Unique constraint with JPA and Bean Validation

Question

I'd like to have a @Unique constraint with Bean Validation, but that is not provided by the standard. If I would use JPA's @UniqueConstraint I wouldn't have a unique validation and error reporting mechanism.

Is there a way to define @Unique as a Bean Validation constraint and combine it with JPA, such that JPA creates a column with an unique constraint and checks wheter a value is unique or not?

Answer 1

Unless you acquire a lock on a whole table, it is basically not possible to check for unicity using a SQL query (any concurrent transaction could modify data after a manual check but before the commit of the ongoing transaction). In other words, it isn't possible to implement a valid unique verification at the Java level and thus to provide a validation implementation. The only reliable way to check for unicity is while committing the transaction.

The BV spec summarizes it like this:

Appendix D. Java Persistence 2.0 integration

Question: should we add @Unique that would map to @Column(unique=true)?

@Unique cannot be tested at the Java level reliably but could generate a database unique constraint generation. @Unique is not part of the BV spec today.

So while I agree that it would be nice to have unique (and non null) constraint violations wrapped in a Bean Validation exception, this is currently not the case.

References

• Bean Validation specification (JSR 303)
  • Appendix D. Java Persistence 2.0 integration
• Question about validation and persistence constraints