Java Classloader - how to reference different versions of a jar

Question

This is a common problem. I'm using 2 libraries A.jar and B.jar and these depend on different versions of the same jar.
Let's say that at runtime I need THIS.x.x.x.jar

MY.jar   
     -> A.jar -> THIS.1.0.0.jar
     -> B.jar -> C.jar -> THIS.5.0.0.jar

I can compile the specific jar (A.jar/B.jar) against its dependency but at runtime I've to load only 1 version. Which one?
Loading only 1 dependency (the latest version) means that my code will probably throw runtime exceptions if the libraries are not Backward Compatible (are there Backward Compatible libraries out there?).

Anyway I know that something like OSGi can fix this issue.
I'm wondering what's the old way to fix this kind of problems...

Thanks a lot

Answer 1

"Old way" you mentioned (and the one OSGI certainly uses under the hood) is to install your own ClassLoader for both branches of your dependencies. That's how, for instance, application servers are able to run both older and newer versions of the same application inside the same JVM.

Read about classloader hierarchy.

In your setup, the tricky part is the joint point, where classes from both branches meet. Neither branches can use classes loaded into another one. The way to make it work is to make sure only classes loaded by boot classloader (JRE classes) or classloader of MY.jar are passed down to both branches.

Answer 2

OSGi can fix this problem. An OSGi bundle is nothing more than a jar with additional metadata detailing versions. A bundle has a version number, and will detail version numbers (or ranges) of dependent jars.

Take a look at this introductory Javaworld article for more information.

To solve this without OSGi means having to ensure manually that you compile and run with compatible jars. As you've discovered that's not necessarily a trivial task. Since jars don't necessarily identify their versions, the only sure way to do this to record/compare checksums or signatures.