MulticastSocket

AI-generated Key Takeaways

• MulticastSocket enables sending and receiving datagrams to a group of subscribers using UDP within a specified network.

• It provides methods to join and leave multicast groups, manage Time-To-Live (TTL) of packets, and control network interface selection.

• Loopback behavior and security permissions are important considerations when working with MulticastSockets.

• Some methods are deprecated; using updated alternatives like setTimeToLive() instead of send(DatagramPacket p, byte ttl) is recommended.

• Applets are restricted from using MulticastSockets due to security constraints.

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);
 byte[] bytes = msg.getBytes(StandardCharsets.UTF_8);
 DatagramPacket hi = new DatagramPacket(bytes, bytes.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.

Public Constructor Summary

	MulticastSocket() Create a multicast socket.
	MulticastSocket(int port) Create a multicast socket and bind it to a specific port.
	MulticastSocket(SocketAddress bindaddr) Create a MulticastSocket bound to the specified socket address.

Public Method Summary

InetAddress	getInterface() Retrieve the address of the network interface used for multicast packets.
boolean	getLoopbackMode() Get the setting for local loopback of multicast datagrams.
NetworkInterface	getNetworkInterface() Get the multicast network interface set.
byte	getTTL() This method is deprecated. use the getTimeToLive method instead, which returns an int instead of a byte.
int	getTimeToLive() Get the default time-to-live for multicast packets sent out on the socket.
void	joinGroup(SocketAddress mcastaddr, NetworkInterface netIf) Joins the specified multicast group at the specified interface.
void	joinGroup(InetAddress mcastaddr) Joins a multicast group.
void	leaveGroup(SocketAddress mcastaddr, NetworkInterface netIf) Leave a multicast group on a specified local interface.
void	leaveGroup(InetAddress mcastaddr) Leave a multicast group.
void	send(DatagramPacket p, byte ttl) This method is deprecated. Use the following code or its equivalent instead: ...... int ttl = mcastSocket.getTimeToLive(); mcastSocket.setTimeToLive(newttl); mcastSocket.send(p); mcastSocket.setTimeToLive(ttl); ......
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	setLoopbackMode(boolean disable) Disable/Enable local loopback of multicast datagrams The option is used by the platform's networking code as a hint for setting whether multicast data will be looped back to the local socket.
void	setNetworkInterface(NetworkInterface netIf) Specify the network interface for outgoing multicast datagrams sent on this socket.
void	setTTL(byte ttl) This method is deprecated. use the setTimeToLive method instead, which uses int instead of byte as the type for ttl.
void	setTimeToLive(int ttl) Set the default time-to-live for multicast packets sent out on this MulticastSocket in order to control the scope of the multicasts.

Inherited Method Summary

From class java.net.DatagramSocket

synchronized void	bind(SocketAddress addr) Binds this DatagramSocket to a specific address and port.
void	close() Closes this datagram socket.
void	connect(InetAddress address, int port) Connects the socket to a remote address for this socket.
void	connect(SocketAddress addr) Connects this socket to a remote socket address (IP address + port number).
void	disconnect() Disconnects the socket.
synchronized boolean	getBroadcast() Tests if SO_BROADCAST is enabled.
DatagramChannel	getChannel() Returns the unique DatagramChannel object associated with this datagram socket, if any.
InetAddress	getInetAddress() Returns the address to which this socket is connected.
InetAddress	getLocalAddress() Gets the local address to which the socket is bound.
int	getLocalPort() Returns the port number on the local host to which this socket is bound.
SocketAddress	getLocalSocketAddress() Returns the address of the endpoint this socket is bound to.
int	getPort() Returns the port number to which this socket is connected.
synchronized int	getReceiveBufferSize() Get value of the SO_RCVBUF option for this DatagramSocket, that is the buffer size used by the platform for input on this DatagramSocket.
SocketAddress	getRemoteSocketAddress() Returns the address of the endpoint this socket is connected to, or null if it is unconnected.
synchronized boolean	getReuseAddress() Tests if SO_REUSEADDR is enabled.
synchronized int	getSendBufferSize() Get value of the SO_SNDBUF option for this DatagramSocket, that is the buffer size used by the platform for output on this DatagramSocket.
synchronized int	getSoTimeout() Retrieve setting for SO_TIMEOUT.
synchronized int	getTrafficClass() Gets traffic class or type-of-service in the IP datagram header for packets sent from this DatagramSocket.
boolean	isBound() Returns the binding state of the socket.
boolean	isClosed() Returns whether the socket is closed or not.
boolean	isConnected() Returns the connection state of the socket.
synchronized void	receive(DatagramPacket p) Receives a datagram packet from this socket.
void	send(DatagramPacket p) Sends a datagram packet from this socket.
synchronized void	setBroadcast(boolean on) Enable/disable SO_BROADCAST.
synchronized static void	setDatagramSocketImplFactory(DatagramSocketImplFactory fac) Sets the datagram socket implementation factory for the application.
synchronized void	setReceiveBufferSize(int size) Sets the SO_RCVBUF option to the specified value for this DatagramSocket.
synchronized void	setReuseAddress(boolean on) Enable/disable the SO_REUSEADDR socket option.
synchronized void	setSendBufferSize(int size) Sets the SO_SNDBUF option to the specified value for this DatagramSocket.
synchronized void	setSoTimeout(int timeout) Enable/disable SO_TIMEOUT with the specified timeout, in milliseconds.
synchronized void	setTrafficClass(int tc) Sets traffic class or type-of-service octet in the IP datagram header for datagrams sent from this DatagramSocket.

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

From interface java.io.Closeable

abstract void

close() Closes this stream and releases any system resources associated with it.

From interface java.lang.AutoCloseable

abstract void

close() Closes this resource, relinquishing any underlying resources.

Public Constructors

public MulticastSocket ()

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.

When the socket is created the DatagramSocket.setReuseAddress(boolean) method is called to enable the SO_REUSEADDR socket option.

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)
• DatagramSocket.setReuseAddress(boolean)

public MulticastSocket (int port)

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.

When the socket is created the DatagramSocket.setReuseAddress(boolean) method is called to enable the SO_REUSEADDR socket option.

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)
• DatagramSocket.setReuseAddress(boolean)

public MulticastSocket (SocketAddress bindaddr)

Create a MulticastSocket bound to the specified socket address.

Or, if the address is null, create an unbound socket.

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

When the socket is created the DatagramSocket.setReuseAddress(boolean) method is called to enable the SO_REUSEADDR socket option.

Parameters

bindaddr	Socket address to bind to, or null for an unbound socket.

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)
• DatagramSocket.setReuseAddress(boolean)

Public Methods

public InetAddress getInterface ()

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)

public boolean getLoopbackMode ()

Get the setting for local loopback of multicast datagrams.

Returns

• true if the LoopbackMode has been disabled

Throws

SocketException	if an error occurs while getting the value

See Also

• setLoopbackMode(boolean)

public NetworkInterface getNetworkInterface ()

Get the multicast network interface set.

Returns

• the multicast NetworkInterface currently set

Throws

SocketException	if there is an error in the underlying protocol, such as a TCP error.

See Also

• setNetworkInterface(NetworkInterface)

public byte getTTL ()

This method is 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.

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

• setTTL(byte)

public int getTimeToLive ()

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)

public void joinGroup (SocketAddress mcastaddr, NetworkInterface netIf)

Joins the specified multicast group at the specified interface.

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
netIf	specifies the local interface to receive multicast datagram packets, or null to defer to the interface set by setInterface(InetAddress) or setNetworkInterface(NetworkInterface)

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.
IllegalArgumentException	if mcastaddr is null or is a SocketAddress subclass not supported by this socket

See Also

• SecurityManager.checkMulticast(InetAddress)

public void joinGroup (InetAddress mcastaddr)

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

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)

public void leaveGroup (SocketAddress mcastaddr, NetworkInterface netIf)

Leave a multicast group on a specified local interface.

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
netIf	specifies the local interface or null to defer to the interface set by setInterface(InetAddress) or setNetworkInterface(NetworkInterface)

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.
IllegalArgumentException	if mcastaddr is null or is a SocketAddress subclass not supported by this socket

See Also

• SecurityManager.checkMulticast(InetAddress)

public void leaveGroup (InetAddress mcastaddr)

Leave a multicast group. Its behavior may be affected by setInterface or setNetworkInterface.

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)

public void send (DatagramPacket p, byte ttl)

This method is deprecated.
Use the following code or its equivalent instead: ...... int ttl = mcastSocket.getTimeToLive(); mcastSocket.setTimeToLive(newttl); mcastSocket.send(p); mcastSocket.setTimeToLive(ttl); ......

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

public void setInterface (InetAddress inf)

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()

public void setLoopbackMode (boolean disable)

Disable/Enable local loopback of multicast datagrams The option is used by the platform's networking code as a hint for setting whether multicast data will be looped back to the local socket.

Because this option is a hint, applications that want to verify what loopback mode is set to should call getLoopbackMode()

Parameters

disable	true to disable the LoopbackMode

Throws

SocketException	if an error occurs while setting the value

See Also

• getLoopbackMode()

public void setNetworkInterface (NetworkInterface netIf)

Specify the network interface for outgoing multicast datagrams sent on this socket.

Parameters

netIf	the interface

Throws

SocketException	if there is an error in the underlying protocol, such as a TCP error.

See Also

• getNetworkInterface()

public void setTTL (byte ttl)

This method is 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 MulticastSocket in order to control the scope of the multicasts.

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()

public void setTimeToLive (int ttl)

Set the default time-to-live for multicast packets sent out on this MulticastSocket in order to control the scope of the multicasts.

The ttl must be in the range 0 <= ttl <= 255 or an IllegalArgumentException will be thrown. Multicast packets sent with a TTL of 0 are not transmitted on the network but may be delivered locally.

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()