Netty样例之如何使用LengthFieldBasedFrameDecoder

一  客户端　

public class NewClient {

    private  int port;
    private  String address;

    public NewClient(int port,String address) {
        this.port = port;
        this.address = address;
    }

    public void start(){
        EventLoopGroup group = new NioEventLoopGroup();

        Bootstrap bootstrap = new Bootstrap();
        bootstrap.group(group)
                .channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                .handler(new NewClientChannelInitializer());

        try {
            ChannelFuture future = bootstrap.connect(address,port).sync();
            new Thread(new Runnable() {
                
                @Override
                public void run() {
                    while (true) {
                        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
                        String message;
                        try {
                            message = br.readLine();
                            Message msg = new Message((byte)0xCA, message.length(), message);
                            future.channel().writeAndFlush(msg);
                        } catch (IOException e) {
                            e.printStackTrace();
                        }finally {
                            try {
                                br.close();
                            } catch (IOException e) {
                                e.printStackTrace();
                            }
                        }
                    }
                    
                    
                }
            }).start();
            
            
            future.channel().closeFuture().sync();
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            group.shutdownGracefully();
        }
    }

    public static void main(String[] args) {
        NewClient client = new NewClient(7788,"127.0.0.1");
        client.start();
    }
}

public class NewClientChannelInitializer extends ChannelInitializer<SocketChannel> {

    protected void initChannel(SocketChannel socketChannel) throws Exception {
        ChannelPipeline pipeline = socketChannel.pipeline();

        pipeline.addLast(new NewEncoder());
        pipeline.addLast(new NewClientHandler());
    }
}

public class NewClientHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        String m = "你好啊,Netty。昂昂";
        Message msg = new Message((byte)0xCA, m.length(), m);
        ctx.writeAndFlush(msg);
    }
}

public class NewEncoder extends MessageToByteEncoder<Message> {

    @Override
    protected void encode(ChannelHandlerContext channelHandlerContext, Message message, ByteBuf byteBuf) throws Exception {
        if(message == null){
            throw new Exception("未获得消息内容");
        }


        String msgBody = message.getMsgBody();
        byte[] b = msgBody.getBytes(Charset.forName("utf-8"));
        byteBuf.writeByte(message.getType());
        byteBuf.writeByte(b.length);
        byteBuf.writeBytes(b);


    }
}

二 服务端

public class NewServer {
    private static final int MAX_FRAME_LENGTH = 1024 * 1024;
    private static final int LENGTH_FIELD_LENGTH = 4;
    private static final int LENGTH_FIELD_OFFSET = 1;
    private static final int LENGTH_ADJUSTMENT = 0;
    private static final int INITIAL_BYTES_TO_STRIP = 0;

    private int port;

    public NewServer(int port) {
        this.port = port;
    }

    public void start(){
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap sbs = new ServerBootstrap()
                    .group(bossGroup,workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .localAddress(new InetSocketAddress(port))
                    .childHandler(new NewServerChannelInitializer(MAX_FRAME_LENGTH,LENGTH_FIELD_LENGTH,LENGTH_FIELD_OFFSET,LENGTH_ADJUSTMENT,INITIAL_BYTES_TO_STRIP))
                    .option(ChannelOption.SO_BACKLOG, 128)
                    .childOption(ChannelOption.SO_KEEPALIVE, true);

            ChannelFuture future = sbs.bind(port).sync();

            System.out.println("Server start listen at " + port );
            future.channel().closeFuture().sync();
        } catch (Exception e) {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }

    public static void main(String[] args) {
        NewServer server = new NewServer(7788);
        server.start();
    }

}

　　

public class NewServerChannelInitializer extends ChannelInitializer<SocketChannel> {
    private  final int MAX_FRAME_LENGTH;
    private  final int LENGTH_FIELD_LENGTH;
    private  final int LENGTH_FIELD_OFFSET;
    private  final int LENGTH_ADJUSTMENT;
    private  final int INITIAL_BYTES_TO_STRIP;

    public NewServerChannelInitializer(int MAX_FRAME_LENGTH, int LENGTH_FIELD_LENGTH, int LENGTH_FIELD_OFFSET, int LENGTH_ADJUSTMENT, int INITIAL_BYTES_TO_STRIP) {
        this.MAX_FRAME_LENGTH = MAX_FRAME_LENGTH;
        this.LENGTH_FIELD_LENGTH = LENGTH_FIELD_LENGTH;
        this.LENGTH_FIELD_OFFSET = LENGTH_FIELD_OFFSET;
        this.LENGTH_ADJUSTMENT = LENGTH_ADJUSTMENT;
        this.INITIAL_BYTES_TO_STRIP = INITIAL_BYTES_TO_STRIP;
    }

    @Override
    protected void initChannel(SocketChannel socketChannel) throws Exception {
        ChannelPipeline pipeline = socketChannel.pipeline();

        pipeline.addLast(new NewDecoder(MAX_FRAME_LENGTH,LENGTH_FIELD_LENGTH,LENGTH_FIELD_OFFSET,LENGTH_ADJUSTMENT,INITIAL_BYTES_TO_STRIP,false));
        // 自己的逻辑Handler
        pipeline.addLast("handler", new NewServerHandler());
    }

}

public class NewDecoder extends LengthFieldBasedFrameDecoder {

    /**
     * 我们在Message类中定义了type和length，这都放在消息头部
     * type占1个字节，length占4个字节所以头部总长度是5个字节
     */
    private static final int HEADER_SIZE = 5;
    private byte type;
    private int length;
    private String msgBody;


    /**
     *
     * @param maxFrameLength   网络字节序，默认为大端字节序
     * @param lengthFieldOffset 消息中长度字段偏移的字节数
     * @param lengthFieldLength 数据帧的最大长度
     * @param lengthAdjustment 该字段加长度字段等于数据帧的长度
     * @param initialBytesToStrip 从数据帧中跳过的字节数
     * @param failFast 如果为true，则表示读取到长度域，TA的值的超过maxFrameLength，就抛出一个 TooLongFrameException
     */
    public NewDecoder(int maxFrameLength, int lengthFieldOffset,
                      int lengthFieldLength, int lengthAdjustment, int initialBytesToStrip,
                      boolean failFast) {
        super(maxFrameLength, lengthFieldOffset, lengthFieldLength,
                lengthAdjustment, initialBytesToStrip, failFast);
    }

    @Override
    protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
        if(in == null){
            return null;
        }
        if(in.readableBytes() < HEADER_SIZE){
            throw new Exception("错误的消息");
        }
        in = (ByteBuf) super.decode(ctx,in);
        /**
         * 通过源码我们能看到在读的过程中
         * 每读一次读过的字节即被抛弃
         * 即指针会往前跳
         */
        type = in.readByte();

        length = in.readByte();


        if(in.readableBytes() < length){
            throw new Exception("消息不正确");
        }

        ByteBuf buf = in.readBytes(length);
        byte[] b = new byte[buf.readableBytes()];
        buf.readBytes(b);

        msgBody = new String(b,"UTF-8");
        Message msg = new Message(type,length,msgBody);
        return msg;
    }
}

public class NewServerHandler extends SimpleChannelInboundHandler<Object> {

    @Override
    protected void channelRead0(ChannelHandlerContext channelHandlerContext, Object o) throws Exception {
        if(o instanceof Message) {
            Message msg = (Message)o;
            System.out.println("Client->Server:"+channelHandlerContext.channel().remoteAddress()+" send "+msg.getMsgBody());
        }
    }
}

三  LengthFieldBasedFrameDecoder介绍

　　首先要知道它是一个InBoundHandler，也就是用来把字节转成对象的

　　他有四个参数

public LengthFieldBasedFrameDecoder(
            int maxFrameLength,
            int lengthFieldOffset, int lengthFieldLength,
            int lengthAdjustment, int initialBytesToStrip) 

　　maxFrameLength 可以设的稍微大点，比如1M

　　lengthFieldOffset 在字节中表示长度的偏移

　　lengthFieldLength 表示长度的部分占用几个字节

　　lengthAdjustment 如果代表长度字段后立刻跟的是content，此值为0

　　initialBytesToStrip 跳过的字节，解码后会把原字节数组进行截断

　　源代码中就有注释 其实一般我们都是这么设计的，也够了

* <pre>
* <b>lengthFieldOffset</b>   = <b>2</b> (= the length of Header 1)
* <b>lengthFieldLength</b>   = <b>3</b>
* lengthAdjustment    = 0
* initialBytesToStrip = 0
*
* BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
* +----------+----------+----------------+      +----------+----------+----------------+
* | Header 1 |  Length  | Actual Content |----->| Header 1 |  Length  | Actual Content |
* |  0xCAFE  | 0x00000C | "HELLO, WORLD" |      |  0xCAFE  | 0x00000C | "HELLO, WORLD" |
* +----------+----------+----------------+      +----------+----------+----------------+

　　稍微复杂点的，代表长度字段后不是content

　　

* lengthFieldOffset   = 0
* lengthFieldLength   = 3
* <b>lengthAdjustment</b>    = <b>2</b> (= the length of Header 1)
* initialBytesToStrip = 0
*
* BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
* +----------+----------+----------------+      +----------+----------+----------------+
* |  Length  | Header 1 | Actual Content |----->|  Length  | Header 1 | Actual Content |
* | 0x00000C |  0xCAFE  | "HELLO, WORLD" |      | 0x00000C |  0xCAFE  | "HELLO, WORLD" |
* +----------+----------+----------------+      +----------+----------+----------------+