Why should I specify @Column( nullable = false )?
I have an entity annotated with @Entity.
If I am responsible for creating the CREATE TABLE scripts why should I specify @Column( nullable = false ) when I can create a column in the database with the NOT NULL keywords? Is there any example that shows the benefits of using this property in a field?
Answer
Better error messages and error handling, especially if you also add the JSR303 @NotNull annotation.
If you create the column as NOT NULL but don't tell JPA it's not null, JPA will assume that null values are OK. When you try to save an object with nulls, it'll proceed to send that to the DB and you'll get a DB level error. This increases log spam in the database and it's much harder to determine from the error which column(s) of which table(s) were the problem, let alone map them back to their JPA names.
If you annotate them as not null, JPA will throw an exception before saving, avoiding DB log spam and usually giving you a better error. In particular, if your JPA provider supports JSR303 and either translates nullable=false into @NotNull internally or you've added @NotNull too, it'll give you a data structure you can examine to see exactly what fields of what objects were rejected for what reasons, along with customisable templated error messages.
That's why you should tell JPA about NOT NULL fields. That, and it's easier for others working on your code to understand without also having to read the DB schema.