### WHAT IS SOCKET PROGRAMMING ?

* Sockets programming is the fundamental technology behind communications on TCP/IP networks. A socket is one endpoint of a two-way link between two programs running on a network. The socket provides a bidirectional communication endpoint to send and receive data with another socket.

* Socket connections normally run between two different computers on a local area network (LAN) or across the internet, but they can also be used for interprocess communication on a single computer.

### HOW SERVER SOCKET WORKS ::

* Typically, a server runs on one computer and accesses a socket that is bound to a specific port. The server waits for a different computer to make a connection request. The client computer knows the hostname of the server computer and the port number on which the server is listening. The client computer identifies itself, and — if everything goes right — the server permits the client computer to connect.

// The java.net package provides support for the two common network protocols −:

TCP − TCP stands for Transmission Control Protocol, which allows for reliable communication between two applications. TCP is typically used over the Internet Protocol, which is referred to as TCP/IP.

UDP − UDP stands for User Datagram Protocol, a connection-less protocol that allows for packets of data to be transmitted between applications.

###Socket Client Example

// File Name GreetingClient.java

import java.net.*;

import java.io.*;

public class GreetingClient {

public static void main(String [] args) {

String serverName = args[0];

int port = Integer.parseInt(args[1]);

try {

System.out.println("Connecting to " + serverName + " on port " + port);

Socket client = new Socket(serverName, port);

System.out.println("Just connected to " + client.getRemoteSocketAddress());

OutputStream outToServer = client.getOutputStream();

DataOutputStream out = new DataOutputStream(outToServer);

out.writeUTF("Hello from " + client.getLocalSocketAddress());

InputStream inFromServer = client.getInputStream();

DataInputStream in = new DataInputStream(inFromServer);

System.out.println("Server says " + in.readUTF());

client.close();

} catch (IOException e) {

e.printStackTrace();

}

}

}

###Socket Server Example

// File Name GreetingServer.java

import java.net.*;

import java.io.*;

public class GreetingServer extends Thread {

private ServerSocket serverSocket;

public GreetingServer(int port) throws IOException {

serverSocket = new ServerSocket(port);

serverSocket.setSoTimeout(10000);

}

public void run() {

while(true) {

try {

System.out.println("Waiting for client on port " +

serverSocket.getLocalPort() + "...");

Socket server = serverSocket.accept();

System.out.println("Just connected to " + server.getRemoteSocketAddress());

DataInputStream in = new DataInputStream(server.getInputStream());

System.out.println(in.readUTF());

DataOutputStream out = new DataOutputStream(server.getOutputStream());

out.writeUTF("Thank you for connecting to " + server.getLocalSocketAddress()

+ "\nGoodbye!");

server.close();

} catch (SocketTimeoutException s) {

System.out.println("Socket timed out!");

break;

} catch (IOException e) {

e.printStackTrace();

break;

}

}

}

public static void main(String [] args) {

int port = Integer.parseInt(args[0]);

try {

Thread t = new GreetingServer(port);

t.start();

} catch (IOException e) {

e.printStackTrace();

}

}

}

###Compile the client and the server and then start the server as follows −

$ java GreetingServer 6066

Waiting for client on port 6066...

###Check the client program also

###OUTPUT

$ java GreetingClient localhost 6066

Connecting to localhost on port 6066

Just connected to localhost/127.0.0.1:6066

Server says Thank you for connecting to /127.0.0.1:6066

Goodbye!