private static short nextId() {
    
        // If the ID we are about to return is already in use
        // then skip it.
        while (idRegistrations.containsKey(nextAvailableId) ) {
            nextAvailableId--;   
        }
        
        // Return the available ID and post-decrement to get
        // ready for next time.    
        return nextAvailableId--;
    }
When set to true, classes that do not have intrinsic IDs in their
 {@code @Serializable } will not be auto-registered during write.  Defaults
 to true since this is almost never desired behavior with the way
 this code works.  Set to false to get the old permissive behavior.
jme3-networking/src/main/java/com/jme3/network/serializing/Serializer.java