Solution is to make JAVA_HOME == dir above bin where javac lives as in
type javac
javac is /usr/bin/javac # now check if its just a symlink
ls -la /usr/bin/javac
/usr/bin/javac -> /etc/alternatives/javac # its a symlink so check again
ls -la /etc/alternatives/javac # now check if its just a symlink
/etc/alternatives/javac -> /usr/lib/jvm/java-8-openjdk-amd64/bin/javac
OK so finally found the bin above actual javac so do this
export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH
above can be simplified and generalized to
which javac >/dev/null 2>&1 || die "ERROR: no 'javac' command could be found in your PATH"
export JAVA_HOME=$(dirname $(dirname $(readlink -f $(which javac) )))