What Is @Joincolumn and How It Is Used in Hibernate

What is the point of @JoinColumn in hibernate?

@JoinColumn indicates that this entity is the owner of the relationship, that the corresponding table has a column with a foreign key to the referenced table.

@JoinColumn annotation is not mandatory. If you will not specify it framework will perform default configuration instead. Hibernate and JPA follows convention over configuration. This means that the developer only needs to specify not standard aspects some custom definitions.

In case of @JoinColumn, the default column name will be generated like: [field_name]_[id_column_name].

In your case:

[field_name] - is person field of Passport entity.

[id_column_name] - is @Id column of related entity. It is id field of Person entity.

According to documentation:

2.2.5. Mapping entity associations/relationships

If no @JoinColumn is declared on the owner side, the defaults apply. A
join column(s) will be created in the owner table and its name will be
the concatenation of the name of the relationship in the owner side, _
(underscore), and the name of the primary key column(s) in the owned
side.

So if you need to override the default convention then that's exactly what @JoinColumn is for.

It covering:

• specify the name of the foreign key column
• specify the name of the column referenced by this foreign key column
• specify column definition
• specify or control the generation of a foreign key definition
• specify the name of the table that contains the column
• whether the property is a unique key
• whether the column is included in SQL UPDATE statements generated by the persistence provider

JPA JoinColumn vs mappedBy

@JoinColumn could be used on both sides of the relationship. The question was about using @JoinColumn on the @OneToMany side (rare case). And the point here is in physical information duplication (column name) along with not optimized SQL query that will produce some additional UPDATE statements.

According to documentation:

Since many to one are (almost) always the owner side of a bidirectional relationship in the JPA spec, the one to many association is annotated by @OneToMany(mappedBy=...)

@Entity
public class Troop {
    @OneToMany(mappedBy="troop")
    public Set<Soldier> getSoldiers() {
    ...
}

@Entity
public class Soldier {
    @ManyToOne
    @JoinColumn(name="troop_fk")
    public Troop getTroop() {
    ...
} 

Troop has a bidirectional one to many relationship with Soldier through the troop property. You don't have to (must not) define any physical mapping in the mappedBy side.

To map a bidirectional one to many, with the one-to-many side as the owning side, you have to remove the mappedBy element and set the many to one @JoinColumn as insertable and updatable to false. This solution is not optimized and will produce some additional UPDATE statements.

@Entity
public class Troop {
    @OneToMany
    @JoinColumn(name="troop_fk") //we need to duplicate the physical information
    public Set<Soldier> getSoldiers() {
    ...
}

@Entity
public class Soldier {
    @ManyToOne
    @JoinColumn(name="troop_fk", insertable=false, updatable=false)
    public Troop getTroop() {
    ...
}

Is @JoinColumn annotation mandatory in Hibernate?

It is not necessary, JPA follows convention over configuration principle which means there are allways some default values that you can override with annotations.

In case of @JoinColumn, the default column name is generated like this: <field_name>_<id_column_name>

field_name is address in your case, and id_column_name is referring to the related entity's id, which is id. Thus, you get address_id by default.

JPA: difference between @JoinColumn and @PrimaryKeyJoinColumn?

What happens if I promote the column to be a/the PK, too (a.k.a. identifying relationship)? As the column is now the PK, I must tag it with @Id (...).

This enhanced support of derived identifiers is actually part of the new stuff in JPA 2.0 (see the section 2.4.1 Primary Keys Corresponding to Derived Identities in the JPA 2.0 specification), JPA 1.0 doesn't allow Id on a OneToOne or ManyToOne. With JPA 1.0, you'd have to use PrimaryKeyJoinColumn and also define a Basic Id mapping for the foreign key column.

Now the question is: are @Id + @JoinColumn the same as just @PrimaryKeyJoinColumn?

You can obtain a similar result but using an Id on OneToOne or ManyToOne is much simpler and is the preferred way to map derived identifiers with JPA 2.0. PrimaryKeyJoinColumn might still be used in a JOINED inheritance strategy. Below the relevant section from the JPA 2.0 specification:

11.1.40 PrimaryKeyJoinColumn Annotation

The PrimaryKeyJoinColumn annotation
specifies a primary key column that is
used as a foreign key to join to
another table.

The PrimaryKeyJoinColumn annotation
is used to join the primary table of
an entity subclass in the JOINED
mapping strategy to the primary table
of its superclass; it is used within a
SecondaryTable annotation to join a
secondary table to a primary table;
and it may be used in a OneToOne
mapping in which the primary key of
the referencing entity is used as a
foreign key to the referenced
entity^[108].

...

If no PrimaryKeyJoinColumn
annotation is specified for a subclass
in the JOINED mapping strategy, the
foreign key columns are assumed to
have the same names as the primary key
columns of the primary table of the
superclass.

...

Example: Customer and ValuedCustomer subclass

@Entity
@Table(name="CUST")
@Inheritance(strategy=JOINED)
@DiscriminatorValue("CUST")
public class Customer { ... }

@Entity
@Table(name="VCUST")
@DiscriminatorValue("VCUST")
@PrimaryKeyJoinColumn(name="CUST_ID")
public class ValuedCustomer extends Customer { ... }

_{[108] The derived id mechanisms
described in section 2.4.1.1 are now
to be preferred over
PrimaryKeyJoinColumn for the
OneToOne mapping case.}

See also

• Primary Keys through OneToOne Relationships

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This source http://weblogs.java.net/blog/felipegaucho/archive/2009/10/24/jpa-join-table-additional-state states that using @ManyToOne and @Id works with JPA 1.x. Who's correct now?

The author is using a pre release JPA 2.0 compliant version of EclipseLink (version 2.0.0-M7 at the time of the article) to write an article about JPA 1.0(!). This article is misleading, the author is using something that is NOT part of JPA 1.0.

For the record, support of Id on OneToOne and ManyToOne has been added in EclipseLink 1.1 (see this message from James Sutherland, EclipseLink comitter and main contributor of the Java Persistence wiki book). But let me insist, this is NOT part of JPA 1.0.

How to use the Hibernate Join Column?

You cannot point to a database column in a mappedBy.
You have to point to an entity field that is the owning side of the relationship and that is:

 @ManyToOne(fetch = FetchType.LAZY)
 @JoinColumn(name = "prizv_id")
 private Prisvishche abiturients_pr;

So your owned side should be:

@OneToMany(mappedBy = "abiturients_pr")
private List<Abiturient> abiturients = new ArrayList();

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