2.3.2. TCP Server (ServerSocket)

import java.io.*;
import java.net.*;
public class TCPServer {
    public static void main(String[] args) {
        try {
            ServerSocket serverSocket = new ServerSocket(5000);
            System.out.println("Server is running...");
            Socket clientSocket = serverSocket.accept();
            ...
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

In this chunk, we focus on creating a TCP server using the ServerSocket class. The server runs on a specified port—in this case, port 5000. When the server starts, it listens for incoming connections through the ServerSocket's accept() method. This method will block the program execution until a client attempts to connect. Once a client connects, a new Socket object is created to communicate with the client.

Think of the ServerSocket as a host at a restaurant who waits at the entrance. The host's job is to greet customers (clients) as they arrive. The host (ServerSocket) stands by until a customer walks in (connects), at which point the host engages with the customer (Socket) to take their order (manage communication).

BufferedReader reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
System.out.println("Client says: " + reader.readLine());

Once the server has accepted a connection from the client, the server uses a BufferedReader to read data sent by the client. The InputStreamReader converts the byte stream from the client socket into a character stream. The server then calls readLine() on the BufferedReader to read a line of text sent by the client, which it prints to the console.

Imagine the server is now chatting with the customer at the restaurant. The server listens carefully to the customer's order and writes it down. In this analogy, the BufferedReader is like the server’s notepad that helps jot down what the customer (client) is saying.

OutputStream os = clientSocket.getOutputStream();
PrintWriter writer = new PrintWriter(os, true);
writer.println("Hello Client");

After reading the client's message, the server needs to reply. It obtains an OutputStream from the client's Socket, allowing it to send data back. The PrintWriter is used to write text responses easily. By sending "Hello Client", the server can confirm it has received the message and is replying back. The 'true' argument enables automatic flushing of the stream, ensuring that the output is sent immediately.

Continuing our restaurant analogy, once the server has taken the customer’s order, the server responds by saying, 'Hello there, your order will be ready shortly!' This response, like the one sent by the server, reassures the customer that their request has been acknowledged.

clientSocket.close();
serverSocket.close();

After the server has sent its response, it needs to properly close both the client and server sockets to free up system resources and prevent memory leaks. The close() method on both sockets achieves this, signaling that the connection has been terminated safely.

Once the server has finished chatting with the customer, it politely says goodbye and clears the table (closes the socket). This ensures that both the server and the customer can move on without leaving unresolved business.

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