JavaTM 2 Platform
Std. Ed. v1.3.1

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.
 byte getTTL()
          Deprecated. use the getTimeToLive method instead, which returns an int instead of a byte.
 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.
 void setTTL(byte ttl)
          Deprecated. use the setTimeToLive method instead, which uses int instead of byte as the type for ttl.
 
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

setTTL

public void setTTL(byte ttl)
            throws IOException
Deprecated. use the setTimeToLive method instead, which uses int instead of byte as the type for ttl.

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 an unsigned 8-bit quantity, and so must be in the range 0 <= ttl <= 0xFF .

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

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()

getTTL

public byte getTTL()
            throws IOException
Deprecated. use the getTimeToLive method instead, which returns an int instead of a byte.

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

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)

JavaTM 2 Platform
Std. Ed. v1.3.1

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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