I loaded Optifine 1.9 on to MC 1.9 last night with no problems!
The steps I used were:
Duplicate 1.9 folder, rename folder .jar and .json, set same name in .json file, copy optifine 1.9a content into .jar, delete MOJANGCS.RSA and MOJANGCS.SF, create a profile for it and launch!
Worked a treat, no java environment needed (important).
So I load it up again this morning and *poof*, all gone, back to Vanilla! D=
What can I do to prevent this from happened?
I've actually uninstalled and tried it again from scratch but its still doing it, I do notice when I launch now it quickly flashes up a 'downloading snapshot *something*' and then downloading 1.9 - OF.jar (my mod name).
You can run the installer with a non OS registered Java distro if you launch it manually from command prompt.
I thought that too! And it does run, but for some reason wasn't detecting the client install after running vanilla.
I used MultiMC in the end, and pointed to it to Java. Added the optifine .jar to the mods and added the mod to an instance!