How does Hibernate handle the life cycle of associated objects?

Hibernate uses different life cycle strategies to handle associated objects based on the association type. These strategies include one-way one-to-many cascading, two-way one-to-many bidirectional cascading, many-to-one orphan deletion, two-way one-to-one cascading, etc. In practical applications, you can choose an appropriate strategy according to your needs. For example, cascade deletion ensures that all associated child entities are deleted at the same time when the parent entity is deleted. In addition, Hibernate also provides rich mapping configuration options, such as orphanRemoval and mappedBy, to further customize the processing of associated objects.

How Hibernate handles the life cycle of associated objects

Hibernate is a popular Java object-relational mapping (ORM) framework that can automatically map Java objects to a relational database table. When dealing with related objects, Hibernate uses different life cycle strategies based on different relationship types.

Association types

In Hibernate, common association types include:

• One-way one-to-many: One entity references multiple entity.
• Bidirectional one-to-many: One entity refers to multiple entities and is referenced by multiple entities at the same time.
• Many-to-one: Multiple entities refer to one entity.
• Two-way one-to-one: An entity refers to an entity and is referenced by the entity at the same time.

Life cycle strategies

Hibernate provides a variety of life cycle strategies to handle the life cycle of associated objects. Each policy has its own unique behavior, depending on the type of association and mapping configuration.

One-to-one-to-many

• Cascade deletion: When the parent entity is deleted, the cascade deletion policy will be automatically deleted All associated child entities.
• Cascading persistence: When the parent entity is persisted, the cascading persistence strategy will automatically persist all associated child entities.
• Cascade Merge: When a parent entity is merged, the cascade merge strategy automatically merges all associated child entities.
• Cascading Refresh: When the parent entity is refreshed, the cascading refresh strategy automatically refreshes all associated child entities.
• Cascading detachment: The cascading detachment strategy will not cascade any operations to the associated child entities.

Two-way one-to-many

• Bidirectional cascading: This strategy is similar to one-way cascading, but it will Cascading operations in parent and child entities.
• Collection cascading: This strategy only cascades operations in child entity collections and does not cascade to parent entities.
• Mapping cascading: This strategy only cascades operations in the parent entity mapping and does not cascade to the child entity collection.

Many-to-One

• Orphan deletion: When a child entity is no longer referenced by any parent entity, orphan The deletion policy will automatically delete the child entity.
• FK Foreign Key Enforcement: When the parent entity is deleted, this policy will set the foreign key of the child entity to NULL without automatically deleting the child entity. .
• FK Foreign Key Cancellation: When the parent entity is deleted, this strategy will set the foreign key of the child entity to NULL, and also automatically delete the child entity. .

Two-way one-to-one

• Cascade: This strategy is similar to the two-way cascade in two-way one-to-many Strategy.
• Main cascading: This strategy only cascades operations in the main entity and does not cascade to associated entities.

Practical case

One-way one-to-many cascade deletion

@Entity
public class Parent {

    @Id
    private Long id;

    @OneToMany(mappedBy = "parent", cascade = CascadeType.ALL)
    private List<Child> children;
    // ...

}

@Entity
public class Child {

    @Id
    private Long id;

    @ManyToOne
    private Parent parent;
    // ...

}

In this example, if we delete the parent entity, all Associated child entities will also be automatically deleted.

Bidirectional One-to-Many Bidirectional Cascade

@Entity
public class Parent {

    @Id
    private Long id;

    @OneToMany(mappedBy = "parent")
    private List<Child> children;
    // ...

}

@Entity
public class Child {

    @Id
    private Long id;

    @ManyToOne
    @JoinColumn(name = "parent_id", nullable = false)
    private Parent parent;
    // ...

}

In this example, if we update the parent entity or delete the child entity, the other end of the association will also be affected.

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