IO 原理（一）：从 BIO 到 NIO

    public static void main(String[] args) throws IOException {        //socket()        try (ServerSocket serverSocket = new ServerSocket(8888)) {            Socket socket;            //loop1:connect()            while (true) {                //阻塞1：connect()                System.out.println("waiting for connect...");                socket = serverSocket.accept();                System.out.println("connected...");
                try (InputStream inputStream = socket.getInputStream();) {                    StringBuilder req = new StringBuilder();                    byte[] recvByteBuf = new byte[1024];                    int len;                    //loop2:read()                    //阻塞2：read()                    while ((len = inputStream.read(recvByteBuf)) != -1) {                        String append = new String(recvByteBuf, 0, len, StandardCharsets.UTF_8);                        req.append(append);                        System.out.println("read buf.." + append);                    }                    System.out.println("connect closed,read string.." + req);                }            }        }    }

    public static void main(String[] args) throws Exception {        // server socket channel:        try (ServerSocketChannel serverSocketChannel = ServerSocketChannel.open()) {            serverSocketChannel.bind(new InetSocketAddress(InetAddress.getLocalHost(), 8888));            while (true) {                SocketChannel socketChannel = serverSocketChannel.accept();                ByteBuffer buffer = ByteBuffer.allocateDirect(1024);                int readBytes = socketChannel.read(buffer);                if (readBytes > 0) {                    // 从写数据到buffer翻转为从buffer读数据                    buffer.flip();                    byte[] bytes = new byte[buffer.remaining()];                    buffer.get(bytes);                    String body = new String(bytes, StandardCharsets.UTF_8);                    System.out.println("server 收到：" + body);                }            }        }    }

public class NIOTestServer {
    public static void main(String[] args) throws Exception {        NIOTestServer server = new NIOTestServer();
        Selector selector = server.register(new InetSocketAddress("127.0.0.1", 8888));        server.listen(selector);    }
    public Selector register(InetSocketAddress address) throws IOException {        //1.初始化selector        Selector selector = Selector.open();
        //2.初始化ServerSocketChannel        ServerSocketChannel chanel = ServerSocketChannel.open();        chanel.bind(address);//绑定地址和端口        //搭配channel时需要设置为false非阻塞，否则抛异常        //selector在感知事件时，需要不断轮询channel，如果为阻塞就会堵住主线程，这样就没有使用selector多路复用的意义了        chanel.configureBlocking(false);
        //3.注册ServerSocketChannel到selector到上，关注连接事件        chanel.register(selector, SelectionKey.OP_ACCEPT);
        return selector;    }
    public void listen(Selector selector) {        try {            while (true) {                //1.获取事件，这一步是阻塞的                selector.select();
                //返回就绪事件列表，SelectionKey包含事件信息                Set<SelectionKey> selectionKeys = selector.selectedKeys();                Iterator<SelectionKey> iterator = selectionKeys.iterator();                //2.遍历事件                while (iterator.hasNext()) {                    SelectionKey selectionKey = iterator.next();                    //3.处理事件业务                    processKey(selector, selectionKey);                    //4.需要手动移除事件                    iterator.remove();                }            }        } catch (Exception e) {            e.printStackTrace();        }    }
    private void processKey(Selector selector, SelectionKey key) throws IOException {        //1.客户端第一次进来，先处理accept事件        if (key.isAcceptable()) {//事件类型为接受连接            ServerSocketChannel ssc = (ServerSocketChannel) key.channel(); //这里对应的是ServerSocketChannel            //为每一个连接创建一个SocketChannel，用来读写数据            //完成该操作意味着完成TCP三次握手，TCP物理链路正式建立            SocketChannel client = ssc.accept();            client.configureBlocking(false);//原因同上            client.register(selector, SelectionKey.OP_READ);//连接建立后关注读事件        }
        //2.accept后请求进来，可以处理关注的事件        if (key.isReadable()) {            SocketChannel sc = (SocketChannel) key.channel();//这里对应的是SocketChannel
            //读取数据            ByteBuffer buffer = ByteBuffer.allocate(1024);//buffer            while (sc.read(buffer) > 0) {                //前面read(buffer)操作是写数据到缓冲区中                //这里要将buffer由写模式变成读模式，用于后续buffer读                buffer.flip();                //读取数据                String content = new String(buffer.array(), StandardCharsets.UTF_8);                //处理业务                System.out.println(content);            }        }    }}