My son and I have been working our way through the book, "Minecraft Modding with Forge", only to get stuck on page six for quite a while. We found lots of people encountering the same error, but no solutions. Tonight, I was determined to get it to work, and here is how I did it!
The first step can be done with your regular user account:
1. Download Minecraft Forge from http://files.minecraftforge.net (get the "recommended version"--at the time of this post, 22.214.171.1243 (the Mdk one)).
2. You should now have a 'Forge' directory in your 'Downloads' folder (mine was actually named 'Forge-1'). Make a folder on your Desktop called 'forge' and copy the contents of the Download 'Forge' directory into the 'forge' directory on your Desktop.
Note: These next few steps need to be run under an Administrator account:
3. Install "brew" from http://brew.sh, which you can use to easily install an older version of Java SDK (1.6)
4. Install Java SDK 1.6 by typing the following into the 'Terminal' program:
brew tap cask room/versions
brew cask install java6
5. Delete the 'vecmath.jar' file in /System/Library/Java/Extensions (if you don't have a copy of 'vecmath.jar' at this location, that is ok)
Now go back to your regular user account
6. Set your JAVA_HOME environment variable, again using the "Terminal" program:
export JAVA_HOME=$(/usr/libexec/java_home -v 1.6)
7. Change directories to your Desktop 'forge' directory
8. Edit the 'gradlew' file. Change the DEFAULT_JVM_OPTS="" line to DEFAULT_JVM_OPTS="-Xmx1024m"
9. You're now ready to run the command:
./gradlew setupDecompWorkspace eclipse
I got the desired "BUILD SUCCESSFUL" message after about 5 minutes and 30 seconds. If you get past the 'recompileMc' task, you should be home free.
I really hope this helps others who are working with the book, or trying to get Forge to work on OSX. It can be done! Don't give up!
[My System Specs]
OS X Yosemite (version 10.10.5)
Mac mini (Late 2009)
Processor 2.66 GHz Intel Core 2 Duo
Memory 8GB 1067 Mhz DDR3
Graphics NVIDIA GeForce 9400 256MB
Using Java SDK 1.6 co-installed with Java SDK 1.8
JAVA_HOME set using /usr/libexec/java_home -v 1.6