To implement a simple TCP client and server in Java, you use the ServerSocket class for the server and the Socket class for the client.

Here is a straightforward example of both, where they exchange simple text messages.

1. The TCP Server

The server listens on a specific port and waits for a client to connect. Once connected, it opens input and output streams to communicate.

package org.kodejava.net;

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

public class SimpleTCPServer {
    public static void main(String[] args) {
        int port = 8080;

        try (ServerSocket serverSocket = new ServerSocket(port)) {
            System.out.println("Server is listening on port " + port);

            // Wait for a client connection
            try (Socket socket = serverSocket.accept()) {
                System.out.println("Client connected!");

                // Setup streams for communication
                BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                PrintWriter writer = new PrintWriter(socket.getOutputStream(), true);

                // Read message from a client
                String clientMessage = reader.readLine();
                System.out.println("Received from client: " + clientMessage);

                // Send a response back
                writer.println("Hello from Server!");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

2. The TCP Client

The client connects to the server’s IP address (or localhost) and the same port number.

package org.kodejava.net;

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

public class SimpleTCPClient {
    public static void main(String[] args) {
        String hostname = "localhost";
        int port = 8080;

        try (Socket socket = new Socket(hostname, port)) {
            // Setup streams for communication
            PrintWriter writer = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));

            // Send a message to the server
            writer.println("Hello from Client!");

            // Read the server's response
            String response = reader.readLine();
            System.out.println("Server says: " + response);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

How to run it:

1. Start the Server first: Run SimpleTCPServer. It will sit and wait (“block”) at the accept() method.
2. Start the Client: Run SimpleTCPClient. It will connect, send the message, and then both programs will finish.

Key Concepts:

• ServerSocket: Used by the server to “listen” for incoming connection requests.
• Socket: Represents the actual connection. Both the client and the server use a Socket object to talk to each other once the connection is established.
• Try-with-resources: Using try (...) ensures that the sockets and streams are automatically closed, even if an error occurs.
• Blocking: The accept() and readLine() methods are “blocking,” meaning the program pauses there until a connection is made or data is received.