public static void setReadOnly( boolean b ) {
        locked = b;
    }
Can put the registry in a read-only state such that additional attempts
 to register classes will fail.  This can be used by servers to lock the
 registry to avoid accidentally registering classes after a full registry
 set has been compiled.
jme3-networking/src/main/java/com/jme3/network/serializing/Serializer.java