Keep Coding

I'm trying to parse RDF data and load it to a model instance. However, I don't want to read the data from a file, but rather get it through an URI via http get.
What I did untik now ist this:
Model model = ModelFactory.createDefaultModel();
model.read(rdfdata, null);
model.write(System.out);
rdfdata is the data that I received (String). But I get the following Exception:
Exception in thread "main" org.apache.jena.riot.RiotNotFoundException: Not found: <?xml version="1.0" encoding="UTF-8" ?> [...]
Where is the problem and how can I solve it?
Is it right to just read the data into the model or do I have to do something before?
Sorry for my bad english. :)

Your error message says:
Not found: <?xml version="1.0" encoding="UTF-8" ?> [...]
Staring at that, it appears that it's reporting that the file or URI named <?xml version="1.0" encoding="UTF-8" ?>... can't be opened. Which makes sense, since that's unlikely to be a correct file name.
Check the documentation of model.read(), and in particular check what its first argument is. Also check what rdfdata is: is it the path to a file or a URI? (it looks like it's the contents of an RDF/XML file).
(If this guess is right, then) Moral of the story: recall that when writing an error message, a programmer is typically at least trying to be helpful, thus ‘what is this gibberish^Werror message trying to tell me?’