Improved Javadoc of LengthFieldBasedFrameDecoder
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Trustin Lee
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@ -27,15 +27,21 @@ import org.jboss.netty.channel.ChannelHandlerContext;
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* <p>
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* {@link LengthFieldBasedFrameDecoder} has many configuration parameters so
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* that it can decode any message with a length field, which is often seen in
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* proprietary client-server protocols. Here are some example that will give
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* proprietary client-server protocols. Here are some example that will give
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* you the basic idea on which option does what.
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*
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* <h3>2 bytes length field at offset 0, do not strip header</h3>
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*
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* The value of the length field in this example is <tt>10 (0x0C)</tt> which
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* represents the length of "HELLO, WORLD". By default, the decoder assumes
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* that the length field represents the number of the bytes that follows the
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* length field. Therefore, it can be decoded with the simplistic parameter
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* combination.
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* <pre>
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* <b>lengthFieldOffset</b> = <b>0</b>
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* <b>lengthFieldLength</b> = <b>2</b>
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* <b>lengthAdjustment</b> = <b>0</b> (default)
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* <b>initialBytesToStrip</b> = <b>0</b> (default)
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* lengthAdjustment = 0
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* initialBytesToStrip = 0 (= do not strip header)
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*
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* BEFORE DECODE (14 bytes) AFTER DECODE (14 bytes)
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* +--------+----------------+ +--------+----------------+
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@ -45,11 +51,16 @@ import org.jboss.netty.channel.ChannelHandlerContext;
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* </pre>
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*
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* <h3>2 bytes length field at offset 0, strip header</h3>
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*
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* Because we can get the length of the content by calling
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* {@link ChannelBuffer#readableBytes()}, you might want to strip the length
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* field by specifying <tt>initialBytesToStrip</tt>. In this example, we
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* specified <tt>2</tt>, that is same with the length of the length field.
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* <pre>
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* <b>lengthFieldOffset</b> = <b>0</b>
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* <b>lengthFieldLength</b> = <b>2</b>
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* <b>lengthAdjustment</b> = <b>0</b>
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* <b>initialBytesToStrip</b> = <b>2</b>
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* lengthFieldOffset = 0
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* lengthFieldLength = 2
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* lengthAdjustment = 0
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* <b>initialBytesToStrip</b> = <b>2</b> (= the length of the Length field)
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*
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* BEFORE DECODE (14 bytes) AFTER DECODE (12 bytes)
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* +--------+----------------+ +----------------+
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@ -58,25 +69,80 @@ import org.jboss.netty.channel.ChannelHandlerContext;
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* +--------+----------------+ +----------------+
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* </pre>
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*
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* <h3>3 bytes length field at the end of 5 bytes header, strip header</h3>
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* <pre>
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* <b>lengthFieldOffset</b> = <b>2</b> (= 5 - 3)
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* <b>lengthFieldLength</b> = <b>3</b>
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* <b>lengthAdjustment</b> = <b>0</b>
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* <b>initialBytesToStrip</b> = <b>5</b>
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* <h3>2 bytes length field at offset 0, do not strip header, the length field
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* represents the length of the whole message</h3>
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*
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* BEFORE DECODE (17 bytes) AFTER DECODE (12 bytes)
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* +----------+----------+----------------+ +----------------+
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* | Header 1 | Length | Actual Content |----->| Actual Content |
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* | 0xCAFE | 0x00000C | "HELLO, WORLD" | | "HELLO, WORLD" |
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* +----------+----------+----------------+ +----------------+
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* In most cases, the length field represents the length of the message body
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* only, as shown in the previous examples. However, in some protocols, the
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* length field represents the length of the whole message, including the
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* message header. In such a case, we specify a non-zero
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* <tt>lengthAdjustment</tt>. Because the length value in this example message
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* is always greater than the body length by <tt>2</tt>, we specify <tt>-2</tt>
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* as <tt>lengthAdjustment</tt> for compensation.
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* <pre>
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* lengthFieldOffset = 0
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* lengthFieldLength = 2
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* <b>lengthAdjustment</b> = <b>-2</b> (= the length of the Length field)
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* initialBytesToStrip = 0
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*
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* BEFORE DECODE (14 bytes) AFTER DECODE (14 bytes)
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* +--------+----------------+ +--------+----------------+
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* | Length | Actual Content |----->| Length | Actual Content |
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* | 0x000E | "HELLO, WORLD" | | 0x000E | "HELLO, WORLD" |
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* +--------+----------------+ +--------+----------------+
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* </pre>
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*
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* <h3>3 bytes length field at the end of 5 bytes header, do not strip header</h3>
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*
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* The following message is a simple variation of the first example. An extra
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* header value is prepended to the message. <tt>lengthAdjustment</tt> is zero
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* again because the decoder always counts the length of the prepended data into
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* frame length calculation.
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* <pre>
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* <b>lengthFieldOffset</b> = <b>2</b> (= the length of Header 1)
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* <b>lengthFieldLength</b> = <b>3</b>
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* lengthAdjustment = 0
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* initialBytesToStrip = 0
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*
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* BEFORE DECODE (17 bytes) AFTER DECODE (17 bytes)
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* +----------+----------+----------------+ +----------+----------+----------------+
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* | Header 1 | Length | Actual Content |----->| Header 1 | Length | Actual Content |
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* | 0xCAFE | 0x00000C | "HELLO, WORLD" | | 0xCAFE | 0x00000C | "HELLO, WORLD" |
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* +----------+----------+----------------+ +----------+----------+----------------+
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* </pre>
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*
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* <h3>3 bytes length field at the beginning of 5 bytes header, do not strip header</h3>
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*
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* This is an advanced example that shows a case where there is an extra header
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* between the length field and the message body. You have to specify a
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* positive <tt>lengthAdjustment</tt> so that the decoder counts the extra
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* header into the frame length calculation.
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* <pre>
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* lengthFieldOffset = 0
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* lengthFieldLength = 3
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* <b>lengthAdjustment</b> = <b>2</b> (= the length of Header 1)
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* initialBytesToStrip = 0
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*
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* BEFORE DECODE (17 bytes) AFTER DECODE (17 bytes)
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* +----------+----------+----------------+ +----------+----------+----------------+
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* | Length | Header 1 | Actual Content |----->| Length | Header 1 | Actual Content |
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* | 0x00000C | 0xCAFE | "HELLO, WORLD" | | 0x00000C | 0xCAFE | "HELLO, WORLD" |
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* +----------+----------+----------------+ +----------+----------+----------------+
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* </pre>
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*
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* <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
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* strip the first header field and the length field</h3>
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*
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* This is a combination of all the examples above. There are the prepended
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* header before the length field and the extra header after the length field.
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* The prepended header affects the <tt>lengthFieldOffset</tt> and the extra
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* header affects the <tt>lengthAdjustment</tt>. We also specified a non-zero
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* <tt>initialBytesToStrip</tt> to strip the length field and the prepended
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* header from the frame. If you don't want to strip the prepended header, you
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* could specify <tt>0</tt> for <tt>initialBytesToSkip</tt>.
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* <pre>
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* <b>lengthFieldOffset</b> = <b>1</b>
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* <b>lengthFieldLength</b> = <b>2</b>
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* lengthFieldOffset</b> = 1 (= the length of HDR1)
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* lengthFieldLength</b> = 2
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* <b>lengthAdjustment</b> = <b>1</b> (= the length of HDR2)
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* <b>initialBytesToStrip</b> = <b>3</b> (= the length of HDR1 + LEN)
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*
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@ -89,10 +155,17 @@ import org.jboss.netty.channel.ChannelHandlerContext;
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*
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* <h3>2 bytes length field at offset 1 in the middle of 4 bytes header,
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* strip the first header field and the length field, the length field
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* includes the header length</h3>
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* represents the length of the whole message</h3>
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*
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* Let's give another twist to the previous example. The only difference from
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* the previous example is that the length field represents the length of the
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* whole message instead of the message body, just like the third example.
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* We have to count the length of HDR1 and Length into <tt>lengthAdjustment</tt>.
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* Please note that we don't need to take the length of HDR2 into account
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* because the length field already includes the whole header length.
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* <pre>
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* <b>lengthFieldOffset</b> = <b> 1</b>
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* <b>lengthFieldLength</b> = <b> 2</b>
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* lengthFieldOffset = 1
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* lengthFieldLength = 2
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* <b>lengthAdjustment</b> = <b>-3</b> (= the length of HDR1 + LEN, negative)
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* <b>initialBytesToStrip</b> = <b> 3</b>
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*
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