Question

Can a class add a method to itself at runtime (like from a static block), so that if someone is performing reflection on this class, they’ll see the new method, even though it wasn’t defined at compile time?

Background:

A framework I’m using expects Action classes to be defined that have a doAction(...) method, by convention. The framework inspects these classes at runtime to see what type of parameters are available in their doAction() method. For example: doAction(String a, Integer b)

I’d like each class to be able to programatically generate its doAction() method with various parameters, just-in-time when it is inspected. The body of the method can be empty.

Answer 1

It’s not simple. Once a class is loaded by a classloader, there is no way to change the methods of loaded classes. When a class is requested, a classloader will load it and link it. And there is no way (with Java) to change the linked code or to add/remove methods.

The only trick that comes to my mind is playing with classloaders. If we delete a custom classloader, then the classes loaded by that classloader should be deleted or inaccessible too. The idea that comes to my mind is to

1. implement one custom classloader
2. load the dynamic class with that custom classloader
3. if we have an updated version of this class,
4. remove the custom classloader and
5. load the new version of this class with a new instance of the custom classloader

I leave that as food for thought, can’t prove, if this leads to a solution or if we have pitfalls.

As a simple answer to the question: No, we can’t change a loaded class like we can change the content of fields with reflection. (we can’t add or remove fields too).