====== 01 - Sockets ======

===== Sockets =====

Un socket es un “enchufe” de software que permite a dos programas comunicarse por red. En Java (vía TCP):

  * **Servidor**: Usa la clase ([[https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/net/ServerSocket.html|ServerSocket]]), espera conexiones con el método ([[https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/net/ServerSocket.html#accept()|accept()]]), y habla con cada cliente a través de un [[https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/net/Socket.html|Socket]].\\ \\
  * **Cliente**: se conecta a host:puerto y envía/recibe datos.\\ \\

TCP garantiza entrega y orden de los bytes; tú decides el protocolo de mensajes (por ejemplo, “una línea por mensaje”).

===== Ciclo de vida clásico =====

**Servidor**

  - **new ServerSocket(puerto)** → queda “escuchando”.\\ \\
  - **accept() (bloqueante)** → devuelve un Socket por cliente.\\ \\
  - Lee/escribe por los streams de ese Socket.\\ \\
  - Cierra Socket (y luego ServerSocket si termina).\\ \\

**Cliente**

  - **new Socket(host, puerto)** → intenta conectarse.\\ \\
  - Escribe/lee datos.\\ \\
  - Cierra Socket.\\ \\

===== Ejemplo =====

**Servidor**

<sxh java; title:Server.java>
package es.cesguiro.socket;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;

public class Server {

    private final int port = 12345;

    public Server() {
    }

    public void start() {
        System.out.println("Starting server on port " + port + "...");
        try {
            ServerSocket serverSocket = new ServerSocket(port);
            System.out.println("Server started. Waiting for client connections...");

            Socket socket = serverSocket.accept();
            System.out.println("Client connected: " + socket.getInetAddress());

            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            String message = in.readLine();
            System.out.println("Received message: " + message);

            socket.close();
            serverSocket.close();
        } catch (Exception e) {
            System.out.println("Error starting server: " + e.getMessage());
            throw new RuntimeException("Failed to start server", e);
        }
    }
}
</sxh>

<sxh java; title:Main.java>
package es.cesguiro.socket;


public class Main {

    public static void main(String[] args) {

        Server server = new Server();
        server.start();
    }
}
</sxh>

**Cliente**

<sxh java; title:Client.java>
package es.cesguiro.socket;

import java.io.IOException;
import java.io.PrintWriter;
import java.net.Socket;

public class Client {

    private final String host = "localhost";
    private final int port = 12345;

    public void connect() throws IOException {
        System.out.println("Connecting to server at " + host + ":" + port + "...");
        Socket socket = new Socket(host, port);
        System.out.println("Connected to server: " + socket.getInetAddress());
        PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
        out.println("Hello from client!");
        System.out.println("Message sent to server.");
        out.close();
    }
}
</sxh>

<sxh java; title:Main.java>
package es.cesguiro.socket;

import java.io.IOException;

public class Main {

    public static void main(String[] args) {
        Client client = new Client();
        try {
            client.connect();
        } catch (IOException e) {
            System.err.println("Error connecting to server: " + e.getMessage());
            e.printStackTrace();
        }
    }
}
</sxh>