Primary keys

A primary key field is mapped by using the @Id annotation as in the following example:

@Entity 
public class Movie { 
    @Id 
    private long id; 
} 

You can also use the primary key field on a getter method in the following example:

@Entity 
public class Movie { 
     
private long id; 
 
    @Id 
public long getId() { 
        return id; 
    } 
 
} 

The difference between the two approaches is that if you used the @Id annotation on a class field directly, the persistence provider will read all other mapping annotations from class fields. Otherwise, it will read all other mappings from the getter methods. You cannot mix annotations on both fields and getters, and the @Id annotation is a descriminator here. The choice between either approaches is the developer's preference. I personally prefer using field mapping in order to make all mapping information available at the beginning of the class.