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From Daniel Noll <dan...@nuix.com>
Subject How are we supposed to use Blob.getBinaryStream(long, long) ?
Date Mon, 16 Feb 2009 05:17:17 GMT
Hi all.

I'm baffled.  I'm trying to use Blob.getBinaryStream(long, long) to read 
a slice of data out of a blob, and I'm sure this is just another of 
Sun's conspiracies to make our life harder than it needs to be.

The Javadoc is as follows:

http://java.sun.com/javase/6/docs/api/java/sql/Blob.html#getBinaryStream()

     InputStream getBinaryStream(long pos, long length)
         throws SQLException

     Returns an InputStream object that contains a partial Blob value,
     starting with the byte specified by pos, which is length bytes in
     length.

     Parameters:

         pos - the offset to the first byte of the partial value to be
               retrieved. The first byte in the Blob is at position 1

         length - the length in bytes of the partial value to be
                  retrieved

     Returns:

         InputStream through which the partial Blob value can be read.

     Throws:

         SQLException - if pos is less than 1 or if pos is greater than
         the number of bytes in the Blob or if pos + length is greater
         than the number of bytes in the Blob

     (cutting the rest)

If everything in this Javadoc is true then it is in fact *impossible* to 
read the final byte of the blob.

Suppose you have a blob of length 20, and you want to read bytes 11 
through 20.  You pass 11 as the starting position (indexed from 1, so 
it's actually byte 10 of the original byte array) and length 10.  This 
then throws an SQLException, because 11 + 10 = 21, which is greater than 
20, so you can't do that.

And Derby is completely faithful to this seemingly ridiculous rule:

     java.sql.SQLException: Sum of position('11') and length('10') is
         greater than the size of the LOB.

So what are we supposed to do, use this method to read all chunks 
*except* the last one, and then use getBytes(long,long) to read the last 
chunk?

Daniel


P.S. you can pass length 9 here to make Derby happy.  Then you receive 
an InputStream where you can only read 9 bytes before the read loop goes 
into an infinite loop, due to Derby's read(byte[],int,int) returning 0 
at EOS instead of -1.  I assume that this last bit is a bug -- 
everything above this paragraph appears to be an error in the JDBC API 
itself, which Derby has blindingly followed, leading to a nearly useless 
method.



-- 
Daniel Noll                            Forensic and eDiscovery Software
Senior Developer                              The world's most advanced
Nuix                                                email data analysis
http://nuix.com/                                and eDiscovery software

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