Networking

By Md. Jehadul Karim

Prerequisite:

  • Threading

Recommended:

  • Input & Output Stream

So, how do you do Networking. Lets dive into the basics of networking.

Basics of Networking

First of all you need to understand how networking is done. There are two types of connection (as far as I know):

  1. TCP
  2. UDP

In this tutorial, I am only going to talk about TCP Networking. In TCP networking there exists a server which creates connection between two sockets. A network connection should contain two sockets, one at Server-End and other at Client-End.

Socket Networking

A Socket exposes some interface to send and receive data from one socket to other when a network is established. To relate, you can consider cin as the interface to get (read) and cout as the interface to write (send).

Server-Client Networking

A client can not directly communicate with other client directly. They need to get connected first. After getting connected, there will be two Sockets at both ends (Server and Client).

There is no difference between a server-side socket and client-side socket.

The socket read/write interface are relative to the side socket is in. These are not absolute. For example, If I write on one of the sockets (eg. Client-SIde), then I must read from the other socket (eg. Server-Side). You should not confuse to use write interface on both side to access the data.

Socket On One Side Socket On other Side
write(data) –> read() : returns data
read() : returns data <– write(data)

Socket Networking in JAVA

Prework

Before using socket, we have to connect two sockets at two ends. The basic way of doing this is to wait for other to connect (listening). It is done at the server side. To listen for a connection in server side, we have to use a java class, ServerSocket, Using this class we can start a connection and two sockets to communicate with.

Initialization

To start a ServerSocket, you first have to create an instance of it with the port you want to serve to. ServerSocket server = new ServerSocket(PORT);

PORT can be any positive integer from 0 to 2^16-1, But my suggestion would be using port higher than 10000. You have to note down the PORT as the same PORT will be used in the client side.

Connecting…

After the instance is being created, you can now wait for client to be connected on the same port. Socket clientSocket = server.accept();

Connected

When a client is connected to the server, we will have the the socket in the clientSocket and use it to send and receive data to and from client.

Close ServerSocket

You should always close the ServerSocket to release the PORT to be used by other applications by calling: server.close()

Connect from Client Side

Just initialize a Socket instance, just use 127.0.0.1 as hostname and the same PORT that was used in Server Side. Socket socketServer = new Socket("127.0.0.1", PORT);

Current Code

Server.java

import java.net.ServerSocket;
import java.net.Socket;

public class Server {
    static final int PORT = 12345;
    
    public static void main(String[] args) {
        ServerSocket server = null;
        Socket clientSocket = null;
        try{
        	server = new ServerSocket(PORT);
        	clientSocket = server.accept();
            server.close();
           	System.out.println("Client Connected");
            // DO YOUR WORK WITH SOCKET HERE
            // ...
            // THEN CLOSE IT
            clientSocket.close();
        } catch (Exception e) {
            // Ignored for Simplicity
        } finally {
            // Free up Resources by CLOSING them
        }
    }
}

Client.java

import java.net.Socket;

public class Client {
    static final int PORT = 12345;
	
    public static void main(String[] args) {
    	try {
            Socket socketServer = new Socket("127.0.0.1", PORT);
            // DO YOUR WORK WITH SOCKET HERE
            // ...
            // THEN CLOSE IT
            socketServer.close();
        } catch (Exception e) {
            // Ignored for Simplicity
        } finally {
            // Free up Resources by CLOSING them
        }
	}
}

You must allow parallel run from [ Run ] > [ Edit Configuration ] and run two classes in separate projects as they will have be run simultaneously

  1. Run Server First
  2. Then Run Client

Socket

Networking through Socket includes Sending and Receiving data as I have mentioned earlier. This can be done using OutputStream and InputStream. Socket class has methods for directly accessing these steams.

InputSteam in = clientSocket.getInputSteam(); OutputStream out = serverSocket.getOutputSteam();

OutputStream and InputStream has limited numbers of methods to send and receive data. Simple in.read() will read an int and out.write(INT) will send an INT to the other side.

Steams must be closed by calling close()

Current Code

Server.java

import java.net.ServerSocket;
import java.net.Socket;

public class Server {
    static final int PORT = 12345;
    
    public static void main(String[] args) {
        ServerSocket server = null;
        Socket clientSocket = null;
        try{
        	server = new ServerSocket(PORT);
        	clientSocket = server.accept();
            server.close();
           	System.out.println("Client Connected");
            // DO YOUR WORK WITH SOCKET HERE
            // Getting I/O Stream
            OutputStream out = clientSocket.getOutputSteam();
            InputStream in = clientSocket.getInputSteam();
            
            // Simple Integer Value Sending...
            int DATA = 12345;
            out.write(DATA); // CHANGE 12345 to test
            System.out.println("DATA sent: " + DATA);
            
            // THEN CLOSE IT
            in.close();
            out.close();
            clientSocket.close();
        } catch (Exception e) {
            // Ignored for Simplicity
        } finally {
            // Free up Resources by CLOSING them
        }
    }
}

Client.java

import java.net.Socket;

public class Client {
    static final int PORT = 12345;
	
    public static void main(String[] args) {
    	try {
            Socket socketServer = new Socket("127.0.0.1", PORT);
            // DO YOUR WORK WITH SOCKET HERE
            
            // Getting I/O Stream
            OutputStream out = socketClient.getOutputSteam();
            InputStream in = socketClient.getInputSteam();
            
            // Simple Integer Value Sending...
            int DATA;
            DATA = in.read();
            System.out.println("DATA received: " + DATA);
            
            // THEN CLOSE IT
            in.close();
            out.close();
            socketServer.close();
        } catch (Exception e) {
            // Ignored for Simplicity
        } finally {
            // Free up Resources by CLOSING them
        }
	}
}

Reader & Writer

You normally don’t use Input & Output Stream directly. We wrap them within another class to get some extra functionalities. For example, we could have directly used System.in instead of using new Scanner(System.in), But the Scanner provides much more functionalities to access data.

Here we will use BufferedReader and BufferedWriter. They provide functions to directly send and receive String over the I/O Streams.

BufferedReader

This one is used with InputStream, to get an instance of this, use: BufferedReader reader = new BufferedReader(new InputStreamReader(in));

BufferedReader has a method readLine() to directly read a String from the stream. String input = reader.readLine();

BufferedWriter

This one is used with OutputStream, to get an instance of this, use: BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(out));

BufferedReader has a method write(String) to directly send a String from the stream. Be careful to include a \n in the end of the String or call newLine() method, else readLine() from the otherside will not be able to read the steam,

writer.write("Hello World!"); write.newLine()

Current Code

Server.java

import java.net.ServerSocket;
import java.net.Socket;

public class Server {
    static final int PORT = 12345;
    
    public static void main(String[] args) {
        ServerSocket server = null;
        Socket clientSocket = null;
        try{
        	server = new ServerSocket(PORT);
        	clientSocket = server.accept();
            server.close();
           	System.out.println("Client Connected");
            // DO YOUR WORK WITH SOCKET HERE
            // Getting I/O Stream
            OutputStream out = clientSocket.getOutputSteam();
            InputStream in = clientSocket.getInputSteam();
            
            // Simple String Value Sending...
            BufferedWriter writer = new BufferedWriter(new InputStreamWriter(out));
            writer.write("Hello World from Server.");
            writer.newLine();
            
            // THEN CLOSE IT
            in.close();
            out.close();
            clientSocket.close();
        } catch (Exception e) {
            // Ignored for Simplicity
        } finally {
            // Free up Resources by CLOSING them
        }
    }
}

Client.java

import java.net.Socket;

public class Client {
    static final int PORT = 12345;
	
    public static void main(String[] args) {
    	try {
            Socket socketServer = new Socket("127.0.0.1", PORT);
            // DO YOUR WORK WITH SOCKET HERE
            
            // Getting I/O Stream
            OutputStream out = socketClient.getOutputSteam();
            InputStream in = socketClient.getInputSteam();
            
            // Simple Integer Value Sending...
            BufferedReader reader = new BufferedReader(new InputStreamReader(in));
            // Read String Value
            System.out.println(reader.readLine());
            
            // THEN CLOSE IT
            in.close();
            out.close();
            socketServer.close();
        } catch (Exception e) {
            // Ignored for Simplicity
        } finally {
            // Free up Resources by CLOSING them
        }
	}
}

You can try sending different string, or even multiple strings. Just use these methods and you are good to go…

Multi-client Network

This is where threading comes in. A server might need to be connected to multiple client at the same time. To connect with other client, server must always listen using server.accept(), it is a BLOCKING function. So, to perform other actions with other connected clients, All clients must be in different threads from server.accept().

**BLOCKING: **Example, scanner.readLine(), program will not proceed until input has been given. **Blocks **the program.

To connect multiple client, we can use java Therad like below:

Current Code

Server.java

import java.net.ServerSocket;
import java.net.Socket;

public class Server {
    static final int PORT = 12345;
    
    public static void main(String[] args) {
        ServerSocket server = null;
        Socket clientSocket = null;
        try{
        	server = new ServerSocket(PORT);
            for(int i=1; true; i++) {
                clientSocket = server.accept();
                new Thread(){
                    Socket socket = clientSocket;
                    System.out.println("Client #"+i" Connected");
            		// DO YOUR WORK WITH SOCKET HERE
         		   	// Getting I/O Stream
            		OutputStream out = socket.getOutputSteam();
            		InputStream in = socket.getInputSteam();
            
            		// Simple String Value Sending...
        	    	BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(out));
    	        	writer.write("Hello World from Server to Client #"+i);
	            	writer.newLine();
            
            		// THEN CLOSE IT
            		in.close();
            		out.close();
            		socket.close();
                }.start();
            }
        	
            server.close();
        } catch (Exception e) {
            // Ignored for Simplicity
        } finally {
            // Free up Resources by CLOSING them
        }
    }
}

Client.java

import java.net.Socket;

public class Client {
    static final int PORT = 12345;
	
    public static void main(String[] args) {
    	try {
            Socket socketServer = new Socket("127.0.0.1", PORT);
            // DO YOUR WORK WITH SOCKET HERE
            
            // Getting I/O Stream
            OutputStream out = socketClient.getOutputSteam();
            InputStream in = socketClient.getInputSteam();
            
            // Simple Integer Value Sending...
            BufferedReader reader = new BufferedReader(new InputStreamReader(in));
            // Read String Value
            System.out.println(reader.readLine());
            
            // THEN CLOSE IT
            in.close();
            out.close();
            socketServer.close();
        } catch (Exception e) {
            // Ignored for Simplicity
        } finally {
            // Free up Resources by CLOSING them
        }
	}
}

Thats all for Today.

Happy Coding