NIO服务器示例（使用 select( )来为多个通道提供服务）

package com.nio.demo;

import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.nio.ByteBuffer;
import java.nio.channels.SelectableChannel;
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 SelectSockets {
	public static int PORT_NUMBER = 1234;

	public static void main(String[] args) throws Exception {
		new SelectSockets().go(args);

	}

	public void go(String[] argv) throws Exception {
		int port = PORT_NUMBER;
		if (argv.length > 0) { // Override default listen port
			port = Integer.parseInt(argv[0]);
		}
		System.out.println("Listening on port " + port);
		// 打开一个 socket channel。注：通道不能复用，一个通道只能对应一个连接，这里的ServerSocketChannel只是通道的监听类。
		ServerSocketChannel serverChannel = ServerSocketChannel.open();
		// 获取绑定在Channel上的socket
		ServerSocket serverSocket = serverChannel.socket();
		// 创建一个选择器
		Selector selector = Selector.open();
		// 监听端口
		serverSocket.bind(new InetSocketAddress(port));
		// 设置为非阻塞模式
		serverChannel.configureBlocking(false);
		// 将serverChannel注册到选择器上，需要监听的操作为OP_ACCEPT
		serverChannel.register(selector, SelectionKey.OP_ACCEPT);
		while (true) {
			//返回选择器上就绪的通道数量。注：每个通道可能会注册多个操作，任一操作就绪：n>0
			int n = selector.select();
			if (n == 0) {
				continue; // nothing to do
			}
			// Get an iterator over the set of selected keys
			Iterator it = selector.selectedKeys().iterator();
			// Look at each key in the selected set
			while (it.hasNext()) {
				SelectionKey key = (SelectionKey) it.next();
				// 如果就绪的通道为serverChannel（因为只有serverChannel注册了ACCEPT操作），即意味着有新连接到来
				if (key.isAcceptable()) {
					ServerSocketChannel server = (ServerSocketChannel) key.channel();
					//创建一个对应新连接的通道。注：因为已经通过选择器的检查，accept方法的会立即返回，并且不会为null
					SocketChannel channel = server.accept();
					//将此通道注册到选择器上
					registerChannel(selector, channel, SelectionKey.OP_READ);
					//模拟客户端向通道发送消息
					sayHello(channel);
				}
				// 如果就绪的通道为连接通道
				if (key.isReadable()) {
					//读取数据
					readDataFromSocket(key);
				}
				// 从就绪集合中移除
				it.remove();
			}
		}
	}

	protected void registerChannel(Selector selector, SelectableChannel channel, int ops) throws Exception {
		if (channel == null) {
			return; // could happen
		}
		// Set the new channel nonblocking
		channel.configureBlocking(false);
		// Register it with the selector
		channel.register(selector, ops);
	}

	// ----------------------------------------------------------
	// Use the same byte buffer for all channels. A single thread is
	// servicing all the channels, so no danger of concurrent acccess.
	private ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
	protected void readDataFromSocket(SelectionKey key) throws Exception {
		SocketChannel socketChannel = (SocketChannel) key.channel();
		int count;
		buffer.clear(); // Empty buffer
// socketChannel是以非阻塞模式运行，count可能会为0；但因为使用了选择器，第一次读取肯定不会为0。
		while ((count = socketChannel.read(buffer)) > 0) {
			buffer.flip(); 
			while (buffer.hasRemaining()) {
				socketChannel.write(buffer);
			}
			buffer.clear(); // Empty buffer
		}
		if (count < 0) {
// Close channel on EOF, invalidates the key
			socketChannel.close();
		}
	}

// ----------------------------------------------------------
	/**
	 * Spew a greeting to the incoming client connection.
	 *
	 * @param channel The newly connected SocketChannel to say hello to.
	 */
	private void sayHello(SocketChannel channel) throws Exception {
		buffer.clear();
		buffer.put("Hi there!\r\n".getBytes());
		buffer.flip();
		channel.write(buffer);
	}

}