Meaning of java.lang.ClassCastException: someClass incompatible with someClass

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Philippe Riand explaned this by email:

This class cast happens because the same class had been loaded twice by 2 different class loaders. Thus, from a Java standpoint, they are different and the cast fails.

Now, each XPages application is having its own classloader. But this class loader is discarded each time a design change happens to the application, through Domino Designer for example. This is required as a change to an XPages generates a new Java class that should then be loaded instead of the previous one. When this happens, the classloader is discarded and a new is created. Then all the application related classes are reloaded, as they are needed, even though they didn’t change.This is a common behavior implemented by J2EE servers.
That said, if your code is caching an object in a scope that is not discarded when a design change occurs, then this is likely to happen. For example, the applicationScope & sessionScope are currently not discarded when a design change happens, which might lead to this problem. This was a design choice as discarding the scopes sometimes provides a bad developer experience, but with this drawback.

Finally, saving faces-config.xml works as a workaround. When this file is saved, then the entire module is discarded from memory, including the scopes, This explains why it works. Making a change to your custom Java class should reload the module and remove the issue.

So it seems putting beans (even indirectly) into sessionScope or applicationScope is the cause.

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