Accessing a class with the exact same name

I have a tricky question and I'm not sure if it is even possible in Java. I'm in the following situation:

I got a class A that uses another class, let's call it B. Now, I'm trying to write a third class (and I don't call it C), I call it B again (to be sure which class 'B' I mean, I will call it B1 and B2 in the rest of this post, ok?). All three classes resides in three different JAR-files. So far, so good.

Normaly, class A finds class B(1) on the classpath and will use it. But now I'm putting the JAR-file that contains B(2) at the very beginning of the classpath, so class A will find this class instead of the old one. So my class B(2) can act like the old class B(1) (which is a library in reality, that I can't customize in any other way. that's why I am doing that...).

And here comes my problem: In my class B(2) I want to load the real class B(1) and use it. I can do this so far by using reflection. I can even invoke methods via reflection, but I can't cast an instance of the loaded class to B reference. Here is the exception:

java.lang.ClassCas开发者_JAVA百科tException: my.a.ClassB incompatible with my.a.ClassB

Has anybody an idea how I can use class B(1) in class B(2)? I am happy with any workarround...

Thanx, Thomas.

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You can only do this through reflection.

As far as the runtime system is concerned, your two ClassB classes are complete separate entities and cannot be cast to each-other.

This sometimes happens in OSGi environments (which have complex classloader setups) or if you somehow manage to pass data between web application contexts.

If you want a common interface to call methods on both without reflection, then you need to create just that: a common interface (or parent class). And that interface needs to reside in a jar file that both these ClassB can see.

Example:

first jar: interface I

second jar: class B implements I

third jar: class B implements I

Now you have two versions of class B, but they can both be cast to I.

Needless to say, you should find a better solution to whatever problem you have here.

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Maybe you can use CDI to @Inject the desired class into class A. Doing so helps you to mock classes and provide alternatives. See also the @Alternative annotation of CDI. Basically you are using the interface methods, the implementation behind it is chosen at runtime or injectione time, resp.