Please consider the following scenario: A Java Framework and a bunch of application-plugins are deployed as a Java SE Client Application. The Client Runtime is Java SE 6, whereas the framework as well as the application-plugins is compiled with Java SE 5.
Now the framework is recompiled with Java SE 6, the application-plugins remain unchanged (compiled with Java SE 5).
Assuming the framework “interface” (classes/interfaces directly used by the application-plugins) remains compilable with Java SE 5 (language level and API) the question is: Do the application-plugins need to be recompiled (against the recompiled framework) in order to be usable together at runtime (with Java SE 6)?
Maybe the question comes down to this: Under what circumstance may it make any problem when “SE6-classfiles” (v50) are used by “SE5-classfiles” (v49) within an SE6-runtime?
If there are no (binary) incompatible changes in the interfaces/classes and the runtime VM supports the new classfile version, then there should be no issues.