Question

I am learning the NIO package. I refer the NioServer example from here. The selector thread in NioServer.java blocks on

this.selector.select(); 
Iterator<SelectionKey> selectedKeys = this.selector.selectedKeys().iterator();
while (selectedKeys.hasNext()) {
    SelectionKey key = selectedKeys.next();
    selectedKeys.remove();
    if (!key.isValid()) {
        continue;
    }

    if (key.isAcceptable()) {
        this.accept(key);
    } else if (key.isReadable()) {
        this.read(key);
    } else if (key.isWritable()) {
        this.write(key);
    }

When a remote client connects, this.accept(key) is called and in this method the interest
interestOps is changed to Read and awakes the selector.
is this what causes the selector to select this channel? So we signal in this way for the channel to be picked?

Now suppose in write to a socket channel selector is signalled by changing the
interest that the channel is ready for write.
But suppose in write did not complete due to socket buffer full as shown in code, then we don’t change the interest and keep it as is as write only.
then when will the selector select this channel?

Answer 1

1. this.accept(key) calls serverSocketChannel.accept() which returns a new socket channel for communication with the client. It’s this channel that is registered with the selector for “read” operations, i.e. the selector now has two registrations:

   • the original ServerSocketChannel, with OP_ACCEPT
   • the SocketChannel for the new client, with OP_READ

2. If a write cannot complete due to the buffer filling up, the corresponding SocketChannel remains registered with OP_WRITE. Once the client reads some data off the other end, the channel will be selected again, allowing us to write the remaining data before flipping the interest set back to OP_READ.