@OneToOne(optional=false) and @JoinColumn(nullable=false) used together

Question

I've bumped into this example in JPA 2.0 FR Specification, 11.1.37. OneToOne Annotation, page 403:

@OneToOne(optional=false)
@JoinColumn(name="CUSTREC_ID", unique=true, nullable=false, updatable=false)
public CustomerRecord getCustomerRecord() { return customerRecord; }

Is there any reason that I should put @OneToOne(optional=false) and at that same time put @JoinColumn(... nullable=false)?

Aren't these two declarations the same? Isn't one of them redundant?
Are both of them used in DDL schema generation?

Answer 1

Formally optional=false is a runtime instruction to the JPA implementation, and nullable=false is an instruction to the DDL generator. So they are not strictly redundant.

The difference can become significant when there is entity inheritance involved. If a particular mapping exists only on a subclass, and you have single table table per-hierarchy strategy, then the OneToOne mapping may be optional=false on the particular subclass that contains the mapping. However, the actual join column cannot be made not-null, since then other sub classes that share the table can't be inserted!

In practice different versions of different providers may or may not interpret either one at either time, caveat emptor.