public static class ABCMarshallingExternalizer implements AdvancedExternalizer<ABCMarshalling> { @Override public void writeObject(ObjectOutput output, ABCMarshalling object) throws IOException { MapExternalizer ma = new MapExternalizer(); ma.writeObject(output, object.getMap()); } @Override public ABCMarshalling readObject(ObjectInput input) throws IOException, ClassNotFoundException { ABCMarshalling hi = new ABCMarshalling(); MapExternalizer ma = new MapExternalizer(); hi.setMap((ConcurrentHashMap<Long, Long>) ma.readObject(input)); return hi; } ... }