Ahh you are referencing this thread:
I dont know if there is a way to update the jave on the jail that you have and then change the location that java is envoked from in FreeNAS. ( I dont have any knowledge of freenas).
But if you can install java JDK 8 into that jail, I would assume you would be able to change it to envoke JDK 8. You would need to change the path that the jail tries to execute java from.
Something like this:
\jails\mineos_1\usr\local\openjdk7\jre\lib
would be like this:
\jails\mineos_1\usr\local\openjdk8\jre\lib
Again, how to do that in the jail with freenas isnt something that I can answer at this time, I dont have any working knowledge of jails and freenas.
If I find something Ill post it here for you to look at.