Module java.base
Package java.net

Class MulticastSocket

java.lang.Object
java.net.DatagramSocket
java.net.MulticastSocket
All Implemented Interfaces:
Closeable, AutoCloseable
public class MulticastSocket extends DatagramSocket
A MulticastSocket is a datagram socket that is convenient for sending and receiving IP multicast datagrams. The MulticastSocket constructors create a socket with appropriate socket options enabled that make it suitable for receiving multicast datagrams. The MulticastSocket class additionally defines convenient setter and getter methods for socket options that are commonly used by multicasting applications.

Joining one or more multicast groups makes it possible to receive multicast datagrams sent to these groups.

An IPv4 multicast group is specified by a class D IP address and by a standard UDP port number. Class D IP addresses are in the range 224.0.0.0 to 239.255.255.255, inclusive. The address 224.0.0.0 is reserved and should not be used.

One would join a multicast group by first creating a MulticastSocket with the desired port, then invoking the joinGroup method, specifying the group address and the network interface through which multicast datagrams will be received: 


 // join a Multicast group and send the group salutations
 ...
 String msg = "Hello";
 InetAddress mcastaddr = InetAddress.getByName("228.5.6.7");
 InetSocketAddress group = new InetSocketAddress(mcastaddr, 6789);
 NetworkInterface netIf = NetworkInterface.getByName("bge0");
 MulticastSocket s = new MulticastSocket(6789);

 s.joinGroup(new InetSocketAddress(mcastaddr, 0), netIf);
 byte[] msgBytes = msg.getBytes(StandardCharsets.UTF_8);
 DatagramPacket hi = new DatagramPacket(msgBytes, msgBytes.length, group);
 s.send(hi);
 // get their responses!
 byte[] buf = new byte[1000];
 DatagramPacket recv = new DatagramPacket(buf, buf.length);
 s.receive(recv);
 ...
 // OK, I'm done talking - leave the group...
 s.leaveGroup(group, netIf);
When one sends a message to a multicast group, all subscribing recipients to that host and port receive the message (within the time-to-live range of the packet, see below). The socket needn't be a member of the multicast group to send messages to it.

When a socket subscribes to a multicast group/port, it receives datagrams sent by other hosts to the group/port, as do all other members of the group and port. A socket relinquishes membership in a group by the leaveGroup(SocketAddress mcastaddr, NetworkInterface netIf) method. Multiple MulticastSockets may subscribe to a multicast group and port concurrently, and they will all receive group datagrams.

The DatagramSocket and MulticastSocket classes define convenience methods to set and get several socket options. Like DatagramSocket this class also supports the setOption and getOption methods to set and query socket options. The set of supported socket options is defined in DatagramSocket. Additional (implementation specific) options may also be supported.

API Note:
DatagramSocket may be used directly for sending and receiving multicast datagrams. DatagramChannel implements the MulticastChannel interface and provides an alternative API for sending and receiving multicast datagrams. The MulticastChannel API supports both any-source and source-specific multicast. Consider using DatagramChannel for multicasting.
Since:
1.1