上一節(jié)中提到了對象反序列化解析器ObjectDecoder,ObjectDecoder是繼承自LengthFieldBasedFrameDecoder构捡，LengthFieldBasedFrameDecoder是Netty通用的拆包器

LengthFieldBasedFrameDecoder用法

LengthFieldBasedFrameDecoder源碼解釋中是這么說的：
通過長度域lengthFieldLength對ByteBuf進行動態(tài)拆分的decoder，特別適合當(dāng)有一個int做了消息包體頭部進行反序列化
LengthFieldBasedFrameDecoder有很多配置參數(shù)所以可以對任何指定長度域lengthFieldLength的消息進行反序列化她我，所以經(jīng)常用戶客戶端和服務(wù)端的通信協(xié)議。
LengthFieldBasedFrameDecoder的幾個重要參數(shù)：
lengthFiledLength: 長度域大小
maxFrameLength：最大幀長度迫横，即單個包最大的長度
lengthFieldOffset：表示數(shù)據(jù)長度字段開始的偏移量
lengthAdjustment：長度調(diào)整值
initialBytesToStrip：表示從整個包第一個字節(jié)開始鸦难，向后忽略的字節(jié)數(shù)
1、基于長度的拆包

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長度域2個字節(jié)员淫，偏移量0合蔽，initialBytesToStrip為0
2、過濾包頭Header介返，只獲取包體數(shù)據(jù)

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總之衡瓶，反序列首先會跳過initialBytesToStrip個字節(jié)讀取buyBuf,lengthAdjustment表示長度域后還有多少個字節(jié)才是真正的數(shù)據(jù)長度。lengthFieldOffset表示還有多少偏移才是真正的長度域

源碼分析

構(gòu)造函數(shù)

 public LengthFieldBasedFrameDecoder(
            ByteOrder byteOrder, int maxFrameLength, int lengthFieldOffset, int lengthFieldLength,
            int lengthAdjustment, int initialBytesToStrip, boolean failFast) {
        if (byteOrder == null) {
            throw new NullPointerException("byteOrder");
        }

        if (maxFrameLength <= 0) {
            throw new IllegalArgumentException(
                    "maxFrameLength must be a positive integer: " +
                    maxFrameLength);
        }

        if (lengthFieldOffset < 0) {
            throw new IllegalArgumentException(
                    "lengthFieldOffset must be a non-negative integer: " +
                    lengthFieldOffset);
        }

        if (initialBytesToStrip < 0) {
            throw new IllegalArgumentException(
                    "initialBytesToStrip must be a non-negative integer: " +
                    initialBytesToStrip);
        }

        if (lengthFieldOffset > maxFrameLength - lengthFieldLength) {
            throw new IllegalArgumentException(
                    "maxFrameLength (" + maxFrameLength + ") " +
                    "must be equal to or greater than " +
                    "lengthFieldOffset (" + lengthFieldOffset + ") + " +
                    "lengthFieldLength (" + lengthFieldLength + ").");
        }

        this.byteOrder = byteOrder;
        this.maxFrameLength = maxFrameLength;
        this.lengthFieldOffset = lengthFieldOffset;
        this.lengthFieldLength = lengthFieldLength;
        this.lengthAdjustment = lengthAdjustment;
        lengthFieldEndOffset = lengthFieldOffset + lengthFieldLength;
        this.initialBytesToStrip = initialBytesToStrip;
        this.failFast = failFast;
    }

lengthFieldEndOffset = lengthFieldOffset + lengthFieldLength;表示長度域后第一個字節(jié)在整個包的偏移量
具體的拆包協(xié)議：

@Override
    protected final void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        Object decoded = decode(ctx, in);
        if (decoded != null) {
            out.add(decoded);
        }
    }

protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
      //是否丟棄超出長度的數(shù)據(jù)幀 默認為false
        if (discardingTooLongFrame) {
            long bytesToDiscard = this.bytesToDiscard;
            int localBytesToDiscard = (int) Math.min(bytesToDiscard, in.readableBytes());
            in.skipBytes(localBytesToDiscard);
            bytesToDiscard -= localBytesToDiscard;
            this.bytesToDiscard = bytesToDiscard;

            failIfNecessary(false);
        }
      //如果buf中可讀字節(jié)數(shù)小于長度域最小偏移量 說明肯定讀不到長度域 直接返回
        if (in.readableBytes() < lengthFieldEndOffset) {
            return null;
        }
    //bytebuf讀位置索引+長度域偏移量=長度域在整個buf中實際偏移量
        int actualLengthFieldOffset = in.readerIndex() + lengthFieldOffset;
    //獲取實際未調(diào)整的包長度
        long frameLength = getUnadjustedFrameLength(in, actualLengthFieldOffset, lengthFieldLength, byteOrder);
      //如果包長度小于0 拋異常
        if (frameLength < 0) {
            in.skipBytes(lengthFieldEndOffset);
            throw new CorruptedFrameException(
                    "negative pre-adjustment length field: " + frameLength);
        }
      //整個包的長度 = 未調(diào)整的包長度+長度域調(diào)整值+長度域偏移量
        frameLength += lengthAdjustment + lengthFieldEndOffset;
      //如果整個包長度小于長度域偏移量 異常
        if (frameLength < lengthFieldEndOffset) {
            in.skipBytes(lengthFieldEndOffset);
            throw new CorruptedFrameException(
                    "Adjusted frame length (" + frameLength + ") is less " +
                    "than lengthFieldEndOffset: " + lengthFieldEndOffset);
        }
    //整個包長度大于最大定義的數(shù)據(jù)包長度 丟棄模式
        if (frameLength > maxFrameLength) {
            //多余的字節(jié)數(shù) = 數(shù)據(jù)包長度-buf可讀字節(jié)數(shù)
            long discard = frameLength - in.readableBytes();
            tooLongFrameLength = frameLength;
          // 多余的字節(jié)數(shù)小于0牲证，表示當(dāng)前可讀字節(jié)已達到frameLength哮针，直接跳過frameLength個字節(jié)，丟棄之后坦袍，后面有可能就是一個合法的數(shù)據(jù)包
            if (discard < 0) {
                // buffer contains more bytes then the frameLength so we can discard all now
                in.skipBytes((int) frameLength);
            } else {
                // Enter the discard mode and discard everything received so far.
                discardingTooLongFrame = true;
               // 當(dāng)前可讀字節(jié)未達到frameLength十厢，說明后面未讀到的字節(jié)也需要丟棄，進入丟棄模式捂齐，先把當(dāng)前累積的字節(jié)全部丟棄
                bytesToDiscard = discard;
                in.skipBytes(in.readableBytes());
            }
            failIfNecessary(true);
            return null;
        }

        // never overflows because it's less than maxFrameLength
        int frameLengthInt = (int) frameLength;
        if (in.readableBytes() < frameLengthInt) {
            return null;
        }
      //如果需要跳過的字節(jié)數(shù)大于數(shù)據(jù)包長度 直接拋異常
        if (initialBytesToStrip > frameLengthInt) {
            in.skipBytes(frameLengthInt);
            throw new CorruptedFrameException(
                    "Adjusted frame length (" + frameLength + ") is less " +
                    "than initialBytesToStrip: " + initialBytesToStrip);
        }
    //跳過字節(jié)數(shù)
        in.skipBytes(initialBytesToStrip);

        // extract frame
      //拿到當(dāng)前buf的讀偏移量
        int readerIndex = in.readerIndex();
      //實際數(shù)據(jù)包長度 = 整個包長度減去忽略的字節(jié)數(shù)
        int actualFrameLength = frameLengthInt - initialBytesToStrip;
    //拿到待抽取的數(shù)據(jù)幀
        ByteBuf frame = extractFrame(ctx, in, readerIndex, actualFrameLength);
      //移動讀指針
        in.readerIndex(readerIndex + actualFrameLength);
        return frame;
    }

getUnadjustedFrameLength獲取未調(diào)整數(shù)據(jù)長度的源碼：

protected long getUnadjustedFrameLength(ByteBuf buf, int offset, int length, ByteOrder order) {
//入?yún)ffsset是in.readerIndex() + lengthFieldOffset蛮放，即buf讀指針+長度偏移量，
//那么獲取長度字段所在的位置奠宜，然后讀取消息長度字段所占字節(jié)數(shù)的字節(jié)（消息長度字段占用的字節(jié)數(shù)是協(xié)議約定好的）
//讀取出來的數(shù)值就是消息長度的值了包颁。
       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:
           frameLength = buf.getUnsignedInt(offset);
           break;
       case 8:
           frameLength = buf.getLong(offset);
           break;
       default:
           throw new DecoderException(
                   "unsupported lengthFieldLength: " + lengthFieldLength + " (expected: 1, 2, 3, 4, or 8)");
       }
       return frameLength;
   }