Hibernate : How to build a primary key with annotations ?

You can use the @GeneratedValue annotation.

The agreement of the strategy type that you choose this annotation will generate automatically a unique values for this column.

Example :

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

Possible Types :

public enum GenerationType {
    TABLE,
    SEQUENCE,
    IDENTITY,
    AUTO;

    private GenerationType() {
    }
}

More Info ( Documentation ) : 
https://docs.oracle.com/javaee/5/api/javax/persistence/GenerationType.html