Netty LengthFieldBasedFrameDecoder

先看看LengthFieldBasedFrameDecoder的官方API 
http://docs.jboss.org/netty/3.1/api/org/jboss/netty/handler/codec/frame/LengthFieldBasedFrameDecoder.html 

API举例说明了LengthFieldBasedFrameDecoder的解析机制，如下： 

实际消息内容是“HELLO, WORLD”，长度是12 bytes（注意逗号后面有一个空格） 

实例1 
lengthFieldLength   = 2表示“Length”的长度，而“Length”的值 
就是“Actual Content”的长度（0x000C， 也就是12）： 

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

实例2 
initialBytesToStrip = 2 表示在decode时，要去掉多少个字节 
在这个例子，表示要去掉“Length”（2个字节） 
可以看到，AFTER DECODE后，“Length”没有了，只剩下“Actual Content”： 

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

实例3 
与实例1不同，这里“Length”的值不是“Actual Content”的长度，而是 
整个消息的长度（0x000E，14  = 2  + 12）。用lengthAdjustment=-2来表示 
“Actual Content”的长度要减2： 
wholeLength = valueOf(Length) = 14 
actualContentLength = wholeLength + lengthAdjustment = 14 + (-2)=12 

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

实例4 
这个例子多了一个“Header 1”（oxCAFE，长度为2 ） 
用 lengthFieldOffset   = 2表示“Length”从第3个字节开始 
“Length”的值仍然是“Actual Content ”的长度，不过“Length”自身的 
长度是3 （值是0x00000C，而不是上面的0x000C） 

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

实例5 
与实例4不同的地方是，“Length”和“Header 1”的位置调换了 
wholeLength = valueOf(Length) = 14 
actualContentLength = wholeLength + lengthAdjustment = 14 + 2=16 
因此在decode时，会认为“Header 1”也是“Actual Content”的一部分 

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

实例6 
看起来要比之前的例子复杂一些，但其实是上面例子的组合 
lengthFieldOffset = 1表示“Length”从第2个字节开始 
lengthFieldLength   = 2表示“Length”的长度是2 
lengthAdjustment    = 1表示“HDR2”的长度是1： 
wholeLength = valueOf(Length) = 0x000C = 12 
actualContentLength = wholeLength + lengthAdjustment = 12 + 1=13 
decode时，会认为“Length”后面的13个字节都是“Actual Content”， 
因此会认为“HDR2”也是“Actual Content”的一部分 
initialBytesToStrip = 3 表示decode时，去掉3个字节 

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

实例7 
与实例6不同的是，lengthAdjustment = -3，是负数 
因此， 
wholeLength = valueOf(Length) = 0x0010 = 16 
actualContentLength = wholeLength + lengthAdjustment = 16 + (-3)=13 
decode时，会认为“Length”后面的13个字节都是“Actual Content”， 
最终效果与实例6一样 

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

API看完，我们来看看源码（只保留关键代码）： 

public class LengthFieldBasedFrameDecoder extends FrameDecoder {

    private final int maxFrameLength;   //超出此长度的Frame将被丢弃
    private final int lengthFieldOffset;
    private final int lengthFieldLength;
    private final int lengthFieldEndOffset;     //这个值等于lengthFieldOffset + lengthFieldLength
    private final int lengthAdjustment;
    private final int initialBytesToStrip;

    protected Object decode(
            ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {

        //数据未完整，先不处理
        if (buffer.readableBytes() < lengthFieldEndOffset) {
            return null;
        }

        /*
        先读取“Length”的值
        在LengthFieldBasedFrameDecoder的构造函数中，限定了“Length”的长度：
        “lengthFieldLength must be either 1, 2, 3, 4, or 8”
        单位是bytes。这个限定不知从何而来，先不管
        由于接收到的数据的类型是ChannelBuffer，也就是byte[]，那么在读取时，
        就应该根据长度来分割数据
        例如，lengthFieldLength=3，说明读取前3个字节就得到“Length”的值
        读取时，用到了位操作，ByteOrder是BIG_ENDIAN，因此高位在前，要左移位：
        public int getUnsignedMedium(int index) {
            return  (array[index]     & 0xff) << 16 |
                    (array[index + 1] & 0xff) <<  8 |
                    (array[index + 2] & 0xff) <<  0;
        }
        注意到，这个方法不会改变readerIndex
        下面代码的frameLength，其实就是上面API分析时提到的valueOf(Length)
        */
        int actualLengthFieldOffset = buffer.readerIndex() + lengthFieldOffset;
        long frameLength;
        switch (lengthFieldLength) {
        case 1:
            frameLength = buffer.getUnsignedByte(actualLengthFieldOffset);
            break;
        case 2:
            frameLength = buffer.getUnsignedShort(actualLengthFieldOffset);
            break;
        case 3:
            frameLength = buffer.getUnsignedMedium(actualLengthFieldOffset);
            break;
        case 4:
            frameLength = buffer.getUnsignedInt(actualLengthFieldOffset);
            break;
        case 8:
            frameLength = buffer.getLong(actualLengthFieldOffset);
            break;
        default:
            throw new Error("should not reach here");
        }

        //如分析API时所说，要加上lengthAdjustment
        //那为什么 还要加上lengthFieldEndOffset？
        //加上之后，frameLength就代表整个frame的长度了，包括前缀和“Actual Content”
        frameLength += lengthAdjustment + lengthFieldEndOffset;

        int frameLengthInt = (int) frameLength;

        //数据未完整，先不处理
        if (buffer.readableBytes() < frameLengthInt) {
            return null;
        }

        //readerIndex往后移，跳过指定的字节，不读取
        buffer.skipBytes(initialBytesToStrip);

        // extract frame ，读取消息的内容
        int readerIndex = buffer.readerIndex();
        int actualFrameLength = frameLengthInt - initialBytesToStrip;
        ChannelBuffer frame = extractFrame(buffer, readerIndex, actualFrameLength);

        //extractFrame方法不改变buffer的readerIndex，因此要手动设置
        buffer.readerIndex(readerIndex + actualFrameLength);
        return frame;
    }

    //这个方法创建了新的ChannelBuffer，不影响原buffer
    protected ChannelBuffer extractFrame(ChannelBuffer buffer, int index, int length) {
        ChannelBuffer frame = buffer.factory().getBuffer(length);
        frame.writeBytes(buffer, index, length);
        return frame;
    }
}