DatagramSocket

public class DatagramSocket extends Object
implements Closeable
Known Direct Subclasses

This class represents a socket for sending and receiving datagram packets.

A datagram socket is the sending or receiving point for a packet delivery service. Each packet sent or received on a datagram socket is individually addressed and routed. Multiple packets sent from one machine to another may be routed differently, and may arrive in any order.

Where possible, a newly constructed DatagramSocket has the SO_BROADCAST socket option enabled so as to allow the transmission of broadcast datagrams. In order to receive broadcast packets a DatagramSocket should be bound to the wildcard address. In some implementations, broadcast packets may also be received when a DatagramSocket is bound to a more specific address.

Example: DatagramSocket s = new DatagramSocket(null); s.bind(new InetSocketAddress(8888)); Which is equivalent to: DatagramSocket s = new DatagramSocket(8888); Both cases will create a DatagramSocket able to receive broadcasts on UDP port 8888.

Public Constructor Summary

DatagramSocket()
Constructs a datagram socket and binds it to any available port on the local host machine.
DatagramSocket(SocketAddress bindaddr)
Creates a datagram socket, bound to the specified local socket address.
DatagramSocket(int port)
Constructs a datagram socket and binds it to the specified port on the local host machine.
DatagramSocket(int port, InetAddress laddr)
Creates a datagram socket, bound to the specified local address.

Protected Constructor Summary

DatagramSocket(DatagramSocketImpl impl)
Creates an unbound datagram socket with the specified DatagramSocketImpl.

Public Method Summary

synchronized void
bind(SocketAddress addr)
Binds this DatagramSocket to a specific address & 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.

Inherited Method Summary

Public Constructors

public DatagramSocket ()

Constructs a datagram socket and binds it to any available port on the local host machine. The socket will be bound to the wildcard address, an IP address chosen by the kernel.

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
SocketException if the socket could not be opened, or the socket could not bind to the specified local port.
SecurityException if a security manager exists and its checkListen method doesn't allow the operation.
See Also

public DatagramSocket (SocketAddress bindaddr)

Creates a datagram socket, bound to the specified local socket address.

If, if the address is null, creates an unbound socket.

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

Parameters
bindaddr local socket address to bind, or null for an unbound socket.
Throws
SocketException if the socket could not be opened, or the socket could not bind to the specified local port.
SecurityException if a security manager exists and its checkListen method doesn't allow the operation.
See Also

public DatagramSocket (int port)

Constructs a datagram socket and binds it to the specified port on the local host machine. The socket will be bound to the wildcard address, an IP address chosen by the kernel.

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
SocketException if the socket could not be opened, or the socket could not bind to the specified local port.
SecurityException if a security manager exists and its checkListen method doesn't allow the operation.
See Also

public DatagramSocket (int port, InetAddress laddr)

Creates a datagram socket, bound to the specified local address. The local port must be between 0 and 65535 inclusive. If the IP address is 0.0.0.0, the socket will be bound to the wildcard address, an IP address chosen by the kernel.

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 local port to use
laddr local address to bind
Throws
SocketException if the socket could not be opened, or the socket could not bind to the specified local port.
SecurityException if a security manager exists and its checkListen method doesn't allow the operation.
See Also

Protected Constructors

protected DatagramSocket (DatagramSocketImpl impl)

Creates an unbound datagram socket with the specified DatagramSocketImpl.

Parameters
impl an instance of a DatagramSocketImpl the subclass wishes to use on the DatagramSocket.

Public Methods

public synchronized void bind (SocketAddress addr)

Binds this DatagramSocket to a specific address & port.

If the address is null, then the system will pick up an ephemeral port and a valid local address to bind the socket.

Parameters
addr The address & port to bind to.
Throws
SocketException if any error happens during the bind, or if the socket is already bound.
SecurityException if a security manager exists and its checkListen method doesn't allow the operation.
IllegalArgumentException if addr is a SocketAddress subclass not supported by this socket.

public void close ()

Closes this datagram socket.

Any thread currently blocked in receive(DatagramPacket) upon this socket will throw a SocketException.

If this socket has an associated channel then the channel is closed as well.

public void connect (InetAddress address, int port)

Connects the socket to a remote address for this socket. When a socket is connected to a remote address, packets may only be sent to or received from that address. By default a datagram socket is not connected.

If the remote destination to which the socket is connected does not exist, or is otherwise unreachable, and if an ICMP destination unreachable packet has been received for that address, then a subsequent call to send or receive may throw a PortUnreachableException. Note, there is no guarantee that the exception will be thrown.

If a security manager has been installed then it is invoked to check access to the remote address. Specifically, if the given address is a multicast address, the security manager's checkMulticast method is invoked with the given address. Otherwise, the security manager's checkConnect and checkAccept methods are invoked, with the given address and port, to verify that datagrams are permitted to be sent and received respectively.

When a socket is connected, receive and send will not perform any security checks on incoming and outgoing packets, other than matching the packet's and the socket's address and port. On a send operation, if the packet's address is set and the packet's address and the socket's address do not match, an IllegalArgumentException will be thrown. A socket connected to a multicast address may only be used to send packets.

Parameters
address the remote address for the socket
port the remote port for the socket.
Throws
IllegalArgumentException if the address is null, or the port is out of range.
SecurityException if a security manager has been installed and it does not permit access to the given remote address
See Also

public void connect (SocketAddress addr)

Connects this socket to a remote socket address (IP address + port number).

If given an InetSocketAddress, this method behaves as if invoking connect(InetAddress,int) with the the given socket addresses IP address and port number.

Parameters
addr The remote address.
Throws
SocketException if the connect fails
IllegalArgumentException if addr is null, or addr is a SocketAddress subclass not supported by this socket
SecurityException if a security manager has been installed and it does not permit access to the given remote address

public void disconnect ()

Disconnects the socket. If the socket is closed or not connected, then this method has no effect.

public synchronized boolean getBroadcast ()

Tests if SO_BROADCAST is enabled.

Returns
  • a boolean indicating whether or not SO_BROADCAST is enabled.
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.

public DatagramChannel getChannel ()

Returns the unique DatagramChannel object associated with this datagram socket, if any.

A datagram socket will have a channel if, and only if, the channel itself was created via the DatagramChannel.open method.

Returns
  • the datagram channel associated with this datagram socket, or null if this socket was not created for a channel

public InetAddress getInetAddress ()

Returns the address to which this socket is connected. Returns null if the socket is not connected.

If the socket was connected prior to being closed, then this method will continue to return the connected address after the socket is closed.

Returns
  • the address to which this socket is connected.

public InetAddress getLocalAddress ()

Gets the local address to which the socket is bound.

If there is a security manager, its checkConnect method is first called with the host address and -1 as its arguments to see if the operation is allowed.

Returns
  • the local address to which the socket is bound, null if the socket is closed, or an InetAddress representing wildcard address if either the socket is not bound, or the security manager checkConnect method does not allow the operation

public int getLocalPort ()

Returns the port number on the local host to which this socket is bound.

Returns
  • the port number on the local host to which this socket is bound, -1 if the socket is closed, or 0 if it is not bound yet.

public SocketAddress getLocalSocketAddress ()

Returns the address of the endpoint this socket is bound to.

Returns
  • a SocketAddress representing the local endpoint of this socket, or null if it is closed or not bound yet.

public int getPort ()

Returns the port number to which this socket is connected. Returns -1 if the socket is not connected.

If the socket was connected prior to being closed, then this method will continue to return the connected port number after the socket is closed.

Returns
  • the port number to which this socket is connected.

public 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.

Returns
  • the value of the SO_RCVBUF option for this DatagramSocket
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.

public SocketAddress getRemoteSocketAddress ()

Returns the address of the endpoint this socket is connected to, or null if it is unconnected.

If the socket was connected prior to being closed, then this method will continue to return the connected address after the socket is closed.

Returns
  • a SocketAddress representing the remote endpoint of this socket, or null if it is not connected yet.

public synchronized boolean getReuseAddress ()

Tests if SO_REUSEADDR is enabled.

Returns
  • a boolean indicating whether or not SO_REUSEADDR is enabled.
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.

public 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.

Returns
  • the value of the SO_SNDBUF option for this DatagramSocket
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.

public synchronized int getSoTimeout ()

Retrieve setting for SO_TIMEOUT. 0 returns implies that the option is disabled (i.e., timeout of infinity).

Returns
  • the setting for SO_TIMEOUT
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.

public synchronized int getTrafficClass ()

Gets traffic class or type-of-service in the IP datagram header for packets sent from this DatagramSocket.

As the underlying network implementation may ignore the traffic class or type-of-service set using setTrafficClass(int) this method may return a different value than was previously set using the setTrafficClass(int) method on this DatagramSocket.

Returns
  • the traffic class or type-of-service already set
Throws
SocketException if there is an error obtaining the traffic class or type-of-service value.

public boolean isBound ()

Returns the binding state of the socket.

If the socket was bound prior to being closed, then this method will continue to return true after the socket is closed.

Returns
  • true if the socket successfully bound to an address

public boolean isClosed ()

Returns whether the socket is closed or not.

Returns
  • true if the socket has been closed

public boolean isConnected ()

Returns the connection state of the socket.

If the socket was connected prior to being closed, then this method will continue to return true after the socket is closed.

Returns
  • true if the socket successfully connected to a server

public synchronized void receive (DatagramPacket p)

Receives a datagram packet from this socket. When this method returns, the DatagramPacket's buffer is filled with the data received. The datagram packet also contains the sender's IP address, and the port number on the sender's machine.

This method blocks until a datagram is received. The length field of the datagram packet object contains the length of the received message. If the message is longer than the packet's length, the message is truncated.

If there is a security manager, a packet cannot be received if the security manager's checkAccept method does not allow it.

Parameters
p the DatagramPacket into which to place the incoming data.
Throws
IOException if an I/O error occurs.
SocketTimeoutException if setSoTimeout was previously called and the timeout has expired.
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.
IllegalBlockingModeException if this socket has an associated channel, and the channel is in non-blocking mode.

public void send (DatagramPacket p)

Sends a datagram packet from this socket. The DatagramPacket includes information indicating the data to be sent, its length, the IP address of the remote host, and the port number on the remote host.

If there is a security manager, and the socket is not currently connected to a remote address, 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() as its argument. 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 the DatagramPacket to be sent.
Throws
IOException if an I/O error occurs.
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.
IllegalBlockingModeException if this socket has an associated channel, and the channel is in non-blocking mode.
IllegalArgumentException if the socket is connected, and connected address and packet address differ.

public synchronized void setBroadcast (boolean on)

Enable/disable SO_BROADCAST.

Some operating systems may require that the Java virtual machine be started with implementation specific privileges to enable this option or send broadcast datagrams.

Parameters
on whether or not to have broadcast turned on.
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.
See Also

public static synchronized void setDatagramSocketImplFactory (DatagramSocketImplFactory fac)

Sets the datagram socket implementation factory for the application. The factory can be specified only once.

When an application creates a new datagram socket, the socket implementation factory's createDatagramSocketImpl method is called to create the actual datagram socket implementation.

Passing null to the method is a no-op unless the factory was already set.

If there is a security manager, this method first calls the security manager's checkSetFactory method to ensure the operation is allowed. This could result in a SecurityException.

Parameters
fac the desired factory.
Throws
IOException if an I/O error occurs when setting the datagram socket factory.
SocketException if the factory is already defined.
SecurityException if a security manager exists and its checkSetFactory method doesn't allow the operation.

public synchronized void setReceiveBufferSize (int size)

Sets the SO_RCVBUF option to the specified value for this DatagramSocket. The SO_RCVBUF option is used by the the network implementation as a hint to size the underlying network I/O buffers. The SO_RCVBUF setting may also be used by the network implementation to determine the maximum size of the packet that can be received on this socket.

Because SO_RCVBUF is a hint, applications that want to verify what size the buffers were set to should call getReceiveBufferSize().

Increasing SO_RCVBUF may allow the network implementation to buffer multiple packets when packets arrive faster than are being received using receive(DatagramPacket).

Note: It is implementation specific if a packet larger than SO_RCVBUF can be received.

Parameters
size the size to which to set the receive buffer size. This value must be greater than 0.
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.
IllegalArgumentException if the value is 0 or is negative.

public synchronized void setReuseAddress (boolean on)

Enable/disable the SO_REUSEADDR socket option.

For UDP sockets it may be necessary to bind more than one socket to the same socket address. This is typically for the purpose of receiving multicast packets (See MulticastSocket). The SO_REUSEADDR socket option allows multiple sockets to be bound to the same socket address if the SO_REUSEADDR socket option is enabled prior to binding the socket using bind(SocketAddress).

Note: This functionality is not supported by all existing platforms, so it is implementation specific whether this option will be ignored or not. However, if it is not supported then getReuseAddress() will always return false.

When a DatagramSocket is created the initial setting of SO_REUSEADDR is disabled.

The behaviour when SO_REUSEADDR is enabled or disabled after a socket is bound (See isBound()) is not defined.

Parameters
on whether to enable or disable the
Throws
SocketException if an error occurs enabling or disabling the SO_RESUEADDR socket option, or the socket is closed.

public synchronized void setSendBufferSize (int size)

Sets the SO_SNDBUF option to the specified value for this DatagramSocket. The SO_SNDBUF option is used by the network implementation as a hint to size the underlying network I/O buffers. The SO_SNDBUF setting may also be used by the network implementation to determine the maximum size of the packet that can be sent on this socket.

As SO_SNDBUF is a hint, applications that want to verify what size the buffer is should call getSendBufferSize().

Increasing the buffer size may allow multiple outgoing packets to be queued by the network implementation when the send rate is high.

Note: If send(DatagramPacket) is used to send a DatagramPacket that is larger than the setting of SO_SNDBUF then it is implementation specific if the packet is sent or discarded.

Parameters
size the size to which to set the send buffer size. This value must be greater than 0.
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.
IllegalArgumentException if the value is 0 or is negative.

public synchronized void setSoTimeout (int timeout)

Enable/disable SO_TIMEOUT with the specified timeout, in milliseconds. With this option set to a non-zero timeout, a call to receive() for this DatagramSocket will block for only this amount of time. If the timeout expires, a java.net.SocketTimeoutException is raised, though the DatagramSocket is still valid. The option must be enabled prior to entering the blocking operation to have effect. The timeout must be > 0. A timeout of zero is interpreted as an infinite timeout.

Parameters
timeout the specified timeout in milliseconds.
Throws
SocketException if there is an error in the underlying protocol, such as an UDP error.
See Also

public synchronized void setTrafficClass (int tc)

Sets traffic class or type-of-service octet in the IP datagram header for datagrams sent from this DatagramSocket. As the underlying network implementation may ignore this value applications should consider it a hint.

The tc must be in the range 0 <= tc <= 255 or an IllegalArgumentException will be thrown.

Notes:

For Internet Protocol v4 the value consists of an integer, the least significant 8 bits of which represent the value of the TOS octet in IP packets sent by the socket. RFC 1349 defines the TOS values as follows:

  • IPTOS_LOWCOST (0x02)
  • IPTOS_RELIABILITY (0x04)
  • IPTOS_THROUGHPUT (0x08)
  • IPTOS_LOWDELAY (0x10)
The last low order bit is always ignored as this corresponds to the MBZ (must be zero) bit.

Setting bits in the precedence field may result in a SocketException indicating that the operation is not permitted.

for Internet Protocol v6 tc is the value that would be placed into the sin6_flowinfo field of the IP header.

Parameters
tc an int value for the bitset.
Throws
SocketException if there is an error setting the traffic class or type-of-service