Assigning @MapsId annotation in Hibernate

I'm trying to figure out what's the difference in @MapsId annotation in Hibernate. I have looked at the Hibernate documentation but I am still confused by the explanations given here as I am new to Hibernate.

document says the following:

@Entity
class Customer {
   @EmbeddedId CustomerId id;
   boolean preferredCustomer;

   @MapsId("userId")
   @JoinColumns({
      @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
      @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
   })
   @OneToOne User user;
}

@Embeddable
class CustomerId implements Serializable {
   UserId userId;
   String customerNumber;

   //implements equals and hashCode
}

@Entity 
class User {
   @EmbeddedId UserId id;
   Integer age;
}

@Embeddable
class UserId implements Serializable {
   String firstName;
   String lastName;

   //implements equals and hashCode
}

      

        
        
        
      

    

In the built-in id object, the association is represented as the identifier of the associated object. But you can tie your value to a regular association in essence through the @MapsId annotation. The @MapsId value matches the property name of the embedded identifier object that contains the corresponding object identifier. In a database, this means that the Customer.user and CustomerId.userId properties share the same base column (user_fk in this case).

I don't understand what this explanation says, so I got tired of hbm2ddl

being set like create

in my config file and I noticed the following:

With @MapsId and @JoinColumns on the Customer object, DDL statements are:

Hibernate: create table CUSTOMER (customerNumber varchar2(255 char) not null, preferredCustomer number(1,0) not null, userfirstname_fk varchar2(255 char) not null, userlastname_fk varchar2(255 char) not null, primary key (customerNumber, userfirstname_fk, userlastname_fk))
Hibernate: create table USER (firstName varchar2(255 char) not null, lastName varchar2(255 char) not null, age number(10,0), primary key (firstName, lastName))
Hibernate: alter table CUSTOMER add constraint UK_xxxx  unique (userfirstname_fk, userlastname_fk)
Hibernate: alter table CUSTOMER add constraint FK_xxxx foreign key (userfirstname_fk, userlastname_fk) references TBL_USER

      

        
        
        
      

    

If I remove the @MapsId and @JoinColumns annotations, I see the following:

Hibernate: create table CUSTOMER (customerNumber varchar2(255 char) not null, firstName varchar2(255 char), lastName varchar2(255 char), preferredCustomer number(1,0) not null, user_firstName varchar2(255 char), user_lastName varchar2(255 char), primary key (customerNumber, firstName, lastName))
Hibernate: create table USER (firstName varchar2(255 char) not null, lastName varchar2(255 char) not null, age number(10,0), primary key (firstName, lastName))
Hibernate: alter table CUSTOMER add constraint FK_xxxx foreign key (user_firstName, user_lastName) references TBL_USER

      

        
        
        
      

    

Please help me understand the concept of @MapsId annotation in Hibernate.