> If existing code is using Proxy.getProxyClass and the Constructor.newInstance method to create a proxy instance, it will fail with IllegalAccessException if the caller is not in the same runtime package as the non-public proxy interface. For such code, it requires a source change to either (1) call Constructor.setAccessible to set the accessible flag to true, or (2) use the Proxy.newProxyInstance convenience method.

To end up in that specific condition is fairly difficult and usually means something is wrong with the code to begin with. It's still a very easy fix in any event (adding in 1 extra line of code before the call).