Question

I was reading this book. Explaing about “@OneToOne unidirectional”, the author has taken the following Customer, Address example:

@Entity
public class Customer{
   @Id @GeneratedValue
   private Long id;
   private String name;
   private Address address;
   //few other columns, getters/setters
}

@Entity
public class Address{
   @Id @GeneratedValue
   private Long id;
   private String city;
   //few other columns, getters/setters

}

And was saying that –

• This is the minimum required
  annotaions.
• No @OneToOne annotaion is
  needed.(Because, by default, the
  persistance provider will assume it)
• The @JoinColumn annotation allows you
  to customize the mapping of a foreign
  key. As show below, we can RENAME the
  foreign key column to ADD_FK

And then about this one:

@Entity
public class Customer {    
    @Id    @GeneratedValue    
    private Long id;      
    private String name;    
    @OneToOne       
    @JoinColumn(name="ADD_FK")    
    private Address address;    
    //few other    columns, getters/setters 
}

@Entity
public class Order {   
    ....       
    List<OrderLine> orderLines;   
    ... 
}

• By default, OneToMany relationship is
  assumend when the collection of an
  entity type is being used.

My questions:

1. Are the above statements Valid? Because when I try these examples on Hibernate, I was getting exceptions.

2. Does the statements are made as per the JPA
   standards?

3. Or is it that Hibernate is
   implemented differently?

Kindly clarify.

Answer 1

To my knowledge, relations between entities must be explicitly mapped. From the JPA 1.0 specification (bold is mine):

2.1.7 Entity Relationships

Relationships among entities may be
one-to-one, one-to-many, many-to-one,
or many-to-many. Relationships are
polymorphic.

If there is an association between two
entities, one of the following
relationship modeling annotations
must be applied to the corresponding persistent property or
field of the referencing entity:
OneToOne, OneToMany, ManyToOne,
ManyToMany. For associations that do
not specify the target type (e.g.,
where Java generic types are not used
for collections), it is necessary to
specify the entity that is the target
of the relationship.

(…)

And this didn’t change in JPA 2.0.

I thus annotate relationships between entities. And AFAIK, Hibernate will indeed complain about not being able to persist a complex type when not doing so.

But unless someone can show me the relevant part of the spec, I consider the behavior as correct.

References

• JPA 1.0 specification
  • Section 2.1.7 “Entity Relationships”
• JPA 2.0 specification
  • Section 2.9 “Entity Relationships”