What's the difference between --add-exports and --add-opens in Java 9?

Difference

• With --add-exports the package is exported, meaning all public types and members therein are accessible at compile and run time.
• With --add-opens the package is opened, meaning all types and members (not only public ones!) therein are accessible at run time.

So the main difference at run time is that --add-opens allows “deep reflection”, meaning access of non-public members. You can typically identify this kind of access by the reflecting code making calls to setAccessible(true).

It is worth adding that at runtime -add-opens implicates -add-exports.

For more, please review details through JEP 261.