I have a very old project still running on Jboss 6 AS, it has been on the TODO list to write but since it is of low importance, only used internally and it just works.
After upgrading to Java 8 the following errors started to appear in eclipse.
This is due to a bug in Drools, more specifically the Eclipse compiler which is the default. For more info see the JIRA : DROOLS-329
Now the quick fix is just to use the old Compiler 1.7 or older. This is different to setting the compliance level. This is done in eclipse:
Window -> Preferences
Server -> Runtime Environment
Select the Runtime environment, in my case Jboss 6.x Runtime, click Edit and select the radio button “Alternative JRE”.