How Can You Define Relationships in Hibernate and Spring Using Annotations?

Using Annotations to Define Relationships in Hibernate 4 and Spring

Uni-Directional and Bi-Directional Relationships

One-to-One Relationships

Uni-Directional:

<code class="java">class Foo {
    private Bar bar;
}

class Bar {
}</code>

Bi-Directional (managed by Foo class):

<code class="java">class Foo {
    @OneToOne(cascade = CascadeType.ALL)
    private Bar bar;
}

class Bar {
    @OneToOne(mappedBy = "bar")
    private Foo foo;
}</code>

One-to-Many Relationships

Uni-Directional using User Managed Join Table:

<code class="java">class Foo {
    @OneToMany
    @JoinTable(name = "FOO_BAR",
        joinColumns = {@JoinColumn(name = "fooId")},
        inverseJoinColumns = {@JoinColumn(name = "barId")})
    private List<Bar> bars;
}

class Bar {
    // No corresponding mapping to Foo.class
}

@Entity
@Table(name = "FOO_BAR")
class FooBar {
    private UUID fooBarId;
    private Foo foo;
    private Bar bar;
}</code>

Bi-Directional using Foreign Key Mapping:

<code class="java">class Foo {
    @OneToMany(mappedBy = "bar")
    private List<Bar> bars;
}

class Bar {
    @ManyToOne
    @JoinColumn(name = "fooId")
    private Foo foo;
}</code>

Many-to-Many Relationships

Bi-Directional using Hibernate Managed Join Table:

<code class="java">class Foo {
    @OneToMany
    @JoinTable(name = "FOO_BAR",
        joinColumns = {@JoinColumn(name = "fooId")},
        inverseJoinColumns = {@JoinColumn(name = "barId")})
    private List<Bar> bars;
}

class Bar {
    @OneToMany
    @JoinTable(name = "FOO_BAR",
        joinColumns = {@JoinColumn(name = "barId")},
        inverseJoinColumns = {@JoinColumn(name = "fooId")})
    private List<Foo> foos;
}</code>

Bi-Directional using User Managed Join Table:

<code class="java">class Foo {
    @OneToMany(mappedBy = "bar")
    private List<FooBar> bars;
}

class Bar {
    @OneToMany(mappedBy = "foo")
    private List<FooBar> foos;
}

@Entity
@Table(name = "FOO_BAR")
class FooBar {
    private UUID fooBarId;
    private Foo foo;
    private Bar bar;
}</code>

Determining Relationship Ownership

• Generally, the object with the collection owns the relationship.

Choosing FetchType

• Default: LAZY
• Use EAGER for collections that are often used.
• Consider using Hibernate.initialize() or FetchMode.SUBSELECT for lazily loaded collections needed in a specific method.

Determining Cascade Direction

• Cascade operations only going in one direction unless specified.
• Mark the @OneToMany annotation for cascade in the owning side of the relationship.

Orphan Removal

• OrphanRemoval = true: Removes orphaned objects that are no longer referenced in any other relationship.

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