I've been trying to get a clear story regarding the interaction of these three components before upgrading my system from Lion to Mavericks --- I can't afford a Mathematica disaster.
What I think is the case is: 1. Upgrading to Mavericks removes Java from the system (or perhaps installs Java 7). 2. MMA 9 requires Java 6, and the MMA installer gives you an option to install (downgrade?) to Java 6. 3. Once you have Java 6 installed, MMA works fine.
Does that sound correct? Does Wolfram indeed support MMA 9 on Mavericks?