What is the different between @ManyToOne optional flag vs @JoinColumn nullable flag?

Question

I have the following to define a Company which cannot be null on my entity:

@ManyToOne(optional = false)
@JoinColumn(nullable = false)
private Company company;

What is the difference between 'optional = false' on the @ManyToOne annotation and 'nullable = false' on the @JoinColumn annotation. Or is setting both of them redundant?

Answer 1

@ManyToOne operates on the so called logical model, i.e. the object-oriented side of the object-relational mapping. The semantics of optional=false here are:

Whether the association is optional. If set to false then a non-null relationship must always exist.

So the JPA engine expects that the underlying storage will always provide a value that can be translated to a Company object.

@JoinColumn operates on the physical model, i.e. how things are actually laid down in the datastore (database). Specifying nullable = false will make the DB column non-nullable.

If @JoinColumn(nullable = false) was omitted, the column would be nullable. One could insert a null value there and the DB would happily accept it. However if someone tried to read that value through JPA, the JPA engine would protest because it expects a value that can be translated to a Company object to always be there, as specified by @ManyToOne(optional = false).