java.lang.Object
- io.netty5.channel.ChannelHandlerAdapter
- - io.netty5.handler.codec.ByteToMessageDecoder
  - - io.netty5.handler.codec.LengthFieldBasedFrameDecoder

All Implemented Interfaces:

ChannelHandler

Direct Known Subclasses:

TcpDnsQueryDecoder, TcpDnsResponseDecoder
```
public class LengthFieldBasedFrameDecoder
extends ByteToMessageDecoder
```
A decoder that splits the received Buffers dynamically by the value of the length field in the message. It is particularly useful when you decode a binary message which has an integer header field that represents the length of the message body or the whole message.
LengthFieldBasedFrameDecoder has many configuration parameters so that it can decode any message with a 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.
2 bytes length field at offset 0, do not strip header

The value of the length field in this example is 12 (0x0C) 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.
```
 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" |
 +--------+----------------+      +--------+----------------+
 
```
2 bytes length field at offset 0, strip header

Because we can get the length of the content by calling Buffer.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)

 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

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 the lengthFieldOffset 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

Nested Class Summary
- Nested classes/interfaces inherited from class io.netty5.handler.codec.ByteToMessageDecoder
  ByteToMessageDecoder.Cumulator

Field Summary
- Fields inherited from class io.netty5.handler.codec.ByteToMessageDecoder
  COMPOSITE_CUMULATOR, MERGE_CUMULATOR

Constructor Summary

Constructors
Constructor	Description
`LengthFieldBasedFrameDecoder(int maxFrameLength, int lengthFieldOffset, int lengthFieldLength)`	Creates a new instance.
`LengthFieldBasedFrameDecoder(int maxFrameLength, int lengthFieldOffset, int lengthFieldLength, int lengthAdjustment, int initialBytesToStrip)`	Creates a new instance.
`LengthFieldBasedFrameDecoder(int maxFrameLength, int lengthFieldOffset, int lengthFieldLength, int lengthAdjustment, int initialBytesToStrip, boolean failFast)`	Creates a new instance.
`LengthFieldBasedFrameDecoder(ByteOrder byteOrder, int maxFrameLength, int lengthFieldOffset, int lengthFieldLength, int lengthAdjustment, int initialBytesToStrip, boolean failFast)`	Creates a new instance.

Method Summary

All Methods Instance Methods Concrete Methods
Modifier and Type	Method	Description
`protected void`	`decode(ChannelHandlerContext ctx, Buffer in)`	Decode the from one `Buffer` to another.
`protected Object`	`decode0(ChannelHandlerContext ctx, Buffer buffer)`	Create a frame out of the `Buffer` and return it.
`protected Buffer`	`extractFrame(ChannelHandlerContext ctx, Buffer buffer, int length)`	Extract the sub-region of the specified buffer.
`protected long`	`getUnadjustedFrameLength(Buffer buffer, int offset, int length, ByteOrder byteOrder)`	Decodes the specified region of the buffer into an unadjusted frame length.

Methods inherited from class io.netty5.handler.codec.ByteToMessageDecoder
actualReadableBytes, channelInactive, channelRead, channelReadComplete, channelShutdown, decodeLast, discardSomeReadBytes, handlerAdded, handlerAdded0, handlerRemoved, handlerRemoved0, internalBuffer, isSharable, isSingleDecode, setSingleDecode

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface io.netty5.channel.ChannelHandler
bind, channelActive, channelExceptionCaught, channelInboundEvent, channelRegistered, channelUnregistered, channelWritabilityChanged, close, connect, deregister, disconnect, flush, pendingOutboundBytes, read, register, sendOutboundEvent, shutdown, write

- Constructor Detail
  - LengthFieldBasedFrameDecoder
```
public LengthFieldBasedFrameDecoder(int maxFrameLength,
                                    int lengthFieldOffset,
                                    int lengthFieldLength)
```
    Creates a new instance.
    
    Parameters:
    
    maxFrameLength - the maximum length of the frame. If the length of the frame is greater than this value, TooLongFrameException will be thrown.
    
    lengthFieldOffset - the offset of the length field
    
    lengthFieldLength - the length of the length field
  - LengthFieldBasedFrameDecoder
```
public LengthFieldBasedFrameDecoder(int maxFrameLength,
                                    int lengthFieldOffset,
                                    int lengthFieldLength,
                                    int lengthAdjustment,
                                    int initialBytesToStrip)
```
    Creates a new instance.
    
    Parameters:
    
    maxFrameLength - the maximum length of the frame. If the length of the frame is greater than this value, TooLongFrameException will be thrown.
    
    lengthFieldOffset - the offset of the length field
    
    lengthFieldLength - the length of the length field
    
    lengthAdjustment - the compensation value to add to the value of the length field
    
    initialBytesToStrip - the number of first bytes to strip out from the decoded frame
  - LengthFieldBasedFrameDecoder
```
public LengthFieldBasedFrameDecoder(int maxFrameLength,
                                    int lengthFieldOffset,
                                    int lengthFieldLength,
                                    int lengthAdjustment,
                                    int initialBytesToStrip,
                                    boolean failFast)
```
    Creates a new instance.
    
    Parameters:
    
    maxFrameLength - the maximum length of the frame. If the length of the frame is greater than this value, TooLongFrameException will be thrown.
    
    lengthFieldOffset - the offset of the length field
    
    lengthFieldLength - the length of the length field
    
    lengthAdjustment - the compensation value to add to the value of the length field
    
    initialBytesToStrip - the number of first bytes to strip out from the decoded frame
    
    failFast - If true, a TooLongFrameException is thrown as soon as the decoder notices the length of the frame will exceed maxFrameLength regardless of whether the entire frame has been read. If false, a TooLongFrameException is thrown after the entire frame that exceeds maxFrameLength has been read.
  - LengthFieldBasedFrameDecoder
```
public LengthFieldBasedFrameDecoder(ByteOrder byteOrder,
                                    int maxFrameLength,
                                    int lengthFieldOffset,
                                    int lengthFieldLength,
                                    int lengthAdjustment,
                                    int initialBytesToStrip,
                                    boolean failFast)
```
    Creates a new instance.
    
    Parameters:
    
    byteOrder - the ByteOrder of the length field
    
    maxFrameLength - the maximum length of the frame. If the length of the frame is greater than this value, TooLongFrameException will be thrown.
    
    lengthFieldOffset - the offset of the length field
    
    lengthFieldLength - the length of the length field
    
    lengthAdjustment - the compensation value to add to the value of the length field
    
    initialBytesToStrip - the number of first bytes to strip out from the decoded frame
    
    failFast - If true, a TooLongFrameException is thrown as soon as the decoder notices the length of the frame will exceed maxFrameLength regardless of whether the entire frame has been read. If false, a TooLongFrameException is thrown after the entire frame that exceeds maxFrameLength has been read.
- Method Detail
  - decode
```
protected final void decode(ChannelHandlerContext ctx,
                            Buffer in)
                     throws Exception
```
    Description copied from class: ByteToMessageDecoder
    
    Decode the from one Buffer to another. This method will be called till either the input Buffer has nothing to read when return from this method or till nothing was read from the input Buffer.
    
    Specified by:
    
    decode in class ByteToMessageDecoder
    
    Parameters:
    
    ctx - the ChannelHandlerContext which this ByteToMessageDecoder belongs to
    
    in - the Buffer from which to read data
    
    Throws:
    
    Exception - is thrown if an error occurs
  - decode0
```
protected Object decode0(ChannelHandlerContext ctx,
                         Buffer buffer)
                  throws Exception
```
    Create a frame out of the Buffer and return it.
    
    Parameters:
    
    ctx - the ChannelHandlerContext which this ByteToMessageDecoder belongs to
    
    buffer - the Buffer from which to read data
    
    Returns:
    
    the Buffer which represent the frame or null if no frame could be created.
    
    Throws:
    
    Exception
  - getUnadjustedFrameLength
```
protected long getUnadjustedFrameLength(Buffer buffer,
                                        int offset,
                                        int length,
                                        ByteOrder byteOrder)
```
    Decodes the specified region of the buffer into an unadjusted frame length. The default implementation is capable of decoding the specified region into an unsigned 8/16/24/32/64 bit integer. Override this method to decode the length field encoded differently. Note that this method must not modify the state of the specified buffer (e.g. readerOffset, writerOffset, and the content of the buffer.)
    
    Throws:
    
    DecoderException - if failed to decode the specified region
  - extractFrame
```
protected Buffer extractFrame(ChannelHandlerContext ctx,
                              Buffer buffer,
                              int length)
```
    Extract the sub-region of the specified buffer. This method must modify the state of the buffer to continue after the frame (e.g. by splitting the buffer or by increasing readerOffset).

Class LengthFieldBasedFrameDecoder

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

2 bytes length field at offset 0, strip header

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

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

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

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

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

Nested Class Summary

Nested classes/interfaces inherited from class io.netty5.handler.codec.ByteToMessageDecoder

Field Summary

Fields inherited from class io.netty5.handler.codec.ByteToMessageDecoder

Constructor Summary

Method Summary

Methods inherited from class io.netty5.handler.codec.ByteToMessageDecoder

Methods inherited from class java.lang.Object

Methods inherited from interface io.netty5.channel.ChannelHandler

Constructor Detail

LengthFieldBasedFrameDecoder

LengthFieldBasedFrameDecoder

LengthFieldBasedFrameDecoder

LengthFieldBasedFrameDecoder

Method Detail

decode

decode0

getUnadjustedFrameLength

extractFrame