Java API for XML Web Services (JAX-WS) is an enabler for achieving SOA and is a key component of ESBs. Web Services along with the associated XML based stack that it is built on is required to be carefully studied before one can delve into SOA and ESBs.
This new article that I have published on my web site takes a user onto a journey that ends with a clear and cogent elucidation of the differences between the various styles of SOAP bindings for web services. The styles covered are RPC Literal versus Document Literal versus Document Wrapped. We also talk about WS-I Basic Profile that web services need to be compliant of in order to achieve interoperability with consumers on a different platform, technology stack etc.
The article assumes familiarity with XML, WSDL, SOAP as well as Java (upwards of Java 5 including annotations). You can run any of the examples with JDK 1.6 without downloading any extensions or any other libraries.
Here is the link.