I am writing a plugin API for a Java application, the idea being that eventually third parties will provide their own plugin extensions for the application and all the user needs to do is place the plugin jar into a plugins directory of the application. For the impatient, my question in short is how to handle possible version conflicts when the plugin relates to a different API than that on the system, see below for details of my situation and what I have thought about.
I have read a number of articles using service provider interfaces and have something working with that. The problem comes when considering how to deal with differing version combinations.
I am aware of the technique of when adding to an API adding extension interfaces, rather than changing the existing interface (eg. API 1.0 having MyInterface, API 1.1 adding MyInterface2 with the new methods, etc). With this technique if the user has the latest API then older plugins should work fine, but what happens if the user has an old API and newer plugins?
So as an example the user has API 1.0 only with MyInterface but installs binary plugin compiled against API 1.1 where the provider class implements MyInterface2. Whilst the application may only ever call plugins using MyInterface, what happens if the plugin internally calls MyInterface2? Will this cause an error or exception and when (IE. when the class is loaded or when the method from MyInterface2 is called). Also is this standard across JVMs or may it differ depending on the JVM used?
Finally, would it be better to use a plugin framework, would that be able to check version requirements? Searching the internet I find PF4J on github. A quick look in the source code seems to show it may support some sort of version checks.
