import java.io.IOException;

import java.net.InetAddress;

import java.net.InetSocketAddress;

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.ArrayList;

import java.util.Iterator;

import java.util.Random;

import java.util.Set;

import java.util.concurrent.ConcurrentHashMap;

public class Server {

    private static int PORT = 8000;

    private static final long PAUSE_BETWEEEN_MSGS = 10; // millisecs

    private static ByteBuffer echoBuffer = ByteBuffer.allocate(4);

    private static ByteBuffer sendBuffer = ByteBuffer.allocate(256);

    private static ConcurrentHashMap<Integer, SocketChannel> chm

            = new ConcurrentHashMap<Integer, SocketChannel>();

    private static int msg = 0;

public static void main(String args[]) throws Exception {

        Selector selector = Selector.open();

        // Open a listener on each port, and register each one

        ServerSocketChannel ssc = ServerSocketChannel.open();

        ssc.configureBlocking(false);

        InetSocketAddress address = new InetSocketAddress("localhost",PORT);

        ssc.bind(address);

        ssc.register(selector, SelectionKey.OP_ACCEPT);

        System.out.println("Going to listen on " + PORT);

        while (true){

            selector.select();

            Set<SelectionKey> selectedKeys = selector.selectedKeys();

            Iterator<SelectionKey> it = selectedKeys.iterator();

            System.out.println(selectedKeys.size());

            while (it.hasNext()){

                String msg = new String();

                SelectionKey key = (SelectionKey) it.next();

                if(key.isAcceptable()){

                    ServerSocketChannel sscNew = (ServerSocketChannel) key

                            .channel();

                    SocketChannel sc = sscNew.accept();

                    sc.configureBlocking(false);

                    // Add the new connection to the selector

                    sc.register(selector, SelectionKey.OP_READ);

                    // Add the socket channel to the list

                    chm.put(sc.hashCode(), sc);

                    it.remove();

                }else if(key.isReadable()){

                    SocketChannel sc = (SocketChannel) key.channel();

                    int code = 0;

                    while ((code = sc.read(echoBuffer)) > 0) {

                        byte b[] = new byte[echoBuffer.position()];

                        echoBuffer.flip();

                        echoBuffer.get(b);

                        msg+=new String(b, "UTF-8");

                    }

//                    if(msg.length()>1)

//                        msg = msg.substring(0, msg.length()-2);

                    //client关闭时，收到可读事件，code = -1

                    if (code == -1 ||

                            msg.toUpperCase().indexOf("BYE")>-1){

                        chm.remove(sc.hashCode());

                        sc.close();

                    } else {

                        //code=0，消息读完或者echoBuffer空间不够时，部分消息内容下一次select后收到

                        echoBuffer.clear();

                    }

                    System.out.println("msg: " + msg  + " from: " + sc + "code:  " + code );

                    it.remove();

                    //注册可写通知

                    sc.register(selector,SelectionKey.OP_WRITE);

                }else if(key.isWritable()){

                    SocketChannel client = (SocketChannel) key.channel();

                    String sendTxt = "Message from Server";

                    sendBuffer.put(sendTxt.getBytes());

                    sendBuffer.flip();

                    int code = 0;

                    //如果sendBuffer内容一次没有写完，会在下一次事件中处理吗？

                    while (client.write(sendBuffer) != 0){

                    }

                    if (code == -1 ){

                        chm.remove(client.hashCode());

                        client.close();

                    } else {

                        //code=0，消息写完

                        sendBuffer.clear();

                    }

                    it.remove();

                    System.out.println("Send message to client ");

                    //在读通知里面注册为写事件，所以这里还需要注册为读，否则不在接受客户端消息

                    client.register(selector,SelectionKey.OP_READ);

                }

            }

            Thread.sleep(5000);

        }

    }

}

while (buffer.hasRemaining()){

                socketChannel.write(buffer);

            }