Chapter 15
The java.net Package

MulticastSocket

Name

MulticastSocket

Synopsis

Class Name:

java.net.MulticastSocket

Superclass:

java.net.DatagramSocket

Immediate Subclasses:

None

Interfaces Implemented:

None

Availability:

New as of JDK 1.1

The MulticastSocket class implements packet-oriented, connectionless, multicast data communication. In Internet parlance, this is the User Datagram Protocol (UDP) with additional functionality for joining and leaving groups of other multicast hosts on the Internet. A multicast group is specified by a Class D address, which means that the four highest-order bits are set to 1110. In other words, multicast addresses are in the range 224.0.0.1 through 239.255.255.255 inclusive.

MulticastSocket inherits most of its functionality from DatagramSocket; it adds the ability to join and leave multicast groups. When a MulticastSocket joins a group, it receives all of the packets destined for the group. Any DatagramSocket or MulticastSocket can send packets to a multicast group.

Class Summary

public final class java.net.MulticastSocket 
                   extends java.net.DatagramSocket {
  // Constructors
  public MulticastSocket();
  public MulticastSocket(int port);
  // Instance Methods
  public InetAddress getInterface();
  public byte getTTL();
  public void joinGroup(InetAddress mcastaddr);
  public void leaveGroup(InetAddress mcastaddr) 
  public synchronized void send(DatagramPacket p, byte ttl);
  public void setInterface(InetAddress inf);
  public void setTTL(byte ttl);
}

Constructors

MulticastSocket

public MulticastSocket() throws IOException

Throws

IOException

If any kind of I/O error occurs.

SecurityException

If the application is not allowed to listen on the port.

Description

This constructor creates a MulticastSocket that is bound to any available port on the local host machine.

public MulticastSocket(int port) throws IOException

Parameters

port: A port number.

Throws

IOException

If any kind of I/O error occurs.

SecurityException

If the application is not allowed to listen on the given port.

Description

This constructor creates a MulticastSocket that is bound to the given port on the local host machine.

Instance Methods

getInterface

public InetAddress getInterface() throws SocketException

Returns

The address of the network interface used for outgoing multicast packets.

Throws

SocketException: If any kind of socket error occurs.

Description

This method returns the IP address that this MulticastSocket uses to send out packets to multicast destinations.

getTTL

public byte getTTL() throws IOException

Returns

The time-to-live (TTL) value for this socket.

Throws

IOException: If any kind of I/O error occurs.

Description

This method returns the TTL value for this socket. This value is the number of hops an outgoing packet can traverse before it is discarded.

joinGroup

public void joinGroup(InetAddress mcastaddr) throws IOException

Parameters

mcastaddr: The IP address of the group to join.

Throws

IOException

If any kind of I/O error occurs.

SecurityException

If the application is not allowed to access the given multicast address.

Description

This method is used to join a multicast group. An exception is thrown if the given address is not a multicast address. While the socket is part of a group, it receives all the packets that are sent to the group.

leaveGroup

public void leaveGroup(InetAddress mcastaddr) throws IOException

Parameters

mcastaddr: The IP address of the group to leave.

Throws

IOException

If any kind of I/O error occurs.

SecurityException

If the application is not allowed to access the given multicast address.

Description

This method is used to leave a multicast group. An exception is thrown if the given address is not a multicast address.

send

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

Parameters

p

The DatagramPacket to be sent.

ttl

The time-to-live (TTL) value for this packet.

Throws

IOException

If any kind of I/O error occurs.

SecurityException

If the application is not allowed to send data to the packet's destination.

Description

This method sends a packet from this socket using the given TTL value. The packet data, packet length, destination address, and destination port number are specified by the given DatagramPacket.

Generally, it is easier to use setTTL() to set the TTL value for the socket, then use send(DatagramPacket) to send data. This method is provided for special cases.

setInterface

public void setInterface(InetAddress inf) throws SocketException

Parameters

inf: The new address of the network interface for multicast packets.

Throws

SocketException: If any kind of socket error occurs.

Description

This method is used to set the address that is used for outgoing multicast packets.

setTTL

public void setTTL(byte ttl) throws IOException

Parameters

ttl: The new time-to-live (TTL) value for this socket.

Throws

IOException: If any kind of I/O error occurs.

Description

This method is used to set the TTL value of the socket. The TTL value is the number of hops an outgoing packet can traverse before it is discarded.

Inherited Methods

Method	Inherited From	Method	Inherited From
`clone()`	`Object`	`close()`	`DatagramSocket`
`equals(Object)`	`Object`	`finalize()`	`Object`
`getClass()`	`Object`	`getLocalAddress()`	`DatagramSocket`
`getLocalPort()`	`DatagramSocket`	`getSoTimeout()`	`DatagramSocket`
`hashCode()`	`Object`	`notify()`	`Object`
`notifyAll()`	`Object`	`receive(DatagramPacket)`	`DatagramSocket`
`send(DatagramPacket)`	`DatagramSocket`	`setSoTimeout(int)`	`DatagramSocket`
`toString()`	`Object`	`wait()`	`Object`
`wait(long)`	`Object`	`wait(long, int)`	`Object`