Selectors使用时注意事项

1.ServerSocketChannel和SocketChannel必须配置成非阻塞的；
2.每次轮询后，得到SelectionKey后，必须从选择集里remove掉

Iterator<SelectionKey> keys = selector.selectedKeys().iterator();
while (keys.hasNext()) {
       SelectionKey key = keys.next();
        keys.remove();
       //...其他处理
}

3.如果在客户端也配置了Selector，如：

SocketChannel sc = SocketChannel.open();
			Selector selector = Selector.open();
			sc.configureBlocking(false);
			sc.connect(new InetSocketAddress("localhost", 7777));
			sc.register(selector, SelectionKey.OP_CONNECT);

在建立连接时，需要调用finishConnect方法，如：

Iterator<SelectionKey> keys = selector.selectedKeys()
						.iterator();
				while (keys.hasNext()) {
					SelectionKey key = keys.next();
					keys.remove();
					
					if (key.isConnectable()) {
						SocketChannel socketChannel = (SocketChannel) key.channel();
						
						// Finish the connection. If the connection operation failed
						// this will raise an IOException.
						try {
							socketChannel.finishConnect();
						} catch (IOException e) {
							// Cancel the channel's registration with our selector
							System.out.println(e);
							return;
						}
						System.out.println("Connected...");
						key.interestOps(SelectionKey.OP_READ);
					} 
}