Use classes in unnamed module that are also contained in a JDK platform/runtime module

[Alan Bateman]

On 04/07/2017 08:02, Langer, Christoph wrote:
> Hi experts,
>
> probably this was already asked or discussed here but I don't find an exact answer for my type of issue, so I'm asking again.
>
> I have some piece of software that we ship as a jar file and which will hence run on a JDK 9 in the unnamed module. However, this jar file contains a package that is also contained in our JDK image in a module that is always part of the runtime. So, when my app wants to use one of these classes, I get a java.lang.IllegalAccessError because the class is not exported from the runtime module to my app. And, also if I have a class in that package which is not part of the runtime module, I get a java.lang.NoClassDefFoundError because probably the package is loaded from the runtime module.
>
> As I don't want to maintain 2 copies of the same code just to have different package names, I'm wondering if I could solve this somehow by implementing my own class loader for the app that would first try to load these classes from the classpath before delegating to the default application class loader. Would that work? I also did some googling on implementing a non-delegating class loader but to me that seems not so easy and has pitfalls.
Yes, that should work.

An alternative is to patch the module (with --patch-module) with the 
additional classes in the JAR file. This assumes of course that the 
contents of the JAR file are indeed a compatible superset.


> Is there a good reference or even something in the JDK that I could subclass?
>
The trampoline code in sun.reflect.misc.MethodUtil is roughly in this 
area but might be too specialized. There may be some test code that you 
could use - one example is in the patch for JDK-8183503 which is 
currently waiting for review on hotspot-runtime-dev [1].

-Alan.

[1] 
http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2017-July/023841.html