NIO通信示例

读者可以将代码拷贝到编译器上跑一跑，如果看了注释还有不懂的地方，可以参考：
NIO基础（一）之Buffer
NIO基础（二）之Channel
NIO基础（三）之Selector
第三篇尤为重要对看懂大体框架有很大意义。

public class Const {
    public static int DEFAULT_PORT = 12345;
    public static String DEFAULT_SERVER_IP = "127.0.0.1";

    public static String response(String msg){
        return "Hello,"+msg+",Now is "+new java.util.Date(
                System.currentTimeMillis()).toString() ;
    }
}

/**
 * 类说明：nio通信客户端
 */
public class NioClient {
    private static NioClientHandle nioClientHandle;

    public static void start(){
        if(nioClientHandle !=null)
            nioClientHandle.stop();
        nioClientHandle = new NioClientHandle(DEFAULT_SERVER_IP,DEFAULT_PORT);
        new Thread(nioClientHandle,"Server").start();
    }
    //向服务器发送消息
    public static boolean sendMsg(String msg) throws Exception{
        nioClientHandle.sendMsg(msg);
        return true;
    }
    public static void main(String[] args) throws Exception {
        start();
        Scanner scanner = new Scanner(System.in);
        while(NioClient.sendMsg(scanner.next()));
    }
}

/**
 * 类说明：nio通信客户端处理器
 */
public class NioClientHandle implements Runnable{
    private String host;
    private int port;
    private volatile boolean started;
    private Selector selector;
    private SocketChannel socketChannel;


    public NioClientHandle(String ip, int port) {
        this.host = ip;
        this.port = port;
        try {
            /*创建选择器*/
            this.selector = Selector.open();
            /*打开监听通道*/
            socketChannel = SocketChannel.open();
            /*如果为 true，则此通道将被置于阻塞模式；
            * 如果为 false，则此通道将被置于非阻塞模式
            * 缺省为true*/
            socketChannel.configureBlocking(false);
            started = true;
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(-1);
        }
    }
    public void stop(){
        started = false;
    }


    @Override
    public void run() {
        //连接服务器
        try {
            doConnect();
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(-1);
        }
        /*循环遍历selector*/
        while(started){
            try {
                /*阻塞方法,当至少一个注册的事件发生的时候就会继续*/
                selector.select();
                /*获取当前有哪些事件可以使用*/
                Set<SelectionKey> keys = selector.selectedKeys();
                /*转换为迭代器*/
                Iterator<SelectionKey> it = keys.iterator();
                SelectionKey key = null;
                while(it.hasNext()){
                    key = it.next();
                    /*我们必须首先将处理过的 SelectionKey 从选定的键集合中删除。
                    如果我们没有删除处理过的键，那么它仍然会在事件集合中以一个**
                    的键出现，这会导致我们尝试再次处理它。*/
                    it.remove();
                    try {
                        handleInput(key);
                    } catch (Exception e) {
                        if(key!=null){
                            key.cancel();
                            if(key.channel()!=null){
                                key.channel().close();
                            }
                        }
                    }
                }

            } catch (IOException e) {
                e.printStackTrace();
                System.exit(-1);
            }
        }

        if(selector!=null){
            try {
                selector.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

    }

    /*具体的事件处理方法*/
    private void handleInput(SelectionKey key) throws IOException {
        if(key.isValid()){
            /*获得关心当前事件的channel*/
            SocketChannel sc =(SocketChannel)key.channel();
            /*处理连接就绪事件
            * 但是三次握手未必就成功了，所以需要等待握手完成和判断握手是否成功*/
            if(key.isConnectable()){
                /*finishConnect的主要作用就是确认通道连接已建立，
                方便后续IO操作（读写）不会因连接没建立而
                导致NotYetConnectedException异常。*/
                if(sc.finishConnect()){
                    /*连接既然已经建立，当然就需要注册读事件，
                    写事件一般是不需要注册的。*/
                    socketChannel.register(selector,SelectionKey.OP_READ);
                }else System.exit(-1);
            }

            /*处理读事件，也就是当前有数据可读*/
            if(key.isReadable()){
                /*创建ByteBuffer，并开辟一个1k的缓冲区*/
                ByteBuffer buffer = ByteBuffer.allocate(1024);
                /*将通道的数据读取到缓冲区，read方法返回读取到的字节数*/
                int readBytes = sc.read(buffer);
                if(readBytes>0){
                    buffer.flip();
                    byte[] bytes = new byte[buffer.remaining()];
                    buffer.get(bytes);
                    String result = new String(bytes,"UTF-8");
                    System.out.println("客户端收到消息："+result);
                }
                /*链路已经关闭，释放资源*/
                else if(readBytes<0){
                    key.cancel();
                    sc.close();
                }

            }
        }
    }

    /*进行连接*/
    private void doConnect() throws IOException {
        /*如果此通道处于非阻塞模式，则调用此方法将启动非阻塞连接操作。
        如果连接马上建立成功，则此方法返回true。
        否则，此方法返回false，
        因此我们必须关注连接就绪事件，
        并通过调用finishConnect方法完成连接操作。*/
        if(socketChannel.connect(new InetSocketAddress(host,port))){
            /*连接成功，关注读事件*/
            socketChannel.register(selector,SelectionKey.OP_READ);
        }
        else{
            socketChannel.register(selector,SelectionKey.OP_CONNECT);
        }
    }

    /*写数据对外暴露的API*/
    public void sendMsg(String msg) throws IOException {
        doWrite(socketChannel,msg);
    }

    private void doWrite(SocketChannel sc,String request) throws IOException {
        byte[] bytes = request.getBytes();
        ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length);
        writeBuffer.put(bytes);
        writeBuffer.flip();
        sc.write(writeBuffer);
    }
}

/**
 * 类说明：nio通信服务端
 */
public class NioServer {
    private static NioServerHandle nioServerHandle;

    public static void start(){
        if(nioServerHandle !=null)
            nioServerHandle.stop();
        nioServerHandle = new NioServerHandle(DEFAULT_PORT);
        new Thread(nioServerHandle,"Server").start();
    }
    public static void main(String[] args){
        start();
    }
}

/**
 * 类说明：nio通信服务端处理器
 */
public class NioServerHandle implements Runnable{
    private Selector selector;
    private ServerSocketChannel serverChannel;
    private volatile boolean started;
    /**
     * 构造方法
     * @param port 指定要监听的端口号
     */
    public NioServerHandle(int port) {
        try{
            //创建选择器
            selector = Selector.open();
            //打开监听通道
            serverChannel = ServerSocketChannel.open();
            //如果为 true，则此通道将被置于阻塞模式；
            // 如果为 false，则此通道将被置于非阻塞模式
            serverChannel.configureBlocking(false);//开启非阻塞模式
            serverChannel.socket().bind(new InetSocketAddress(port));
            serverChannel.register(selector,SelectionKey.OP_ACCEPT);

            //标记服务器已开启
            started = true;
            System.out.println("服务器已启动，端口号：" + port);
        }catch(IOException e){
            e.printStackTrace();
            System.exit(1);
        }
    }
    public void stop(){
        started = false;
    }

    @Override
    public void run() {
        //循环遍历selector
        while(started){
            try{
                //阻塞,只有当至少一个注册的事件发生的时候才会继续.
				selector.select();
                Set<SelectionKey> keys = selector.selectedKeys();
                Iterator<SelectionKey> it = keys.iterator();
                SelectionKey key = null;
                while(it.hasNext()){
                    key = it.next();
                    it.remove();
                    try{
                        handleInput(key);
                    }catch(Exception e){
                        if(key != null){
                            key.cancel();
                            if(key.channel() != null){
                                key.channel().close();
                            }
                        }
                    }
                }
            }catch(Throwable t){
                t.printStackTrace();
            }
        }
        //selector关闭后会自动释放里面管理的资源
        if(selector != null)
            try{
                selector.close();
            }catch (Exception e) {
                e.printStackTrace();
            }
    }
    private void handleInput(SelectionKey key) throws IOException{
        if(key.isValid()){
            //处理新接入的请求消息
            if(key.isAcceptable()){
                ServerSocketChannel ssc = (ServerSocketChannel)key.channel();
                SocketChannel sc = ssc.accept();
                System.out.println("=======建立连接===");
                sc.configureBlocking(false);
                sc.register(selector,SelectionKey.OP_READ);
            }

            //读消息
            if(key.isReadable()){
                System.out.println("======socket channel 数据准备完成，" +
                        "可以去读==读取=======");
                SocketChannel sc = (SocketChannel) key.channel();
                //创建ByteBuffer，并开辟一个1M的缓冲区
                ByteBuffer buffer = ByteBuffer.allocate(1024);
                //读取请求码流，返回读取到的字节数
                int readBytes = sc.read(buffer);
                //读取到字节，对字节进行编解码
                if(readBytes>0){
                    //将缓冲区当前的limit设置为position,position=0，
                    // 用于后续对缓冲区的读取操作
                    buffer.flip();
                    //根据缓冲区可读字节数创建字节数组
                    byte[] bytes = new byte[buffer.remaining()];
                    //将缓冲区可读字节数组复制到新建的数组中
                    buffer.get(bytes);
                    String message = new String(bytes,"UTF-8");
                    System.out.println("服务器收到消息：" + message);
                    //处理数据
                    String result = response(message) ;
                    //发送应答消息
                    doWrite(sc,result);
                }
                //链路已经关闭，释放资源
                else if(readBytes<0){
                    key.cancel();
                    sc.close();
                }
            }
        }
    }
    //发送应答消息
    private void doWrite(SocketChannel channel,String response)
            throws IOException {
        //将消息编码为字节数组
        byte[] bytes = response.getBytes();
        //根据数组容量创建ByteBuffer
        ByteBuffer writeBuffer = ByteBuffer.allocate(bytes.length);
        //将字节数组复制到缓冲区
        writeBuffer.put(bytes);
        //flip操作
        writeBuffer.flip();
        //发送缓冲区的字节数组
        channel.write(writeBuffer);
    }
}