FixedLengthFrameDecoder是固定长度解码器,它根据指定的长度自动对消息进行解码,开发者不需要考虑TCP拆包/粘包问题。
下面通过一个例子进行说明。

服务端:
在服务端的ChannelPipleline中增加FixedLengthFrameDecoder,长度设置为20,然后依次增加字符串解码器和自定义的EchoServerHandler。

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/**
* 使用FixedLengthFrameDecoder解码器解决TCP粘包/拆包问题。
*
* @author j.tommy
* @version 1.0
* @date 2017/11/18
*/
public class EchoServer {
public static void main(String[] args) {
EventLoopGroup bossGroup = new NioEventLoopGroup();
EventLoopGroup workerGroup = new NioEventLoopGroup();
ServerBootstrap b = new ServerBootstrap();
b.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_BACKLOG, 1024)
.handler(new LoggingHandler(LogLevel.INFO))
.childHandler(new ChannelInitializer<SocketChannel>() {
protected void initChannel(SocketChannel socketChannel) throws Exception {
socketChannel.pipeline().addLast(new FixedLengthFrameDecoder(20)); // 固定长度解码器,长度设置为20
socketChannel.pipeline().addLast(new StringDecoder());
socketChannel.pipeline().addLast(new EchoServerHandler());
}
});
try {
ChannelFuture f = b.bind(8899).sync();
f.channel().closeFuture().sync();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
}
class EchoServerHandler extends ChannelHandlerAdapter {
private int counter;
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
ctx.close();
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
String body = (String) msg;
System.out.println("接收到客户端请求:" + body + ",counter=" + ++counter);
// 响应客户端
ByteBuf resp = Unpooled.copiedBuffer(body.getBytes());
ctx.write(resp);
}
@Override
public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
ctx.flush();
}
}

客户端:
在客户端的ChannelPipleline中增加FixedLengthFrameDecoder,长度设置为20,然后依次增加字符串解码器和自定义的EchoClientHandler。

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/**
* @author j.tommy
* @version 1.0
* @date 2017/11/18
*/
public class EchoClient {
public static void main(String[] args) {
EventLoopGroup group = new NioEventLoopGroup();
Bootstrap b = new Bootstrap();
b.group(group).channel(NioSocketChannel.class).option(ChannelOption.TCP_NODELAY,true).handler(new ChannelInitializer<SocketChannel>() {
protected void initChannel(SocketChannel socketChannel) throws Exception {
socketChannel.pipeline().addLast(new FixedLengthFrameDecoder(20));
socketChannel.pipeline().addLast(new StringDecoder());
socketChannel.pipeline().addLast(new EchoClientHandler());
}
});
try {
ChannelFuture f = b.connect("127.0.0.1",8899).sync();
f.channel().closeFuture().sync();
} catch (InterruptedException e) {
e.printStackTrace();
}
finally {
group.shutdownGracefully();
}
}
}
class EchoClientHandler extends ChannelHandlerAdapter {
private int counter = 0;
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
ctx.close();
}
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
String req = "hello,this message is from client.";
ByteBuf sendBuf = null;
for (int i=0;i<100;i++) {
sendBuf = Unpooled.copiedBuffer(req.getBytes());
ctx.writeAndFlush(sendBuf);
}
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
String resp = (String) msg;
System.out.println("接收到服务端响应:" + resp + ",counter=" + ++counter);
}
}

示例中,客户端发送的消息是hello,this message is from client.
因为按照固定长度截取,所以服务端接收到的是这样的:

参考《Netty权威指南》