Java Socket Programming - TCP

COMP211 Lectures 2021-10-12

Compared to UDP there are some differences:

The client must contact the server:

• The server process must first be running.
• The server must have created a socket that welcomes the client’s contact.

Clients contact the server by:

• Creating TCP socket, specifying IP address, port number of server process.
• When the cloent creates a socket the slcinet TCP establishes connection to server TCP.

When contacted by the client, the server TCP creates a new socket for the server process to communcate with that particular client:

• This allows the server to talk iwht multiple clients.
• Source port number are used to distinguish clients.

TCP provides a reliable, in-order byte-stream transfer (pipe).

Example Program

Like the UDP example the client sends a string the the server over TCP. The server then converts the string to upper-case and then sends it back.

Java TCP Client

import java.io.*;
import java.net.*;
class TCPClient {
	public static void main(String argv[]) throws Exception {
		String sentence;
		String modifiedSentence;
		
		// create input stream
		BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
		
		// create client socket and connect to server
		Socket clientSocket = new Socket("hostname", 6789);
		
		// create output stream attached to socket
		DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
		
		// create input stream attached to socket
		Buffered Reader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
		
		sentence inFromUser.readLine();
		
		// send line to server
		outToServer.writeBytes(sentence +'\n');
		
		// read line from server
		modifiedSentence = inFromServer.readLine();
		
		System.out.println("FROM SERVER: " + modifiedSentence);
		
		clientSocket.close();
	}
}

Java TCP Server

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

class TCPServer {
	public static void main(String argv[]) throws Exception{
		String clientSentence;
		String capitalisedSentence;
		
		// create welcoming socket at port 6789
		ServerSocket welcomeSocket = new ServerSocket(6789);
		
		while(true) {
			// wait on welcoming socket for contact by client
			Socket connectionSocket = welcomeSocket.accept();
			
			// create input stream attached to socket
			BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
			
			// create output stream attached to socket
			DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
			
			// read in line from socket
			clientSentence = inFromClient.readLine();
			
			capitalisedSentence = clientSentence.toUpperCase() +'\n';
			
			// write out line to socket
			outToClient.writeBytes(capitalisedSentence);
		}
	}
}