Socket programming

Socket programming

Sockets for Clients

A socket is a connection between two hosts.

Operations

•       The program creates a new socket with a Socket ( ) constructor.

•       The socket attempts to connect to the remote host.

•       Once the connection is established, the local and remote hosts get input and output streams from the socket and use those streams to send data to each other. This connection is full-duplex; both hosts can send and receive data simultaneously.

•       When the transmission of data is complete, one or both sides close the connection.

Constructors

1. public Socket (String host, int port) throws UnknownHostException, IOException

This constructor creates a TCP socket to the specified port on the specified host and attempts to connect to the remote host.

2. public Socket(InetAddress host, int port) throws IOException

This constructor creates a TCP socket to the specified port on the specified host and tries to connect. It differs by using an InetAddress object to specify the host rather than a hostname.

Example: Find out which of the Ports at or Above 1,024 Seem to Be Hosting TCP Server

import java.net.*; import java.io.*;

public class PortScanner {

public static void main(String[] args) { String host = "localhost";

if (args.length > 0) { host = args[0];

}

try {

InetAddress theAddress = InetAddress.getByName(host); for (int i = 1024; i < 65536; i++) {

try {

Socket theSocket = new Socket(theAddress, i);

System.out.println("There is a server on port "

+ i + " of " + host);

}

catch (IOException e) {

// must not be a server on this port }} // end for

} // end try

catch (UnknownHostException e) { System.err.println(e);

}

} // end main } // end PortScanner

Example: Echo Client import java.net.*; import java.io.*;

public class EchoClient {

public static void main(String[] args) { String hostname = "localhost"; if (args.length > 0) {

hostname = args[0];

}

PrintWriter out = null; BufferedReader br = null; try {

Socket s= new Socket (hostname, 7);

br= new BufferedReader(new InputStreamReader(s.getInputStream( )));

BufferedReader in = new BufferedReader(new InputStreamReader(System.in)); out = new PrintWriter(s.getOutputStream( ));

System.out.println ("Connected to echo server"); while (true) {

String theLine = in.readLine( ); if (theLine.equals(".")) break; out.println(theLine);

out.flush( ); System.out.println(networkIn.readLine( ));

}

} // end try

catch (IOException e) { System.err.println (e);

}

} // end main

} // end EchoClient

Sockets for Servers

• A ServerSocket runs on the server and listens for incoming TCP connections. Each ServerSocket listens on a particular port on the server machine. When a client Socket on a remote host attempts to connect to that port, the server wakes up, negotiates the connection between the client and the server, and opens a regular Socket between the two hosts.

Operations

A new ServerSocket is created on a particular port using a ServerSocket( ) constructor.

•       The ServerSocket listens for incoming connection attempts on that port using its accept( ) method. accept( ) blocks until a client attempts to make a connection, at which point accept( ) returns a Socket object connecting the client and the server.

•       Depending on the type of server, either the Socket's getInputStream( ) method, getOutputStream( ) method, or both are called to get input and output streams that communicate with the client.

•       The server and the client interact according to an agreed-upon protocol until it is time to close the connection.

•       The server, the client, or both close the connection.

•       The server returns to step 2 and waits for the next connection.

Constructors

1. public ServerSocket(int port) throws IOException, BindException

2. public ServerSocket(int port, int queueLength) throws IOException, BindException

3. public ServerSocket(int port, int queueLength, InetAddress bindAddress) throws IOException

Example: Look for Local Ports

import java.net.*; import java.io.*;

public class LocalPortScanner {

public static void main(String[] args) {

for (int port = 1; port <= 65535; port++)

{ try {

// the next line will fail and drop into the catch block if

//  there is already a server running on the port ServerSocket server = new ServerSocket(port);

}

catch (IOException e) {

System.out.println("There is a server on port " + port + ".");

} // end try } // end for

}

}

The program checks for ports on the local machine by attempting to create ServerSocket objects on them and seeing on which ports that fails.

Datagram Packet

UDP datagram is represented by an instance of the DatagramPacket class public final class DatagramPacket extends Object

Constructors

Receiving datagram

1. public DatagramPacket(byte[] buffer, int length)

When a socket receives a datagram, it stores the datagram's data part in buffer beginning at buffer[0] and continuing until the packet is completely stored or until length bytes have been written into the buffer.

2. public DatagramPacket(byte[] buffer, int offset, int length)

When a socket receives a datagram, it stores the datagram's data part in buffer beginning at buffer[offset] and continuing until the packet is completely stored or until length bytes have been written into the buffer.

Sending datagram

1. public DatagramPacket(byte[] data, int length, InetAddress destination, int port)

2. public DatagramPacket(byte[] data, int offset, int length, InetAddress destination, int port)

Each constructor creates a new DatagramPacket to be sent to another host. The packet is filled with length bytes of the data array starting at offset or if offset is not used. If you try to construct a DatagramPacket with a length that is greater than data.length, the constructor throws an IllegalArgumentException.

Datagram Socket

To send or receive a Datagram Packet, need to open a datagram socket.

Constructors

1. public DatagramSocket( ) throws SocketException

This constructor creates a socket that is bound to an anonymous port.

2. public DatagramSocket(int port) throws SocketException

This constructor creates a socket that listens for incoming data grams on a specific port, specified by the port argument.

Sending and Receiving Data grams

1. public void send(DatagramPacket dp) throws IOException

Once a DatagramPacket is created and a DatagramSocket is constructed, send the packet by passing it to the socket's send( ) method.

2. public void receive(DatagramPacket dp) throws IOException

This method receives a single UDP datagram from the network and stores it in the pre-existing DatagramPacket object