上一节中提到了对象反序列化解析器ObjectDecoder,ObjectDecoder是继承自LengthFieldBasedFrameDecoder，LengthFieldBasedFrameDecoder是Netty通用的拆包器
LengthFieldBasedFrameDecoder用法
LengthFieldBasedFrameDecoder源码解释中是这么说的：
通过长度域lengthFieldLength对ByteBuf进行动态拆分的decoder，特别适合当有一个int做了消息包体头部进行反序列化
LengthFieldBasedFrameDecoder有很多配置参数所以可以对任何指定长度域lengthFieldLength的消息进行反序列化，所以经常用户客户端和服务端的通信协议。
LengthFieldBasedFrameDecoder的几个重要参数：
lengthFiledLength: 长度域大小
maxFrameLength：最大帧长度，即单个包最大的长度
lengthFieldOffset：表示数据长度字段开始的偏移量
lengthAdjustment：长度调整值
initialBytesToStrip：表示从整个包第一个字节开始，向后忽略的字节数
1、基于长度的拆包
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长度域2个字节，偏移量0，initialBytesToStrip为0
2、过滤包头Header，只获取包体数据
image.png
lengthFieldLength = 2 //数据长度为2个字节
initialBytesToStrip=2 //等于数据的长度，意思就是跳过数据长度字节
lengthFieldOffset=0 //偏移量为0
3、lengthFieldLength代表数据Header+Body的长度，
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4、基于可调整长度的拆包
image.png
总之，反序列首先会跳过initialBytesToStrip个字节读取buyBuf,lengthAdjustment表示长度域后还有多少个字节才是真正的数据长度。lengthFieldOffset表示还有多少偏移才是真正的长度域
源码分析
构造函数
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;表示长度域后第一个字节在整个包的偏移量
具体的拆包协议：
@Override
protected final void decode(ChannelHandlerContext ctx, ByteBuf in, Listprotected Object decode(ChannelHandlerContext ctx, ByteBuf in) throws Exception {
//是否丢弃超出长度的数据帧 默认为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中可读字节数小于长度域最小偏移量 说明肯定读不到长度域 直接返回
if (in.readableBytes() < lengthFieldEndOffset) {
return null;
}
//bytebuf读位置索引+长度域偏移量=长度域在整个buf中实际偏移量
int actualLengthFieldOffset = in.readerIndex() + lengthFieldOffset;
//获取实际未调整的包长度
long frameLength = getUnadjustedFrameLength(in, actualLengthFieldOffset, lengthFieldLength, byteOrder);
//如果包长度小于0 抛异常
if (frameLength < 0) {
in.skipBytes(lengthFieldEndOffset);
throw new CorruptedFrameException(
"negative pre-adjustment length field: " + frameLength);
}
//整个包的长度 = 未调整的包长度+长度域调整值+长度域偏移量
frameLength += lengthAdjustment + lengthFieldEndOffset;
//如果整个包长度小于长度域偏移量 异常
if (frameLength < lengthFieldEndOffset) {
in.skipBytes(lengthFieldEndOffset);
throw new CorruptedFrameException(
"Adjusted frame length (" + frameLength + ") is less " +
"than lengthFieldEndOffset: " + lengthFieldEndOffset);
}
//整个包长度大于最大定义的数据包长度 丢弃模式
if (frameLength > maxFrameLength) {
//多余的字节数 = 数据包长度-buf可读字节数
long discard = frameLength - in.readableBytes();
tooLongFrameLength = frameLength;
// 多余的字节数小于0，表示当前可读字节已达到frameLength，直接跳过frameLength个字节，丢弃之后，后面有可能就是一个合法的数据包
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;
// 当前可读字节未达到frameLength，说明后面未读到的字节也需要丢弃，进入丢弃模式，先把当前累积的字节全部丢弃
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;
}
//如果需要跳过的字节数大于数据包长度 直接抛异常
if (initialBytesToStrip > frameLengthInt) {
in.skipBytes(frameLengthInt);
throw new CorruptedFrameException(
"Adjusted frame length (" + frameLength + ") is less " +
"than initialBytesToStrip: " + initialBytesToStrip);
}
//跳过字节数
in.skipBytes(initialBytesToStrip);
// extract frame
//拿到当前buf的读偏移量
int readerIndex = in.readerIndex();
//实际数据包长度 = 整个包长度减去忽略的字节数
int actualFrameLength = frameLengthInt - initialBytesToStrip;
//拿到待抽取的数据帧
ByteBuf frame = extractFrame(ctx, in, readerIndex, actualFrameLength);
//移动读指针
in.readerIndex(readerIndex + actualFrameLength);
return frame;
}
getUnadjustedFrameLength获取未调整数据长度的源码：
protected long getUnadjustedFrameLength(ByteBuf buf, int offset, int length, ByteOrder order) {
//入参offsset是in.readerIndex() + lengthFieldOffset，即buf读指针+长度偏移量，
//那么获取长度字段所在的位置，然后读取消息长度字段所占字节数的字节（消息长度字段占用的字节数是协议约定好的）
//读取出来的数值就是消息长度的值了。
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;
}