Entity and common annotations

Entities in JPA are nothing but POJOs representing data that can be persisted to the database. An entity represents a table stored in a database. Every instance of an entity represents a row in the table.

The @Entity Annotation

Let's say we have a POJO called Student, which represents the data of a student, and we would like to store it in the database:

public class Student {
    
    // fields, getters and setters
    
}

So let's define it by making use of the @Entity annotation. We must specify this annotation at the class level. We must also ensure that the entity has a no-arg constructor and a primary key:

@Entity
public class Student {
    
    // fields, getters and setters
    
}

The @Id Annotation

Each JPA entity must have a primary key that uniquely identifies it. The @Id annotation defines the primary key. We can generate the identifiers in different ways, which are specified by the @GeneratedValue annotation.

We can choose from four id generation strategies with the strategy element. The value can be AUTO, TABLE, SEQUENCE, or IDENTITY:

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;
    
    private String name;
    
    // getters and setters
}