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

public class Server{
	final static int serverPort = 8080; // server port number
	
	public static void main(String args[]) {
		ServerSocket sock = null; // server socket
	        Socket clientSocket = null; // Client socket created by accept on Server side
		try{
	            	sock = new ServerSocket(serverPort); // create socket and bind to port
	            	System.out.println("waiting for client to connect");
		
 			while(true)
			{
				clientSocket = sock.accept(); // wait for client to connect. Program will stuck on this statement until a client connects to the server
				multiThread mt = new multiThread(clientSocket); //creating a separate therad to handle the client socket
				mt.start(); //this starts the thread and calls run(). As a separate thread is launched, our loop continues and goes to sock.accept()
					//So now we have one thread( to handle client socket) running separately and this loop.
					// as clients keep coming in we will launch a separate thread for them
			}
		}
            	catch(Throwable e) { // it's better to use try-catch in order to catch exceptions
	            System.out.println("Error " + e.getMessage());
	            e.printStackTrace();
		}
		//sock.close(); 
		// I have commented sock.close() because I wanted my server to be always up. 
		//You can set a condition to break the loop and then you should close the socket.
	}

}

class multiThread extends Thread //separate thread class to handle client socket 
{
	Socket clientSocket = null; // socket created by accept
	PrintWriter pw = null; // socket output stream for sending data to client
	BufferedReader br = null; // socket input stream for receiving data from client
	public multiThread(Socket s) //constructor for thread to set its client socket
	{
		clientSocket = s;
	}
	public void run() //this will be executed when mt.start() is called in other program
	{
		try{
			System.out.println("client has connected");
			pw   = new PrintWriter(clientSocket.getOutputStream(),true); //creating PrintWriter over OutputStream
	        	br   = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); //creating InputStreamReader over InputStream and Bufferreader over InputStreamReader
	        	String msg = br.readLine(); // read msg from client
	        	System.out.println("Message from the client > " + msg);
	        	pw.println("Got it!"); // send msg to client
			// close everything		
			pw.close(); 
			br.close();
			clientSocket.close();
		} catch (Throwable e) {
        	    System.out.println("Error " + e.getMessage());
        	    e.printStackTrace();
		}
	}
}