I found in some old code strange thing (at least for me).
The field which is annotated #ManyToOne is also annotated with #BatchSize.
I always thought that #BatchSize annotation only affects when annotated at class level or on a collection (#OneToMany) and affects pre-fetching when iterating.
But maybe I am wrong and annotating #ManyToOne with #BatchSize affects something. I can't find the answer in the documentation.
Does annotating #ManyToOne with #BatchSize have sense?
I think the question refers to combining #ManyToOne and #BatchSize on the same field, e.g.:
#ManyToOne
#BatchSize(size = 5)
private User owner;
This use case is not supported by Hibernate, at least when using annotations. The only uses of batch fetching mentioned by the documentation are:
On collection fields, i.e., #OneToMany or #ManyToMany (but not #ManyToOne)
On the entity class to be fetched
E.g.:
#Entity
#BatchSize(size = 5)
public class User {
...
}
This latter case enables batching for all relationships of type User, including many-to-one relationships. However, with the annotation on the entity class it is not possible to control the behaviour on a field-by-field basis.
A search through the Hibernate source code for all uses of #BatchSize confirms the lack of support for your usage. From what I see in AnnotationBinder.java, the #BatchSize annotation is only inspected on the entity class and on fields which have some kind of #XxxToMany annotation.
#ManyToOne associated with #BatchSize could make sense only if the corresponding field is marked as lazy (lazy=true).
Indeed, if the field is not lazy, it's by definition already loaded since the enclosing entity is loaded, so the problem of database calls doesn't apply.
Imagine a Person class who has a collection of ShoesPair element (ShoesPair.class) and within this one is present an owner field marked as lazy (since optional and not really bringing an important information when retrieving a specific pair of shoes).
One wants to iterate through 25 pair of shoes (25 ShoesPair objects) in order to retrieve their owner.
If the owner field (corresponding to one person) is only annotated with #ManyToOne, there would be 25 select to database.
However, if annoted with #BatchSize(size=5), there would be merely 5 calls and so increasing performance.
From the Hibernate documentation, it is precised that batch size does not only apply with collections:
You can also enable batch fetching of collections.
Hibenate mentions especially #OneToMany cases, because these one are applied with fields that are in 90% of cases marked as lazy.
Solving N+1 query problem with Hibernate
1 Using Criteria queries with fetchMode
Criteria criteria = session.createCriteria(Customer.class);
criteria.setFetchMode("contact", FetchMode.EAGER);
2 HOL fetch join
3 #BatchSize
The #BatchSize annotation can be used to define how many identical associations to populate in a single database query. If the session has 100 customers attached to it and the mapping of the 'contact' collection is annotated with #BatchSize of size n. It means that whenever Hibernate needs to populate a lazy contact collection it checks the session and if it has more customers which their contact collections need to be populated it fetches up to n collections.
#OneToMany(mappedBy="customer",cascade=CascadeType.ALL, fetch=FetchType.LAZY)
#BatchSize(size=25)
private Set<Contact> contacts = new HashSet<Contact>();
Related
I have a table "class" which is linked to tables "student" and "teachers".
A "class" is linked to multiple students and teachers via foriegn key relationship.
When I use hibernate associations and fetch large number of entities(tried for 5000) i am seeing that it is taking 4 times more memory than if i just use foreign key place holders.
Is there something wrong in hibernate association?
Can i use any memory profiler to figure out what's using too much memory?
This is how the schema is:
class(id,className)
student(id,studentName,class_id)
teacher(id,teacherName,class_id)
class_id is foreign key..
Case #1 - Hibernate Associations
1)in Class Entity , mapped students and teachers as :
#Entity
#Table(name="class")
public class Class {
private Integer id;
private String className;
private Set<Student> students = new HashSet<Student>();
private Set<Teacher> teachers = new HashSet<Teacher>();
#OneToMany(fetch = FetchType.EAGER, mappedBy = "classRef")
#Cascade({ CascadeType.ALL })
#Fetch(FetchMode.SELECT)
#BatchSize(size=500)
public Set<Student> getStudents() {
return students;
}
2)in students and teachers , mapped class as:
#Entity
#Table(name="student")
public class Student {
private Integer id;
private String studentName;
private Class classRef;
#ManyToOne
#JoinColumn(name = "class_id")
public Class getClassRef() {
return classRef;
}
Query used :
sessionFactory.openSession().createQuery("from Class where id<5000");
This however was taking a Huge amount of memory.
Case #2- Remove associations and fetch seperately
1)No Mapping in class entity
#Entity
#Table(name="class")
public class Class {
private Integer id;
private String className;
2)Only a placeholder for Foreign key in student, teachers
#Entity
#Table(name="student")
public class Student {
private Integer id;
private String studentName;
private Integer class_id;
Queries used :
sessionFactory.openSession().createQuery("from Class where id<5000");
sessionFactory.openSession().createQuery("from Student where class_id = :classId");
sessionFactory.openSession().createQuery("from Teacher where class_id = :classId");
Note - Shown only imp. part of the code. I am measuring memory usage of the fetched entities via JAMM library.
I also tried marking the query as readOnly in case #1 as below, which does not improve memory usage very much ; just a very little. So that's not the solve.
Query query = sessionFactory.openSession().
createQuery("from Class where id<5000");
query.setReadOnly(true);
List<Class> classList = query.list();
sessionFactory.getCurrentSession().close();
Below are the heapdump snapshots sorted by sizes. Looks like the Entity maintained by hibernate is creating the problem..
Snapshot of Heapdump for hibernate associations program
Snapshot of heapdump for fetching using separate entities
You are doing a EAGER fetch with the below annotation. This will in turn fetch all the students without even you accessing the getStudents(). Make it lazy and it will fetch only when needed.
From
#OneToMany(fetch = FetchType.EAGER, mappedBy = "classRef")
To
#OneToMany(fetch = FetchType.LAZY, mappedBy = "classRef")
When Hibernate loads a Class entity containing OneToMany relationships, it replaces the collections with its own custom version of them. In the case of a Set, it uses a PersistentSet. As can be seen on grepcode, this PersistentSet object contains quite a bit of stuff, much of it inherited from AbstractPersistentCollection, to help Hibernate manage and track things, particularly dirty checking.
Among other things, the PersistentSet contains a reference to the session, a boolean to track whether it's initialized, a list of queued operations, a reference to the Class object that owns it, a string describing its role (not sure what exactly that's for, just going by the variable name here), the string uuid of the session factory, and more. The biggest memory hog among the lot is probably the snapshot of the unmodified state of the set, which I would expect to approximately double memory consumption by itself.
There's nothing wrong here, Hibernate is just doing more than you realized, and in more complex ways. It shouldn't be a problem unless you are severely short on memory.
Note, incidentally, that when you save a new Class object that Hibernate previously was unaware of, Hibernate will replace the simple HashSet objects you created with new PersistentSet objects, storing the original HashSet wrapped inside the PersistentSet in its set field. All Set operations will be forwarded to the wrapped HashSet, while also triggering PersistentSet dirty tracking and queuing logic, etc. With that in mind, you should not keep and use any external references to the Set from before saving, and should instead fetch a new reference to Hibernate's PersistentSet instance and use that if you need to make any changes (to the set, not to the students or teachers within it) after the initial save.
Regarding the huge memory consumption you are noticing, one potential reason is Hibernate Session has to maintain the state of each entity it has loaded the form of EntityEntry object i.e., one extra object, EntityEntry, for each loaded entity. This is needed for hibernate automatic dirty checking mechanism during the flush stage to compare the current state of entity with its original state (one that is stored as EntityEntry).
Note that this EntityEntry is different from the object that we get to access in our application code when we call session.load/get/createQuery/createCriteria. This is internal to hibernate and stored in the first level cache.
Quoting form the javadocs for EntityEntry :
We need an entry to tell us all about the current state of an object
with respect to its persistent state Implementation Warning: Hibernate
needs to instantiate a high amount of instances of this class,
therefore we need to take care of its impact on memory consumption.
One option, assuming the intent is only to read and iterate through the data and not perform any changes to those entities, you can consider using StatelessSession instead of Session.
The advantage as quoted from Javadocs for Stateless Session:
A stateless session does not implement a first-level cache nor
interact with any second-level cache, nor does it implement
transactional write-behind or automatic dirty checking
With no automatic dirty checking there is no need for Hibernate to create EntityEntry for each entity of loaded entity as it did in the earlier case with Session. This should reduce pressure on memory utilization.
Said that, it does have its own set of limitations as mentioned in the StatelessSession javadoc documentation.
One limitation that is worth highlighting is, it doesn't lazy loading the collections. If we are using StatelessSession and want to load the associated collections we should either join fetch them using HQL or EAGER fetch using Criteria.
Another one is related to second level cache where it doesn't interact with any second-level cache, if any.
So given that it doesn't have any overhead of first-level cache, you may want to try with Stateless Session and see if that fits your requirement and helps in reducing the memory consumption as well.
Yes, you can use a memory profiler, like visualvm or yourkit, to see what takes so much memory. One way is to get a heap dump and then load it in one of these tools.
However, you also need to make sure that you compare apples to apples. Your queries in case#2 sessionFactory.openSession().createQuery("from Student where class_id = :classId");
sessionFactory.openSession().createQuery("from Teacher where class_id = :classId");
select students and teachers only for one class, while in case #1 you select way more. You need to use <= :classId instead.
In addition, it is a little strange that you need one student and one teacher record per one class. A teacher can teach more than one class and a student can be in more than one class. I do not know what exact problem you're solving but if indeed a student can participate in many classes and a teacher can teach more than one class, you will probably need to design your tables differently.
Try #Fetch(FetchMode.JOIN), This generates only one query instead of multiple select queries. Also review the generated queries. I prefer using Criteria over HQL(just a thought).
For profiling, use freewares like visualvm or jconsole. yourkit is good for advanced profiling, but it is not for free. I guess there is a trail version of it.
You can take the heapdump of your application and analyze it with any memory analyzer tools to check for any memory leaks.
BTW, I am not exactly sure about the memory usage for current scenario.
Its likely the reason is the bi-directional link from Student to Class and Class to Students. When you fetch Class A (id 4500), The Class object must be hydrated, in turn this must go and pull all the Student objects (and teachers presumably) associated with this class. When this happens each Student Object must be hydrated. Which causes the fetch of every class the Student is a part of. So although you only wanted class A, you end up with:
Fetch Class A (id 4900)
Returns Class A with reference to 3 students, Student A, B, C.
Student A has ref to Class A, B (id 5500)
Class B needs hydrating
Class B has reference to Students C,D
Student C needs hydrating
Student C only has reference to Class A and B
Student C hydration complete.
Student D needs hydrating
Student D only has reference to Class B
Student B hydration complete
Class B hydration complete
Student B needs hydrating (from original class load class A)
etc... With eager fetching, this continues until all links are hydrated. The point being that its possible you end up with Classes in memory that you didn't actually want. Or whose id is not less than 5000.
This could get worse fast.
Also, you should make sure you are overriding the hashcode and equals methods. Otherwise you may be getting redundant objects, both in memory and in your set.
One way to improve is either change to LAZY loading as other have mentioned or break the bidirectional links. If you know you will only ever access students per class, then don't have the link from student back to class. For student/class example it makes sense to have the bidirectional link, but maybe it can be avoided.
as you say you "I want "all" the collections". so lazy-loading won't help.
Do you need every field of every entity? In which case use a projection to get just the bits you want. See when to use Hibernate Projections.
Alternatively consider having minimalist Teacher-Lite and Student-Lite entity that the full-fat versions extend.
I'm having difficulties with proxied objects in Grails.
Assuming I've got the following
class Order {
#ManyToMany(fetch = FetchType.EAGER)
#JoinTable(name="xxx", joinColumns = {#JoinColumn(name = "xxx")}, inverseJoinColumns = {#JoinColumn(name = "yyy")})
#OrderBy("id")
#Fetch(FetchMode.SUBSELECT)
private List<OrderItem> items;
}
class Customer {
#ManyToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY, optional = true)
#JoinColumn(name = "xxx",insertable = false, nullable = false)
private OrderItem lastItem;
private Long lastOrderId;
}
And inside some controller class
//this all happens during one hibernate session.
def currentCustomer = Customer.findById(id)
//at this point currentCustomer.lastItem is a javassist proxy
def lastOrder = Order.findById(current.lastOrderId)
//lastOrder.items is a proxy
//Some sample actions to initialise collections
lastOrder.items.each { println "${it.id}"}
After the iteration lastOrder.items still contains a proxy of currentCustomer.lastItem. For example if there are 4 items in the lastOrder.items collection, it looks like this:
object
object
javassist proxy (all fields are null including id field). This is the same object as in currentCustomer.lastItem.
object
Furthermore, this proxy object has all properties set to null and it's not initialized when getters are invoked. I have to manually call GrailsHibernateUtils.unwrapIdProxy() on every single element inside lastOrder.items to ensure that there are no proxies inside (which basically leads to EAGER fetching).
This one proxy object leads to some really weird Exceptions, which are difficult to track on testing phase.
Interesting fact: if I change the ordering of the operations (load the order first and the customer second) every element inside lastOrder.items is initialized.
The question is: Is there a way to tell Hibernate that it should initialize the collections when they are touched, no matter if any elements from the collection is already proxied in the session?
I think what's happening here is an interesting interaction between the first level cache (stored in Hibernate's Session instance) and having different FetchType on related objects.
When you load Customer, it gets put in to the Session cache, along with any objects that are loaded with it. This includes a proxy object for the OrderItem object, because you've got FetchType.LAZY. Hibernate only allows one instance to be associated with any particular ID, so any further operations that would be acting on the OrderItem with that ID would always be using that proxy. If you asked the same Session to get that particular OrderItem in another way, as you are by loading an Order containing it, that Order would have the proxy, because of Session-level identity rules.
That's why it 'works' when you reverse the order. Load the Order first, it's collection is FetchType.EAGER, and so it (and the first level cache) have fully realized instances of OrderItem. Now load a Customer which has it's lastItem set to one of the already-loaded OrderItem instances and presto, you have a real OrderItem, not a proxy.
You can see the identity rules documented in the Hibernate manual:
For objects attached to a particular Session... JVM identity for database identity is guaranteed by Hibernate.
All that said, even if you get an OrderItem proxy, it should work fine as long as the associated Session is still active. I wouldn't necessarily expect the proxy ID field to show up as populated in the debugger or similar, simply because the proxy handles things in a 'special' way (ie, it's not a POJO). But it should respond to method calls the same way it's base class would. So if you have an OrderItem.getId() method, it should certainly return the ID when called, and similarly on any other method. Because it's lazily initialized though, some of those calls may require a database query.
It's possible that the only real problem here is simply that it's confusing to have it so that any particular OrderItem could be a proxy or not. Maybe you want to simply change the relationships so that they're either both lazy, or both eager?
For what it's worth, it's a bit odd that you've got the ManyToMany relationship as EAGER and the ManyToOne as LAZY. That's exactly the reverse of the usual settings, so I would at least think about changing it (although I obviously don't know your entire use case). One way to think about it: If an OrderItem is so expensive to fetch completely that it's a problem when querying for Customer, surely it's also too expensive to load all of them at once? Or conversely, if it's cheap enough to load all of them, surely it's cheap enough to just grab it when you get a Customer?
I think you can force eager loading this way or using
def lastOrder = Order.withCriteria(uniqueResult: true) {
eq('id', current.lastOrderId)
items{}
}
or using HQL query with 'fetch all'
I've bumped into this example in JPA 2.0 FR Specification, 11.1.37. OneToOne Annotation, page 403:
#OneToOne(optional=false)
#JoinColumn(name="CUSTREC_ID", unique=true, nullable=false, updatable=false)
public CustomerRecord getCustomerRecord() { return customerRecord; }
Is there any reason that I should put #OneToOne(optional=false) and at that same time put #JoinColumn(... nullable=false)?
Aren't these two declarations the same? Isn't one of them redundant?
Are both of them used in DDL schema generation?
Formally optional=false is a runtime instruction to the JPA implementation, and nullable=false is an instruction to the DDL generator. So they are not strictly redundant.
The difference can become significant when there is entity inheritance involved. If a particular mapping exists only on a subclass, and you have single table table per-hierarchy strategy, then the OneToOne mapping may be optional=false on the particular subclass that contains the mapping. However, the actual join column cannot be made not-null, since then other sub classes that share the table can't be inserted!
In practice different versions of different providers may or may not interpret either one at either time, caveat emptor.
I have the following two annotated classes that I use to build a graph:
#Entity
#Table(name = "Edge")
public class Edge
{
/* some code omitted for brevity */
#ManyToOne
#JoinColumn(name = "ixNodeFrom", nullable = false)
private Node _nodFrom;
#ManyToOne
#JoinColumn(name = "ixNodeTo", nullable = false)
private Node _nodTo;
/* some code omitted for brevity */
}
#Entity
#Table(name = "Node")
public class Node
{
/* some code omitted for brevity */
#OneToMany(mappedBy = "_nodTo")
private Set<Edge> _rgInbound;
#OneToMany(mappedBy = "_nodFrom")
private Set<Edge> _rgOutbound;
/* some code omitted for brevity */
}
Now, when I build the graph, I issue two queries to fetch all rows from either table and set up the child / parent references, for which I need the ids stored in the Edge table.
Because I have defined the relation between the two tables in JPA, accessing the edge object to get the two nodes' ids triggers two SQL statements per edge, when the JPA provider lazily * loads the associated nodes. Since I already have the node objects, and the ids have already been loaded from the edge table, I want to skip those queries, as they take an awfully long time for larger graphs.
I tried adding these lines to the Edge class, but then my JPA provider wants me to make one mapping read-only, and I can't seem to find a way how to do that:
#Column(name = "ixNodeTo")
private long _ixNodeTo;
#Column(name = "ixNodeFrom")
private long _ixNodeFrom;
I'm using Eclipselink and MySQL, if it matters.
**The default behaviour for #ManyToOne actually is eager loading, see Pascal's answer*
I got three good answers that were equally helpful, and by now none percolated to the top by public vote, so I'm merging them together here for a single comprehensive answer:
a) Change the query
You can load the whole graph at once by changing the query, thereby giving the JPA provider a chance to realize that it already has everything in memory and doesn't need to go back to the DB:
List<Node> nodes = em.createQuery(
"SELECT DISTINCT n FROM Node n LEFT JOIN FETCH n._rgOutbound")
.getResultList();
(via axtavt)
b) Use read-only fields for the FKs
Loading the FKs into their own fields, as described in the question, will also work if, as the JPA provider is demanding, the fields are declared to be readonly, which is done like this:
#Column(name = "ixNodeTo", insertable = false, updatable = false)
(via bravocharlie)
c) Use property access
If you are using property access instead of field access, the JPA provider also gets a chance to realize it already has the FK and doesn't need to fetch the referenced object. In short, property access means that you put the JPA annotations on the getter, thereby "promising" the JPA provider that your getter won't go and access the rest of the object. More details in this question. This will work for Hibernate, and for Eclipselink, it will work (assumed in the original answer, experimentally confirmed by me) with weaving enabled. (via Pascal Thivent)
Additionally, as Pascal points out in his answer, #ManyToOne, contrary to my original post, is not lazy-loading, but eager-loading by default, and changing that will require weaving as well.
Have you tried
#Column(name = "ixNodeTo", insertable = false, updatable = false)
How can I retrieve the foreign key from a JPA ManyToOne mapping without hitting the target table?
In theory, a JPA provider should be able to not trigger a query when calling
someEdge.getNodeFrom().getId()
as it already has the id (as FK).
I'm 100% sure Hibernate can (assuming you're using property access). In the case of EclipseLink, I don't know (if it does, it will probably requires weaving).
Because I have defined the relation between the two tables in JPA, accessing the edge object to get the two nodes' ids triggers two SQL statements per edge, when the JPA provider lazily loads the associated nodes. Since I already have the node objects, and the ids have already been loaded from the edge table, I want to skip those queries, as they take an awfully long time for larger graphs.
Note that #ManyToOne uses an EAGER strategy by default. If you want to make it LAZY, you have to decalre it explicitly (but again, this will require weaving of your classes with EclipseLink).
I think you should try to optimize your query rather than change the mapping. For example, the following query fetches the whole graph at once (tested in Hibernate):
List<Node> nodes = em.createQuery(
"SELECT DISTINCT n FROM Node n LEFT JOIN FETCH n._rgOutbound")
.getResultList();
How about using getReference()?
For example:
Node fkNode = em.getReference(edge.getNodeFrom()); // [1]
fkNode.getId()
[1] This will not trigger a SQL query to retrieve the nodeFrom
I have two entity models, Customer and Order. Each customer could have thousands of orders. I have a OneToMany and ManyToOne relationship between these two entities.
How do I restrict this relationship's list to only top 10 orders?
Is it possible to apply 'WHERE' condition as an attribute on #OneToMany or not?
Like:
#OneToMany("Where Order.orderNo > 100")
My problem is when the object created by Entity Manager all Orders are created in memory.
Lazy loading can not solve my consideration, because I need to get top 10 orders in default construction.
I mean if it is possible to apply 'WHERE' condition as an attribute on #OneToMany or not?
Not with standard JPA. But some providers have extensions for this. For example, Hibernate does have a #Where annotation:
#OneToMany(cascade=CascadeType.ALL, fetch=FetchType.EAGER)
#Where(clause="1=1")
public Set<Ticket> getTickets() {
return tickets;
}
References
Hibernate Annotations Reference Guide
2.4.6. Collection related annotations
JPA does not support this. But in EclipseLink you can use an Expression to filter a relationship.
See,
http://wiki.eclipse.org/EclipseLink/Examples/JPA/MappingSelectionCriteria