How to write only once or "on some command" using NIO sockets?

Question

This is an example of chargen server I found in book that I was reading:

import java.nio.*;
import java.nio.channels.*;
import java.net.*;
import java.util.*;
import java.io.IOException;

public class ChargenServer {

  public static int DEFAULT_PORT = 19;

  public static void main(String[] args) {

    int port;
    try {
      port = Integer.parseInt(args[0]);
    }
    catch (Exception ex) {
      port = DEFAULT_PORT;
    }
    System.out.println("Listening for connections on port " + port);

    byte[] rotation = new byte[95*2];
    for (byte i = ' '; i <= '~'; i++) {
        rotation[i-' '] = i;
        rotation[i+95-' '] = i;
    }

    ServerSocketChannel serverChannel;
    Selector selector;
    try {
      serverChannel = ServerSocketChannel.open();
      ServerSocket ss = serverChannel.socket();
      InetSocketAddress address = new InetSocketAddress(port);
      ss.bind(address);
      serverChannel.configureBlocking(false);
      selector = Selector.open();
      serverChannel.register(selector, SelectionKey.OP_ACCEPT);
    }
    catch (IOException ex) {
      ex.printStackTrace();
      return;
    }

    while (true) {

      try {
        selector.select();
      }
      catch (IOException ex) {
        ex.printStackTrace();
        break;
      }

      Set readyKeys = selector.selectedKeys();
      Iterator iterator = readyKeys.iterator();
      while (iterator.hasNext()) {

        SelectionKey key = (SelectionKey) iterator.next();
        iterator.remove();
        try {
          if (key.isAcceptable()) {
            ServerSocketChannel server = (ServerSocketChannel) key.channel();
            SocketChannel client = server.accept();
            System.out.println("Accepted connection from " + client);
            client.configureBlocking(false);
            SelectionKey key2 = client.register(selector, SelectionKey.OP_WRITE);
            ByteBuffer buffer = ByteBuffer.allocate(74);
            buffer.put(rotation, 0, 72);
            buffer.put((byte) '\r');
            buffer.put((byte) '\n');
            buffer.flip();
            key2.attach(buffer);
          }
          else if (key.isWritable()) {
            SocketChannel client = (SocketChannel) key.channel();
            ByteBuffer buffer = (ByteBuffer) key.attachment();
            if (!buffer.hasRemaining()) {
              // Refill the buffer with the next line
              buffer.rewind();
              // Get the old first character
              int first = buffer.get();
              // Get ready to change the data in the buffer
              buffer.rewind();
              // Find the new first characters position in rotation
              int position = first - ' ' + 1;
              // copy the data from rotation into the buffer
              buffer.put(rotation, position, 72);
              // Store a line break at the end of the buffer
              buffer.put((byte) '\r');
              buffer.put((byte) '\n');
              // Prepare the buffer for writing
              buffer.flip();
            }
            client.write(buffer);
          }
        }
        catch (IOException ex) {
          key.cancel();
          try {
            key.channel().close();
          }
          catch (IOException cex) {}
        }

      }

    }

  }

}

Once the client is connected to the server it keeps sending a stream of characters to the client and doesn't stop.

I am curious if I want to send out the buffer once and still keep the server connected to client. What should I do?

Answer 1

Socket channels are almost always writable. You should only select on OP_WRITE when you've had a prior zero length write. If you have somethung to write, just write it, and then check the return value.