Lifecycle and half-close

Sockets have a half-close: socket.shutdownOutput() sends EOF to the peer while your read direction stays open. It is the precise way to say "my request is complete" — a one-shot command client writes its payload, half-closes, reads the entire response until EOF. close() closes BOTH directions and releases the descriptor.

The server side is ServerSocket.accept() — each accepted connection gets its own Socket, and the classic model is a thread (or virtual thread) per connection. With virtual threads (Module 5), thread-per-connection returns to respectability: 10k concurrent connections with blocking-style code.

Accept/close hygiene: close the SERVER socket in finally (stops new connections), close each client socket in finally (frees descriptors), and never share one Socket object across threads without synchronization — the streams are not thread-safe.