the best practise of udpchannel with selector??

I wrote an module which can send and receive datagrams through UDP.
I use a selector to manage it,but I come with a problem of 100%cpu.

the code is as follows:

private ConcurrentLinkedQueue<ToSendData> resps = new
ConcurrentLinkedQueue<ToSendData>();
...

while (run) {
                selector.select();//why it does not block?
                if (key.isValid() && key.isReadable()) {
    ByteBuffer buffer =
ByteBuffer.allocate(Constant.MAX_RECEIVE_BUFFER_SIZE);
    buffer.order(ByteOrder.LITTLE_ENDIAN);
    InetSocketAddress sock = (InetSocketAddress) channel.receive(buffer);
    if (sock == null)
                 continue;
    receivedDatagramCount++;
    log.info("received No." + receivedDatagramCount+ " datagram");
    String ip = sock.getAddress().getHostAddress();
    int port = sock.getPort();
    if (!Crypt.decrypt(buffer.array(), buffer.position())) {
    log.warn("=D0=A3=D1=E9=CA=A7=B0=DC=A3=A1");
    return;
    }
    buffer.flip();
    P2IHeaderInfo header = P2IHeaderInfo.getInstance(buffer);
                buffer.rewind();
    DispatchData data = new DispatchData(header, buffer, ip,port,
receivedDatagramCount);
    Task task = new Task(Task.DISPATCH, data);
    server.sendMessage(task);
    }
    if (key.isValid() && key.isWritable()) {
    if (resps.size() > 0) {
    for (ToSendData data = resps.poll(); data != null; data =
resps.poll()) {
        if (data.getIp().equals("e")) {
            break;
        }
        String ip = data.getIp();
        int port = data.getPort();
        ByteBuffer buffer = data.getBuffer();
        Crypt.encrypt(buffer.array(), buffer.remaining());
        byte[] x = new byte[buffer.remaining()];
        System.arraycopy(buffer.array(), 0, x, 0, buffer.remaining());
        sendDatagramCount++;
        log.info("Server sends No." + sendDatagramCount+ " datagram: ");
        channel.send(buffer,new InetSocketAddress(ip, port));
    }
    }
// If uncomment these block,the cpu goes 100%
// else{
// Thread.sleep(1);
// }
    }

}

It seems that udpchannel is always writable and it won't block these
thread any bit,
I think maybe I need another thread to send datagrams and use a
LinkedBlockingQueue instead of ConcurrentLinkedQueue.

What is the best practice of making this module?