. However, nothing in the SOAP 1.1 specification (which can be obtained from the W3C web site at http://www.w3c.org/TR/SOAP^[3]) prevents the use of other transport mechanisms, such as FTP, SMTP or even JMS; in fact, both Apache SOAP and its successor, Axis, support the use of SMTP as the carrier for SOAP messages.

A basic SOAP message consists of an envelope that may contain any number of headers plus a body. These parts are delimited by XML elements called Envelope, Header, and Body, which belong to a namespace defined by the SOAP specification. Although the specification defines rules that implementations must follow when creating the structure of a SOAP message, it says nothing about the application-dependent information that the message may contain, apart from the fact that any content conveyed within the envelope must be valid XML. ...