问题
I have identified a random series of bytes inserted into any blob field when the database is manipulated via Adobe AIR. (from my results it appear to always start with bytes[12, ...] but I'm not sure of that)
I think it's a sizeinfo of the bytes, let me explain how I came to this conclusion.
First my context : I manipulate sqlite databases through Adobe AIR (client-side) and System.data.sqlite (C# server-side)
With System.data.sqlite if I read a Sqlite db filled with BLOB by Adobe AIR I have to get ride of those bytes appended in the beginning by AIR and then I have the binary data all well shaped. PERFECT!
With Adobe AIR if I tried to read a sqlite db filled with BLOB by System.data.Sqlite the data are corrupted I get an error! Obviously because I don't have the missing bytes researched by AIR.
Of course I tried to add those bytes by just copying a series of 3 bytes that I removed in my first case but then it returned the data partially and in the cases of images the last rows of pixels go all gray and in some images I have more or less grays lines. Because the data were corresponding to a series of images of the same ~4k size and I added 3 bytes from one of them and I had this result.
And Air will also sometimes throw this error :
Error: Error #2030: End of file was encountered.
So obviously those bytes give an info on the size but I don't really get how it does!?!
I tried to add a bytearray with 4k length it tends to add 3 bytes but I tried to add 4M and it goes up to 5 bytes.
I found this Question How do you convert 3 bytes into a 24 bit number in C#? and I thought that could be how the size info is stored.
But I still don't get it...
回答1:
Thanks to FluorineFX (AMF for .NET) opensource project here is the answer.
Because in my Adobe AIR project I have to pass my air.ByteArray object as a parameter to store everything in the sqlite Blob field; AIR will serialize everything into AMF, a compact binary actionscript message format.
Page 11 Secion 3.14 ByteArray type
http://opensource.adobe.com/wiki/download/attachments/1114283/amf3_spec_05_05_08.pdf
The doc stipulate :
AMF 3 serializes this type using a variable length encoding 29-bit integer for the byte-length prefix followed by the raw bytes of the ByteArray.
But thats not all, I searched for an AMF .NET opensource project and founded FluorineFX. Looking into the code I identified that every AMF binaries is prefixed with a byte TypeCode which is 12 for a ByteArray:
/// <summary>
/// AMF ByteArray data type.
/// </summary>
public const byte ByteArray = 12;
Further search and again I found in the FluorineFX sources AMFReader.ReadAMF3ByteArray() and AMFWriter.WriteByteArray()
Which help me to quickly build what I need :
private static byte[] RemoveAMF3ByteArrayPrefixBytes(byte[] ar)
{
var ms = new MemoryStream(ar);
var br = new BinaryReader(ms);
// if first byte is AMF TypeCode for ByteArray
if (br.Read() != 12)
return ar;
int handle = ReadAMF3IntegerData(br);
bool inline = ((handle & 1) != 0);
handle = handle >> 1;
if (inline)
{
int length = handle;
byte[] buffer = br.ReadBytes(length);
return buffer;
}
return ar;
}
private static byte[] AddAMF3ByteArrayPrefixBytes(byte[] ar)
{
var ms = new MemoryStream();
var bw = new BinaryWriter(ms);
bw.Write((byte)12); // AMF TypeCode for ByteArray
var handle = (int)ar.Length;
handle = handle << 1;
handle = handle | 1;
WriteAMF3IntegerData(bw, handle);
bw.Write(ar);
return ms.ToArray();
}
/// <summary>
/// Handle decoding of the variable-length representation which gives seven bits of value per serialized byte by using the high-order bit
/// of each byte as a continuation flag.
/// </summary>
/// <returns></returns>
private static int ReadAMF3IntegerData(BinaryReader br)
{
int acc = br.ReadByte();
if(acc < 128)
return acc;
else
{
acc = (acc & 0x7f) << 7;
int tmp = br.ReadByte();
if(tmp < 128)
acc = acc | tmp;
else
{
acc = (acc | tmp & 0x7f) << 7;
tmp = br.ReadByte();
if(tmp < 128)
acc = acc | tmp;
else
{
acc = (acc | tmp & 0x7f) << 8;
tmp = br.ReadByte();
acc = acc | tmp;
}
}
}
//To sign extend a value from some number of bits to a greater number of bits just copy the sign bit into all the additional bits in the new format.
//convert/sign extend the 29bit two's complement number to 32 bit
int mask = 1 << 28; // mask
int r = -(acc & mask) | acc;
return r;
//The following variation is not portable, but on architectures that employ an
//arithmetic right-shift, maintaining the sign, it should be fast.
//s = 32 - 29;
//r = (x << s) >> s;
}
private static void WriteAMF3IntegerData(BinaryWriter bw, int value)
{
//Sign contraction - the high order bit of the resulting value must match every bit removed from the number
//Clear 3 bits
value &= 0x1fffffff;
if (value < 0x80)
bw.Write((byte)value);
else
if (value < 0x4000)
{
bw.Write((byte)(value >> 7 & 0x7f | 0x80));
bw.Write((byte)(value & 0x7f));
}
else
if (value < 0x200000)
{
bw.Write((byte)(value >> 14 & 0x7f | 0x80));
bw.Write((byte)(value >> 7 & 0x7f | 0x80));
bw.Write((byte)(value & 0x7f));
}
else
{
bw.Write((byte)(value >> 22 & 0x7f | 0x80));
bw.Write((byte)(value >> 15 & 0x7f | 0x80));
bw.Write((byte)(value >> 8 & 0x7f | 0x80));
bw.Write((byte)(value & 0xff));
}
}
I hope that will help someone else.
回答2:
Thanks a lot, that helped me solve the problem. Here is how to do it with Java code:
Add org.granite granite-core dependency to your project
Init GDS with boilerplate code
GraniteConfig graniteConfig = new GraniteConfig(null, null, null, null);
ServicesConfig servicesConfig = new ServicesConfig(null, null, false);
Map<String, Object> applicationMap = new HashMap<String, Object>();
SimpleGraniteContext.createThreadIntance(graniteConfig, servicesConfig, applicationMap);
In my example, i read an image file that i will insert into a BLOB:
fis = new FileInputStream(file);
ps = connection.prepareStatement(INSERT_PICTURE);
ps.setString(1, key);
byte[] fileBytes = FileUtils.readFileToByteArray(file);
ByteArrayOutputStream out = new ByteArrayOutputStream();
AMF3Serializer ser = new AMF3Serializer(out);
ser.writeObject(fileBytes);
ser.flush();
ps.setBytes(2, out.toByteArray());
Works like a charm, thanks for the hint :)
Fabien
来源：https://stackoverflow.com/questions/6388456/what-is-the-first-bytes-in-a-blob-column-sqlite-adobe-air-blob-sizeinfo