LengthFieldBasedFrameDecoder解码器

1参数详解

1.1.1 new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 2)

lengthFieldOffset = 0
lengthFieldLength = 2
lengthAdjustment = 0
initialBytesToStrip = 0 (= do not strip header)

 BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
 +--------+----------------+      +--------+----------------+
 | Length | Actual Content |----->| Length | Actual Content |
 | 0x000C | "HELLO, WORLD" |      | 0x000C | "HELLO, WORLD" |
 +--------+----------------+      +--------+----------------+
假设请求报文的前两个字节为报文体的长度
并希望服务端能够收到完整的报文
则加入下面这个解码器

pipeline.addLast(new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 2));
当加入这个解码器时会调用这个构造方法
public LengthFieldBasedFrameDecoder(
            int maxFrameLength,
            int lengthFieldOffset, int lengthFieldLength) {
        this(maxFrameLength, lengthFieldOffset, lengthFieldLength, 0, 0);
    }
则实际的参数为
 maxFrameLength：Integer.MAX_VALUE(整体报文的最大长度，包括报文头和报文体)
 lengthFieldOffset：0（长度字段的起始索引）
 lengthFieldLength ：2（长度字段的长度）
 lengthAdjustment ： 0    
 initialBytesToStrip ：0

1.组装请求体：
String request = "HELLO, WORLD";
//创建一个字节缓冲区
ByteBuf buffer = Unpooled.buffer();
//写入两个字节的报文体长度，长度为12
buffer.writeShort(request.length());
//写入报文体
buffer.writeBytes(request.getBytes());
//发送
ch.writeAndFlush(buffer);
实际请求体为：
信息: [id: 0x87221f3a, L:/127.0.0.1:52524 - R:/127.0.0.1:8023] WRITE: 14B
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 00 0c 48 45 4c 4c 4f 2c 20 57 4f 52 4c 44       |..HELLO, WORLD  |
+--------+-------------------------------------------------+----------------+

2.在服务端解析
lengthFieldOffset：0（长度字段的起始索引）
 lengthFieldLength ：2（长度字段的长度）
从0索引开始读两个字节为报文体的长度，则长度为 00 0c 即长度为12
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
        int length = msg.readableBytes();
        System.out.println("请求总长度 = " + length);
        //首先读取两个字节的长度位
        short len = msg.readShort();
        System.out.println("请求报文体的长度 = " + len);
        System.out.println("读取两个字节后的ByteBuf = " + msg);
        //读取报文内容
        byte[] bytes = new byte[len];
        msg.readBytes(bytes, 0, len);
        String request = new String(bytes, Charset.forName("GBK"));
        System.out.println("请求报文体 = " + request);

    }
请求总长度 = 14
请求报文体的长度 = 12
读取两个字节后的ByteBuf = PooledSlicedByteBuf(ridx: 2, widx: 14, cap: 14/14, unwrapped: PooledUnsafeDirectByteBuf(ridx: 14, widx: 14, cap: 1024))
请求报文体 = HELLO, WORLD

1.1.2new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 2,0,2)

lengthFieldOffset = 0
lengthFieldLength = 2
lengthAdjustment = 0
initialBytesToStrip = 2 (= the length of the Length field)

 BEFORE DECODE (14 bytes)         AFTER DECODE (12 bytes)
 +--------+----------------+      +----------------+
 | Length | Actual Content |----->| Actual Content |
 | 0x000C | "HELLO, WORLD" |      | "HELLO, WORLD" |
 +--------+----------------+      +----------------+

假设请求报文的前两个字节为报文体的长度
并希望服务端只接收报文体，丢弃掉长度字节
则加入下面这个解码器
new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 2,0,2)
请求报文与2.2.1相同
   @Override
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
        int length = msg.readableBytes();
        System.out.println("请求总长度 = " + length);
        String s = msg.toString(0, length, Charset.defaultCharset());
        System.out.println("请求报文体 = " + s);
    }
请求总长度 = 12
请求报文体 = HELLO, WORLD

1.1.3new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 2,-2,0);

lengthFieldOffset = 0
lengthFieldLength = 2
lengthAdjustment = -2 (= the length of the Length field)
initialBytesToStrip = 0

 BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
 +--------+----------------+      +--------+----------------+
 | Length | Actual Content |----->| Length | Actual Content |
 | 0x000E | "HELLO, WORLD" |      | 0x000E | "HELLO, WORLD" |
 +--------+----------------+      +--------+----------------+

假设请求报文的前两个字节为总体的长度 
并希望服务端接收全部报文
则加入下面这个解码器
new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 2,-2,0)

组装请求体
String request = "HELLO, WORLD";
ByteBuf buffer = Unpooled.buffer();
buffer.writeShort(request.length()+2);
buffer.writeBytes(request.getBytes());
信息: [id: 0xa51d3b17, L:/127.0.0.1:56298 - R:/127.0.0.1:8023] WRITE: 14B
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 00 0e 48 45 4c 4c 4f 2c 20 57 4f 52 4c 44       |..HELLO, WORLD  |
+--------+-------------------------------------------------+----------------+

@Override
    protected void channelRead0(ChannelHandlerContext ctx, ByteBuf msg) throws Exception {
        int length = msg.readableBytes();
        System.out.println("请求总长度 = " + length);
        //读取两个字节的长度位
        short len = msg.readShort();
        System.out.println("报文总长度为 = " + len);
        //报文的请求体为
        String content = msg.toString(2, len-2, Charset.defaultCharset());
        System.out.println("报文请求体为 = " + content);
    }
请求总长度 = 14
报文总长度为 = 14
报文请求体为 = HELLO, WORLD

其他几个参数如下：

 lengthFieldOffset   = 2 (= the length of Header 1)
 lengthFieldLength   = 3
 lengthAdjustment    = 0
 initialBytesToStrip = 0

 BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
 +----------+----------+----------------+      +----------+----------+----------------+
 | Header 1 |  Length  | Actual Content |----->| Header 1 |  Length  | Actual Content |
 |  0xCAFE  | 0x00000C | "HELLO, WORLD" |      |  0xCAFE  | 0x00000C | "HELLO, WORLD" |
 +----------+----------+----------------+      +----------+----------+----------------+

lengthFieldOffset   = 0
 lengthFieldLength   = 3
 lengthAdjustment    = 2 (= the length of Header 1)
 initialBytesToStrip = 0

 BEFORE DECODE (17 bytes)                      AFTER DECODE (17 bytes)
 +----------+----------+----------------+      +----------+----------+----------------+
 |  Length  | Header 1 | Actual Content |----->|  Length  | Header 1 | Actual Content |
 | 0x00000C |  0xCAFE  | "HELLO, WORLD" |      | 0x00000C |  0xCAFE  | "HELLO, WORLD" |
 +----------+----------+----------------+      +----------+----------+----------------+

lengthFieldOffset   = 1 (= the length of HDR1)
 lengthFieldLength   = 2
 lengthAdjustment    = 1 (= the length of HDR2)
 initialBytesToStrip = 3 (= the length of HDR1 + LEN)

 BEFORE DECODE (16 bytes)                       AFTER DECODE (13 bytes)
 +------+--------+------+----------------+      +------+----------------+
 | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
 | 0xCA | 0x000C | 0xFE | "HELLO, WORLD" |      | 0xFE | "HELLO, WORLD" |
 +------+--------+------+----------------+      +------+----------------+
 

lengthFieldOffset   =  1
 lengthFieldLength   =  2
 lengthAdjustment    = -3 (= the length of HDR1 + LEN, negative)
 initialBytesToStrip =  3

 BEFORE DECODE (16 bytes)                       AFTER DECODE (13 bytes)
 +------+--------+------+----------------+      +------+----------------+
 | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
 | 0xCA | 0x0010 | 0xFE | "HELLO, WORLD" |      | 0xFE | "HELLO, WORLD" |
 +------+--------+------+----------------+      +------+----------------+
 

2解析长度位为字符串的报文

• 还有的报文长度是用字符串表示的例如：0002AD 前四个字节表示长度位2，AD为报文内容
  则需要自定义解码器，重写getUnadjustedFrameLength
• 将上述代码做如下修改
   

String request = "HELLO, WORLD";
            ByteBuf buffer = Unpooled.buffer();
            //将长度转化为4位字符串
            String strLen = "00"+request.length();
            buffer.writeBytes(strLen.getBytes());
            buffer.writeBytes(request.getBytes());
            ch.writeAndFlush(buffer);

信息: [id: 0x82110268, L:/127.0.0.1:57369 - R:/127.0.0.1:8023] WRITE: 16B
         +-------------------------------------------------+
         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
+--------+-------------------------------------------------+----------------+
|00000000| 30 30 31 32 48 45 4c 4c 4f 2c 20 57 4f 52 4c 44 |0012HELLO, WORLD|
+--------+-------------------------------------------------+----------------+

2.2自定义解码器

package cn.itbin.decode.server;

import io.netty.buffer.ByteBuf;
import io.netty.handler.codec.DecoderException;
import io.netty.handler.codec.LengthFieldBasedFrameDecoder;

import java.nio.ByteOrder;

/**
 * @author chenxiaogao
 * @className MyLFBFD
 * @description TODO
 * @date 2020/11/8
 **/
public class MyLFBFD extends LengthFieldBasedFrameDecoder {
    public MyLFBFD(int maxFrameLength, int lengthFieldOffset, int lengthFieldLength) {
        super(maxFrameLength, lengthFieldOffset, lengthFieldLength);
    }

    @Override
    protected long getUnadjustedFrameLength(ByteBuf buf, int offset, int length, ByteOrder order) {
        buf = buf.order(order);
        long frameLength;
        switch (length) {
            case 1:
                frameLength = buf.getUnsignedByte(offset);
                break;
            case 2:
                frameLength = buf.getUnsignedShort(offset);
                break;
            case 3:
                frameLength = buf.getUnsignedMedium(offset);
                break;
            case 4:
                //获取四个字节长度位
                byte[] bytes = new byte[4];
//注意此处使用getBytes,不会改变原有的ByteBuf
                buf.getBytes(0, bytes);
                String strLen = new String(bytes);
                frameLength = Long.valueOf(strLen);
                break;
            case 8:
                frameLength = buf.getLong(offset);
                break;
            default:
                throw new DecoderException(
                        "unsupported lengthFieldLength: " + length + " (expected: 1, 2, 3, 4, or 8)");
        }
        return frameLength;
    }
}