You can use the @RequestHeader annotation with HttpHeaders method parameter to gain access to all request headers:
@RequestMapping(value = "/restURL")
public String serveRest(@RequestBody String body, @RequestHeader HttpHeaders headers) {
// Use headers to get the information about all the request headers
long contentLength = headers.getContentLength();
// ...
StreamSource source = new StreamSource(new StringReader(body));
YourObject obj = (YourObject) jaxb2Mashaller.unmarshal(source);
// ...
}