NIO: Send message and then disconnect immediately

Question

In some circumstances I wish to send an error message from a server to client using non-blocking I/O ( SocketChannel.write(ByteBuffer) ) and then disconnect the client. Assuming I write the full contents of the message and then immediately disconnect I presume the client may not receive this message as I'm guessing that the OS hasn't actually sent the data at this point.

Is this correct, and if so is there a recommended approach to dealing with this situation?

I was thinking of using a timer whereby if I wish to disconnect a client I send a message and then close their connection after 1-2 seconds.

Answer 1

SocketChannel.write will in non-blocking mode return the number of bytes which could immediately be sent to the network without blocking. Your question makes me think that you expect the write method to consume the entire buffer and try asynchronously to send additional data to the network, but that is not how it's working.

If you really need to make sure that the error message is sent to the client before disconnecting the socket, I would simply enable blocking before calling the write method. Using non-blocking mode, you would have to call write in a loop, counting the number of bytes being sent by each invocation and exit the loop when you've succeeded to pass the entire message to the socket (bad solution, I know, unnecessary code, busy wait and so on).

Answer 2

you may be better off launching a thread and synchronously write data to the channel. the async api is more geared toward "one thread dispatching multiple channels" and not really intended for fire and forget communications.

Answer 3

The close() method of sockets makes sure, everything sent using write before is actually sent before the socket is really closed. However this assumes that your write() was able to copy all data to the tcp stacks output window, which will not always work. For solutions to this see the other answers.