OS X Yosemite Deletes Java / JDK, but Photoshop Needs It
OS X Yosemite deletes an installed java, which MPG uses for writing server code and various other tasks. Not a very nice behavior at all.
The problem is, some parts of Photoshop (some types of scripting) require java.
JDK 1.6 for OS X (works OK on 10.10)
See also Information and system requirements for installing and using Oracle Java on Mac OS X and How can I find which version of Java is installed without running an applet in Windows or Mac?.
The whole thing is a mess in terms of transition: JDK 1.6 goes into the right directories and shows up on the command line but JDK 1.8 does not, and does not have the familiar directory structure, breaking scripts and builds (no doubt solvable but very confusing and annoying). MPG needs 1.6 for development purposes so it works out for now. Photoshop users should be fine either way.
- Disable java in web browsers (strongly recommended).