How Can I Reliably Detect a Remote Socket Closure in Java?

Determining Connection Closure with Java Socket API

When working with Java's socket API, it can be challenging to detect when a connection has been severed. Unlike methods like isConnected() and isClosed(), which only indicate the state of the local socket, there's no built-in API to determine the remote socket's status.

To address this limitation, the recommended approach involves monitoring I/O operations. If read() returns -1, readLine() returns null, or readXXX() throws an EOFException, it's likely that the peer has closed the connection gracefully.

In cases where the connection has terminated abruptly, attempts to write may result in an IOException, indicating a "connection reset by peer." Reading operations may also fail with similar exceptions.

Alternatively, a read timeout can be employed to detect when the peer remains connected but inactive.

It's important to note that ClosedChannelException and SocketException: socket closed do not signify a closed connection but rather an attempt to use a channel after it has been closed locally.

In specific scenarios with Java 7 on Windows XP, monitoring the validity of a SelectionKey can reveal connection resets. However, this behavior is not guaranteed across different platforms or JRE versions.

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