Is there a way to get something I catch with a proxy (like Fiddler) to java code?

I have this question: a Java backend sends a http post request to somewhere, and inside it holds an object with some fields. I know there is Fiddler which has GUI. But can I get (I can't get it from the destination, I need it before it has been sent) whatever is sent with that Apache HttpRequest into my java test code and use it in the test? I'm interested in one String value only, but getting the whole object is ok too.