Netty4实战 - TCP粘包&拆包解决方案

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。

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>

1.1 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);
}
}

参考资料

Netty API Reference

Netty系列之Netty编解码框架分析

源码下载

https://github.com/TiFG/netty4-in-action/tree/master/netty4-unpack-demo