Multicasting with NIO
By user12820862 on Apr 28, 2007
A long standing issue for many developers is that the java.nio.channels package has lacked support for Internet Protocol (IP) multicasting. In the NIO.2 early review draft specification you will see that we have addressed this issue by adding multicast support to DatagramChannel. Here's a small example that opens a DatagramChannel, binds the channel's socket to a local port, sets the network interface for multicast datagrams sent via the channel, and then joins a multicast group on the same interface:
NetworkInterface interf = NetworkInterface.getByName("eth0"); InetAddress group = InetAddress.getByName("126.96.36.199"); DatagramChannel dc = DatagramChannel.open(ProtocolFamily.INET) .setOption(SocketOption.SO_REUSEADDR, true) .bind(new InetSocketAddress(5000)) .setOption(SocketOption.IP_MULTICAST_IF, interf); MembershipKey key = dc.join(group, interf);
Once the channel has joined the group then it reads or receives multicast datagrams in the same manner that it reads or receives unicast datagrams.
The most significant thing in this example is the static factory method to open the channel specifies a protocol family. In this example, the channel is to an IPv4 socket. In the existing APIs, the protocol family is transparent and all sockets created by the java.net or java.nio.channels packages are either all IPv4 or all IPv6. For IP multicasting it is important that the protocol family corresponds to the address type of the multicast groups that the socket joins; otherwise it is highly operating system specific if the socket can join the group, configure options, or receive multicast datagrams. Legacy java.net.MulticastSocket has suffered greatly from problems in this area. Another interesting thing to point out is that the channel's socket is bound and socket options are configured directly. The awkward and counterintuitive socket adaptor isn't required so it isn't necessary to mix java.net socket APIs when configuring the channel's socket.
In the example, the MembershipKey that is returned by the join method is a token to represent membership of the group. It defines methods to query information about the membership and defines the "drop" method to drop membership of the group.
Developers tracking multicast standards will know that the RFCs in this area have been updated in recent years to add source filtering and this is now supported by almost all modern operating systems. In the NIO.2 draft specification we have included basic support for source filtering. The following code fragment shows a channel joining a multicast group to only receive multicast datagrams sent by a specific IP source address (otherwise known as "include-mode" filtering):
MembershipKey key = dc.join(group, interf, source);
"Exclusive-mode" filtering is where a group is joined to receive all multicast datagrams except those from specific IP source address:
MembershipKey key = dc.join(group, interf).block(source1).block(source2);
So that's a brief introduction to the multicast support that we propose to add to the java.nio.channels package. There's a lot more detail in the draft specification for those interested in this topic.