java.net
Class MulticastSocket

java.lang.Object
  |
  +--java.net.DatagramSocket
        |
        +--java.net.MulticastSocket

public class MulticastSocket

extends DatagramSocket

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 to use multicast sockets.

Since:

JDK1.1

Constructor Summary

MulticastSocket()
Create a multicast socket.

MulticastSocket(int port)
Create a multicast socket and bind it to a specific port.

Method Summary

InetAddress	getInterface() Retrieve the address of the network interface used for multicast packets.
int	getTimeToLive() Get the default time-to-live for multicast packets sent out on the socket.
byte	getTTL() Deprecated. use the getTimeToLive method instead, which allows you to get time-to-live values from 0 (excluded) to 255 (included).
void	joinGroup(InetAddress mcastaddr) Joins a multicast group.
void	leaveGroup(InetAddress mcastaddr) Leave a multicast group.
void	send(DatagramPacket p, byte ttl) Sends a datagram packet to the destination, with a TTL (time- to-live) other than the default for the socket.
void	setInterface(InetAddress inf) Set the outgoing network interface for multicast packets on this socket, to other than the system default.
void	setTimeToLive(int ttl) Set the default time-to-live for multicast packets sent out on this socket.
void	setTTL(byte ttl) Deprecated. use the setTimeToLive method instead, which allows you to set time-to-live values from 0 (excluded) to 255 (included).

Methods inherited from class java.net.DatagramSocket

close, connect, disconnect, getInetAddress, getLocalAddress, getLocalPort, getPort, getReceiveBufferSize, getSendBufferSize, getSoTimeout, receive, send, setReceiveBufferSize, setSendBufferSize, setSoTimeout

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

MulticastSocket

public MulticastSocket()
                throws IOException

Create a multicast socket.

If there is a security manager, its checkListen method is first called with 0 as its argument to ensure the operation is allowed. This could result in a SecurityException.

Throws:

SecurityException - if a security manager exists and its checkListen method doesn't allow the operation.

See Also:

SecurityManager.checkListen(int)

MulticastSocket

public MulticastSocket(int port)
                throws IOException

Create a multicast socket and bind it to a specific port.

If there is a security manager, its checkListen method is first called with the port argument as its argument to ensure the operation is allowed. This could result in a SecurityException.

Parameters:

port - port to use

Throws:

SecurityException - if a security manager exists and its checkListen method doesn't allow the operation.

See Also:

SecurityManager.checkListen(int)

Method Detail

setTTL

public void setTTL(byte ttl)
            throws IOException

Deprecated. use the setTimeToLive method instead, which allows you to set time-to-live values from 0 (excluded) to 255 (included).

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.

This method may only be used to set time-to-live value between 1 and 127. The behavior if the value is outside that range is undefined.

Parameters:

ttl - the time-to-live

setTimeToLive

public void setTimeToLive(int ttl)
                   throws IOException

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 must be in the range 0 < ttl <= 255 or an IllegalArgumentException will be thrown.

Parameters:

ttl - the time-to-live

getTTL

public byte getTTL()
            throws IOException

Deprecated. use the getTimeToLive method instead, which allows you to get time-to-live values from 0 (excluded) to 255 (included).

Get the default time-to-live for multicast packets sent out on the socket. This method will truncate any time to live values greater than 127 to 127.

getTimeToLive

public int getTimeToLive()
                  throws IOException

Get the default time-to-live for multicast packets sent out on the socket.

joinGroup

public void joinGroup(InetAddress mcastaddr)
               throws IOException

Joins a multicast group.

If there is a security manager, this method first calls its checkMulticast method with the mcastaddr argument as its argument.

Parameters:

mcastaddr - is the multicast address to join

Throws:

IOException - if there is an error joining or when the address is not a multicast address.

SecurityException - if a security manager exists and its checkMulticast method doesn't allow the join.

See Also:

SecurityManager.checkMulticast(InetAddress)

leaveGroup

public void leaveGroup(InetAddress mcastaddr)
                throws IOException

Leave a multicast group.

If there is a security manager, this method first calls its checkMulticast method with the mcastaddr argument as its argument.

Parameters:

mcastaddr - is the multicast address to leave

Throws:

IOException - if there is an error leaving or when the address is not a multicast address.

SecurityException - if a security manager exists and its checkMulticast method doesn't allow the operation.

See Also:

SecurityManager.checkMulticast(InetAddress)

setInterface

public void setInterface(InetAddress inf)
                  throws SocketException

Set the outgoing network interface for multicast packets on this socket, to other than the system default. Useful for multihomed hosts.

getInterface

public InetAddress getInterface()
                         throws SocketException

Retrieve the address of the network interface used for multicast packets.

send

public void send(DatagramPacket p,
                 byte ttl)
          throws IOException

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.

If there is a security manager, this method first performs some security checks. First, if p.getAddress().isMulticastAddress() is true, this method calls the security manager's checkMulticast method with p.getAddress() and ttl as its arguments. If the evaluation of that expression is false, this method instead calls the security manager's checkConnect method with arguments p.getAddress().getHostAddress() and p.getPort(). Each call to a security manager method could result in a SecurityException if the operation is not allowed.

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.

Throws:

IOException - is raised if an error occurs i.e error while setting ttl.

SecurityException - if a security manager exists and its checkMulticast or checkConnect method doesn't allow the send.

See Also:

DatagramSocket.send(java.net.DatagramPacket), DatagramSocket.receive(java.net.DatagramPacket), SecurityManager.checkMulticast(java.net.InetAddress, byte), SecurityManager.checkConnect(java.lang.String, int)