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.