Netty 包头
LengthFieldBasedFrameDecoder
常用的处理大数据分包传输问题的解决类，先对构造方法LengthFieldBasedFrameDecoder中的参数做以下解释说明
maxFrameLength：解码的帧的最大长度
lengthFieldOffset ：长度属性的起始位（偏移位），包中存放有整个大数据包长度的字节，这段字节的其实位置
lengthFieldLength：长度属性的长度，即存放整个大数据包长度的字节所占的长度
lengthAdjustmen：长度调节值，在总长被定义为包含包头长度时，修正信息长度。initialBytesToStrip：跳过的字节数，根据需要我们跳过lengthFieldLength个字节，以便接收端直接接受到不含“长度属性”的内容
failFast ：为true，当frame长度超过maxFrameLength时立即报TooLongFrameException异常，为false，读取完整个帧再报异常
下面对各种情况分别描述：
1. 2 bytes length field at offset 0, do not strip header
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" |
+--------+----------------+ +--------+----------------+
此时数据格式不做任何改变（没有跳过任何字节）
2. 2 bytes length field at offset 0, strip header
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" |
+--------+----------------+ +----------------+
此时帧长度为14个字节，但由于前（lengthFieldOffset = 0）两个（lengthFieldLength = 2）字节是表示帧长度的字节，不计入数据，故真实的数据长度为12个字节。
3. 2 bytes length field at offset 0, do not strip header, the length field represents the length of the whole message
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" |
+--------+----------------+ +--------+----------------+
此处定义的Length为0x000E共占了两个字节，表示的帧长度为14个字节，前（lengthFieldOffset = 0）两个（lengthFieldLength = 2）字节为Length，由于设置的lengthAdjustment = -2 (= the length of the Length field)，故修正的信息实际长度补2，即解码时往前推2个字节，解码后还是14个字节长度（此种情况是把整个长度封装，一般来讲，我们只封装数据长度）
4. 3 bytes length field at the end of 5 bytes header, do not strip header
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"|
+---------+---------+--------------+ +----------+--------+-----------+
此处lengthFieldOffset = 2，从第3个字节开始表示数据长度，长度占3个字节，真实数据长度为0x00000C 即12个字节，而lengthAdjustment=0，initialBytesToStrip = 0，故解码后的数据与解码前的数据相同。
4. 3 bytes length field at the beginning of 5 bytes header, do not strip header
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" |
+----------+----------+----------------+ +----------+----------+----------------+
此处由于修正的字节数是2，且initialBytesToStrip = 0，故整个数据的解码数据保持不变
总字节数是17，开始的三个字节表示字节长度：12，修正的字节是2,（即从第三个字节开始，再加两个开始是真正的数据，其中跳过的字节数是0）
5. 2 bytes length field at offset 1 in the middle of 4 bytes header, strip the first header field and the length field
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" |
+------+--------+------+----------------+ +------+----------------+
从第2个字节开始解码，取两个字节作为帧长度，为12个字节，然后，修正一个字节，从第5个字节到最后表示帧数据，解码时，由于initialBytesToStrip=3，表示跳过前三个字节（去掉），故从第四个字节开始解析，解析出来后，如右图所示。
6. 2 bytes length field at offset 1 in the middle of 4 bytes header, strip the first header field and the length field, the length field represents the length of the whole message
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" |
+------+--------+------+----------------+ +------+----------------+
从第二个字节开始，取两个字节作为帧长度，为16个字节，然后补3个字节，故往前找三个字节，从HDP1开始解码，而又因为initialBytesToStrip=3，解码时忽略掉前三个字节，故从第四个字节开始解析，解析结果如右图所示。
总结：一般来讲，当lengthAdjustment 为负数时，Length表示的是整个帧的长度，当lengthAdjustment为正数或0时，表示真实数据长度。
(6) LengthFieldPrepender
编码类，自动将
+----------------+
| "HELLO, WORLD" |
+----------------+
格式的数据转换成
+--------+----------------+
+ 0x000C | "HELLO, WORLD" |
+--------+----------------+
格式的数据，
如果lengthIncludesLengthFieldLength设置为true,则编码为
+--------+----------------+
+ 0x000E | "HELLO, WORLD" |
+--------+----------------+
格式的数据
应用场景：自定义pipelineFactory类: MyPipelineFactory implements ChannelPipelineFactory
中
pipeline.addLast("frameEncode", new LengthFieldPrepender(4, false));
