1.TCP粘包和拆包基本介绍
(1)TCP是面向连接的,面向流的,提供高可靠性服务。收发两端(客户端和服务器端)都要有一一成对的socket,因此,发送端为了将多个发给接收端的包,更有效的发给对方,使用了优化方法(Nagle算法),将多次间隔较小且数据量小的数据合并成一个大的数据块,然后进行封包。这样做虽然提高了效率,但是接收端就难于分辨出完整的数据包了,因此面向流的通信是无消息保护边界的。
(2)由于TCP无消息保护边界,需要在接收端处理消息边界问题,也就是我们所说的粘包、拆包问题。TCP粘包、拆包图解如下所示。
假设客户端分别发送了两个数据包 D1 和 D2 给服务端,由于服务端一次读取到字节数是不确定的,故可能存在以下四种情况:
(1)服务端分两次读取到了两个独立的数据包,分别是 D1 和 D2,没有粘包和拆包;
(2)服务端一次接受到了两个数据包,D1 和 D2 粘合在一起,称之为 TCP 粘包;
(3)服务端分两次读取到了数据包,第一次读取到了完整的 D1 包和 D2 包的部分内容,第二次读取到了 D2 包的剩余内容,这称之为 TCP 拆包;
(4)服务端分两次读取到了数据包,第一次读取到了 D1 包的部分内容 D1_1,第二次读取到了 D1 包的剩余部分内容 D1_2 和完整的 D2 包。
2.TCP粘包和拆包现象实例
在编写Netty 程序时,如果没有做处理,就会发生粘包和拆包的问题。具体看下面一个实例。
服务端代码:
public class MyServer {
public static void main(String[] args) throws InterruptedException {
NioEventLoopGroup bossGroup = new NioEventLoopGroup(1);
NioEventLoopGroup workerGroup = new NioEventLoopGroup(8);
try {
ServerBootstrap serverBootstrap = new ServerBootstrap();
serverBootstrap.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.childHandler(new MyServerInitializer());
ChannelFuture channelFuture = serverBootstrap.bind(7000).sync();
channelFuture.channel().closeFuture().sync();
}finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
}
public class MyServerInitializer extends ChannelInitializer<SocketChannel> {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline pipeline = ch.pipeline();
pipeline.addLast(new MyServerHandler());
}
}
public class MyServerHandler extends SimpleChannelInboundHandler<ByteBuf> {
private int count;
@Override
protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
byte[] buffer = new byte[msg.readableBytes()];
msg.readBytes(buffer);
// 将 buffer 转成字符串
String message = new String(buffer, Charset.forName("utf-8"));
System.out.println("服务器接收到数据 " + message);
System.out.println("服务器接收到消息量= " + (++this.count));
// 服务器回送数据给客户端,回送一个随机id
ByteBuf response = Unpooled.copiedBuffer(UUID.randomUUID().toString() + " ", Charset.forName("utf-8"));
ctx.writeAndFlush(response);
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
cause.printStackTrace();
ctx.close();
}
}
客户端代码:
public class MyClinet {
public static void main(String[] args) throws InterruptedException {
NioEventLoopGroup group = new NioEventLoopGroup();
try {
Bootstrap bootstrap = new Bootstrap();
bootstrap.group(group)
.channel(NioSocketChannel.class)
.handler(new MyClientInitializer());
ChannelFuture channelFuture = bootstrap.connect("localhost", 7000).sync();
channelFuture.channel().closeFuture().sync();
}finally {
group.shutdownGracefully();
}
}
}
public class MyClientInitializer extends ChannelInitializer<SocketChannel> {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline pipeline = ch.pipeline();
pipeline.addLast(new MyClientHandler());
}
}
public class MyClientHandler extends SimpleChannelInboundHandler<ByteBuf> {
private int count;
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
// 使用客户端发送10条数据 hello,Server 编号
for (int i = 0; i < 1000; i++) {
ByteBuf byteBuf = Unpooled.copiedBuffer(" hello,Server " + i, Charset.forName("utf-8"));
ctx.writeAndFlush(byteBuf);
}
}
@Override
protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
byte[] buffer = new byte[msg.readableBytes()];
msg.readBytes(buffer);
String message = new String(buffer, Charset.forName("utf-8"));
System.out.println("客户端接收的消息= " + message);
System.out.println("客户端接收消息数量=" + (++this.count));
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
cause.printStackTrace();
ctx.close();
}
}
结果如下所示。
3.TCP粘包和拆包解决方案
(1)使用自定义协议 + 编解码器 来解决;
(2)关键就是要解决 服务端每次读取数据长度的问题,这个问题解决,就不会出现服务器多度或少读数据的问题,从而避免的TCP粘包、拆包。
看一个具体的实例:
(1)要求客户端发送5个 Message 对象,客户端每次发送一个 Message 对象。
(2)服务器端每次接收一个 Message,分5次进行解码,每读取到一个 Message,会回复一个 Message 对象给客户端。
协议包对象代码:
/**
* 协议包
*/
public class MessageProtocol {
private int len;
private byte[] content;
public int getLen() {
return len;
}
public void setLen(int len) {
this.len = len;
}
public byte[] getContent() {
return content;
}
public void setContent(byte[] content) {
this.content = content;
}
}
编解码器代码:
public class MyMessageEncoder extends MessageToByteEncoder<MessageProtocol> {
@Override
protected void encode(ChannelHandlerContext ctx, MessageProtocol msg, ByteBuf out) throws Exception {
System.out.println("MyMessageEncoder encode()方法被调用");
out.writeInt(msg.getLen());
out.writeBytes(msg.getContent());
}
}
public class MyMessageDecoder extends ReplayingDecoder<Void> {
@Override
protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
System.out.println("MyMessageDecoder decode()方法被调用~~~~");
// 将得到的二进制字节码转成 MessageProtocol 数据包
int length = in.readInt();
byte[] content = new byte[length];
in.readBytes(content);
// 封装成 MessageProtocol 对象,放入到 out,传递给下一个 handler进行业务处理
MessageProtocol messageProtocol = new MessageProtocol();
messageProtocol.setContent(content);
messageProtocol.setLen(length);
out.add(messageProtocol);
}
}
服务端代码:
public class MyServer {
public static void main(String[] args) throws InterruptedException {
NioEventLoopGroup bossGroup = new NioEventLoopGroup(1);
NioEventLoopGroup workerGroup = new NioEventLoopGroup(8);
try {
ServerBootstrap serverBootstrap = new ServerBootstrap();
serverBootstrap.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.childHandler(new MyServerInitializer());
ChannelFuture channelFuture = serverBootstrap.bind(7000).sync();
channelFuture.channel().closeFuture().sync();
}finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
}
public class MyServerInitializer extends ChannelInitializer<SocketChannel> {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline pipeline = ch.pipeline();
pipeline.addLast(new MyMessageDecoder());
pipeline.addLast(new MyMessageEncoder());
pipeline.addLast(new MyServerHandler());
}
}
/**
* 处理业务
*/
public class MyServerHandler extends SimpleChannelInboundHandler<MessageProtocol> {
private int count;
@Override
protected void channelRead0(ChannelHandlerContext ctx, MessageProtocol msg) throws Exception {
// 接收到数据,并处理
int len = msg.getLen();
byte[] content = msg.getContent();
System.out.println();
System.out.println("服务端接收到信息如下");
System.out.println("长度=" + len);
System.out.println("内容=" + new String(content, Charset.forName("utf-8")));
System.out.println("服务器接收到消息包数量=" + (++this.count));
// 回复消息
String responseContent = UUID.randomUUID().toString();
int responseLen = responseContent.getBytes(Charset.forName("utf-8")).length;
// 构建一个协议包
MessageProtocol messageProtocol = new MessageProtocol();
messageProtocol.setLen(responseLen);
messageProtocol.setContent(responseContent.getBytes(Charset.forName("utf-8")));
ctx.writeAndFlush(messageProtocol);
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
cause.printStackTrace();
ctx.close();
}
}
客户端代码:
public class MyClinet {
public static void main(String[] args) throws InterruptedException {
NioEventLoopGroup group = new NioEventLoopGroup();
try {
Bootstrap bootstrap = new Bootstrap();
bootstrap.group(group)
.channel(NioSocketChannel.class)
.handler(new MyClientInitializer());
ChannelFuture channelFuture = bootstrap.connect("localhost", 7000).sync();
channelFuture.channel().closeFuture().sync();
}finally {
group.shutdownGracefully();
}
}
}
public class MyClientInitializer extends ChannelInitializer<SocketChannel> {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ChannelPipeline pipeline = ch.pipeline();
// 加入编码器
pipeline.addLast(new MyMessageEncoder());
// 加入解码器
pipeline.addLast(new MyMessageDecoder());
pipeline.addLast(new MyClientHandler());
}
}
public class MyClientHandler extends SimpleChannelInboundHandler<MessageProtocol> {
private int count;
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
// 使用客户端发送5条数据 "今天天气冷,吃火锅" 编号
for (int i = 0; i < 5; i++) {
String msg = "今天天气冷,吃火锅";
byte[] content = msg.getBytes(Charset.forName("utf-8"));
int length = msg.getBytes(Charset.forName("utf-8")).length;
// 创建协议包对象
MessageProtocol messageProtocol = new MessageProtocol();
messageProtocol.setLen(length);
messageProtocol.setContent(content);
ctx.writeAndFlush(messageProtocol);
}
}
@Override
protected void channelRead0(ChannelHandlerContext ctx, MessageProtocol msg) throws Exception {
int len = msg.getLen();
byte[] content = msg.getContent();
System.out.println("客户端接收到消息如下");
System.out.println("长度=" + len);
System.out.println("内容=" + new String(content, Charset.forName("utf-8")));
System.out.println("客户端接收消息包数量=" + (++this.count));
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
cause.printStackTrace();
ctx.close();
}
}
测试结果如下。
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