Lazy loading with Rich datatable and datascroller

[bennybottema]

Hey guys,

Recently I was looking for a solution to the common lazy loading problem in datatables and datascrollers. I found that in JSF land, there aren't many solutions, no clear cut solution, and no elegant alternatives... or so my investigation indicated.

I've encountered some attempts of integrating JSF specific solutions (extended datamodel, extended datatable etc.) as well as the infamous OpenSessionInView pattern where you keep a Hibernate-like session open in the view, lazily fetching data as required. These (vendor-specific) solutions seemed either too complex or too fragile.

I thought I'd share my solution as well.

My rationale: circumvent everything by simply implementing a transparent paging mechanism into a generic Java List. Similar to Apache's lazy list, except smarter... yet simpler, just take a look at the code yourselves.

I've written a blogpost about it with a example implementation for JSF views and JPA dao's. But it might as well work with JMS queue or other type of data accessors.

http://blog.projectnibble.org/2009/07/22/java-generic-paged-lazy-list-with-jsfjpa-example-implementation/

Just let me know what you think.

[nbelaevski]

Hi Benny,

First let me thank you for the interesting article and blog!

Here are some comments on it:

1. It's not clear for me how can lazy-loaded list solve the LazyInitializationException problem - objects in list can contain lazy-loaded items also.

2. Here is the data model that we've created for livedemo - employs Seam and allows to do filtering/sorting just in DB. It's very easy to use: http://fisheye.jboss.org/browse/RichFaces/branches/community/3.3.X/samples/richfaces-demo/src/main/java/org/richfaces/demo/modifiableModel/HibernateDataModel.java?r=14764
http://fisheye.jboss.org/browse/RichFaces/branches/community/3.3.X/samples/richfaces-demo/src/main/java/org/richfaces/demo/modifiableModel/BaseModifiableHibernateDataModel.java?r=14764

[jbalunas]

Hi Benny,

Thank you for sharing your blog and approach to this problem. I was looking at your code and you seem to be effectively skipping all lazy loading completely. You are requesting new result sets for every new page, and making another query for the data set size.

Although this may work for your situation you'll find that you are not solving the LazyInitializationException (LIE) issue, you are avoiding it by not relying on your ORM for managing your data.

This will eventually - depending on the size and scope of your application - cause the various management and performance issues that is the reason ORM was made to manage this data loading in the first place.

I would recommend as Nick has to review what Seam can do for you to avoid LIE, and get the benefits of the ORM system at the same time.