What is the exact meaning of the JPA @Entity annotation?

Question

I am studying JPA in Spring application and I have some doubts related to the @Entity annotation.

So I have a model class like this:

@Entity
@Table(name= “T_CUSTOMER”)
public class Customer {

    @Id
    @Column(name=“cust_id”)
    private Long id;

    @Column(name=“first_name”)
    private String firstName;

    @Transient
    private User currentUser;

    ...........................
    ...........................
    ...........................
}

Ok, I know that the @Entity annotation is on the class level and it means that the fields of the object that are instances of this class are to be mapped to the field of the T_CUSTOMER database table.

But why in JPA it is mandatory to use @Entity annotation and I cannot only use the @Table annotation to map a model object to a specific database table? It have some other meaning\behavior that actually I am missing?

What am I missing? What is the exact meaning of the @Entity annotation?

Answer 1

@Entity annotation defines that a class can be mapped to a table. And that is it, it is just a marker, like for example Serializable interface.

And why @Entity annotation is mandatory? ... well, it is the way how JPA is designed. When you create a new entity you have to do at least two things

1. annotated it with @Entity

2. create an id field and annotate it with @Id

Anything else is optional, for example table name is derived from entity class name (and therefore @Table annotation can be optional), table's columns are derived from entities variables (and therefore @Column annotation can be optional), and so on ...

JPA is trying to provide a fast and easy start to developers who want to learn/use this API, and giving developers option to configure as few things as possible to make something functional is one of the ways how this API wants to achieve this "easy to use/learn" goal. Hence the @Entity annotation (together with @Id annotation) is the minimum you have to do in order to create an entity.