javasockets.md

Java socket programming (WIP)

Classes for socket programming can be found in the java.net package. All class names below are relative to this package.

The Socket class

The Socket class represents an endpoint of a TCP connection between two machines (for UDP connections there is the DatagramSocket class). Sockets are created using one of the below constructors, and they are also returned by various methods, see [TODO serversocket methods]

Constructors

Constructor	Description
Socket()	Creates an unconnected socket. See below for how to connect it to a server.
Socket(InetAddress address, int port)	Connect to a server with IP address address on port port. See below for the InetAddress class.
Socket(String host, int port)	Connect to a server with hostname host (e.g. www.google.com) on port port.

Instance methods

Type	Method	Description
void	close()	Closes the socket.
void	bind(SocketAddress bindpoint)	Binds the socket to a local address. We describe the SocketAddress class below. It is not advised to manually bind client sockets.
void	connect(SocketAddress endpoint)	Connects the socket to the server with address endpoint.
InputStream	getInputStream()	Returns an input stream for the socket.
OutputStream	getOutputStream()	Returns an output stream for the socket.

The InetAddress class

The InetAddress class represents an IP address. IPv4 (IPv6) addresses are represented using the subclass Inet4Address (Inet6Address). None of these classes have constructors, but various static methods can be used to construct InetAddress objects, as seen below.

Static methods

Type	Method	Description
InetAddress	getByAddress(byte[] addr)	Returns an InetAddress object given the raw IP address addr, e.g. {127, 0, 0, 1}.
InetAddress	getByName(String host)	Returns an InetAddress object given the host name host. This can either be a domain name (e.g. "www.google.com") or an IP address (e.g. "127.0.0.1").

The InetSocketAddress class

The InetSocketAddress class is a direct subclass of the SocketAddress abstract class, and instances can thus e.g. be passed to the connect method of the Socket class (see above). It represents an IP socket address, consisting of a port number and either an IP address or a hostname (in which case an attempt will be made to resolve the hostname).

[TODO wildcard address]

Constructors

Constructor	Description
InetSocketAddress(int port)	Creates a socket address from a port number, where the IP address is the wildcard address.
InetSocketAddress(InetAddress addr, int port)	Creates a socket address from an IP address and a port number.
InetSocketAddress(String hostname, int port)	Creates a socket address from a hostname and a port number.

The ServerSocket class

The ServerSocket class represents TCP server sockets. A server socket waits for an incoming request over the network.

Constructors

Constructor	Description
ServerSocket()	Creates an unbound server socket.
ServerSocket(int port)	Creates a server socket, bound to the specified port.

Instance methods

Type	Method	Description
Socket	accept()	Listens for a connection to be made to the server socket and accepts it. Blocks until a connection is made, after which it returns.
void	bind(SocketAddress endpoint)	Binds the ServerSocket to a specific address. TODO wildcard IP
void	close()	Closes the socket.

The DatagramPacket class

The DatagramPacket class represents UDP packets that are either to be sent or received. To construct a DatagramPacket object for sending packets, we pass an address to the constructor. To construct an object for receiving packets, we omit the address.

Constructors

Constructor	Description
DatagramPacket(byte[] buf, int length)	Constructs a packet for receiving packets of length length into the buffer buf.
DatagramPacket(byte[] buf, int length, InetAddress address, int port)	Constructs a datagram packet for sending packets to the address address:port.
DatagramPacket(byte[] buf, int length, SocketAddress address)	Constructs a datagram packet for sending packets to the address address.

Getter methods

Type	Method	Description
InetAddress	getAddress()	Returns the IP address of the machine to/from which this datagram is being sent/was received.
byte[]	getData()	Returns the data buffer.
int	getLength()	Returns the length of the data in the buffer.
int	getPort()	Returns the port number on the remote host to/from which this datagram is being sent/was received.
SocketAddress	getSocketAddress()	Returns the socket address of the remote host to/from which this datagram is being sent/was received.

Setter methods

Type	Method	Description
void	setAddress(InetAddress address)	Sets the IP address of the machine to which this datagram is being sent.
void	setData(byte[] buf)	Set the data buffer for this packet.
void	setLength(int length)	Set the length for this packet.
void	setPort(int port)	Sets the port number on the
void	setSocketAddress(SocketAddress address)	Sets the SocketAddress (usually IP address + port number) of the remote host to which this datagram is being sent.

The DatagramSocket class

The DatagramSocket class represents endpoints for sending and receiving UDP packets.

Constructors

Constructor	Description
DatagramSocket()	Constructs a datagram socket bound to the wildcard address and any available port.
DatagramSocket(int port)	Constructs a datagram socket bound to the wildcard address and the port port.
DatagramSocket(int port, InetAddress address)	Constructs a datagram socket bound to address:port.
DatagramSocket(SocketAddress address)	Constructs a datagram socket bound to address.

Getter methods

Type	Method	Description
InetAddress	getInetAddress()	Returns the remote address to which the socket is connected.
InetAddress	getLocalAddress()	Returns the local address to which the socket is bound.
int	getLocalPort()	Returns the local port number to which the socket is bound.
SocketAddress	getLocalSocketAddress()	Returns the local address to which socket is bound.
int	getPort()	Returns the remote port number to which the socket is connected.
SocketAddress	getRemoteSocketAddress()	Returns the remote address the socket is connected to, or null if it is unconnected.

Action methods

Type	Method	Description
void	bind(SocketAddress address)	Binds the socket to address.
void	close()	Closes the socket.
void	connect(InetAddress address, int port)	Connects the socket to the remote address address:port.
void	connect(SocketAddress address)	Connects the socket to the remove address address.
void	disconnect()	Disconnects the socket.
void	receive(DatagramPacket p)	Receives a datagram packet from this socket and saves it in p.
void	send(DatagramPacket p)	Sends the datagram packet p from this socket.