Can someone point me to a particularly good resource on JPA/Hibernate lazy/eager fetching?

Question

Looking for a really good article that includes strategies/bugs/workarounds. I would prefer a pure JPA solution but I know Hibernate offers a lot of extensions.

Answer 1

I'm not sure of what you're looking for exactly. But to put is simply, LAZY simply means that a child association won't be loaded while loading the parent, it will be loaded when explicitly asked by the application. EAGER means that a child association will be loaded while loading the parent.

In general, LAZY is better performance wise (when you just don't need or want to load a whole objects graph). But depending on the situation, it might just be suboptimal or lead to the famous "N+1 SELECT" problem (while iterating over a list of N entities, accessing a LAZY association will fire another SELECT, resulting in 1+N SELECT at the end). Depending on the situation, an EAGER association - or a FETCH JOIN to prefetch the association - is preferable.

So I don't know what part is unclear but here are some resources that might help:

• Hibernate wiki
  • A Short Primer On Fetching Strategies (from the pre JPA era but the concepts still apply)
  • Some explanations on lazy loading (one-to-one)
• Articles
  • Hibernate: Understanding Lazy Fetching
  • Hibernate: Tuning Lazy Fetching

See also:

• Hibernate Core documentation
  • 14.3. Associations and joins
  • 19.1. Fetching strategies
• JPA 1.0 specification
  • Section 4.4.5.3 "Fetch Joins"

Answer 2

Book "Java Persistence with Hibernate" by Bauer and King, Chapter 13 "Optimizing fetching and caching". You should be able to find a copy of the book online.