Working with Spring WebServiceMessage

Question

I have code that uses Spring's WebServiceTemplate to invoke a SOAP service.

Currently it uses:

public Object marshalSendAndReceive(String uri, final Object requestPayload)

But I would like to manipulate the content both inbound and outbound. The most suitable public method appears to be:

public <T> T sendAndReceive(String uriString,
    WebServiceMessageCallback requestCallback,
    WebServiceMessageExtractor<T> responseExtractor) 

... for which I would provide a custom callback and extractor.

The interfaces for those both specify methods that work with a WebServiceMessage.

public interface WebServiceMessage {

    Source getPayloadSource();
    Result getPayloadResult();
    void writeTo(OutputStream outputStream) throws IOException;

}

Source and Result are both interfaces from the javax.xml.transform, and they don't seem to expose message content. Both just have a setter and getter for String systemId, and nothing else.

I can see that there are various implementations of Source, all providing different access to the payload. I could do something like this in my callback:

   StreamSource source = (StreamSource) message.getPayloadSource();

But casting is dirty, and anyway, how do I know which implementations I should support?

How can I get at this content (ideally the raw XML)?