The JPA hashCode() / equals() dilemma

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There have been some discussions here about JPA entities and which hashCode()/equals() implementation should be used for JPA entity classes. Most (if not all) of them depend on Hibernate, but I’d like to discuss them JPA-implementation-neutrally (I am using EclipseLink, by the way).

All possible implementations are having their own advantages and disadvantages regarding:

  • hashCode()/equals() contract conformity (immutability) for List/Set operations
  • Whether identical objects (e.g. from different sessions, dynamic proxies from lazily-loaded data structures) can be detected
  • Whether entities behave correctly in detached (or non-persisted) state

As far I can see, there are three options:

  1. Do not override them; rely on Object.equals() and Object.hashCode()
    • hashCode()/equals() work
    • cannot identify identical objects, problems with dynamic proxies
    • no problems with detached entities
  2. Override them, based on the primary key
    • hashCode()/equals() are broken
    • correct identity (for all managed entities)
    • problems with detached entities
  3. Override them, based on the Business-Id (non-primary key fields; what about foreign keys?)
    • hashCode()/equals() are broken
    • correct identity (for all managed entities)
    • no problems with detached entities

My questions are:

  1. Did I miss an option and/or pro/con point?
  2. What option did you choose and why?

UPDATE 1:

By “hashCode()/equals() are broken”, I mean that successive hashCode() invocations may return differing values, which is (when correctly implemented) not broken in the sense of the Object API documentation, but which causes problems when trying to retrieve a changed entity from a Map, Set or other hash-based Collection. Consequently, JPA implementations (at least EclipseLink) will not work correctly in some cases.

UPDATE 2:

Thank you for your answers — most of them have remarkable quality.
Unfortunately, I am still unsure which approach will be the best for a real-life application, or how to determine the best approach for my application. So, I’ll keep the question open and hope for some more discussions and/or opinions.

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