Netty4.1 - TCP粘包拆包解决方案及案例代码

末蓝、 2022-06-08 10:15 153阅读 0赞

[Netty][]是目前业界最流行的NIO框架之一，它的健壮性、高性能、可定制和可扩展性在同类框架中都是首屈一指。它已经得到了成百上千的商业项目的验证，例如Hadoop的RPC框架Avro就使用了Netty作为底层通信框架，其他的业界主流RPC框架，例如：[Dubbo][]、Google 开源的[gRPC][]、新浪微博开源的[Motan][]、Twitter 开源的[ finagle][finagle]也使用Netty来构建高性能的异步通信能力。另外，阿里巴巴开源的消息中间件[RocketMQ][]也使用Netty作为底层通信框架。

## TCP黏包/拆包 ##

TCP是一个“流”协议，所谓流，就是没有界限的一长串二进制数据。TCP作为传输层协议并不不了解上层业务数据的具体含义，它会根据TCP缓冲区的实际情况进行数据包的划分，所以在业务上认为是一个完整的包，可能会被TCP拆分成多个包进行发送，也有可能把多个小的包封装成一个大的数据包发送，这就是所谓的TCP粘包和拆包问题。

**主要四种方案**

**LineBasedFrameDecoder是回车换行解码器**

**DelimiterBasedFrameDecoder是分隔符解码器**

**FixedLengthFrameDecoder是固定长度解码器**

**LengthFieldBasedFrameDecoder解码器-\-自动屏蔽TCP底层的拆包和粘包问题，只需要传入正确的参数，即可轻松解决“读半包“问题**

## 粘包问题的解决策略 ##

由于底层的TCP无法理解上层的业务数据，所以在底层是无法保证数据包不被拆分和重组的，这个问题只能通过上层的应用协议栈设计来解决。业界的主流协议的解决方案，可以归纳如下：   
1. 消息定长，报文大小固定长度，例如每个报文的长度固定为200字节，如果不够空位补空格；   
2. 包尾添加特殊分隔符，例如每条报文结束都添加回车换行符（例如FTP协议）或者指定特殊字符作为报文分隔符，接收方通过特殊分隔符切分报文区分；   
3. 将消息分为消息头和消息体，消息头中包含表示信息的总长度（或者消息体长度）的字段；   
4. 更复杂的自定义应用层协议。

## Netty粘包和拆包解决方案 ##

Netty提供了多个解码器，可以进行分包的操作，分别是：   
\* LineBasedFrameDecoder   
\* DelimiterBasedFrameDecoder（添加特殊分隔符报文来分包）   
\* FixedLengthFrameDecoder（使用定长的报文来分包）   
\* LengthFieldBasedFrameDecoder

### LineBasedFrameDecoder解码器 ###

LineBasedFrameDecoder是回车换行解码器，如果用户发送的消息以回车换行符作为消息结束的标识，则可以直接使用Netty的LineBasedFrameDecoder对消息进行解码，只需要在初始化Netty服务端或者客户端时将LineBasedFrameDecoder正确的添加到ChannelPipeline中即可，不需要自己重新实现一套换行解码器。

#### Netty依赖 ####

<dependency>
                <groupId>io.netty</groupId>
                <artifactId>netty-all</artifactId>
                <version>4.1.9.Final</version>
            </dependency>

*  1
 *  2
 *  3
 *  4
 *  5

#### 1.1 Server端 ####

import io.netty.bootstrap.ServerBootstrap;
    import io.netty.channel.*;
    import io.netty.channel.nio.NioEventLoopGroup;
    import io.netty.channel.socket.SocketChannel;
    import io.netty.channel.socket.nio.NioServerSocketChannel;
    import io.netty.handler.codec.LineBasedFrameDecoder;
    import io.netty.handler.codec.string.StringDecoder;
    import io.netty.handler.codec.string.StringEncoder;
    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;
    
    /** * * @author Ricky * */
    public class LineBasedServer {
        private Logger logger = LoggerFactory.getLogger(getClass());
    
        public void bind(int port) throws Exception {
    
            EventLoopGroup bossGroup = new NioEventLoopGroup();
            EventLoopGroup workerGroup = new NioEventLoopGroup();
            try {
                ServerBootstrap b = new ServerBootstrap();
                b.group(bossGroup, workerGroup)
                .channel(NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 1024)
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    public void initChannel(SocketChannel ch) throws Exception {
    
                        ChannelPipeline p = ch.pipeline();
                        p.addLast(new LineBasedFrameDecoder(1024));
                        p.addLast(new StringDecoder());
                        p.addLast(new StringEncoder());
    
                        p.addLast(new LineServerHandler());
                    }
                });
    
                // Bind and start to accept incoming connections.
                ChannelFuture f = b.bind(port).sync(); // (7)
    
                logger.info("server bind port:{}", port);
    
                // Wait until the server socket is closed.
                f.channel().closeFuture().sync();
    
            } finally {
                bossGroup.shutdownGracefully();
                workerGroup.shutdownGracefully();
            }
        }
    
        public static void main(String[] args) throws Exception {
    
            new LineBasedServer().bind(Constants.PORT);
        }
    }

import io.netty.channel.ChannelHandlerContext; import io.netty.channel.ChannelInboundHandlerAdapter; import org.slf4j.Logger; import org.slf4j.LoggerFactory; public class LineServerHandler extends ChannelInboundHandlerAdapter { private Logger logger = LoggerFactory.getLogger(getClass()); private int count = 0; @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { count++; String body = (String) msg; logger.info("server read msg:{}, count:{}", body, count); String response = "hello from server"+System.getProperty("line.separator"); ctx.writeAndFlush(response); } @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { logger.error("server caught exception", cause); ctx.close(); } }

#### 1.2 Client ####

import io.netty.bootstrap.Bootstrap;
    import io.netty.channel.*;
    import io.netty.channel.nio.NioEventLoopGroup;
    import io.netty.channel.socket.SocketChannel;
    import io.netty.channel.socket.nio.NioSocketChannel;
    import io.netty.handler.codec.LineBasedFrameDecoder;
    import io.netty.handler.codec.string.StringDecoder;
    import io.netty.handler.codec.string.StringEncoder;
    
    public class LineBasedClient {
    
        public void connect(String host, int port) throws InterruptedException {
    
            EventLoopGroup group = new NioEventLoopGroup();
            try {
                Bootstrap b = new Bootstrap();
                b.group(group)
                .channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
    
                        ChannelPipeline p = ch.pipeline();
                        p.addLast(new LineBasedFrameDecoder(1024));
                        p.addLast(new StringDecoder());
                        p.addLast(new StringEncoder());
    
                        p.addLast(new LineClientHandler());
                    }
                });
    
                ChannelFuture future = b.connect(Constants.HOST, Constants.PORT).sync();
    
                future.channel().closeFuture().sync();
            } finally {
                group.shutdownGracefully();
            }
        }
    
        public static void main(String[] args) throws Exception {
    
            new LineBasedClient().connect(Constants.HOST, Constants.PORT);
        }
    }

import io.netty.channel.ChannelHandlerContext; import io.netty.channel.ChannelInboundHandlerAdapter; import org.slf4j.Logger; import org.slf4j.LoggerFactory; public class LineClientHandler extends ChannelInboundHandlerAdapter { private Logger logger = LoggerFactory.getLogger(getClass()); private int count =0; @Override public void channelActive(ChannelHandlerContext ctx) throws Exception { // Send the message to Server for(int i=0; i<100; i++){ String msg = "hello from client "+i; logger.info("client send message:{}", msg); ctx.writeAndFlush(msg+System.getProperty("line.separator")); } } @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { String body = (String) msg; count++; logger.info("client read msg:{}, count:{}", body, count); } @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { logger.error("client caught exception", cause); ctx.close(); } }

### DelimiterBasedFrameDecoder解码器 ###

DelimiterBasedFrameDecoder是分隔符解码器，用户可以指定消息结束的分隔符，它可以自动完成以分隔符作为码流结束标识的消息的解码。回车换行解码器实际上是一种特殊的DelimiterBasedFrameDecoder解码器。

import io.netty.bootstrap.ServerBootstrap;
    import io.netty.buffer.Unpooled;
    import io.netty.channel.*;
    import io.netty.channel.nio.NioEventLoopGroup;
    import io.netty.channel.socket.SocketChannel;
    import io.netty.channel.socket.nio.NioServerSocketChannel;
    import io.netty.handler.codec.DelimiterBasedFrameDecoder;
    import io.netty.handler.codec.string.StringDecoder;
    import io.netty.handler.codec.string.StringEncoder;
    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;
    
    /** * * @author Ricky * */
    public class DelimiterServer {
        private Logger logger = LoggerFactory.getLogger(getClass());
    
    
        public void bind(int port) throws Exception {
    
            EventLoopGroup bossGroup = new NioEventLoopGroup(1);
            EventLoopGroup workerGroup = new NioEventLoopGroup();
            try {
                ServerBootstrap b = new ServerBootstrap();
                b.group(bossGroup, workerGroup)
                .channel(NioServerSocketChannel.class)
                .option(ChannelOption.SO_BACKLOG, 1024)
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    public void initChannel(SocketChannel ch) throws Exception {
    
                        ChannelPipeline p = ch.pipeline();
                        p.addLast(new DelimiterBasedFrameDecoder(1024, Unpooled.copiedBuffer(Constants.DELIMITER.getBytes())));
                        p.addLast(new StringDecoder());
                        p.addLast(new StringEncoder());
    
                        p.addLast(new DelimiterServerHandler());
                    }
                });
    
                // Bind and start to accept incoming connections.
                ChannelFuture f = b.bind(port).sync(); // (7)
    
                logger.info("server bind port:{}", port);
    
                // Wait until the server socket is closed.
                f.channel().closeFuture().sync();
    
            } finally {
                bossGroup.shutdownGracefully();
                workerGroup.shutdownGracefully();
            }
        }
    
        public static void main(String[] args) throws Exception {
    
            new DelimiterServer().bind(Constants.PORT);
        }
    }

Client：

import io.netty.bootstrap.Bootstrap;
    import io.netty.buffer.Unpooled;
    import io.netty.channel.*;
    import io.netty.channel.nio.NioEventLoopGroup;
    import io.netty.channel.socket.SocketChannel;
    import io.netty.channel.socket.nio.NioSocketChannel;
    import io.netty.handler.codec.DelimiterBasedFrameDecoder;
    import io.netty.handler.codec.string.StringDecoder;
    import io.netty.handler.codec.string.StringEncoder;
    
    public class DelimiterClient {
    
        public void connect(String host, int port) throws InterruptedException {
    
            EventLoopGroup group = new NioEventLoopGroup();
            try {
                Bootstrap b = new Bootstrap();
                b.group(group)
                .channel(NioSocketChannel.class)
                .option(ChannelOption.TCP_NODELAY, true)
                .handler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    protected void initChannel(SocketChannel ch) throws Exception {
    
                        ChannelPipeline p = ch.pipeline();
                        p.addLast(new DelimiterBasedFrameDecoder(1024, Unpooled.copiedBuffer(Constants.DELIMITER.getBytes())));
                        p.addLast(new StringDecoder());
                        p.addLast(new StringEncoder());
    
                        p.addLast(new DelimiterClientHandler());
                    }
                });
    
                ChannelFuture future = b.connect(Constants.HOST, Constants.PORT).sync();
    
                future.channel().closeFuture().sync();
            } finally {
                group.shutdownGracefully();
            }
        }
    
        public static void main(String[] args) throws Exception {
    
            new DelimiterClient().connect(Constants.HOST, Constants.PORT);
        }
    }

### FixedLengthFrameDecoder解码器 ###

FixedLengthFrameDecoder是固定长度解码器，它能够按照指定的长度对消息进行自动解码，开发者不需要考虑TCP的粘包/拆包等问题，非常实用。

对于定长消息，如果消息实际长度小于定长，则往往会进行补位操作，它在一定程度上导致了空间和资源的浪费。但是它的优点也是非常明显的，编解码比较简单，因此在实际项目中仍然有一定的应用场景。

import io.netty.bootstrap.ServerBootstrap; import io.netty.buffer.ByteBuf; import io.netty.buffer.Unpooled; import io.netty.channel.*; import io.netty.channel.nio.NioEventLoopGroup; import io.netty.channel.socket.SocketChannel; import io.netty.channel.socket.nio.NioServerSocketChannel; import io.netty.handler.codec.FixedLengthFrameDecoder; import io.netty.handler.codec.string.StringDecoder; import io.netty.handler.codec.string.StringEncoder; import io.netty.handler.logging.LogLevel; import io.netty.handler.logging.LoggingHandler; import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** * @author Ricky Fung */ public class NettyServer { private Logger logger = LoggerFactory.getLogger(getClass()); public void bind(int port) throws InterruptedException { EventLoopGroup bossGroup = new NioEventLoopGroup(1); EventLoopGroup workerGroup = new NioEventLoopGroup(); try { //配置服务器启动类 ServerBootstrap b = new ServerBootstrap(); b.group(bossGroup, workerGroup) .channel(NioServerSocketChannel.class) .option(ChannelOption.SO_BACKLOG, 100) .handler(new LoggingHandler(LogLevel.INFO))//配置日志输出 .childHandler(new ChannelInitializer<SocketChannel>() { @Override protected void initChannel(SocketChannel ch) throws Exception { ch.pipeline().addLast(new FixedLengthFrameDecoder(1<<5)); ch.pipeline().addLast(new StringDecoder()); ch.pipeline().addLast(new StringEncoder()); ch.pipeline().addLast(new ServerHandler()); } }); ChannelFuture f = b.bind(port).sync(); //等待服务器退出 f.channel().closeFuture().sync(); } finally { //释放线程资源 bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); } } private class ServerHandler extends ChannelInboundHandlerAdapter { private int counter = 0; @Override public void channelActive(ChannelHandlerContext ctx) throws Exception { super.channelActive(ctx); } @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { logger.info("接收客户端msg:{}", msg); ByteBuf echo = Unpooled.copiedBuffer(String.format("Hello from server:", counter).getBytes()); ctx.writeAndFlush(echo); } } public static void main(String[] args) throws InterruptedException { new NettyServer().bind(Constants.PORT); } } import io.netty.bootstrap.Bootstrap;

import io.netty.channel.*; import io.netty.channel.nio.NioEventLoopGroup; import io.netty.channel.socket.SocketChannel; import io.netty.channel.socket.nio.NioSocketChannel; import io.netty.handler.codec.FixedLengthFrameDecoder; import io.netty.handler.codec.string.StringDecoder; import io.netty.handler.codec.string.StringEncoder; import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** * @author Ricky Fung */ public class NettyClient { private Logger logger = LoggerFactory.getLogger(getClass()); public void connect(String host, int port) throws InterruptedException { EventLoopGroup group = new NioEventLoopGroup(); try { Bootstrap b = new Bootstrap(); b.group(group) .channel(NioSocketChannel.class) .option(ChannelOption.TCP_NODELAY, true) .handler(new ChannelInitializer<SocketChannel>() { @Override protected void initChannel(SocketChannel ch) throws Exception { ch.pipeline().addLast(new FixedLengthFrameDecoder(1<<5)); ch.pipeline().addLast(new StringDecoder()); ch.pipeline().addLast(new StringEncoder()); ch.pipeline().addLast(new ClientHandler()); } }); ChannelFuture future = b.connect(host, port).sync(); future.channel().closeFuture().sync(); } finally { group.shutdownGracefully(); } } private class ClientHandler extends ChannelInboundHandlerAdapter { @Override public void channelActive(ChannelHandlerContext ctx) throws Exception { for(int i=0; i<100; i++){ String msg = "hello from client "+i; logger.info("client send message:{}", msg); ctx.writeAndFlush(msg); } } @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { logger.info("接收服务端msg:{}", msg); } @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { cause.printStackTrace(); ctx.close(); } } public static void main(String[] args) throws InterruptedException { new NettyClient().connect(Constants.HOST, Constants.PORT); } }

### LengthFieldBasedFrameDecoder解码器 ###

大多数的协议（私有或者公有），协议头中会携带长度字段，用于标识消息体或者整包消息的长度，例如SMPP、HTTP协议等。由于基于长度解码需求的通用性，以及为了降低用户的协议开发难度，Netty提供了[LengthFieldBasedFrameDecoder][]，自动屏蔽TCP底层的拆包和粘包问题，只需要传入正确的参数，即可轻松解决“读半包“问题。

Message.java

import java.nio.charset.Charset; /** * @author Ricky Fung */ public class Message { private final Charset charset = Charset.forName("utf-8"); private byte magicType; private byte type;//消息类型 0xAF 表示心跳包 0xBF 表示超时包 0xCF 业务信息包 private long requestId; //请求id private int length; private String body; public Message(){ } public Message(byte magicType, byte type, long requestId, byte[] data) { this.magicType = magicType; this.type = type; this.requestId = requestId; this.length = data.length; this.body = new String(data, charset); } public Message(byte magicType, byte type, long requestId, String body) { this.magicType = magicType; this.type = type; this.requestId = requestId; this.length = body.getBytes(charset).length; this.body = body; } ...setter/getter }

MessageDecoder.java

import com.mindflow.netty4.unpack.model.Message; import io.netty.buffer.ByteBuf; import io.netty.channel.ChannelHandlerContext; import io.netty.handler.codec.LengthFieldBasedFrameDecoder; import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** * @author Ricky Fung */ public class MessageDecoder extends LengthFieldBasedFrameDecoder { private Logger logger = LoggerFactory.getLogger(getClass()); //头部信息的大小应该是 byte+byte+int = 1+1+8+4 = 14 private static final int HEADER_SIZE = 14; public MessageDecoder(int maxFrameLength, int lengthFieldOffset, int lengthFieldLength) { super(maxFrameLength, lengthFieldOffset, lengthFieldLength); } @Override protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception { if (in == null) { return null; } if (in.readableBytes() <= HEADER_SIZE) { return null; } in.markReaderIndex(); byte magic = in.readByte(); byte type = in.readByte(); long requestId = in.readLong(); int dataLength = in.readInt(); // FIXME 如果dataLength过大，可能导致问题 if (in.readableBytes() < dataLength) { in.resetReaderIndex(); return null; } byte[] data = new byte[dataLength]; in.readBytes(data); String body = new String(data, "UTF-8"); Message msg = new Message(magic, type, requestId, body); return msg; } }

MessageEncoder.java

import com.mindflow.netty4.unpack.model.Message;
    import io.netty.buffer.ByteBuf;
    import io.netty.channel.ChannelHandlerContext;
    import io.netty.handler.codec.MessageToByteEncoder;
    import java.nio.charset.Charset;
    
    /** * @author Ricky Fung */
    public class MessageEncoder extends MessageToByteEncoder<Message> {
        private final Charset charset = Charset.forName("utf-8");
    
        @Override
        protected void encode(ChannelHandlerContext ctx, Message msg, ByteBuf out) throws Exception {
    
            //
            out.writeByte(msg.getMagicType());
            out.writeByte(msg.getType());
            out.writeLong(msg.getRequestId());
    
            byte[] data = msg.getBody().getBytes(charset);
            out.writeInt(data.length);
            out.writeBytes(data);
        }
    }

服务端：

import com.mindflow.netty4.unpack.model.Message; import io.netty.bootstrap.ServerBootstrap; import io.netty.channel.*; import io.netty.channel.nio.NioEventLoopGroup; import io.netty.channel.socket.SocketChannel; import io.netty.channel.socket.nio.NioServerSocketChannel; import org.slf4j.Logger; import org.slf4j.LoggerFactory; /** * @author Ricky Fung */ public class NettyServer { private Logger logger = LoggerFactory.getLogger(this.getClass()); public void bind(int port) throws InterruptedException { EventLoopGroup bossGroup = new NioEventLoopGroup(1); EventLoopGroup workerGroup = new NioEventLoopGroup(); try { ServerBootstrap b = new ServerBootstrap(); b.group(bossGroup, workerGroup) .channel(NioServerSocketChannel.class) .option(ChannelOption.SO_BACKLOG, 128) .childHandler(new ChannelInitializer<SocketChannel>() { @Override public void initChannel(SocketChannel ch) throws Exception { ChannelPipeline p = ch.pipeline(); p.addLast(new MessageDecoder(1<<20, 10, 4)); p.addLast(new MessageEncoder()); p.addLast(new ServerHandler()); } }); // Bind and start to accept incoming connections. ChannelFuture future = b.bind(port).sync(); // (7) logger.info("server bind port:{}", port); // Wait until the server socket is closed. future.channel().closeFuture().sync(); } finally { bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); } } private class ServerHandler extends SimpleChannelInboundHandler<Message> { @Override protected void channelRead0(ChannelHandlerContext ctx, Message msg) throws Exception { logger.info("server read msg:{}", msg); Message resp = new Message(msg.getMagicType(), msg.getType(), msg.getRequestId(), "Hello world from server"); ctx.writeAndFlush(resp); } } public static void main(String[] args) throws Exception { new NettyServer().bind(Constants.PORT); } }

客户端：

import com.mindflow.netty4.unpack.model.Message; import io.netty.bootstrap.Bootstrap; import io.netty.channel.*; import io.netty.channel.nio.NioEventLoopGroup; import io.netty.channel.socket.SocketChannel; import io.netty.channel.socket.nio.NioSocketChannel; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import java.util.concurrent.TimeUnit; /** * @author Ricky Fung */ public class NettyClient { public void connect(String host, int port) throws InterruptedException { EventLoopGroup group = new NioEventLoopGroup(); try { Bootstrap b = new Bootstrap(); b.group(group) .channel(NioSocketChannel.class) .option(ChannelOption.TCP_NODELAY, true) .handler(new ChannelInitializer<SocketChannel>() { @Override protected void initChannel(SocketChannel ch) throws Exception { ChannelPipeline p = ch.pipeline(); p.addLast(new MessageDecoder(1<<20, 10, 4)); p.addLast(new MessageEncoder()); p.addLast(new ClientHandler()); } }); ChannelFuture future = b.connect(host, port).sync(); future.awaitUninterruptibly(2000, TimeUnit.MILLISECONDS); if(future.channel().isActive()){ for(int i=0; i<100; i++) { String body = "Hello world from client:"+ i; Message msg = new Message((byte) 0XAF, (byte) 0XBF, i, body); future.channel().writeAndFlush(msg); } } future.channel().closeFuture().sync(); } finally { group.shutdownGracefully(); } } private class ClientHandler extends ChannelInboundHandlerAdapter { private Logger logger = LoggerFactory.getLogger(getClass()); @Override public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception { logger.info("client read msg:{}, ", msg); } @Override public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception { logger.error("client caught exception", cause); ctx.close(); } } public static void main(String[] args) throws Exception { new NettyClient().connect(Constants.HOST, Constants.PORT); } }

[Netty]: https://netty.io/
[Dubbo]: http://dubbo.io/
[gRPC]: http://www.grpc.io/
[Motan]: https://github.com/weibocom/motan
[finagle]: https://github.com/twitter/finagle
[RocketMQ]: https://github.com/alibaba/RocketMQ
[LengthFieldBasedFrameDecoder]: https://netty.io/4.1/api/io/netty/handler/codec/LengthFieldBasedFrameDecoder.html