LengthFieldBasedFrameDecoder

public class LengthFieldBasedFrameDecoder
extends FrameDecoder

A decoder that splits the received ChannelBuffers dynamically by the value of the length field in the message. It is

这个解码器分割动态的buffer更加消息的长度

particularly useful when you decode a binary message which has an integer header field that represents the length of the

有一个代表消息体的长度的头部当你解码一个2进制消息是很有用的

message body or the whole message.

LengthFieldBasedFrameDecoder has many configuration parameters so that it can decode any message with a

  有很多参数所以有一个固定长度的域能解码然后消息

length field, which is often seen in proprietary client-server protocols. Here are some example that will give you the basic idea on which option does what.

下面是一些例子

2 bytes length field at offset 0, do not strip header

2个字节长度 ，不会去掉头部

The value of the length field in this example is 12 (0x0C) which represents the length of "HELLO, WORLD". By default, the

值是12代表“HELLO, WORLD”的长度

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.

 lengthFieldOffset   = 0
 lengthFieldLength   = 2
 lengthAdjustment    = 0
 initialBytesToStrip = 0 (= do not strip header) // 属性

 BEFORE DECODE (14 bytes)         AFTER DECODE (14 bytes)
 +--------+----------------+      +--------+----------------+
 | Length | Actual Content |----->| Length | Actual Content |
 | 0x000C | "HELLO, WORLD" |      | 0x000C | "HELLO, WORLD" |
 +--------+----------------+      +--------+----------------+
 

2 bytes length field at offset 0, strip header

去掉头部

Because we can get the length of the content by calling ChannelBuffer.readableBytes(), you might want to strip the length field by specifying initialBytesToStrip. In this example, we specified 2, that is same with the length of the length field, to strip the first two bytes.

 lengthFieldOffset   = 0
 lengthFieldLength   = 2
 lengthAdjustment    = 0
 initialBytesToStrip = 2 (= the length of the Length field)// 去掉头部字节

 BEFORE DECODE (14 bytes)         AFTER DECODE (12 bytes)
 +--------+----------------+      +----------------+
 | Length | Actual Content |----->| Actual Content |
 | 0x000C | "HELLO, WORLD" |      | "HELLO, WORLD" |
 +--------+----------------+      +----------------+
 

2 bytes length field at offset 0, do not strip header, the length field represents the

length of the whole message 

2个字节代表整个消息的长度

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 lengthAdjustment. Because the length value in this example message is always greater than the body length by 2, we specify -2 as lengthAdjustment for compensation.

 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" |
 +--------+----------------+      +--------+----------------+
 

3 bytes length field at the end of 5 bytes header, do not strip header

The following message is a simple variation of the first example. An extra header value is prepended to the message. lengthAdjustment is zero again because the decoder always takes the length of the prepended data into account during frame length calculation.

 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" |
 +----------+----------+----------------+      +----------+----------+----------------+
 

3 bytes length field at the beginning of 5 bytes header, do not strip header

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 lengthAdjustment so that the decoder counts the extra header into the frame length calculation.

 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" |
 +----------+----------+----------------+      +----------+----------+----------------+
 

2 bytes length field at offset 1 in the middle of 4 bytes header, strip the first header field and the length field

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 thelengthFieldOffset and the extra header affects the lengthAdjustment. We also specified a non-zero initialBytesToStrip 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 0 for initialBytesToSkip.

 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" |
 +------+--------+------+----------------+      +------+----------------+
 

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

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 lengthAdjustment. 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.

 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" |
 +------+--------+------+----------------+      +------+----------------+
 

See Also:

LengthFieldPrepender