When we accept a connection in NioServer, we set the of key of NioConnection to the one associated with the ServerSocketChannel. Later, when key.interestOps(key.interestOps() | SelectionKet.OP_WRITE) is called, an IllegalArgumentException will be thrown because a ServerSocketChannel doesn't support this op (only OP_ACCEPT).
SOLUTION: grab the SocketChannel returned from ServerSocketChannel.accept() and use it instead of the ServerSocketChannel to call interestOps().