Sample code to represent my problem.
Let's say there are entity classes like that
public class User {
...
#ManyToOne
#JoinColumn(name = "users_statuts_id", referencedColumnName = "id", nullable = false)
private UserStatus status;
}
public class UserStatus {
...
#OneToMany(mappedBy = "status")
private List<User> users = new LinkedList<>();
}
This works as expected but, problem starts when I want to use MapStruct generated mapper and DTO object. I will get StackOverflowError becouse of Cyclic references User->UserStatus->List< User>-> .....
And take into account that not necessary it will be in pattern A->B->A (User->UserStatus->User)
sometimes it will be User->ClassA->ClassB->...->User
I tired adding Context (CycleAvoidingMappingContext) to mapper as stated in other threads to break cycle but I failed and still get StackOverflowError. Tried too with AfterMappings and Mapping(ignore) but still not worked and I would like to avoid settings manually nulls to break the cycle.
I wonder if is possible to break cycle similar to #JsonIdentityInfo but on Hibernate level ?
My classes are a bit more complicated and bidirectional relation would help a lot later. Someone have some tips how should I make it done to work properly ?
I am using JPA in my application, and take one model for example:
public class Project {
#Id
private String uuid;
private String name;
.................
#OneToMany(mappedBy = "project", cascade = CascadeType.ALL)
private List<ProjectDetails> details;
}
As shown,there is a one-to-many association between the Project and the ProjectDetail, once a project is fetched, its details will be populated by the jpa provider which is exactly what I want.
However once the authentication is added to the project, the details will be only available for specified users, which means the auto-fetching details is not necessary.
I know I can use the
project.setDetails(null);
to remove the details information for un-authenticated user in the application level. But I wonder if this is a waste of sql resource? So it would be better if I can set the cascade type at runtime.
How do you solve this kind of problem?
By default, since #OneToMany relationships are lazily loaded, you may not necessarily access the getter from your code if the user has not yet been authenticated thereby preventing the relationship from been loaded.
When the entity eventually gets loaded, I think you should just decide on what to do from your application rather than setting the relationship to null.
I two entities a User and a Place witch are bound with many to many association.
When I try to get all the places for a given user thought the getter method, an emtpy list is returned but the user is bound to the place in the database and if I change the default fetching strategy to eager I can see all the places just fine.
I am using MySQL for the persistance.
The annotations used are:
for the User entity:
#ManyToMany
#JoinTable(name= "USER_PLACE",
joinColumns = {#JoinColumn(name="USER_ID")},
inverseJoinColumns = {#JoinColumn(name="PLACE_ID")})
private List<Place> places = new ArrayList<Place>();
and for the Place entity:
#ManyToMany(mappedBy = "places")
private List<User> users = new ArrayList<User>(0);
What can it be the cause of this?
To me, it looks like a Mapping issue in Your domain-model. Do the provided entities also map somewhere else? You might run into crazy joins with other tables. Could you provide all relevant entities? Also, the SQL statements generated by Hibernate for User.getPlaces() would be helpful.
I have stumbled upon a really annoying situation: I am using Hibernate & Spring as backend for my app and it seems that in some cases, the entities that are in a relationship with a particular entity are not fetched as normal entity objects from the DB, but as Javassist types. E.g.:
I have the Campaign entity with the following relationships:
#Entity
#Table(name = "campaign")
public class Campaign implements Serializable {
[..]
#ManyToMany(fetch = FetchType.LAZY)
#JoinTable(uniqueConstraints = #UniqueConstraint(columnNames = {
"campaign_id", "dealer_id" }), name = "campaign_has_dealer", joinColumns = { #JoinColumn(name = "campaign_id", nullable = false) }, inverseJoinColumns = { #JoinColumn(name = "dealer_id", nullable = false) })
private List<Dealer> dealers = new ArrayList<Dealer>();
#ManyToMany
// (fetch = FetchType.LAZY)
#JoinTable(uniqueConstraints = #UniqueConstraint(columnNames = {
"campaign_id", "sales_area_id" }), name = "campaign_has_sales_area", joinColumns = { #JoinColumn(name = "campaign_id", nullable = false) }, inverseJoinColumns = { #JoinColumn(name = "sales_area_id", nullable = false) })
private List<SalesArea> salesAreas = new ArrayList<SalesArea>();
}
Upon retrieving the salesAreas connected to this Campaign, I get a list of SalesArea_$$_javassist_56, while for the dealers, I get normal Hibernate entities. Since the client part is based on GWT, we use RequestFactory for retrieving stuff. I initially thought it was a problem with the proxies, locators and so on but I have set a breakpoint in the service where these are retrieved and they are Javassist objects directly after selecting them. It seems that even removing the FetchType.LAZY annotation (although definitely not a desirable solution), the same thing happens. This happened also with other types of relationships, not only #ManyToMany.
We are using GWT 2.3, Spring 3, Hibernate 3.6.3 and JPA 2.0 for annotations.
Any suggestions would be appreciated.
Thanks in advance
As far as I can see the big problem that you're having is not so much the fetch type of your association, but rather that the proxied types don't work well with RequestFactory.
Yes, it could be solved by changing the fetch strategy but that sounds rather like a weak workaround that may break upon weird circumstances.
I don't remember exactly how to solve it, but I did, and as far as I remember there was an extension point in the ServiceLayerDecorator class. Basically there you check if the object you're returning is a Hibernate proxy (check Hibernate and HibernateProxy classes) and then return the non-proxy type instead in ServiceLayerDecorator. (http://code.google.com/p/google-web-toolkit/issues/detail?id=6767)
As for your fetch strategy, I'd largely recommend #BatchSize(N) where N is big (maybe 1000), but this is an independent subject.
Good luck!
If you call to the static method:
HibernateProxyHelper.getClassWithoutInitializingProxy(entity);
you get the class of the proxied entity and the class itself if it wasn't proxied.
With Hibernate's proxy model and now with it's use of Javassist to help avoid the slower traditional Hibernate run time reflection operations things will never quite be as elegant as the clean, intuitive experience people who use full bytecode enhancement solutions like JDO implementations (eg DataNucleus) enjoy.
Personally I can never see the sense in persisting (pardon the pun) with solutions that cause so many problems and fill the web with questions about broken code that requires strange, unintuitive workarounds but still people do...
However, back to the question: one solution to your problem, if you're using JPA, is to use DataNucleus/JPA which brings many of the benefits of DataNucleus/JDO (clean underlying implementation - no proxies, no Javassist classes etc.,) in a JPA compliant implementation - i.e. you don't need to change your existing source code to start using it.
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.
First off, some clarifications to KLE's answer:
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.
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.
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 :-)
To get lazy loading working on nullable one-to-one mappings you need to let hibernate do compile time instrumentation and add a #LazyToOne(value = LazyToOneOption.NO_PROXY) to the one-to-one relation.
Example Mapping:
#OneToOne(fetch = FetchType.LAZY)
#JoinColumn(name="other_entity_fk")
#LazyToOne(value = LazyToOneOption.NO_PROXY)
public OtherEntity getOther()
Example Ant Build file extension (for doing the Hibernate compile time instrumentation):
<property name="src" value="/your/src/directory"/><!-- path of the source files -->
<property name="libs" value="/your/libs/directory"/><!-- path of your libraries -->
<property name="destination" value="/your/build/directory"/><!-- path of your build directory -->
<fileset id="applibs" dir="${libs}">
<include name="hibernate3.jar" />
<!-- include any other libraries you'll need here -->
</fileset>
<target name="compile">
<javac srcdir="${src}" destdir="${destination}" debug="yes">
<classpath>
<fileset refid="applibs"/>
</classpath>
</javac>
</target>
<target name="instrument" depends="compile">
<taskdef name="instrument" classname="org.hibernate.tool.instrument.javassist.InstrumentTask">
<classpath>
<fileset refid="applibs"/>
</classpath>
</taskdef>
<instrument verbose="true">
<fileset dir="${destination}">
<!-- substitute the package where you keep your domain objs -->
<include name="/com/mycompany/domainobjects/*.class"/>
</fileset>
</instrument>
</target>
Unless you are using Bytecode Enhancement, you cannot fetch lazily the parent-side #OneToOne association.
However, most often, you don't even need the parent-side association if you use #MapsId on the child-side:
#Entity(name = "PostDetails")
#Table(name = "post_details")
public class PostDetails {
#Id
private Long id;
#Column(name = "created_on")
private Date createdOn;
#Column(name = "created_by")
private String createdBy;
#OneToOne(fetch = FetchType.LAZY)
#MapsId
private Post post;
public PostDetails() {}
public PostDetails(String createdBy) {
createdOn = new Date();
this.createdBy = createdBy;
}
//Getters and setters omitted for brevity
}
With #MapsId, the id property in the child table serves as both Primary Key and Foreign Key to the parent table Primary Key.
So, if you have a reference to the parent Post entity, you can easily fetch the child entity using the parent entity identifier:
PostDetails details = entityManager.find(
PostDetails.class,
post.getId()
);
This way, you won't have N+1 query issues that could be caused by the mappedBy #OneToOne association on the parent side.
Here's something that has been working for me (without instrumentation):
Instead of using #OneToOne on both sides, I use #OneToMany in the inverse part of the relationship (the one with mappedBy). That makes the property a collection (List in the example below), but I translate it into an item in the getter, making it transparent to the clients.
This setup works lazily, that is, the selects are only made when getPrevious() or getNext() are called - and only one select for each call.
The table structure:
CREATE TABLE `TB_ISSUE` (
`ID` INT(9) NOT NULL AUTO_INCREMENT,
`NAME` VARCHAR(255) NULL,
`PREVIOUS` DECIMAL(9,2) NULL
CONSTRAINT `PK_ISSUE` PRIMARY KEY (`ID`)
);
ALTER TABLE `TB_ISSUE` ADD CONSTRAINT `FK_ISSUE_ISSUE_PREVIOUS`
FOREIGN KEY (`PREVIOUS`) REFERENCES `TB_ISSUE` (`ID`);
The class:
#Entity
#Table(name = "TB_ISSUE")
public class Issue {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
protected Integer id;
#Column
private String name;
#OneToOne(fetch=FetchType.LAZY) // one to one, as expected
#JoinColumn(name="previous")
private Issue previous;
// use #OneToMany instead of #OneToOne to "fake" the lazy loading
#OneToMany(mappedBy="previous", fetch=FetchType.LAZY)
// notice the type isnt Issue, but a collection (that will have 0 or 1 items)
private List<Issue> next;
public Integer getId() { return id; }
public String getName() { return name; }
public Issue getPrevious() { return previous; }
// in the getter, transform the collection into an Issue for the clients
public Issue getNext() { return next.isEmpty() ? null : next.get(0); }
}
The basic idea behing the XToOnes in Hibernate is that they are not lazy in most case.
One reason is that, when Hibernate have to decide to put a proxy (with the id) or a null,
it has to look into the other table anyway to join. The cost of accessing the other table in the database is significant, so it might as well fetch the data for that table at that moment (non-lazy behaviour), instead of fetching that in a later request that would require a second access to the same table.
Edited: for details, please refer to ChssPly76 's answer. This one is less accurate and detailed, it has nothing to offer. Thanks ChssPly76.
In native Hibernate XML mappings, you can accomplish this by declaring a one-to-one mapping with the constrained attribute set to true. I am not sure what the Hibernate/JPA annotation equivalent of that is, and a quick search of the doc provided no answer, but hopefully that gives you a lead to go on.
As already perfectly explained by ChssPly76, Hibernate's proxies don't help with unconstrained (nullable) one-to-one associations, BUT there is a trick explained here to avoid to set up instrumentation. The idea is to fool Hibernate that the entity class which we want to use has been already instrumented: you instrument it manually in the source code. It's easy! I've implemented it with CGLib as bytecode provider and it works (ensure that you configure lazy="no-proxy" and fetch="select", not "join", in your HBM).
I think this is a good alternative to real (I mean automatic) instrumentation when you have just one one-to-one nullable relation that you want to make lazy. The main drawback is that the solution depends on the bytecode provider you are using, so comment your class accurately because you could have to change the bytecode provider in the future; of course, you are also modifying your model bean for a technical reason and this is not fine.
This question is quite old, but with Hibernate 5.1.10, there are some new better comfortable solution.
Lazy loading works except for the parent side of a #OneToOne association. This is because Hibernate has no other way of knowing whether to assign a null or a Proxy to this variable. More details you can find in this article
You can activate lazy loading bytecode enhancement
Or, you can just remove the parent side and use the client side with #MapsId as explained in the article above. This way, you will find that you don’t really need the parent side since the child shares the same id with the parent so you can easily fetch the child by knowing the parent id
.
For Kotlin devs: To allow Hibernate to inherit from the #Entity types that you want to be lazy-loadable they have to be inheritable/open, which they in Kotlin by default are not. To work around this issue we can make use of the all-open compiler plugin and instruct it to also handle the JPA annotations by adding this to our build.gradle:
allOpen {
annotation("javax.persistence.Entity")
annotation("javax.persistence.MappedSuperclass")
annotation("javax.persistence.Embeddable")
}
If you are using Kotlin and Spring like me, you are most probably also using the kotlin-jpa/no-args and kotlin-spring/all-open compiler plugins already. However, you will still need to add the above lines, as that combination of plugins neither makes such classes open.
Read the great article of Léo Millon for further explanations.
Most efficient mapping of a one-to-one association
You can avoid all these problems and get rid of the foreign key column by using the same primary key value for both associated entities. You can do that by annotating the owning side of the association with #MapsId.
#Entity
public class Book {
#Id
#GeneratedValue
private Long id;
#OneToOne(mappedBy = "book", fetch = FetchType.LAZY, optional = false)
private Manuscript manuscript;
...
}
#Entity
public class Manuscript {
#Id
private Long id;
#OneToOne
#MapsId
#JoinColumn(name = "id")
private Book book;
...
}
Book b = em.find(Book.class, 100L);
Manuscript m = em.find(Manuscript.class, b.getId());
More Detail click on this url
If the relation must not be bidirectional then an #ElementCollection might be easier than using a lazy One2Many collection.
If the child entity is used readonly, then it's possible to simply lie and set optional=false.
Then ensure that every use of that mapped entity is preloaded via queries.
public class App {
...
#OneToOne(mappedBy = "app", fetch = FetchType.LAZY, optional = false)
private Attributes additional;
and
String sql = " ... FROM App a LEFT JOIN FETCH a.additional aa ...";
... maybe even persisting would work...