Class LengthFieldBasedFrameDecoder

  • All Implemented Interfaces:
    ChannelHandler, ChannelUpstreamHandler, LifeCycleAwareChannelHandler
    Direct Known Subclasses:
    MarshallingDecoder, ObjectDecoder

    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 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 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)
    
     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
    • Field Detail

      • maxFrameLength

        private final int maxFrameLength
      • lengthFieldOffset

        private final int lengthFieldOffset
      • lengthFieldLength

        private final int lengthFieldLength
      • lengthFieldEndOffset

        private final int lengthFieldEndOffset
      • lengthAdjustment

        private final int lengthAdjustment
      • initialBytesToStrip

        private final int initialBytesToStrip
      • failFast

        private final boolean failFast
      • discardingTooLongFrame

        private boolean discardingTooLongFrame
      • tooLongFrameLength

        private long tooLongFrameLength
      • bytesToDiscard

        private long bytesToDiscard
    • 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.
    • Method Detail

      • decode

        protected java.lang.Object decode​(ChannelHandlerContext ctx,
                                          Channel channel,
                                          ChannelBuffer buffer)
                                   throws java.lang.Exception
        Description copied from class: FrameDecoder
        Decodes the received packets so far into a frame. If an sub-class wants to extract a frame out of the buffer it should use the FrameDecoder.extractFrame(ChannelBuffer, int, int) method, to make optimizations easier later.
        Specified by:
        decode in class FrameDecoder
        Parameters:
        ctx - the context of this handler
        channel - the current channel
        buffer - the cumulative buffer of received packets so far. Note that the buffer might be empty, which means you should not make an assumption that the buffer contains at least one byte in your decoder implementation.
        Returns:
        the decoded frame if a full frame was received and decoded. null if there's not enough data in the buffer to decode a frame.
        Throws:
        java.lang.Exception
      • failIfNecessary

        private void failIfNecessary​(ChannelHandlerContext ctx,
                                     boolean firstDetectionOfTooLongFrame)