Spring batch jpaPagingItemReader why some rows are not read?

Question

I \'m using Spring Batch(3.0.1.RELEASE) / JPA and an HSQLBD server database. I need to browse an entire table (using paging) and update items (one by one). So I used a jpaP

Answer 1

A couple things to note:

1. All entities that are returned from the JpaPagingItemReader are detached. We accomplish this in one of two ways. We either create a transaction before querying for the page, then commit the transaction (which detaches all entities associated with the EntityManager for that transaction) or we explicitly call entityManager.detach. We do this so that features like retry and skip can be correctly performed.
2. While you didn't post all the code in your processor, my hunch is that in the //do some stuff section, your item is getting re-attached which is why the update is occurring. However, without being able to see that code, I can't be sure.
3. In either case, using an explicit ItemWriter should be done. In fact, I consider it a bug that we don't require an ItemWriter when using java config (we do for XML).
4. For your specific issue of missing records, you need to keep in mind that a cursor isn't used by any of the *PagingItemReaders. They all execute independent queries for each page of data. So if you update the underlying data in between each page, it can have an impact on the items returned in future pages. For example, if my paging query specifies where val1 > 4 and I have a record that val1 was 1 to be 5, in chunk 2, that item may be returned since it now meets the criteria. If you need to update values that are in your where clause (thereby impacting what falls into the set of data you'd be processing), it's best to add a processed flag of some kind that you can query by instead.