How Can I Reliably Detect WebSocket Connection Closures Using Java's Socket API?

Java Socket API: Detecting WebSocket Closures

In Java's Socket API, determining if a connection has been closed can be challenging. The isConnected() method returns true even for remotely disconnected sockets, while isClosed() remains false despite remote closures.

Conventional Approach

Many developers resort to continuously writing garbage messages to the output stream and checking for exceptions to detect closures. However, this method is inefficient and pollutes the network.

Alternative Solutions

Unfortunately, there is no API that explicitly reports the connection state. Instead, the following approaches can be used to infer whether a socket has been closed:

1. Read/Write Operations: If the peer has gracefully closed the connection:

   • read() returns -1
   • readLine() returns null
   • Other readXXX() methods throw EOFException
   • Writes eventually result in an IOException with the message "connection reset by peer"

2. Connection Drop: If the connection drops abruptly:

   • Writes will result in an IOException as described above

3. Read Timeout: If the peer is still connected but inactive:

   • Use a read timeout to detect a lack of communication
4. Channel Management: Contrary to popular belief, ClosedChannelException does not indicate a closed connection, but rather a closed channel after usage.
5. Select Key Validity: In Java 7 on Windows XP, a selected key that becomes invalid despite a non-zero select() value may indicate a peer-initiated connection reset.

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