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copied!<p><strong>Sending multicast datagrams</strong></p> <p>In order to send any kind of datagram in Java, be it unicast, broadcast or multicast, one needs a <code>java.net.DatagramSocket</code>:</p> <pre><code>DatagramSocket socket = new DatagramSocket(); </code></pre> <p>One can optionally supply a local port to the DatagramSocket constructor to which the socket must bind. This is only necessary if one needs other parties to be able to reach us at a specific port. A third constructor takes the local port AND the local IP address to which to bind. This is used (rarely) with multi-homed hosts where it is important on which network adapter the traffic is received.</p> <pre><code> DatagramSocket socket = new DatagramSocket(); byte[] b = new byte[DGRAM_LENGTH]; DatagramPacket dgram; dgram = new DatagramPacket(b, b.length, InetAddress.getByName(MCAST_ADDR), DEST_PORT); System.err.println("Sending " + b.length + " bytes to " + dgram.getAddress() + ':' + dgram.getPort()); while(true) { System.err.print("."); socket.send(dgram); Thread.sleep(1000); } </code></pre> <p><strong>Receiving multicast datagrams</strong></p> <p>One can use a normal DatagramSocket to send and receive unicast and broadcast datagrams and to send multicast datagrams. In order to receive multicast datagrams, however, one needs a MulticastSocket. The reason for this is simple, additional work needs to be done to control and receive multicast traffic by all the protocol layers below UDP.</p> <pre><code>byte[] b = new byte[BUFFER_LENGTH]; DatagramPacket dgram = new DatagramPacket(b, b.length); MulticastSocket socket = new MulticastSocket(DEST_PORT); // must bind receive side socket.joinGroup(InetAddress.getByName(MCAST_ADDR)); while(true) { socket.receive(dgram); // blocks until a datagram is received System.err.println("Received " + dgram.getLength() + " bytes from " + dgram.getAddress()); dgram.setLength(b.length); // must reset length field! } </code></pre> <p>For more Information:</p> <ul> <li><a href="https://docs.oracle.com/javase/7/docs/api/java/net/MulticastSocket.html" rel="nofollow noreferrer">MulticastSocket</a></li> <li><a href="https://docs.oracle.com/javase/7/docs/api/java/net/DatagramSocket.html" rel="nofollow noreferrer">DatagramSocket</a></li> </ul>

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