如果未用于DDL生成，则Hibernate @JoinColumn（nullable = false）是否冗余？

Question

Some of our entities declare OneToMany / OneToOne Relationships with nullable = false

@OneToMany
@JoinColumn(name = "FK_ID", referencedColumnName = "ID", nullable = false)
private List<Things> manyThings;

We use Flyway for DDLs. I have read in many sources that the JoinColumn property nullable is only used for DDL generation - so do I understand correctly that when our Flyway Scripts generate Tables with Constraints on the DB directly, the nullable = false property on our Entities is redundant? What exactly is meant by DDL-Generation and what does Hibernate offer in this regard?

I came across this problem because on some tables it turned out that the constraint on the DB Columns were Non-Null = true whereas on the Java Entities the property nullable = false was set. This didn't seem to have any effect on the application at runtime; it would happily insert null values. However, when running a SpringBootTest it would fail with a constraint violation.

Answer 1

It's not only for DDL generation, but also for Hibernate validation annotations.

When you say nullable = false , JPA generates database constraints (not-null) and ALSO generate a JSR 303 Bean Validation that don't let you persist the entity if the nullable = false column is null in Runtime. So, when you write nullable = false , JPA generates those two features for the price of one. @Column(nullable = false) is the JPA way of declaring a column to be not-null.

With nullable = false, the following save shall fail:

ObjectThatContainsListThings a = new ObjectThatContainsListThings();
a.setListOfThings(null)

hibernate.persist(a);
DataIntegrityViolationException: not-null property references a null or transient value   

JPA will throw this exception instead of letting the database find the problem.