The multicast datagram socket class is useful for sending
and receiving IP multicast packets. A MulticastSocket is
a (UDP) DatagramSocket, with additional capabilities for
joining "groups" of other multicast hosts on the internet.
A multicast group is specified by a class D IP address, those
in the range 224.0.0.1 to 239.255.255.255,
inclusive, and by a standard UDP port number. One would join a
multicast group by first creating a MulticastSocket with the desired
port, then invoking the joinGroup(InetAddress groupAddr)
method:
// join a Multicast group and send the group salutations
...
byte[] msg = {'H', 'e', 'l', 'l', 'o'};
InetAddress group = InetAddress.getByName("228.5.6.7");
MulticastSocket s = new MulticastSocket(6789);
s.joinGroup(group);
DatagramPacket hi = new DatagramPacket(msg, msg.length,
group, 6789);
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);
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(InetAddress addr) method.
Multiple MulticastSocket's may subscribe to a multicast group
and port concurrently, and they will all receive group datagrams.
Currently applets are not allowed ot use multicast sockets.
Set the default time-to-live for multicast packets sent out
on this socket. The TTL sets the IP time-to-live for
DatagramPackets sent to a MulticastGroup, which
specifies how many "hops" that the packet will be forwarded
on the network before it expires.
The ttl is an unsigned 8-bit quantity, and so must be
in the range 0 < ttl <= 0xFF .
Sends a datagram packet to the destination, with a TTL (time-
to-live) other than the default for the socket. This method
need only be used in instances where a particular TTL is desired;
otherwise it is preferable to set a TTL once on the socket, and
use that default TTL for all packets. This method does not
alter the default TTL for the socket.
Parameters:
p - is the packet to be sent. The packet should contain
the destination multicast ip address and the data to be sent.
One does not need to be the member of the group to send
packets to a destination multicast address.
ttl - optional time to live for multicast packet.
default ttl is 1.
Submit a bug or feature - Version 1.1.8 of Java Platform API Specification
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