1.key.attachment为空？   sc.register(selector,  SelectionKey.OP_WRITE|SelectionKey.OP_READ,bb);  使用这种方法注册事件时，需要添加第三个参数：attachment对象；或者使用  key.interestOps(SelectionKey.OP_WRITE | SelectionKey.OP_READ)方法;  

2.如何捕捉到客户端断开事件？断开时会触发readable key，所以在isreadable中判断read的返回值是否为-1

3.如果不注册写事件则写操作无疑和同步方式一样了，如果注册了写事件则需要在写完时取消该事件

4.key.cancel会同时取消读与写事件 

package my;import java.io.IOException;import java.io.UnsupportedEncodingException;import java.net.InetSocketAddress;import java.net.ServerSocket;import java.nio.ByteBuffer;import java.nio.channels.SelectionKey;import java.nio.channels.Selector;import java.nio.channels.ServerSocketChannel;import java.nio.channels.SocketChannel;import java.util.Iterator;public class NIOS {	public static final int SIZE = 1024;	void start(int port) {		ByteBuffer buffer = ByteBuffer.allocate(SIZE);		try {			ServerSocketChannel s = ServerSocketChannel.open();			ServerSocket socket = s.socket();			Selector selector = Selector.open();			InetSocketAddress addr = new InetSocketAddress(port);			socket.bind(addr);			s.configureBlocking(false);			s.register(selector, SelectionKey.OP_ACCEPT);			while (true) {				int selCount = selector.select();				Iterator
     
       keys = selector.selectedKeys().iterator();				while (keys.hasNext()) {					System.out.println("select Count:" + selCount);					SelectionKey key = (SelectionKey) keys.next();					keys.remove();					if (key.isAcceptable()) {						System.out.println("isacc...");						ServerSocketChannel ssc = (ServerSocketChannel) key.channel();						SocketChannel sc = ssc.accept();						sc.configureBlocking(false);						sc.register(selector, SelectionKey.OP_READ);					} else if (key.isReadable()) {						System.out.println("readable..");						SocketChannel sc = (SocketChannel) key.channel();						try {														int count = sc.read(buffer);							System.out.println("Read byte:" + count);							//log(buffer);							if(count==SIZE){								//如果count等于缓冲区大小，则说明此条消息没读完								//需要设计复杂点的消息结构															}else							if (count == -1) {								//当客户端断开连接时会触发read事件，并且可以读到-1，这时关闭通道								sc.close();								key.cancel();								continue;							}						} catch (Exception e) {							// TODO: handle exception							sc.close();							key.cancel();							System.out.println("close!");							continue;						}						buffer.flip();						ByteBuffer bb = buffer.duplicate();						key.attach(bb);						buffer.clear();						// sc.register(selector,						// SelectionKey.OP_WRITE|SelectionKey.OP_READ,bb);						//等价于						key.interestOps(SelectionKey.OP_WRITE | SelectionKey.OP_READ);						//通常不会注册写事件，除非写的内容巨大，注册写事件后注意取消											} else if (key.isWritable()) {						System.out.println("writable..");						SocketChannel sc = (SocketChannel) key.channel();						ByteBuffer att = (ByteBuffer) key.attachment();						if (att.hasRemaining()) {							int count = sc.write(att);							System.out.println("write:" + count + " byte,hasRemain:" + att.hasRemaining());						} else {							//写完后取消可写事件，仅监听可读事件							key.interestOps(SelectionKey.OP_READ);						}					}				}			}		} catch (IOException e) {			// TODO Auto-generated catch block			e.printStackTrace();		}	}	public static void log(ByteBuffer buf){		try {			System.out.println(new String(buf.array(),0,buf.limit(),"utf8"));		} catch (UnsupportedEncodingException e) {			// TODO Auto-generated catch block			e.printStackTrace();		}	}	public static void main(String[] args) {		new NIOS().start(802);	}}