If your application's startup script launches the JDK 9 JVM with an unrecognized VM option, it will exit and let you know that it couldn't create a VM because of that unrecognized VM option. Some of the VM flags that were deprecated in JDK 8, have been removed in JDK 9 - like those relating to the permanent generation, which was removed in JDK 8.
For example, if your application's startup script attempts to launch the JDK 9 JVM providing an unrecognized
-XX:MaxPermSize=256m option, the JDK 9 JVM will exit with an error message:
Unrecognized VM option 'MaxPermSize=256m'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.
Run jdeps on your code
In JDK 8 a new Java Dependency Analysis Tool (jdeps) was added to help developers understand the static dependencies of their applications and libraries. It can help you find dependencies on any internal, unsupported or private APIs that your application or its libraries use. A program using such APIs is not guaranteed to work in future versions of JDK 9 or even the same platform. For more information, please see this document.