According to this link poi 3.11 needs JDK 1.6 or higher. Latest release of poi is 3.15 which is a minor release and should be compatible with JDK 1.6

If the project states that JDK 1.5 or higher is required it means that usually no testing is done with lower versions. It might still work (to some degree) as you found out, but likely will stop working with newer version of Apache POI.

Finally nailed it down:

The issues are not (directly) caused by the lib and/or jdk9 itself but by Eclipse (v4.7.2).

I have 2 Classes in my project using the apache poi library to read or write Excel files. One Class is the one that I took the screenshots of the issues from. The second class had 0 errors in eclipse (funny because it imports the same poi classes...).

But the truth was that the second class had 2 errors, too! These errors were caused by the update from v3.14 to 3.17 because of API changes but they where somewhat obfuscated / not visible in Eclipse.

After I fixed those 2 errors in the second class all the Import errors ("...cannot be resolved" and "... not accessible") disappeared in the primary class and the code becomes compilable and runable again (even with JDK9).

I only found out about the real cause by tinkering around on the command line trying to compile the stuff by hand... So yeah. I hope this helps others with simmilar issues :).