Java Database Connectivity (JDBC)

Hy guys,
I 've a problem with my java application.
I use hibernate to interact with a derby database.
I stored a image into blob field (and no problem).
When I try to get blob to array of byte I've this exception:

java.sql.SQLException: You cannot invoke other java.sql.Clob/java.sql.Blob methods after calling the free() method or after the Blob/Clob's transaction has been committed or rolled back.

To get blob I did

Blob cThumnb = ((Allegato) cAllegati.get(i)).getThumb();
byte[] cPrev = toByteArray(cThumnb);

where

private byte[] toByteArray(Blob fromBlob) {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        try {
            return toByteArrayImpl(fromBlob, baos);
        }
        catch (SQLException e)
        {
            throw new RuntimeException(e);
        }
        catch (IOException e)
        {
            throw new RuntimeException(e);
        }
        finally
        {
            if (baos != null)
            {
                try
                {
                    baos.close();
                }
                catch (IOException ex)
                {
                }
            }
        }
    }

    private byte[] toByteArrayImpl(Blob fromBlob, ByteArrayOutputStream baos)  throws SQLException, IOException
    {
        byte[] buf = new byte[4000];
        
        InputStream is = fromBlob.getBinaryStream();
        try
        {
            for (;;)
            {
                int dataSize = is.read(buf);
                if (dataSize == -1)     break;
                baos.write(buf, 0, dataSize);
            }
        }
        catch(IOException ex)
        {
                throw ex;
        }
        
       finally
        {
            if (is != null)
            {
                try
                {
                    is.close();
                }
                catch (IOException ex)
                {
                }
            }
        }
                return  buf;// baos.toByteArray();
    }

Could you help me?
I set also autocommit to false,
Thanks,
Regards