### 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!