Java Socket API: Identifying Closed Connections

Determining the status of remote connections can be challenging using the Java socket API. Unlike isConnected() and isClosed(), which only reflect the local state of the socket, it is not straightforward to ascertain if the connection has been severed by the remote party.

To accurately determine the connection status, you must actively interact with the socket. Here are a few approaches:

1. Data Exchange: Attempting to write data to the output stream and capturing the exception it throws can indicate a closed connection. However, this approach constantly sends unnecessary traffic and is not ideal.
2. Blocking Input/Output: When a connection is closed orderly, blocking operations, such as readLine() or read(), will return predictable values (-1 or null). Unfortunately, this method may not be timely if the connection is dropped abruptly.
3. Read Timeout: Configuring a read timeout can help detect inactive connections. If the timeout expires before receiving data, the connection is likely lost.
4. Invalid SelectionKeys: In Java 7 and above, using select() with OP_READ can detect closed connections when the associated SelectionKey becomes invalid. However, this behavior may vary depending on the platform and JRE version.

Remember that these methods will only detect closed connections promptly if the remote peer follows the proper connection closure etiquette. Otherwise, the connection may enter a half-closed state, making detection more difficult.