这篇文章我们放松一点，只分析基于长度域解码器的几个参数，
lengthFieldOffset ：长度域的偏移量，也就是长度域要从什么地方开始
lengthFieldLength：长度域的长度，也就是长度域占多少个字节
lengthAdjustment：长度域的值的调整，也就是我们去到长度域里面的值，然后还要做多少调整才是符合要求的，这个值可以是正值，可以是负值，正值表示还要加多少，负值表示还要减去多少
initialBytesToStrip：原始需要跳过多少才返回给用户
我们看几个例子，就能知道了，这些例子都是在源码里面：
例子1：
* <h3>2 bytes length field at offset 0, do not strip header</h3>
*
* The value of the length field in this example is <tt>12 (0x0C)</tt> which
* represents the length of "HELLO, WORLD". By default, the decoder assumes
* that the length field represents the number of the bytes that follows the
* length field. Therefore, it can be decoded with the simplistic parameter
* combination.
* <pre>
* <b>lengthFieldOffset</b> = <b>0</b>
* <b>lengthFieldLength</b> = <b>2</b>
* 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" |
* +--------+----------------+ +--------+----------------+
* </pre>太简单了，没有偏移，长度域的长度为2也就是0x000C，也就是12.
例子2：
*
* <h3>2 bytes length field at offset 0, strip header</h3>
*
* Because we can get the length of the content by calling
* {@link ByteBuf#readableBytes()}, you might want to strip the length
* field by specifying <tt>initialBytesToStrip</tt>. In this example, we
* specified <tt>2</tt>, that is same with the length of the length field, to
* strip the first two bytes.
* <pre>
* lengthFieldOffset = 0
* lengthFieldLength = 2
* lengthAdjustment = 0
* <b>initialBytesToStrip</b> = <b>2</b> (= the length of the Length field)
*
* BEFORE DECODE (14 bytes) AFTER DECODE (12 bytes)
* +--------+----------------+ +----------------+
* | Length | Actual Content |----->| Actual Content |
* | 0x000C | "HELLO, WORLD" | | "HELLO, WORLD" |
* +--------+----------------+ +----------------+
* </pre>
*长度域没有偏移，但是返回给用户的数据要跳过两个字节，也就是跳过了长度域才返回。
例子3：
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* <h3>2 bytes length field at offset 0, do not strip header, the length field
* represents the length of the whole message</h3>
*
* In most cases, the length field represents the length of the message body
* only, as shown in the previous examples. However, in some protocols, the
* length field represents the length of the whole message, including the
* message header. In such a case, we specify a non-zero
* <tt>lengthAdjustment</tt>. Because the length value in this example message
* is always greater than the body length by <tt>2</tt>, we specify <tt>-2</tt>
* as <tt>lengthAdjustment</tt> for compensation.
* <pre>
* lengthFieldOffset = 0
* lengthFieldLength = 2
* <b>lengthAdjustment</b> = <b>-2</b> (= 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" |
* +--------+----------------+ +--------+----------------+
* </pre>这个没有偏移，长度域为2，正常。但是长度域的值为14，而真正的内容只有12，所有通过lengthAdjustement来调整。也就是说，大部分情况下，长度域表示的长度是代表有效信息的内容的长度的，但是某些情况下，长度域表示的长度不仅包括了有效信息的长度，还包括了信息头（这里就是长度域），所以需要一个lengthAdjustment来调整。
例子4：
* <h3>3 bytes length field at the end of 5 bytes header, do not strip header</h3>
*
* The following message is a simple variation of the first example. An extra
* header value is prepended to the message. <tt>lengthAdjustment</tt> is zero
* again because the decoder always takes the length of the prepended data into
* account during frame length calculation.
* <pre>
* <b>lengthFieldOffset</b> = <b>2</b> (= the length of Header 1)
* <b>lengthFieldLength</b> = <b>3</b>
* 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" |
* +----------+----------+----------------+ +----------+----------+----------------+
* </pre>这个就是说长度域偏移了两个字节，这偏移的两个字节就是header1的长度，偏移了之后才到长度域。然后长度域的长度是3个字节，也就是0x00000C。
例子5：
* <h3>3 bytes length field at the beginning of 5 bytes header, do not strip header</h3>
*
* This is an advanced example that shows the case where there is an extra
* header between the length field and the message body. You have to specify a
* positive <tt>lengthAdjustment</tt> so that the decoder counts the extra
* header into the frame length calculation.
* <pre>
* lengthFieldOffset = 0
* lengthFieldLength = 3
* <b>lengthAdjustment</b> = <b>2</b> (= 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" |
* +----------+----------+----------------+ +----------+----------+----------------+
* </pre>长度域没有偏移，并且长度域的长度为3。为了把header也算在给用户的数据里面，需要在长度域的长度0x00000C也就是12上面加2才能得到header1加上内容的长度，所以需要在lengthAdjustment调整值上面加2。
例子6：
* <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
* strip the first header field and the length field</h3>
*
* This is a combination of all the examples above. There are the prepended
* header before the length field and the extra header after the length field.
* The prepended header affects the <tt>lengthFieldOffset</tt> and the extra
* header affects the <tt>lengthAdjustment</tt>. We also specified a non-zero
* <tt>initialBytesToStrip</tt> to strip the length field and the prepended
* header from the frame. If you don't want to strip the prepended header, you
* could specify <tt>0</tt> for <tt>initialBytesToSkip</tt>.
* <pre>
* lengthFieldOffset = 1 (= the length of HDR1)
* lengthFieldLength = 2
* <b>lengthAdjustment</b> = <b>1</b> (= the length of HDR2)
* <b>initialBytesToStrip</b> = <b>3</b> (= 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" |
* +------+--------+------+----------------+ +------+----------------+
* </pre>长度域偏移一个字节；长度域长度为2；调整值为1，也就是内容需要加上HER2，长度为1；另外这里返回给 用户的内容跳过了三个字节，也就是把HDR1和Length长度域跳过了。
例子7：
* <h3>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</h3>
*
* Let's give another twist to the previous example. The only difference from
* the previous example is that the length field represents the length of the
* whole message instead of the message body, just like the third example.
* We have to count the length of HDR1 and Length into <tt>lengthAdjustment</tt>.
* Please note that we don't need to take the length of HDR2 into account
* because the length field already includes the whole header length.
* <pre>
* lengthFieldOffset = 1
* lengthFieldLength = 2
* <b>lengthAdjustment</b> = <b>-3</b> (= the length of HDR1 + LEN, negative)
* <b>initialBytesToStrip</b> = <b> 3</b>
*
* BEFORE DECODE (16 bytes) AFTER DECODE (13 bytes)
* +------+--------+------+----------------+ +------+----------------+
* | HDR1 | Length | HDR2 | Actual Content |----->| HDR2 | Actual Content |
* | 0xCA | 0x0010 | 0xFE | "HELLO, WORLD" | | 0xFE | "HELLO, WORLD" |
* +------+--------+------+----------------+ +------+----------------+
* </pre>长度域偏移为1；长度域长度为2；长度域表示的长度需要减去HDR的长度和Length长度域的长度，也就是3，因为长度域表示的内容是整个数据段的内容，需要把这些没必要的内容例如HDR1和长度域减掉；最后返回给用户需要跳过前三个字节，也就是HDR1和Length。
最后我们看一下LengthFieldBasedFrameDecoder的一些成员变量：
public class LengthFieldBasedFrameDecoder extends ByteToMessageDecoder {
private final ByteOrder byteOrder;
private final int maxFrameLength;
private final int lengthFieldOffset;
private final int lengthFieldLength;
private final int lengthFieldEndOffset;
private final int lengthAdjustment;
private final int initialBytesToStrip;
private final boolean failFast;
private boolean discardingTooLongFrame;
private long tooLongFrameLength;
private long bytesToDiscard;除去我们刚刚说的那4个，还有maxFrameLength，我们解析出来的数据不能超过的范围；lengthFieldEndOffset，我们看源码可以知道，lengthFieldEndOffset = lengthFieldOffset + lengthFieldLength，这个就是偏移加上长度域的长度，后续会使用到；failFast是否快速失败模式；discardingTooLongFrame，是否已经进入了丢弃模式；tooLongFrameLength，这个是最终需要丢弃的数据的长度；bytesToDiscard，表示还需要丢弃的字节长度，bytesToDiscard<=tooLongFrameLength，有可能当前可读的数据长度小于tooLongFrameLength，那么就先抛弃当前可读的，后续补充上bytesToDiscard这么多需要丢弃的，也就是tooLongFrameLength=in.readableBytes+bytesToDiscard。