I run into a ClassCastException:
Cannot cast my.package.classA to my.package.classA.
Note that the (canonical) class (name) is the same.
I understand this should be related to different class loaders and/or loading the class from different jars. The project is built using Maven. Can I debug somehow which classloader was used in each case and from which jar the classes have been loaded?
Add a breakpoint for ClassCastException.
For IntelliJ, this looks something like this:
Once the exception is caught, navigate in the debugger call hierarchy until you find the line where the cast should happen and you have a reference to both classes.
For example:
Now, you can inspect both classes (in my case typeClass and instanceClass). The class holds a reference to it's class loader.
In my case, it's ModuleClassLoader for Module "my.ear" from Service Module Loader
and
ModuleClassLoader for Module "my-web-app.war" from Service Module Loader
As you can see, one class is coming from the ear, the other one from the war.
Change the dependency to provided in the web-app pom.xml, that should solve the problem.
Note that the war file will not run anymore outside of the ear, if you need both scenarios, use a Maven profile.