Hi

Our application has a table that store 4 big blob fields (about 12MB each of them). We need to load mutiple records from this tables and display links to the user to download blob objects as binary files.

Our problem is it seems that Hibernate loads in the actual contents of these blob into memory when loading the objects. It means if I load 10 objects from the table, it would take 480MB memory. I think the best way is to delay the loading of these blob fields when loading the object.

I read through the forum, and some suggested to use some helper JDBC code which actually does the reading/writting. Could any one show me how to do it with Hibernate?

Many thanks in advance.

No, Hibernate does not do this, as long as you map it as type "blob".

Thanks for your reply so quickly.

Do you have suggestions on where could I find those helper JDBC codes which do the lazy loading?

If these blob fields should not be mapped as "blob" in Hibernate, which type should I map them?

You should map them as type blob.



ummm. in your JDBC driver.

Do you mean I should map only non-blob fields? For example, use Hibernate to load these non-blob fields, and use JDBC connection to load the blob later?

No.

You should go and read the javadoc for java.sql.Blob.

I finally found a walk around to solve the lazy loading of Blob without requiring the database server and JDBC driver to support Blob locators.

Although Hibernate does not allow lazy loading a column, lazy loading a row is supported. Therefore, I separate the Blob columns into a new table and the original table only stores the key to the new table. I set up the many-to-one mapping from the original table to the new table, and make the new table lazy loaded in Hibernate configuration.

Now I can display hundreds of links in the same web page, each link points to about 12 MB, and keep the momery comsumption low.

Hope this could help some Hibernate users.

Listen, a Blob, if implemented correctly by the JDBC diver is by nature lazy. Please read the Javadoc!

I understand this. However, MySQL 4.0.17 and ConnectorJ 3.0.10 does not support Blob locators yet. It means when I load a Blob, ConnectorJ returns the contents as well, rather than just a locator. That's why I encountered memory problems previously.

The walk around I mentioned is actually not lazy loading a blob, it is lazy loading a row with blob field. It might sound strange, but it solves my problem.

Cloudman did you move that blob into a different java object? or how exactly did you accomplish lazy loading?
thanks,
David

You do need to create a class for the Blob data only. It looks like the following:



In your data record that previously uses Blob fields, just replace the Blob with the above class. You can use a many-to-one mapping from the host data object to the above binary data object, and specify lazy loading in the host data object.

Hope this helps.

Perfect thanks =)