Here are the steps you need to take;
1. Download, patch and build OpenJDK 9You can follow instruction here on how to do it, but I've outlined it below:
2. While compiling, listen to Mozart to prepare for the enlightenment.
When the build is complete, the jdk home will be in the build/macosx-x86_64-normal-server-release/jdk/ folder. Now edit a file under your netbeans nightly folder, in my case
vi /Applications/NetBeans/NetBeans\ Dev\ 201506270401.app/Contents/Resources/NetBeans/etc/netbeans.conf
Add this line at the end of the file (modified for the exact location where you built openJDK).
Now you can start netbeans normally.