How can I make a JPA OneToOne relation lazy

Question

In this application we are developing, we noticed that a view was particularly slow. I profiled the view and noticed that there was one query executed by hibernate which took 10 seconds even if there only were two object in the database to fetch. All OneToMany and ManyToMany relations were lazy so that wasn't the problem. When inspecting the actual SQL being executed, I noticed that there were over 80 joins in the query.

Further inspecting the issue, I noticed that the problem was caused by the deep hierarchy of OneToOne and ManyToOne relations between entity classes. So, I thought, I'll just make them fetched lazy, that should solve the problem. But annotating either @OneToOne(fetch=FetchType.LAZY) or @ManyToOne(fetch=FetchType.LAZY) doesn't seem to work. Either I get an exception or then they are not actually replaced with a proxy object and thus being lazy.

Any ideas how I'll get this to work? Note that I do not use the persistence.xml to define relations or configuration details, everything is done in java code.

Answer 1

First off, some clarifications to KLE's answer:

1. Unconstrained (nullable) one-to-one association is the only one that can not be proxied without bytecode instrumentation. The reason for this is that owner entity MUST know whether association property should contain a proxy object or NULL and it can't determine that by looking at its base table's columns due to one-to-one normally being mapped via shared PK, so it has to be eagerly fetched anyway making proxy pointless. Here's a more detailed explanation.

2. many-to-one associations (and one-to-many, obviously) do not suffer from this issue. Owner entity can easily check its own FK (and in case of one-to-many, empty collection proxy is created initially and populated on demand), so the association can be lazy.

3. Replacing one-to-one with one-to-many is pretty much never a good idea. You can replace it with unique many-to-one but there are other (possibly better) options.

Rob H. has a valid point, however you may not be able to implement it depending on your model (e.g. if your one-to-one association is nullable).

Now, as far as original question goes:

A) @ManyToOne(fetch=FetchType.LAZY) should work just fine. Are you sure it's not being overwritten in the query itself? It's possible to specify join fetch in HQL and / or explicitly set fetch mode via Criteria API which would take precedence over class annotation. If that's not the case and you're still having problems, please post your classes, query and resulting SQL for more to-the-point conversation.

B) @OneToOne is trickier. If it's definitely not nullable, go with Rob H.'s suggestion and specify it as such:

@OneToOne(optional = false, fetch = FetchType.LAZY)

Otherwise, if you can change your database (add a foreign key column to owner table), do so and map it as "joined":

@OneToOne(fetch = FetchType.LAZY)
@JoinColumn(name="other_entity_fk")
public OtherEntity getOther()

and in OtherEntity:

@OneToOne(mappedBy = "other")
public OwnerEntity getOwner()

If you can't do that (and can't live with eager fetching) bytecode instrumentation is your only option. I have to agree with CPerkins, however - if you have 80!!! joins due to eager OneToOne associations, you've got bigger problems then this :-)