Tuesday, February 10, 2009

JPA 2.0: Why AccessType is Relevant

It has long been debated whether field access is superior to property access for JPA entities. Most arguments that I've seen hinge on performance versus encapsulation. While these arguments may be interesting from a theoretical perspective, control over field access is in fact very useful in solving real-world problems.

Field access allows code to distinguish between the JPA provider setting properties while loading an entity versus application code setting a property.

This was very useful for a project that I'm involved with that has de-normalized data or other fields that are computed and persisted. Here's how it works:
  • In the setter for various properties that affect the computation of a de-normalized property, the value of the de-normalized property is set to null. @AccessType(FIELD) is used for these properties.
  • In the getter for the de-normalized property, if the value is null the de-normalized value is recomputed.
  • In a @PrePersist method, the getter is called to ensure that the de-normalized value is properly persisted.
Using this technique maintaining de-normalized data becomes simple and robust.

While this feature has been available within Hibernate since 3.0, I'm glad to see that JPA 2.0 (JSR 317) introduces a standard annotation to control this behaviour.

3 comments:

Anonymous said...

I agree, however i think fat cats that run most companies will still make us use getters/setters.

Alex Cruise said...

This angle had never occurred to me, but I like it! I'll keep it in mind!

Paul said...

I have never understood the "encapsulation" argument. Implementing JPA in the first place is an exercise in breaking encapsulation. You have to know the data-members inside your class so you can map them to columns and use them in queries. Field access is just more of the same.

Also the semantics of changing a property are _not_ the same as restoring it, so the same setter method should not be used for both.