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Class MulticastSocket

java.lang.Object
java.net.DatagramSocket
java.net.MulticastSocket
All Implemented Interfaces:
Closeable, AutoCloseable 
public class MulticastSocket extends DatagramSocket
A MulticastSocket is a datagram socket that is convenient for sending and receiving IP multicast datagrams. The MulticastSocket constructors create a socket with appropriate socket options enabled that make it suitable for receiving multicast datagrams. The MulticastSocket class additionally defines convenient setter and getter methods for socket options that are commonly used by multicasting applications.

Joining one or more multicast groups makes it possible to receive multicast datagrams sent to these groups.

An IPv4 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 method, specifying the group address and the network interface through which multicast datagrams will be received: 


 // join a Multicast group and send the group salutations
 ...
 String msg = "Hello";
 InetAddress mcastaddr = InetAddress.getByName("228.5.6.7");
 InetSocketAddress group = new InetSocketAddress(mcastaddr, 6789);
 NetworkInterface netIf = NetworkInterface.getByName("bge0");
 MulticastSocket s = new MulticastSocket(6789);

 s.joinGroup(new InetSocketAddress(mcastaddr, 0), netIf);
 byte[] msgBytes = msg.getBytes(StandardCharsets.UTF_8);
 DatagramPacket hi = new DatagramPacket(msgBytes, msgBytes.length, group);
 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, netIf);
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(SocketAddress mcastaddr, NetworkInterface netIf) method. Multiple MulticastSockets may subscribe to a multicast group and port concurrently, and they will all receive group datagrams.

The DatagramSocket and MulticastSocket classes define convenience methods to set and get several socket options. Like DatagramSocket this class also supports the setOption and getOption methods to set and query socket options. The set of supported socket options is defined in DatagramSocket. Additional (implementation specific) options may also be supported.

API Note:
DatagramSocket may be used directly for sending and receiving multicast datagrams. DatagramChannel implements the MulticastChannel interface and provides an alternative API for sending and receiving multicast datagrams. The MulticastChannel API supports both any-source and source-specific multicast. Consider using DatagramChannel for multicasting.
Since:
1.1

Constructor Summary

Constructor Description
MulticastSocket()
Constructs a multicast socket and binds it to any available port on the local host machine.
MulticastSocket(int port)
Constructs a multicast socket and binds it to the specified port on the local host machine.
MulticastSocket(SocketAddress bindaddr)
Creates a multicast socket, bound to the specified local socket address.

Method Summary

Modifier and Type Method Description
InetAddress getInterface()
Deprecated.
The network interface may not be uniquely identified by the InetAddress returned.
boolean getLoopbackMode()
Deprecated.
Use DatagramSocket.getOption(SocketOption) with StandardSocketOptions.IP_MULTICAST_LOOP instead.
NetworkInterface getNetworkInterface()
Get the multicast network interface set for outgoing multicast datagrams sent from this socket.
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)
Deprecated.
This method does not accept the network interface on which to join the multicast group.
void joinGroup(SocketAddress mcastaddr, NetworkInterface netIf)
Joins a multicast group.
void leaveGroup(InetAddress mcastaddr)
Deprecated.
This method does not accept the network interface on which to leave the multicast group.
void leaveGroup(SocketAddress mcastaddr, NetworkInterface netIf)
Leave a multicast group on a specified local interface.
void send(DatagramPacket p, byte ttl)
Deprecated.
Use the following code or its equivalent instead:
void setInterface(InetAddress inf)
Deprecated.
The InetAddress may not uniquely identify the network interface.
void setLoopbackMode(boolean disable)
Deprecated.
Use DatagramSocket.setOption(SocketOption, Object) with StandardSocketOptions.IP_MULTICAST_LOOP instead.
void setNetworkInterface(NetworkInterface netIf)
Specify the network interface for outgoing multicast datagrams sent on this socket.
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.
void setTTL(byte ttl)
Deprecated.
use the setTimeToLive(int) method instead, which uses int instead of byte as the type for ttl.

Methods declared in class java.net.DatagramSocket

bind, close, connect, connect, disconnect, getBroadcast, getChannel, getInetAddress, getLocalAddress, getLocalPort, getLocalSocketAddress, getOption, getPort, getReceiveBufferSize, getRemoteSocketAddress, getReuseAddress, getSendBufferSize, getSoTimeout, getTrafficClass, isBound, isClosed, isConnected, receive, send, setBroadcast, setDatagramSocketImplFactory, setOption, setReceiveBufferSize, setReuseAddress, setSendBufferSize, setSoTimeout, setTrafficClass, supportedOptions

Methods declared in class java.lang.Object

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

Constructor Details

MulticastSocket

public MulticastSocket() throws IOException
Constructs a multicast socket and binds it to any available port on the local host machine. The socket will be bound to the wildcard address.

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:

MulticastSocket

public MulticastSocket(int port) throws IOException
Constructs a multicast socket and binds it to the specified port on the local host machine. The socket will be bound to the wildcard address.

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.
IllegalArgumentException - if port is out of range.
See Also:

MulticastSocket

public MulticastSocket(SocketAddress bindaddr) throws IOException
Creates a multicast socket, bound to the specified local socket address.

If the address is null an unbound socket will be created.

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.
Since:
1.4
See Also:

Method Details

setTTL

@Deprecated public void setTTL(byte ttl) throws IOException
Deprecated.
use the setTimeToLive(int) 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:

setTimeToLive

public void setTimeToLive(int ttl) throws IOException
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.

API Note:
This method is equivalent to calling setOption(StandardSocketOptions.IP_MULTICAST_TTL, ttl).
Parameters:
ttl - the time-to-live
Throws:
IOException - if an I/O exception occurs while setting the default time-to-live value
Since:
1.2
See Also:

getTTL

@Deprecated 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.
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:

getTimeToLive

public int getTimeToLive() throws IOException
Get the default time-to-live for multicast packets sent out on the socket.
API Note:
This method is equivalent to calling getOption(StandardSocketOptions.IP_MULTICAST_TTL).
Returns:
the default time-to-live value
Throws:
IOException - if an I/O exception occurs while getting the default time-to-live value
Since:
1.2
See Also:

joinGroup

@Deprecated(since="14") public void joinGroup(InetAddress mcastaddr) throws IOException
Deprecated.
This method does not accept the network interface on which to join the multicast group. Use joinGroup(SocketAddress, NetworkInterface) instead.
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.

API Note:
Calling this method is equivalent to calling joinGroup(new InetSocketAddress(mcastaddr, 0), null).
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, or the platform does not support multicasting
SecurityException - if a security manager exists and its checkMulticast method doesn't allow the join.
See Also:

leaveGroup

@Deprecated(since="14") public void leaveGroup(InetAddress mcastaddr) throws IOException
Deprecated.
This method does not accept the network interface on which to leave the multicast group. Use leaveGroup(SocketAddress, NetworkInterface) instead.
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.

API Note:
Calling this method is equivalent to calling leaveGroup(new InetSocketAddress(mcastaddr, 0), null).
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:

joinGroup

public void joinGroup(SocketAddress mcastaddr, NetworkInterface netIf) throws IOException
Joins a multicast group.

In order to join a multicast group, the caller should specify the IP address of the multicast group to join, and the local network interface to receive multicast packets from.

  • The mcastaddr argument indicates the IP address of the multicast group to join. For historical reasons this is specified as a SocketAddress. The default implementation only supports InetSocketAddress and the port information is ignored.
  • The netIf argument specifies the local interface to receive multicast datagram packets, or null to defer to the interface set for outgoing multicast datagrams. If null, and no interface has been set, the behaviour is unspecified: any interface may be selected or the operation may fail with a SocketException.

It is possible to call this method several times to join several different multicast groups, or join the same group in several different networks. However, if the socket is already a member of the group, an IOException will be thrown.

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

Overrides:
joinGroup in class DatagramSocket
Parameters:
mcastaddr - indicates the multicast address to join.
netIf - specifies the local interface to receive multicast datagram packets, or null.
Throws:
IOException - if there is an error joining, or when the address is not a multicast address, or the platform does not support multicasting
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
Since:
1.4
See Also:

leaveGroup

public void leaveGroup(SocketAddress mcastaddr, NetworkInterface netIf) throws IOException
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.

Overrides:
leaveGroup in class DatagramSocket
API Note:
The mcastaddr and netIf arguments should identify a multicast group that was previously joined by this DatagramSocket.

It is possible to call this method several times to leave multiple different multicast groups previously joined, or leave the same group previously joined in multiple different networks. However, if the socket is not a member of the specified group in the specified network, an IOException will be thrown.

Parameters:
mcastaddr - is the multicast address to leave. This should contain the same IP address than that used for joining the group.
netIf - specifies the local interface or null to defer to the interface set for outgoing multicast datagrams. If null, and no interface has been set, the behaviour is unspecified: any interface may be selected or the operation may fail with a SocketException.
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.
Since:
1.4
See Also:

setInterface

@Deprecated(since="14") public void setInterface(InetAddress inf) throws SocketException
Deprecated.
The InetAddress may not uniquely identify the network interface. Use setNetworkInterface(NetworkInterface) instead.
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

@Deprecated(since="14") public InetAddress getInterface() throws SocketException
Deprecated.
The network interface may not be uniquely identified by the InetAddress returned. Use getNetworkInterface() instead.
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, or if no interface has been set, an InetAddress representing any local address.
Throws:
SocketException - if there is an error in the underlying protocol, such as a TCP error.
See Also:

setNetworkInterface

public void setNetworkInterface(NetworkInterface netIf) throws SocketException
Specify the network interface for outgoing multicast datagrams sent on this socket.
API Note:
This method is equivalent to calling setOption(StandardSocketOptions.IP_MULTICAST_IF, netIf).
Parameters:
netIf - the interface
Throws:
SocketException - if there is an error in the underlying protocol, such as a TCP error.
Since:
1.4
See Also:

getNetworkInterface

public NetworkInterface getNetworkInterface() throws SocketException
Get the multicast network interface set for outgoing multicast datagrams sent from this socket.
API Note:
When an interface is set, this method is equivalent to calling getOption(StandardSocketOptions.IP_MULTICAST_IF).
Returns:
The multicast NetworkInterface currently set. A placeholder NetworkInterface is returned when there is no interface set; it has a single InetAddress to represent any local address.
Throws:
SocketException - if there is an error in the underlying protocol, such as a TCP error.
Since:
1.4
See Also:

setLoopbackMode

@Deprecated(since="14") public void setLoopbackMode(boolean disable) throws SocketException
Deprecated.
Use DatagramSocket.setOption(SocketOption, Object) with StandardSocketOptions.IP_MULTICAST_LOOP instead. The loopback mode is enabled by default, MulticastSocket.setOption(StandardSocketOptions.IP_MULTICAST_LOOP, false) disables it.
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
Since:
1.4
See Also:

getLoopbackMode

@Deprecated(since="14") public boolean getLoopbackMode() throws SocketException
Deprecated.
Use DatagramSocket.getOption(SocketOption) with StandardSocketOptions.IP_MULTICAST_LOOP instead.
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
Since:
1.4
See Also:

send

@Deprecated public void send(DatagramPacket p, byte ttl) throws IOException
Deprecated.
Use the following code or its equivalent instead: 
  ......
  int ttl = mcastSocket.getOption(StandardSocketOptions.IP_MULTICAST_TTL);
  mcastSocket.setOption(StandardSocketOptions.IP_MULTICAST_TTL, newttl);
  mcastSocket.send(p);
  mcastSocket.setOption(StandardSocketOptions.IP_MULTICAST_TTL, 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.
PortUnreachableException - may be thrown if the socket is connected to a currently unreachable destination. Note, there is no guarantee that the exception will be thrown.
IllegalArgumentException - if the socket is connected, and connected address and packet address differ, or if the socket is not connected and the packet address is not set or if its port is out of range.
See Also:

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/net/MulticastSocket.html