Java Selector returns SelectionKey with OP_READ without data in infinity loop after writing to channel

Question

I've trouble with my code: i've written simple SocketChannel client with Selector, after starting it successfully reads messages from server (server sends events). But after writing to socket (see main method) selector starts returning readable socket in infinyty loop, handleKey returns that -1 bytes readed, so selector all time returns OP_READ SelectionKey without data for reading. Sorry for my English.

Thanks.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;

public class SelectorTest
{

    public SelectorTest() throws IOException {
        selector = Selector.open();
    }

    private void runSelector() {

        new Thread(new Runnable() {

            public void run()
            {

                alive = true;
                try {
                    while(alive) {
                        System.out.println("Selector started...");

                        selector.select();

                        Iterator<SelectionKey> keyIter = selector.selectedKeys().iterator();

                        while(keyIter.hasNext()) {

                            SelectionKey key = keyIter.next();

                            keyIter.remove();

                            handleKey(key);
                        }
                    }
                } catch (IOException ex) {
                    ex.printStackTrace();
                }
            }
        }).start();
    }

    private void handleKey(SelectionKey key) throws IOException {

        SocketChannel chan = (SocketChannel) key.channel();
        System.out.println("Processing selected...");

        if(key.isConnectable()) {
            System.out.println("Connecting ...");
            if(chan.finishConnect()) {
                key.interestOps(SelectionKey.OP_READ);
            } else {
                key.channel();
            }
        } else if(key.isReadable()) {
            System.out.println("Processing reading...");

            ByteBuffer buf = ByteBuffer.allocate(1024);
            int readedBytes = chan.read(buf);
            System.out.println("Readed: " + readedBytes);
            buf.flip();

            for(byte b : buf.array()) {
                System.out.print((char) b);
            }
        } else if(key.isWritable()) {
            System.out.println("Finishing writing...");

            key.interestOps(SelectionKey.OP_READ);
        }
    }

    public static void main(String[] args) throws IOException {

        SocketChannel channel = SocketChannel.open();
        channel.configureBlocking(false);
        channel.connect(new InetSocketAddress("t1.sis.lan", 6001));

        SelectorTest ds = new SelectorTest();
        ds.runSelector();

        channel.register(ds.selector, SelectionKey.OP_CONNECT);

        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

        for(;;) {

            String line = in.readLine();
            if(line==null) break;

            if(line.toLowerCase().equals("bye")) break;

            if (line.toLowerCase().equals("write")) {
                String command = "GET_STREAMS\r\n\0";

                ByteBuffer buf = ByteBuffer.allocate(1024);
                buf.put(command.getBytes());
                buf.flip();

                channel.write(buf);
            }

            System.out.println("echo: "+line); // is it alive check
        }

        ds.alive = false;
        ds.selector.wakeup();
        channel.close();
    }

    private Selector selector;
    private boolean alive;

}

Answer 1

read() returns -1 at EOS, which you are completely ignoring. When you get EOS, you must either close the channel or at least deregister interest in OP_READ. Otherwise you will just get another OP_READ and another -1 when you read, as you are doing, forever. Contrary to your comments above, read() returns zero on an empty read. You can ignore that, indeed you won't even see it if you only read when isReadable(), unless you read in a loop, but you must not ignore EOS.

Answer 2

read() returns -1 when it has read EOF. Definition:

read() returns: The number of bytes read, possibly zero, or -1 if the channel has reached end-of-stream

This means you should unregister the interest for OP_READ.