Given an class hierarchy:
A -> B -> C -> instanceOfC
is it possible (and how) to insert a class, temporarily, at runtime, like so:
A -> B -> B' -> C -> instanceOfC?
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4 Answers
AspectJ
It's possible if you use AspectJ. AspectJ has a declare parents statement that enables you to do exactly that, and through Load Time Weaving you should be able to do it at runtime, also. However, you will not be able to do it on already loaded classes (at least not easily, maybe it works if you unload the class first).
Reference:
Inheritance vs Aggregation
But methinks you are trying to solve the wrong problem here. I'd say try to replace inheritance with aggregation, let C have a member of type D to which functionality is delegated and just pass in a different implementation of D. That's called the strategy pattern, and it's a much cleaner way to do things (and it's testable too)
answered Feb 1, 2011 at 6:19
Sean Patrick Floyd
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The API that allows the most invasive changes into a running JVM is probably the JVM Tool Interface. It is designed to develop debuggers that can dynamically change methods during runtime to facilitate fix-and-continue debugging.
The relevant call for replacing the definition of a class would be RedefineClass. However, the documentation says that it can't be used to change the hierarchy of a class:
The redefinition must not add, remove or rename fields or methods, change the signatures of methods, change modifiers, or change inheritance.
Therefore I'd say that you can't sanely modify the type hierarchy of already-loaded classes in an unmodified JVM. Either change the class hierarchy during loading (with a custom ClassLoader and bytecode-instrumentation, possibly using AOP) or find another approach.
Comments
No, you can not.
You can do:
A -> | interface for B and B` | -> C
or
| B | -> C
A ->
| B` | -> C`
Losing some good manners, you could do B + inner class = B'.
For example:
public class B<extraFunctionalityClassOrInterface> extends A {
private extraFunctionalityClassOrInterface eF = null;
// and B contents
}
If instance of b.eF == null then you can conclude b != B', but b == B.
Comments
I'm not 100%, but I think this would only be possible if both B' and C were dynamically loaded.
4 Comments
blueberryfields
It would be awesome if you can point me to some sample code that has this working.
jerluc
Well before I break out my custom class loader, this seems to be a bit of a design flaw. In what type of situation would you need to do this?
blueberryfields
This looks like a potential solution to the problem listed in this question: programmers.stackexchange.com/questions/41925/… - it's also going to help resolve a bit of an argument between two non-java experts here.
jerluc
Hm, well I still don't understand why this would be necessary for you to do without any other design alternatives, but as mentioned in the link you posted, the only way I'd know of would involve plenty of interfaces to build.