In many ebuilds that need javac there is the problem that when a user has (automatically) installed a jre first and a jdk is installed subsequently, the default java provider is the jre. This breaks ebuilds that use javac such as the sys-libs/db ebuild as it cannot get the data of the jdk.
I made a change on java.eclass to set to default, the latest one installed so this should fix bugs on it, or at least its a workaraound for this.