我认为LengthFieldBasedFrameDecoder的正确使用方式

LengthFieldBasedFrameDecoder是Netty提供的便于编写一般基于长度的协议解码器，最近我发现在搜索引擎上搜到的使用方式代码如下

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@Override
protected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
    if (in == null) {
        return null;
    }
    //HEADER_SIZE 包头大小
    if (in.readableBytes() < HEADER_SIZE) {
        throw new Exception("readable bytes length less than header size");
    }
    int length = in.readInt();
    if (in.readableBytes() < length) {
        throw new Exception("body length less than length declared in header");
    }
    ByteBuf buf = in.readBytes(length);
    byte[] req = new byte[buf.readableBytes()];
    buf.readBytes(req);
    String body = new String(req, "UTF-8");
    Message msg = new Message(flag, type, body);
    return msg;
}

关键点在可读数据不足时直接抛出异常是有点违背常理的，一般我们使用netty的ByteToMessageDecoder处理时代码如下

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@Override
protected void decode(ChannelHandlerContext ctx, ByteBuf bufferIn, List<Object> out) throws Exception {
    if (in.readableBytes() < 4) {
        return null;
    }
    int readIndex = in.readerIndex();
    int length = in.readInt();
    if (in.readableBytes()+1 < length) {
        in.readerIndex(readIndex);
        return null;
    }
    ByteBuf buf = in.readBytes(length);
    byte[] req = new byte[buf.readableBytes()];
    buf.readBytes(req);
    String body = new String(req, "UTF-8");
    out.add(body);
}

LengthFieldBasedFrameDecoder也继承自ByteToMessageDecoder主要有两个方法

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@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 {
    if (discardingTooLongFrame) {
        discardingTooLongFrame(in);
    }

    if (in.readableBytes() < lengthFieldEndOffset) {
        return null;
    }

    int actualLengthFieldOffset = in.readerIndex() + lengthFieldOffset;
    long frameLength = getUnadjustedFrameLength(in, actualLengthFieldOffset, lengthFieldLength, byteOrder);

    if (frameLength < 0) {
        failOnNegativeLengthField(in, frameLength, lengthFieldEndOffset);
    }

    frameLength += lengthAdjustment + lengthFieldEndOffset;

    if (frameLength < lengthFieldEndOffset) {
        failOnFrameLengthLessThanLengthFieldEndOffset(in, frameLength, lengthFieldEndOffset);
    }

    if (frameLength > maxFrameLength) {
        exceededFrameLength(in, frameLength);
        return null;
    }

    // never overflows because it's less than maxFrameLength
    int frameLengthInt = (int) frameLength;
    if (in.readableBytes() < frameLengthInt) {
        return null;
    }

    if (initialBytesToStrip > frameLengthInt) {
        failOnFrameLengthLessThanInitialBytesToStrip(in, frameLength, initialBytesToStrip);
    }
    in.skipBytes(initialBytesToStrip);

    // extract frame
    int readerIndex = in.readerIndex();
    int actualFrameLength = frameLengthInt - initialBytesToStrip;
    ByteBuf frame = extractFrame(ctx, in, readerIndex, actualFrameLength);
    in.readerIndex(readerIndex + actualFrameLength);
    return frame;
}

可以看到LengthFieldBasedFrameDecoder的decode方法在可读数据不足是是会返回null的，
由此可见继承LengthFieldBasedFrameDecoder并重写decode方法在可读数据不足时抛出异常时不可取的

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