Question

How do you detect if Socket#close() has been called on a socket on the remote side?

Answer 1

The isConnected method won’t help, it will return true even if the remote side has closed the socket. Try this:

public class MyServer {     public static final int PORT = 12345;     public static void main(String[] args) throws IOException, InterruptedException {         ServerSocket ss = ServerSocketFactory.getDefault().createServerSocket(PORT);         Socket s = ss.accept();         Thread.sleep(5000);         ss.close();         s.close();     } }  public class MyClient {     public static void main(String[] args) throws IOException, InterruptedException {         Socket s = SocketFactory.getDefault().createSocket('localhost', MyServer.PORT);         System.out.println(' connected: ' + s.isConnected());         Thread.sleep(10000);         System.out.println(' connected: ' + s.isConnected());     } } 

Start the server, start the client. You’ll see that it prints ‘connected: true’ twice, even though the socket is closed the second time.

The only way to really find out is by reading (you’ll get -1 as return value) or writing (an IOException (broken pipe) will be thrown) on the associated Input/OutputStreams.