粘包和拆包是什么?
TCP协议是一种字节流协议,没有记录边界,我们在接收消息的时候,不能人为接收到的数据包就是一个整包消息
当客户端向服务器端发送多个消息数据的时候,TCP协议可能将多个消息数据合并成一个数据包进行发送,这就是粘包
当客户端向服务器端发送的消息过大的时候,tcp协议可能将一个数据包拆成多个数据包来进行发送,这就是拆包
以下一netty为例,展示一下tcp粘包和拆包的例子:
ServerBusinessHanler:
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;
import java.nio.charset.Charset;
import java.util.UUID;
public class ServerBusinessHanler extends SimpleChannelInboundHandler<ByteBuf> {
int count = 0;
@Override
protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
byte[] bytes = new byte[msg.readableBytes()];
msg.readBytes(bytes);
String message = new String(bytes, Charset.forName("UTF-8"));
System.out.println("从客户端收到的字符串:" + message);
System.out.println("服务端接收到的请求数:" + (++count));
ctx.writeAndFlush(Unpooled.copiedBuffer(UUID.randomUUID().toString(),Charset.forName("UTF-8")) );
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
System.out.println(cause.getStackTrace());
ctx.channel().close();
}
}
Server:
import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import java.net.InetSocketAddress;
public class Server {
public static void main(String[] args) throws InterruptedException {
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap serverBootstrap = new ServerBootstrap();
serverBootstrap.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new ServerBusinessHanler());
}
});
serverBootstrap.bind(new InetSocketAddress(8899)).sync().channel().closeFuture().sync();
} finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
}
ClientBusinessHandler:
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;
import java.nio.charset.Charset;
public class ClientBusinessHandler extends SimpleChannelInboundHandler<ByteBuf> {
int count = 1;
@Override
protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
byte[] bytes = new byte[msg.readableBytes()];
msg.readBytes(bytes);
System.out.println("从服务器端接收到的信息: " + new String(bytes, Charset.forName("UTF-8")));
System.out.println("从服务器端读到的请求的个数: " + count++);
}
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
for (int i = 0; i < 10; i++) {
ctx.writeAndFlush(Unpooled.copiedBuffer("中国移动".getBytes()));
}
ctx.writeAndFlush(Unpooled.copiedBuffer("jiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyipingjiaoyiping".getBytes()));
}
}
Client:
import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
public class Client {
public static void main(String[] args) throws InterruptedException {
EventLoopGroup eventExecutors = new NioEventLoopGroup();
Bootstrap bootstrap = new Bootstrap();
bootstrap.group(eventExecutors)
.channel(NioSocketChannel.class)
.handler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new ClientBusinessHandler());
}
});
Channel channel = bootstrap.connect("127.0.0.1", 8899).channel();
channel.closeFuture().sync();
}
}
分别运行服务器端和客户端,我们看到服务器端的输出:
相应的,服务器端也只给客户端了两个返回,而不是我们期待的11个
我们从客户端传入的十个 "中国移动"和第二次发送的长的字符串的前一部分发生了粘包,而第二个长字符串的后一部分则被拆分到第二个数据包里了
粘包和拆包的情况都出现了
那怎么解决粘包和拆包的问题呢? 一种方法是在字符串中使用特定的分隔符来分隔,另外一种方法是,我们在发送数据包的时候在前边附加上数据包的长度
netty为我们提供了多种的解码器,比如定长解码器和基于分隔符的解码器等,这里,我么使用 LengthFieldBasedFrameDecoder这个解码器来解决粘包和拆包的问题,关于这个解码器的详细信息,可以参考这个类的java doc文档,上边讲述的很详细,简单来说,就是我们需要在请求中添加一些关于数据字段长度的信息(以下简称length),LengthFieldBasedFrameDecoder的构造方法中,设置一下关于length所占的字节数和要跳过的字节数等信息(我们只需要读取数据内容的话,可以跳过length的信息) 这样的话,我们从客户端发送的每个数据包,都可以正确地解析
首先,我们增加一个编码器,为数据包附件上lenght:
import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.MessageToByteEncoder;
//本类作用,加上四个字节的消息头(int类型),表示数据包长度
public class LengthEncoder extends MessageToByteEncoder<ByteBuf> {
@Override
protected void encode(ChannelHandlerContext ctx, ByteBuf msg, ByteBuf out) throws Exception {
int length = msg.readableBytes();
byte[] bytes = new byte[length];
msg.readBytes(bytes);
out.writeInt(length);
out.writeBytes(bytes);
}
}
然后,将我们的编码器和对应的 LengthFieldBasedFrameDecoder添加到服务器端和客户端的ChannelInitializer里边去
修改之后的服务器端代码:
import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.LengthFieldBasedFrameDecoder;
import java.net.InetSocketAddress;
public class Server {
public static void main(String[] args) throws InterruptedException {
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap serverBootstrap = new ServerBootstrap();
serverBootstrap.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.childHandler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new LengthFieldBasedFrameDecoder(4096,0,4,0,4));
ch.pipeline().addLast(new LengthEncoder());
ch.pipeline().addLast(new ServerBusinessHanler());
}
});
serverBootstrap.bind(new InetSocketAddress(8899)).sync().channel().closeFuture().sync();
} finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
}
new LengthFieldBasedFrameDecoder(4096,0,4,0,4));的意思是:使用LengthFieldBasedFrameDecoder的最大的数据包大小是4096个,其中length占四个字节,读取的时候跳过4个字节(也就是最终解码的时候,忽略掉length字段,只返回真正的数据内容)
修改之后的客户端代码:
import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.LengthFieldBasedFrameDecoder;
public class Client {
public static void main(String[] args) throws InterruptedException {
EventLoopGroup eventExecutors = new NioEventLoopGroup();
Bootstrap bootstrap = new Bootstrap();
bootstrap.group(eventExecutors)
.channel(NioSocketChannel.class)
.handler(new ChannelInitializer<SocketChannel>() {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new LengthFieldBasedFrameDecoder(4096,0,4,0,4));
ch.pipeline().addLast(new LengthEncoder());
ch.pipeline().addLast(new ClientBusinessHandler());
}
});
Channel channel = bootstrap.connect("127.0.0.1", 8899).channel();
channel.closeFuture().sync();
}
}
分别运行服务器端和客户端之后我们得到的结果:
服务器端输出:
客户端输出:
这样就解决了粘包和拆包导致的问题
