简述 BIO 、NIO 模型

• BIO : 同步阻塞I/O（Block IO）

         服务器实现模式为每一个连接一个线程，即客户端有连接请求时服务器就需要启动一个线程进行处理，如果这个连接不做任何事情会造成不必要的线程开销，此处可以通过线程池机制进行优化。

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import java.util.List;/*** 多人聊天室 - 服务端*/
public class BioServer {static List<Socket> clientList = new ArrayList<>();public static void main(String[] args) throws Exception {int port = 8080;ServerSocket serverSocket = new ServerSocket(port);while (true) {Socket client = serverSocket.accept();System.out.println("客户端: " + client.getPort() + " 连接成功!");clientList.add(client);forwardProcess(client);}}/*** 转发处理*/public static void forwardProcess(Socket socket) {new Thread(new Runnable() {@Overridepublic void run() {while (true) {forwardMsg(socket);}}}).start();}/*** 转发消息*/public static void forwardMsg(Socket socket) {try {String msg = readMsg(socket);System.out.println(msg);for (Socket client : clientList) {if (client != socket) {writeMsg(client, msg);}}} catch (IOException e) {throw new RuntimeException(e);}}/*** 写入消息*/public static void writeMsg(Socket socket, String msg) throws IOException {PrintStream ps = new PrintStream(socket.getOutputStream());ps.println(msg);ps.flush();}/*** 读取消息*/public static String readMsg(Socket socket) throws IOException {InputStream inputStream = socket.getInputStream();BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));String msg;if ((msg = br.readLine()) != null) {msg = socket.getPort() + " 说: " + msg;}return msg;}}

import java.io.*;
import java.net.Socket;
import java.util.Scanner;public class BilClient {public static void main(String[] args) throws IOException {String ip = "127.0.0.1";int port = 8080;Socket client = new Socket(ip, port);readProcess(client);OutputStream os = client.getOutputStream();PrintStream ps = new PrintStream(os);Scanner scanner = new Scanner(System.in);while (true) {String input = scanner.nextLine();ps.println(input);ps.flush();}}/*** 读取处理*/public static void readProcess(Socket socket) {new Thread(() -> {while (true) {try {System.out.println(readMsg(socket));} catch (IOException e) {throw new RuntimeException(e);}}}).start();}/*** 读取消息*/public static String readMsg(Socket socket) throws IOException {InputStream inputStream = socket.getInputStream();BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));String msg;if ((msg = br.readLine()) != null) {}return msg;}
}

• ​NIO: 同步非阻塞式IO，服务器实现模式为一个线程处理多个请求(连接)，即客户端发送的连接请求会被注册到多路复用器上，多路复用器轮询到有 I/O 请求就会进行处理。
• Channel，翻译过来就是“通道”，就是数据传输的管道，类似于“流”，但是与“流”又有着区别。
  • 既可以从Channel中读取数据，又可以写数据到Channel，但流的读写通常是单向的——输入输出流
  • 通道可以异步读写
  • 通道中的数据总是先读取到buffer（缓冲区），或者总是需要从一个buffer写入，不能直接访问数据
  • 非阻塞特性：Channel在设计上采用了非阻塞的特性，它不会像传统的流一样在读写操作上阻塞线程，而是立即返回结果，告诉调用者当前的状态。这使得程序可以在等待数据准备的过程中同时进行其他操作，实现了非阻塞IO。
  • 事件通知机制：Channel通常搭配选择器（Selector）来使用，选择器能够检测多个Channel的就绪状态，如是否可读、可写等，并通过事件通知（例如轮询或回调）及时地通知程序哪些Channel处于就绪状态，从而可以进行相应的读写操作。这种机制支持程序实现异步IO模型。
  • 操作系统底层支持：Channel的异步读写也依赖于操作系统底层的异步IO支持。Java NIO中的Channel实际上是对操作系统底层异步IO的封装和抽象，利用了操作系统提供的异步IO机制来实现其自身的异步读写功能。
• Buffer是一个对象，里面是要写入或者读出的数据，在java.nio库中，所有的数据都是用缓冲区处理的。
  • 在读取数据时，它是直接读到缓冲区中的；在写入数据时，也是直接写到缓冲区中，任何时候访问Channel中的数据，都是通过缓冲区进行操作的。
  • 缓冲区实质上是一个数组，通常是一个字节数组ByteBuffer，当然也有其他类型的：
• Selector被称为选择器，Selector会不断地轮询注册在其上的Channel，如果某个Channel上发生读或写事件，这个Channel就被判定处于就绪状态，会被Selector轮询出来，然后通过SelectionKey可以获取到就绪Channel的集合，进行后续的I/O操作。
  • 一个多路复用器Selector可以同时轮询多个Channel，JDK使用了epoll()代替了传统的select实现，所以并没有最大连接句柄的限制，这意味着只需要一个线程负责Selector的轮询，就可以接入成千上万的客户端。

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.*;
import java.nio.charset.Charset;
import java.util.Set;/*** NIO 聊天室 服务端*/
public class NioServer {private Integer port;ByteBuffer readBuffer = ByteBuffer.allocate(1024);ByteBuffer writerBuffer = ByteBuffer.allocate(1024);private Charset charset = Charset.forName("UTF-8");public NioServer(Integer port) {this.port = port;}public static void main(String[] args) {NioServer nioServer = new NioServer(8080);nioServer.start();}private void start() {try {// 开启socketServerSocketChannel server = ServerSocketChannel.open();// 设置非阻塞server.configureBlocking(false);// 绑定端口server.socket().bind(new InetSocketAddress(port));// 开启通道, 得到 Selector (选择器)Selector selector = Selector.open();//  注册 selector 监听事件server.register(selector, SelectionKey.OP_ACCEPT);System.out.println("启动服务器， 监听端口：" + port + "...");while (true) {// 阻塞监控所有注册的通道,当有对应的事件操作时, 会将SelectionKey放入 集合内部并返回事件数量selector.select();// 返回存有SelectionKey的集合Set<SelectionKey> selectionKeys = selector.selectedKeys();for (SelectionKey selectionKey : selectionKeys) {handle(selectionKey, selector);}//  处理后清理 selectionKeysselectionKeys.clear();}} catch (Exception e) {e.printStackTrace();}}/*** 事件处理*/private void handle(SelectionKey key, Selector selector) throws IOException {//  SelectionKey 常用方法//isAcceptable()	是否是连接继续事件//isConnectable()	是否是连接就绪事件//isReadable()	是否是读就绪事件//isWritable()	是否是写就绪事件// SelectionKey 常用事件//SelectionKey.OP_ACCEPT	接收连接继续事件，表示服务器监听到了客户连接，服务器可以接收这个连接了//SelectionKey.OP_CONNECT	连接就绪事件，表示客户端与服务器的连接已经建立成功//SelectionKey.OP_READ	读就绪事件，表示通道中已经有了可读的数据，可以执行读操作了（通道目前有数据，可以进行读操作了）//SelectionKey.OP_WRITE	写就绪事件，表示已经可以向通道写数据了（通道目前可以用于写操作）//处理连接if (key.isAcceptable()) {ServerSocketChannel server = (ServerSocketChannel) key.channel();SocketChannel client = server.accept();client.configureBlocking(false);client.register(selector, SelectionKey.OP_READ);System.out.println(client.socket().getPort() + " 已建立连接 ...");}//  读取消息else if (key.isReadable()) {SocketChannel client = (SocketChannel) key.channel();String msg = client.socket().getPort() + " 说: " + readMsg(client);System.out.println(msg);forwardMsg(msg, client, selector);}}/*** 读取通道消息*/private String readMsg(SocketChannel client) throws IOException {readBuffer.clear();while (client.read(readBuffer) > 0) ;readBuffer.flip();return String.valueOf(charset.decode(readBuffer));}/*** 转发*/private void forwardMsg(String msg, SocketChannel client, Selector selector) throws IOException {for (SelectionKey key : selector.keys()) {Channel connectedClient = key.channel();if (connectedClient instanceof ServerSocketChannel) {continue;}if (key.isValid() && !client.equals(connectedClient)) {writerBuffer.clear();writerBuffer.put(charset.encode(msg));writerBuffer.flip();while (writerBuffer.hasRemaining())((SocketChannel) connectedClient).write(writerBuffer);}}}
}

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.nio.charset.Charset;
import java.util.Scanner;
import java.util.Set;/*** NIO 聊天室 客户端*/
public class NioClient {private String ip;private Integer port;private ByteBuffer writerBuffer = ByteBuffer.allocate(1024);private ByteBuffer readBuffer = ByteBuffer.allocate(1024);private Charset charset = Charset.forName("UTF-8");public NioClient(String ip, Integer port) {this.ip = ip;this.port = port;}public static void main(String[] args) {NioClient nioClient = new NioClient("127.0.0.1", 8080);nioClient.start();}public void start() {try {// 开启通道SocketChannel client = SocketChannel.open();// 设置非阻塞client.configureBlocking(false);Selector selector = Selector.open();client.register(selector, SelectionKey.OP_CONNECT);client.connect(new InetSocketAddress(ip, port));while (true) {selector.select();Set<SelectionKey> selectionKeys = selector.selectedKeys();for (SelectionKey selectionKey : selectionKeys) {handle(selectionKey, selector);}selectionKeys.clear();}} catch (Exception e) {e.printStackTrace();}}private void handle(SelectionKey key, Selector selector) throws IOException {// 处理连接事件if (key.isConnectable()) {SocketChannel client = (SocketChannel) key.channel();if (client.isConnectionPending()) {client.finishConnect();// 处理用户输入new Thread(() -> {Scanner scanner = new Scanner(System.in);while (true) {String msg = scanner.nextLine();writerBuffer.clear();writerBuffer.put(charset.encode(msg));writerBuffer.flip();while (writerBuffer.hasRemaining()) {try {client.write(writerBuffer);} catch (IOException e) {throw new RuntimeException(e);}}}}).start();}client.register(selector, SelectionKey.OP_READ);}// 读取消息信息else if (key.isReadable()) {SocketChannel client = (SocketChannel) key.channel();String s = readMsg(client);System.out.println(s);}}private String readMsg(SocketChannel client) throws IOException {readBuffer.clear();while (client.read(readBuffer) > 0) ;readBuffer.flip();return String.valueOf(charset.decode(readBuffer));}
}