You can't access another java process classloader class definitions. See this question for how to load a jar properly : How to load a jar file at runtime
Once your jar is loaded, you can use Class.forName to access the second jar desired class
EDIT : Here is a little snippet to help you read process standard output.
//open a buffered reader on process std output
InputStreamReader ir = new InputStreamReader(theProcess1.getInputStream());
BufferedReader in = new BufferedReader(ir);
//read it line per line
String line;
while ((line = in.readLine()) != null) {
System.out.println(line);
}