java-socket-programming

Applications often need to communicate over a network — web browsers talking to servers, chat apps exchanging messages, databases sending query results. Without a standard networking API, every application would need to implement its own network protocol handling at the OS level.

Java’s java.net package provides socket programming APIs for network communication. Socket represents a client endpoint, ServerSocket listens for incoming connections. Data is exchanged through InputStream and OutputStream obtained from the socket, following the same patterns as file I/O.

A server creates a ServerSocket bound to a port and calls accept() to wait for connections. A client creates a Socket with the server’s IP and port. TCP establishes a connection via the three-way handshake. Once connected, both sides read from and write to the socket streams.

• TCP-based: Reliable, ordered, connection-oriented communication
• Blocking I/O: accept(), read(), and write() block until data arrives
• Multi-threaded servers: Each connection typically handled by a separate thread
• URL class: Higher-level abstraction for HTTP-specific communication

• Built from: Java File Handling — socket I/O uses the same InputStream/OutputStream patterns
• Built from: Java Multithreading — servers typically handle each client in a separate thread
• Builds into: Java JDBC — JDBC uses sockets under the hood for database communication
• Related: Try-Catch-Finally — sockets must be closed cleanly, often in finally blocks

• Socket timeout: setSoTimeout() prevents threads from blocking indefinitely
• Half-close: shutdownOutput() allows reading after writing is done
• Thread per connection: Doesn’t scale to thousands of connections — use NIO selectors instead
• Firewall/NAT: Socket connections may fail due to firewalls blocking ports