18.3. TCP Programming in Java

import java.net.*;
import java.io.*;
public class TCPServer {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(5000);
        System.out.println("Server started...");
        Socket clientSocket = serverSocket.accept();
        System.out.println("Client connected.");
        BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
        String message = in.readLine();
        System.out.println("Received: " + message);
        out.println("Echo: " + message);
        in.close();
        out.close();
        clientSocket.close();
        serverSocket.close();
    }
}

The TCP Server example illustrates how to create a simple server in Java which listens for incoming connections.

1. It starts by importing necessary classes for networking and I/O operations.
2. The TCPServer class defines the server, and the main method is the entry point.
3. A ServerSocket is created on port 5000, allowing the server to receive connections.
4. The serverSocket.accept() method waits for a client to connect. Once a connection is made, a Socket for the client is created.
5. The server can then read messages sent by the client through an input stream and respond using an output stream.
6. Finally, it sends an echo of the received message and closes the streams and sockets, ending the connection safely.

Think of the TCP Server as a reception desk in a hotel. Guests (clients) come in and check into the hotel. The desk (server) listens for guests, checks them in, takes their requests, and then processes them (echoes their messages) before bidding them farewell (closing connections).

import java.net.*;
import java.io.*;
public class TCPClient {
    public static void main(String[] args) throws IOException {
        Socket socket = new Socket("localhost", 5000);
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
        BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        out.println("Hello Server!");
        System.out.println("Server response: " + in.readLine());
        in.close();
        out.close();
        socket.close();
    }
}

The TCP Client example demonstrates how to create a simple client that connects to the server created in the previous example.

1. It imports the necessary Java networking and I/O classes.
2. The TCPClient class defines the client application, and the main method serves as the entry point.
3. The client establishes a socket connection to the server located at "localhost" on port 5000.
4. It initializes output (to send messages to the server) and input (to read server responses) streams.
5. The client sends a greeting message, "Hello Server!" to the server.
6. It then waits for a response from the server, reads the response, and prints it to the console.
7. Finally, all streams and sockets are closed to clean up resources.

Imagine the TCP Client as a hotel guest sending a message to the reception desk (the server). The guest states, "Hello!" and waits for the receptionist to respond. After getting the response, the guest leaves the desk with a smile, having had their interaction resolved.

--