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java.lang.Object | +----java.net.DatagramSocket | +----java.net.MulticastSocket
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.
Constructor Summary | |
MulticastSocket()
|
|
MulticastSocket(int port)
|
Method Summary | |
InetAddress | getInterface()
|
byte | getTTL()
|
void | joinGroup(InetAddress mcastaddr)
|
void | leaveGroup(InetAddress mcastaddr)
|
void | send(DatagramPacket p,
byte ttl)
|
void | setInterface(InetAddress inf)
|
void | setTTL(byte ttl)
|
Methods inherited from class java.net.DatagramSocket |
close, getLocalAddress, getLocalPort, getSoTimeout, receive, send, setSoTimeout |
Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
public MulticastSocket() throws IOException
public MulticastSocket(int port) throws IOException
local
- port to use
Method Detail |
public void setTTL(byte ttl) throws IOException
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
.
ttl
- the time-to-live
public byte getTTL() throws IOException
public void joinGroup(InetAddress mcastaddr) throws IOException
mcastaddr
- is the multicast address to join
public void leaveGroup(InetAddress mcastaddr) throws IOException
mcastaddr
- is the multicast address to leave
public void setInterface(InetAddress inf) throws SocketException
public InetAddress getInterface() throws SocketException
public void send(DatagramPacket p, byte ttl) throws IOException
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.
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