In one case, I am interested in using Jython to dynamically create a java class at runtime and I would like to explore this with you.
The general idea is that I want to dynamically (at runtime) create a java class that implements a certain interface, then instantiate that class and call its methods.  I'm inventing a contrived example here because it is hard to explain in a few words the exact application.  Suppose the java interface looks something like (its late so please avoid syntax:):
Interface MyCallInterface:
    public int doCall();
Now I want to generate a new class implementing the above interface, and with some specific logic in the doCall() method , based on some properties that occur at runtime (including some user input).  The doCall method has to act on other java objects based on some configuration data, so normally it would have to do reflection to look up the method names (configurable) and then call them.  But, that is slower than I want it to be and so is using dynamic proxies. 
What I would like to do is something like this:
0) I would rather implement an interface than extend and existing java class if possible.
1) Build a string that contains the class definition (that inherits from MyCallInterface) and the method code for the doCall() method.
2) Pass that string into jython in some manner so that I get a class back that I can instantiate the object from.
3) Instantiate the newly defined object.
4) Call its doCall() method repeatedly during program operation until parameters change, at which point I would generate a new class definition and get a new object.
What I am hoping is that the performance using this approach (after the initial class compile) would be faster than reflection and dynamic calls, since they have more overhead than direct method calls.  Would my approach be faster or is there a lot of overhead the jython introduces?
Also, I can't see clearly how to do what I want.  Could anyone post a very short but complete hello world type example of how to do this from within a java program?
This has all kinds of neat implications for custom configurators and dynamic execution policies, but it should be really really fast.
Also, I have heard of some code from a GNU or Apache project that would generate bytecode for classes on the fly (or something like that) and that is the type of functionality I need (without myself having to understand the byte code).  If the jython approach won't work, can someone point me to the code I mentioned here?