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## 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 and by a standard UDP port number. Class D IP addresses are in the range `224.0.0.0` to `239.255.255.255`, inclusive. The address 224.0.0.0 is reserved and should not be used.

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
     ...
     String msg = "Hello";
     InetAddress group = InetAddress.getByName("228.5.6.7");
     MulticastSocket s = new MulticastSocket(6789);
     s.joinGroup(group);
     DatagramPacket hi = new DatagramPacket(msg.getBytes(), 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.
`void` | `joinGroup(InetAddress mcastaddr)` Joins a multicast group.Its behavior may be affected by `setInterface`.
`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 multicast network interface used by methods whose behavior would be affected by the value of the network interface.
`void` | `setTimeToLive(int ttl)` Set the default time-to-live for multicast packets sent out on this socket.

Methods inherited from class java.net.DatagramSocket
`close, connect, disconnect, getInetAddress, getLocalAddress, getLocalPort, getPort, getReceiveBufferSize, getSendBufferSize, getSoTimeout, receive, send, setDatagramSocketImplFactory, 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:
    `IOException` - if an I/O exception occurs while creating the MulticastSocket
    `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:
    `IOException` - if an I/O exception occurs while creating the MulticastSocket
    `SecurityException` - if a security manager exists and its `checkListen` method doesn't allow the operation.
  See Also:
    `SecurityManager.checkListen(int)`

Method Detail

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

  Parameters:
    `ttl` - the time-to-live
  Throws:
    `IOException` - if an I/O exception occurs while setting the default time-to-live value
  See Also:
    `getTimeToLive()`

### getTimeToLive

    public int getTimeToLive()
                      throws IOException

  Get the default time-to-live for multicast packets sent out on the socket.
  Returns:
    the default time-to-live value
  Throws:
    `IOException` - if an I/O exception occurs while getting the default time-to-live value
  See Also:
    `setTimeToLive(int)`

### joinGroup

    public void joinGroup(InetAddress mcastaddr)
                   throws IOException

  Joins a multicast group.Its behavior may be affected by `setInterface`.

  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. Its behavior may be affected by `setInterface`.

  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 multicast network interface used by methods whose behavior would be affected by the value of the network interface. Useful for multihomed hosts.
  Parameters:
    `inf` - the InetAddress
  Throws:
    `SocketException` - if there is an error in the underlying protocol, such as a TCP error.
  See Also:
    `getInterface()`

### getInterface

    public InetAddress getInterface()
                             throws SocketException

  Retrieve the address of the network interface used for multicast packets.
  Returns:
    An `InetAddress` representing the address of the network interface used for multicast packets.
  Throws:
    `SocketException` - if there is an error in the underlying protocol, such as a TCP error.
  See Also:
    `setInterface(java.net.InetAddress)`

### 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. Its behavior may be affected by `setInterface`.

  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)`

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