Hibernate generating SQL queries when accessing as

问题:

I have Hibernate Entities that look something like this (getters and setters left out):

@Entity
public class EntityA {
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "parent_id")
    private EntityB parent;
}

@Entity
public class EntityB extends SuperEntity {
    @OneToMany(mappedBy = "parent")
    @Fetch(FetchMode.SUBSELECT)
    @JoinColumn(name = "parent_id")
    private Set<EntityA> children;
}

@MappedSuperclass
public class SuperEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private long itemId;
}

When I query for EntityA it loads fine, with the parent association being replaced by a Hibernate proxy (as it is Lazy). If I want access to the parent's id I perform the following call:

EntityA entityA = queryForEntityA();
long parentId = entityA.getParent().getItemId();

As I understand that call should NOT make a roundtrip to the database, as the Id is stored in the EntityA table, and the proxy should only return that value. However, in my case this generates a SQL statement which fetches EntityB and only then returns the Id.

How can I investigate the problem? What are some likely causes of this incorrect behaviour?

回答1:

As I understand that call should NOT make a roundtrip to the database, as the Id is stored in the EntityA table, and the proxy should only return that value.

Use property access type. The behavior you're experiencing is a "limitation" of field access type. Here is how Emmanuel Bernard explained it:

That is unfortunate but expected. That's one of the limitations of field level access. Basically we have no way to know that getId() indeed only go and access the id field. So we need to load the entire object to be safe.

So change your code into:

@Entity
public class EntityA {
    private EntityB parent;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "parent_id")
    public EntityB getParent() {
        return parent; 
    }
    ...
}

@MappedSuperclass
public class SuperEntity {
    private long itemId;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    public long getItemId() { 
        return itemId;
    }
    ...
}

Related question

• Hibernate Annotations - Which is better, field or property access?

References

• Proxy loaded on getId-call when using annotations on fields
• proxy getId => why sql is generated !
• HHH-3718 (if this issue can ever be solved)

回答2:

What you say makes sense - that it would not make a DB hit since EntityA contains the parent ID. I am just not sure if the getParent() call actually loads the EntityB object regardless of whether all you're interested in is the ID. You might try marking the children collection (and any other fields) as Lazy if you want to save the DB hit.

@Entity
public class EntityB : SuperEntity {
    @OneToMany(mappedBy = "parent", fetch = FetchType.LAZY)
    @Fetch(FetchMode.SUBSELECT)
    @JoinColumn(name = "parent_id")
    private Set<EntityA> children;
}

回答3:

As for Hibernate:
This behavior has been changed since Hibernate 5.2.12.