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POWhy is it impossible, without attempting I/O, to detect that TCP socket was gracefully closed by peer?
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copied!<p>As a follow up to a <a href="https://stackoverflow.com/questions/151590/java-how-do-detect-a-remote-side-socket-close">recent question</a>, I wonder why it is impossible in Java, without attempting reading/writing on a TCP socket, to detect that the socket has been gracefully closed by the peer? This seems to be the case regardless of whether one uses the pre-NIO <code>Socket</code> or the NIO <code>SocketChannel</code>.</p> <p>When a peer gracefully closes a TCP connection, the TCP stacks on both sides of the connection know about the fact. The server-side (the one that initiates the shutdown) ends up in state <code>FIN_WAIT2</code>, whereas the client-side (the one that does not explicitly respond to the shutdown) ends up in state <code>CLOSE_WAIT</code>. Why isn't there a method in <code>Socket</code> or <code>SocketChannel</code> that can query the TCP stack to see whether the underlying TCP connection has been terminated? Is it that the TCP stack doesn't provide such status information? Or is it a design decision to avoid a costly call into the kernel?</p> <p>With the help of the users who have already posted some answers to this question, I think I see where the issue might be coming from. The side that doesn't explicitly close the connection ends up in TCP state <code>CLOSE_WAIT</code> meaning that the connection is in the process of shutting down and waits for the side to issue its own <code>CLOSE</code> operation. I suppose it's fair enough that <code>isConnected</code> returns <code>true</code> and <code>isClosed</code> returns <code>false</code>, but why isn't there something like <code>isClosing</code>?</p> <p>Below are the test classes that use pre-NIO sockets. But identical results are obtained using NIO.</p> <pre><code>import java.net.ServerSocket; import java.net.Socket; public class MyServer { public static void main(String[] args) throws Exception { final ServerSocket ss = new ServerSocket(12345); final Socket cs = ss.accept(); System.out.println("Accepted connection"); Thread.sleep(5000); cs.close(); System.out.println("Closed connection"); ss.close(); Thread.sleep(100000); } } import java.net.Socket; public class MyClient { public static void main(String[] args) throws Exception { final Socket s = new Socket("localhost", 12345); for (int i = 0; i < 10; i++) { System.out.println("connected: " + s.isConnected() + ", closed: " + s.isClosed()); Thread.sleep(1000); } Thread.sleep(100000); } } </code></pre> <p>When the test client connects to the test server the output remains unchanged even after the server initiates the shutdown of the connection:</p> <pre><code>connected: true, closed: false connected: true, closed: false ... </code></pre>

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