JPA Entity class without "#Column" annotation - spring

Question

I have this piece of code where the author creates a JPA Entity. Somehow, it's successfully working and that seems weird to me since not every field is annotated with "#Column". Based on that, here's my question:
How it is possible for this class to work properly (all data is being successfully recorded in the database) without every field not having a "Column" annotation (excepting "id")?
/**
* #author Ram Alapure
* #since 05-04-2017
*/
#Entity
#Table(name="User")
public class User {
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
#Column(name = "id", updatable = false, nullable = false)
private long id;
private String firstName;
private String lastName;
private LocalDate dob;
private String gender;
private String role;
private String email;
private String password;
#Override
public String toString() {
return "User [id=" + id + ", firstName=" + firstName + ", lastName=" + lastName + ", dob=" + dob + ", email="
+ email + "]";
}
}
For instance, here's another class which is working properly as well. But this time, with a "#Column" annotation in every field. I thought it was a pre-requisite to JPA create a column from the field.
#Entity
#Table(name = "address")
public class Address {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
#Column(name = "address_id")
private long id;
#Column(name = "rua")
private String street;
#Column(name = "number")
private int number;
#Column(name = "complement")
private String complement;
#Column(name = "suburb")
private String suburb;
#Column(name = "city")
private String city;
#Column(name = "state")
private String state;
#Column(name = "country")
private String country;
#Column(name = "cep")
Getters and Setters were hidden from all pieces of code since they just look like regular getters and setters, not being necessary to be shown.

Answer 1

If you don't specify #Column annotation (Optional) then Hibernate uses default naming stretegy by using camel case.
firstName field becomes first_name column in Database.
You can also define your own naming stretegy according to your needs.
From the documentation.
strategy ; Hibernate 5 defines a Physical and Implicit naming strategies. Spring Boot configures SpringPhysicalNamingStrategy by default. This implementation provides the same table structure as Hibernate 4: all dots are replaced by underscores and camel cases are replaced by underscores as well.

Answer 2

this annotation means create table in database #Entity with all field in class same the name
the spring boot scan this annotation #Entity after scan
is inject and create Bean in IOC with fields
but if used #Column this help for mapping with Table field if change colume name in dataBase
and spring boot when scan component and see this annotation the bean is change with Colume name to name in database becouse we need success mapping with data base