socket

// Client.java

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

public final class Client {
    public static final void main(final String[] args) throws IOException {
        final var out = System.out;

        try (
                final var stdin = new BufferedReader(new InputStreamReader(System.in));
                final var socket = new Socket("127.0.0.1", 1337);
                final var socketReader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
                final var socketWriter = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()), true)
        ) {
            out.println("String: ");

            final var s = stdin.readLine();
            socketWriter.println(s.length());
            socketWriter.println(s);

            final var size = Integer.parseInt(socketReader.readLine());
            final var builder = new StringBuilder();

            do { builder.append(socketReader.readLine()); }
            while (builder.length() < size);

            out.println(builder);
        }
    }
}
// Server.java

import java.io.*;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.util.concurrent.Executors;

public final class Server {
    public static final void main(final String[] args) throws IOException {
        final var executor = Executors.newCachedThreadPool();
        final var server = new ServerSocket(1337, 50, InetAddress.getByName("127.0.0.1"));

        while (true) {
            final var client = server.accept();
            final var reader = new BufferedReader(new InputStreamReader(client.getInputStream()));
            final var writer = new PrintWriter(new OutputStreamWriter(client.getOutputStream()), true);

            final var builder = new StringBuilder();

            executor.execute(() -> {
                try {
                    final var size = Integer.parseInt(reader.readLine());

                    do { builder.append(reader.readLine()); }
                    while (builder.length() < size);

                    writer.println(builder.length());
                    writer.println(builder.reverse());
                    client.close();reader.close();writer.close();
                } catch (final Exception ignored) {}
            });
        }

        // server.close(); Unreachable statement
    }
}