demo

public static void method1(){        RandomAccessFile aFile = null;        try{            aFile = new RandomAccessFile("src/nio.txt","rw");            FileChannel fileChannel = aFile.getChannel();            ByteBuffer buf = ByteBuffer.allocate(1024);             int bytesRead = fileChannel.read(buf);            System.out.println(bytesRead);             while(bytesRead != -1)            {                buf.flip();                while(buf.hasRemaining())                {                    System.out.print((char)buf.get());                }                 buf.compact();                bytesRead = fileChannel.read(buf);            }        }catch (IOException e){            e.printStackTrace();        }finally{            try{                if(aFile != null){                    aFile.close();                }            }catch (IOException e){                e.printStackTrace();            }        }    }

分析

从案例2中可以总结出使用Buffer一般遵循下面几个步骤：

• 分配空间（ByteBuffer buf = ByteBuffer.allocate(1024); 还有一种allocateDirector后面再陈述）
• 写入数据到Buffer(int bytesRead = fileChannel.read(buf);)
• 调用filp()方法（ buf.flip();）
• 从Buffer中读取数据（System.out.print((char)buf.get());）
• 调用clear()方法或者compact()方法

Buffer顾名思义：缓冲区，实际上是一个容器，一个连续数组。Channel提供从文件、网络读取数据的渠道，但是读写的数据都必须经过Buffer。如下图：

向Buffer中写数据：

• 从Channel写到Buffer (fileChannel.read(buf))
• 通过Buffer的put()方法 （buf.put(…)）

从Buffer中读取数据：

• 从Buffer读取到Channel (channel.write(buf))
• 使用get()方法从Buffer中读取数据 （buf.get()）

可以把Buffer简单地理解为一组基本数据类型的元素列表，它通过几个变量来保存这个数据的当前位置状态：capacity, position, limit, mark：

Selector简介

服务端

public class ServerSocketChannelTest {    private int size = 1024;    private ServerSocketChannel socketChannel;    private ByteBuffer byteBuffer;    private Selector selector;    private final int port = 8998;    private int remoteClientNum=0;    public ServerSocketChannelTest() {        try {            initChannel();        } catch (Exception e) {            e.printStackTrace();            System.exit(-1);        }    }    public void initChannel() throws Exception {        socketChannel = ServerSocketChannel.open();        socketChannel.configureBlocking(false);        socketChannel.bind(new InetSocketAddress(port));        System.out.println("listener on port:" + port);        selector = Selector.open();        socketChannel.register(selector, SelectionKey.OP_ACCEPT);        byteBuffer = ByteBuffer.allocateDirect(size);        byteBuffer.order(ByteOrder.BIG_ENDIAN);    }    private void listener() throws Exception {        while (true) {            int n = selector.select();            if (n == 0) {                continue;            }            Iterator
    
      ite = selector.selectedKeys().iterator();            while (ite.hasNext()) {                SelectionKey key = ite.next();                //a connection was accepted by a ServerSocketChannel.                if (key.isAcceptable()) {                    ServerSocketChannel server = (ServerSocketChannel) key.channel();                    SocketChannel channel = server.accept();                    registerChannel(selector, channel, SelectionKey.OP_READ);                    remoteClientNum++;                    System.out.println("online client num="+remoteClientNum);                    replyClient(channel);                }                //a channel is ready for reading                if (key.isReadable()) {                    readDataFromSocket(key);                }                ite.remove();//must            }        }    }    protected void readDataFromSocket(SelectionKey key) throws Exception {        SocketChannel socketChannel = (SocketChannel) key.channel();        int count;        byteBuffer.clear();        while ((count = socketChannel.read(byteBuffer)) > 0) {            byteBuffer.flip(); // Make buffer readable            // Send the data; don't assume it goes all at once            while (byteBuffer.hasRemaining()) {                socketChannel.write(byteBuffer);            }            byteBuffer.clear(); // Empty buffer        }        if (count < 0) {            socketChannel.close();        }    }    private void replyClient(SocketChannel channel) throws IOException {        byteBuffer.clear();        byteBuffer.put("hello client!\r\n".getBytes());        byteBuffer.flip();        channel.write(byteBuffer);    }    private void registerChannel(Selector selector, SocketChannel channel, int ops) throws Exception {        if (channel == null) {            return;        }        channel.configureBlocking(false);        channel.register(selector, ops);    }    public static void main(String[] args) {        try {            new ServerSocketChannelTest().listener();        } catch (Exception e) {            e.printStackTrace();        }    }}
    

客户端代码：

public class SocketChannelTest {    private int size = 1024;    private ByteBuffer byteBuffer;    private SocketChannel socketChannel;    public void connectServer() throws IOException {        socketChannel = SocketChannel.open();        socketChannel.connect(new InetSocketAddress("127.0.0.1", 8998));        byteBuffer = ByteBuffer.allocate(size);        byteBuffer.order(ByteOrder.BIG_ENDIAN);        receive();    }    private void receive() throws IOException {        while (true) {            int count;            byteBuffer.clear();            while ((count = socketChannel.read(byteBuffer)) > 0) {                byteBuffer.flip();                while (byteBuffer.hasRemaining()) {                    System.out.print((char) byteBuffer.get());                }                //send("send data to server\r\n".getBytes());                byteBuffer.clear();            }        }    }    private void send(byte[] data) throws IOException {        byteBuffer.clear();        byteBuffer.put(data);        byteBuffer.flip();        socketChannel.write(byteBuffer);    }    public static void main(String[] args) throws IOException {        new SocketChannelTest().connectServer();    }}

链接