Ho fatto così....

================================================== =============unicast??
DatagramSocket dsocket = new DatagramSocket();

ByteArrayOutputStream bostream = new ByteArrayOutputStream();
ObjectOutputStream dostream = new ObjectOutputStream(bostream);
dostream.writeObject(message);
dostream.close();
bostream.close();

byte[] bMessage = bostream.toByteArray();

DatagramPacket messagePacket = new DatagramPacket(bMessage, bMessage.length, recipientAddress, recipientPort);
dsocket.connect(recipientAddress,recipientPort);

dsocket.send(messagePacket);
dsocket.close();
================================================== =============unicast??


Può funzionare?!