Checking if a ClientSocket has disconnected in java hangs

Question

This is a follow up to:

this question

Basically, I have a server loop that manages a connection to one solitary client. At one point in the loop, if a Clien

Answer 1

You are checking correctly, you can should add a try catch with IOException in case it occurs.

There is a way to avoid threading, you can use a Selector with a non-bloking socket.

public void initialize(){
  //create selector
  Selector selector = Selector.open();
  ServerSocketChannel acceptSocket = ServerSocketChannel.open();
  acceptSocket.configureBlocking(false);
  String bindIp = "127.0.0.1";
  int bindPort = 80;
  acceptSocket.socket().bind(new InetSocketAddress(bindIp, bindPort));
  //register socket in selector for ACCEPT operation
  acceptSocket.register(selector, SelectionKey.OP_ACCEPT);
  this.selector = selector;
  this.serverSocketChannel = serverSocketChannel;
}

public void serverStuff() {
   selector.select(maxMillisecondsToWait);
   Set<SelectionKey> selectedKeys = selector.selectedKeys();
   if( selectedKeys.size() > 0 )
   {
      if( key.isAcceptable() ){
        //you can accept a new connection
        SocketChannel clientSk = serverSocketChannel.accept();
        clientSk.configureBlocking(false);
        //register your SocketChannel in the selector for READ operations
    clientSk.register(selector, SelectionKey.OP_READ);
      } else if( key.isReadable() ){
        //you can read from your socket.
        //it will return you -1 if the connection has been closed
      }
   }

   if( shouldSendHeartBeat() ){
     SendHeartBeat
   }
}

Answer 2

When you create your socket, first set a timeout:

private int timeout    = 10000;
private int maxTimeout = 25000;

clientSocket.setSoTimeout(timeout);

With this, if a read times out you'll get java.net.SocketTimeoutException (which you have to catch). Thus, you could do something like this, assuming you've previously set the SO_TIMEOUT as shown above, and assuming that the heartbeat will always get a response from the remote system:

volatile long lastReadTime;

try {
    bufferedReader.read();
    lastReadTime = System.currentTimeMillis();
} catch (SocketTimeoutException e) {
    if (!isConnectionAlive()) {
        logger.info("CONNECTION TERMINATED!");
        clientSocket.close(); 
        setUpSocket(); //sets up the server to reconnect to the client
    } else {
        sendHeartBeat(); //Send a heartbeat to the client
    }
}

public boolean isConnectionAlive() {
    return System.currentTimeMillis() - lastReadTime < maxTimeout;
}

A common way of handling this is setting the timeout to some number (say 10 seconds) and then keeping track of the last time you successfully read from the socket. If 2.5 times your timeout have elapsed, then give up on the client and close the socket (thus sending a FIN packet to the other side, just in case).

If the heartbeat will not get any response from the remote system, but is just a way of ultimately generating an IOException earlier when the connection has fallen down, then you could do this (assuming that the sendHeartBeat itself will not throw an IOException):

try {
    if (bufferedReader.read() == -1) {
        logger.info("CONNECTION TERMINATED with EOF!");
        resetConnection();
    }
} catch (SocketTimeoutException e) {
    // This just means our read timed out ... the socket is still good
    sendHeartBeat(); //Send a heartbeat to the client
} catch (IOException e) {
    logger.info("CONNECTION TERMINATED with Exception " + e.getMessage());
    resetConnection();
}

....

private void resetConnection() {
    clientSocket.close(); 
    setUpSocket(); //sets up the server to reconnect to the client
}

Answer 3

You should add error checking in your disconnection detection. Sometimes an IOException may be thrown when the connection to the other end is lost.

I am afraid that threading is unavoidable here. If you don't want to block the execution of your code, you need to create a separate thread.