How to read blobs?

[epdv]

Hello!
I want to read blobs in a loop (more exact: the length of the blobs). 
I'm using a connection with autocommit(true) and a prepared statement 
with sth. like "select mydata, myblob where mykey=?".

Then I'm using the following loop:

    while (rs.next()) {
        int m = wantedColumnNames.length;
        Blob blob = (Blob)rs.getObject(x);
        if (blob == null) {
            result = 0L;
        } else {
            result = blob.length();
            blob.free();
        }
        ...
    }

I'm always getting an error message saying that the blob cannot be 
released after closing, probably the transaction had already been 
committed. As connection is in autocommit mode, I assume it has already 
been closed after statement execution, but it I try non-autocommit mode, 
the resultset is not open after execute (at least derby says that). How 
can I get the expected result?

BTW, using derby 10.4.2.0 in client/server mode.

Kind regards

Peter