BIO模型
BIO即blocking IO，顾名思义是一种阻塞模型。当没有客户端连接时，服务端会一直阻塞，当有客户端新建连接时，服务端会新开一个线程去响应（不用多线程的话服务端同一时刻最多只能接收一个连接）。但不断的新开线程对服务器的压力是巨大的，为了缓解压力可以采用线程池技术实现线程复用，但这种做法治标不治本，本质还是一个连接一个线程。代码如下：
//服务端public class BIOServer {private final static int PORT = 8888;public static void main(String[] args) throws IOException {ServerSocket server = new ServerSocket(PORT);ExecutorService pool = Executors.newFixedThreadPool(20);while(true) {Socket socket = server.accept();pool.execute(()->{handler(socket);});}}public static void handler(Socket socket) {try {InputStream inputStream = socket.getInputStream();byte[] bytes = new byte[1024];while(true) {int len = inputStream.read(bytes);if(len != -1) {System.out.println("收到来自客户端的消息：" + new String(bytes,0, len));}}} catch (IOException e) {e.printStackTrace();} finally {try {socket.close();} catch (IOException e) {e.printStackTrace();}}}}
//客户端public class BIOClient {private final static String IP = "localhost";private final static int PORT = 8888;public static void main(String[] args) throws IOException {Socket socket = new Socket(IP,PORT);Scanner sc = new Scanner(System.in);BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));while(sc.hasNext()) {String str = sc.nextLine();bw.write(str);bw.flush();}}}
NIO模型
NIO即non-blocking IO，顾名思义是一种非阻塞模型。NIO的目的就是实现一个线程处理多个连接，故引入了几个重要的核心概念：
Channel，管道。Channel可以理解为连接，与BIO中Sokcet类似，一个连接对应一个Channel，但Channel中仍内置了一个Socket，可以调用socket()获取。Selector，选择器。Selector类似一个调度中心，所有Channel都需要注册到选择器中，并绑定一个SelectionKey，绑定时还会指定要监听的事件，如：连接就绪、读就绪、写就绪等。可以调用Selector提供的API实现对发生监听事件的连接进行处理。Buffer，缓冲区。Buffer底层是一个数组，供Channel实现对数据的读写。Buffer的position、limit、capacity分别指当前索引、读/写上限索引、数组容量。
三者之间的关系如下图所示（图比较乱…序号表示NIO流程执行的大概步骤，部分连线只为了便于理解，不等于实际调用）：
下面是用NIO实现的简易多人聊天室代码：
//服务端public class NIOServer {private final String IP = "localhost";private final int PORT = 8888;private ServerSocketChannel serverChannel;private Selector selector;private int count;public NIOServer init() {try {serverChannel = ServerSocketChannel.open();selector = Selector.open();serverChannel.bind(new InetSocketAddress(IP,PORT));serverChannel.configureBlocking(false);serverChannel.register(selector, SelectionKey.OP_ACCEPT);count = 0;} catch (IOException e) {System.out.println("初始化失败...");} finally {return this;}}public void start() {while(true) {try {//阻塞直到有任意通道发生任一事件selector.select();Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();while (iterator.hasNext()) {SelectionKey key = iterator.next();//监听到新用户连接if (key.isAcceptable()) {userConnect(key);}//监听到有用户发消息if (key.isReadable()) {userSendMsg(key);}iterator.remove();}} catch (IOException e) {System.out.println("未知的bug...");}}}public void userConnect(SelectionKey key) throws IOException {SocketChannel channel = serverChannel.accept();channel.configureBlocking(false);channel.register(selector, SelectionKey.OP_READ);String msg = "有新的用户接入：" + channel.socket().getRemoteSocketAddress() +"，在线用户总数：" + ++count + "个";System.out.println(msg);transfer(msg, channel);}public void userSendMsg(SelectionKey key) throws IOException {SocketChannel channel = (SocketChannel) key.channel();ByteBuffer buffer = ByteBuffer.allocate(1024);String msg = null;try {//用户非正常关闭会直接在read这里抛IO异常int len = channel.read(buffer);//用户正常关闭连接需要手动抛IO异常，不然会一直空轮询分发读就绪事件if (len == -1)throw new IOException();msg = channel.socket().getPort() + "：" + new String(buffer.array(), 0, len);} catch (IOException e) {//有用户断开连接msg = channel.socket().getPort() + "断开连接，在线用户总数：" + --count + "个";key.cancel();channel.socket().close();channel.close();} finally {System.out.println(msg);transfer(msg, channel);}}public void transfer(String msg, SocketChannel self) throws IOException{Iterator<SelectionKey> iterator = selector.keys().iterator();ByteBuffer buffer = ByteBuffer.wrap(msg.getBytes());while(iterator.hasNext()) {SelectionKey key = iterator.next();if(key.channel() instanceof ServerSocketChannel)continue;SocketChannel channel = (SocketChannel) key.channel();//转发用户消息if(self != key.channel()) {channel.write(buffer);buffer.clear();}}}public static void main(String[] args) {new NIOServer().init().start();}}
public class NIOClient {private final static String IP = "localhost";private final static int PORT = 8888;private Selector selector;private SocketChannel channel;public void init() {try {selector = Selector.open();InetSocketAddress address = new InetSocketAddress(IP, PORT);channel = SocketChannel.open();channel.connect(address);channel.configureBlocking(false);channel.register(selector, SelectionKey.OP_READ);} catch (IOException e) {System.out.println("初始化失败...");}}public void sendMsg(String msg) {try {ByteBuffer buffer = ByteBuffer.wrap(msg.getBytes());channel.write(buffer);} catch (IOException e) {System.out.println("服务器异常...");}}public void readMsg(ByteBuffer buffer) {try {//只绑定了一个channel且监听的读事件...if(selector.select() > 0) {int len = channel.read(buffer);if (len != -1) {buffer.flip();System.out.println(new String(buffer.array(), 0, len));}//这一步一定不能少，不然只能读一次消息selector.selectedKeys().clear();}} catch (IOException e) {System.out.println("服务器异常...");}}public static void main(String[] args) {NIOClient client = new NIOClient();client.init();ByteBuffer buffer = ByteBuffer.allocate(1024);new Thread(() -> {while (true) {buffer.clear();client.readMsg(buffer);}}).start();Scanner sc = new Scanner(System.in);while(sc.hasNext()) {client.sendMsg(sc.nextLine());}try {client.channel.socket().close();client.channel.close();} catch (IOException e) {System.out.println("退出异常...");}}}
