In the last chapter, you saw how to use JAX-RPC to create web services based around the use of remote procedure calls. Even though JAX-RPC uses an XML-based protocol, there was hardly any XML in evidence in Chapter 2. For the most part, JAX-RPC does a very good job of hiding the details of messages that are exchanged between a web service and its client during a remote procedure call. However, most of the more advanced JAX-RPC features require an understanding of SOAP, the XML-based protocol on which JAX-RPC in particular, and web services in general, are based. This chapter paves the way for a more in-depth examination of JAX-RPC by introducing SOAP, together with SAAJ, which is a Java API for creating, sending, and receiving SOAP messages.