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.
Related
one quick question for java hibernate/jpa users.
I have two tables(entities) A and B with relations as A has many B (one to many). Entity A has Set of values B in java.
Due to read performance issue i want to implement master-details denormalization, so i want to store raw Set object (maybe serialized) directly in entity A (because many to one relation cost me to much cpu time because of read by jpa (update is not an issue)).
The problem is, can i achieve something like that getBs always returns me denormalized object (so its fast) and addB adds new B to Set and updates denormalized object with new raw data that is prepared for faster read?
its oracle db.
entity example:
class A {
Long id,
String name;
Set<B> arrayOfBs;
byte[] denormalizedArrayOfB;
getArrayOfBs() {
return (Set<B>) denormalizedArrayOfB;
}
addArrayOfBs(B b) {
//persist b
// update and persist denormalizedArray with new b
}
//getters and setters...
}
class B {
Long id;
A reference;
String x;
String y;
//getters and setters...
}
That's complicated. There are better approaches to your problem:
You can simply replace the one-to-many association with a DAO query. So whenever you fetch the parent entities you won't be able to get the children collection (maybe they are way too many). But when you want to get a parent's children, you simply run a DAO query, which is also easier to filter.
You leave the children collection, but you use an in-memory cache to save the fully initialized object graph. This might sounds like a natural choice, but most likely you're going to trade consistency for performance.
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'
In my java application I am using equal objects multiple times at different places. That means the equals method returns true, when comparing theses objects. Now I want to update one object and make the changes to all objects that are equal. Do you know if there is a pattern for that?
My concrete use case is:
I am using JSF, JPA and CDI. A user is on web page that allows him to edit the detached entity EntityA. The page is sessionscoped. EntityA has two references to an EntityB (also detached). These objects can be same. Not the same reference, but they may be equal.
#Entity
public class EntityA {
#OneToOne()
private EntityB entity1;
#OneToOne();
private EntityB entity2;
}
The JSF view lets the uses select entity1 and entity2 from a selection list. It also shows some details of theses EntityBs and the user is allowed to edit entity1 and entity2 seperately. Everything works fine, except the user has choses the same (equal) EntityB for entity1 and entity2. Then, only the references to these objects are updated. Of course entity1 and entity2 are two different JPA entites, and are not the same reference. But I want to distribute the changes to all detached instances of EntityB. I have this situation hundreds of times in my application, so I dont want to take care about, which objects have to be updated in which situations. I need some solutation the does it for me automatically. One Idea was to keep all objects I use in this session in special list and every time a request was submitted and processed iterate over this map and change alle equal objects. But his sounds very dirty. Maybe there is a build in JPA function to make all equal objects the same reference. I dont know if this is possible. Do you have a solution for this? Thanks.
I'm going to abstract your problem out a bit here: if a change to one object requires changing a number of other objects, consider putting the field that you're changing in a separate object, and have all those objects reference it.
For example, if you have:
class MyClass {
String info;
int id;
}
and two instances of MyClass with the same 'id' should both be updated when the 'info' field changes then use this:
class myClass {
myInfoClass info;
int id
}
class myInfoClass {
String value;
}
and give all instances of myClass that are equal the same instances of myInfoClass. Changing myClass.info.value will effectively change all instances of myClass, because they all hold the same instance of myInfoClass.
Sorry if I've got the syntax slightly wrong, I jump between languages a lot.
I use this technique in a game I wrote recently where a switch activates a door- both the switch and door have a Circuit object that holds a boolean powered field. The doors 'isOpen()' method simply returned circuit.powered, and when the switch is activated I just call switch.circuit.powered = true, and the door is automatically considered 'open'. Previously, I had it searching the game's map for all doors with the same circuit id, and changing the powered field on each.
this is classic form handling logic
if the user clicks the save button manipulate the data in the database
reload the data every time you create the web page
you should not cache the data in the web session
if you need caching, activate it in the persistence layer (ex. hibernate cache)
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>();
Given a classic example of Student and Subject
where they have a many-to-many relationship,
is there any way to map them using POJOs w/o the use of Collections?
e.g.
Student.java
#Entity
class Student{
#id
int id;
String name;
}
Subject.java
#Entity
class Subject{
#id
int id;
String desc;
}
Tables
student (id,name)
subject (id,desc)
student_subject (student_id, subject_id) /* both foreign keys */
How will you query all subjects of a student?
Is it possible to generate these tables with the given beans?
Thanks in advance!
UPDATE: (I'll just give a background why I ask this ?)
My reason for avoiding Collections is that I would like my Service Layer to return data that is not tied to the Persistence Layer. Returning a Student object that has a list of Subjects will make my Service Layer Clients assume that they can get the subjects from the returned student object (then they'll get a LazyLoadException). If I make it EAGER loading, it would be overkill, since in many situations the client would only like the info about the Student and not get all his subjects.
To get all subjects, you need to join the tables like so:
select *
from subject
join student_subject
on subject.id = student_subject.subject_id
where
student_subject.student_id = ?
Is it possible to generate these tables with the given beans?
If you use the many-to-many mapping from Hibernate, it will create the query for you if you add the collections in the POJOs. Without the collection, you have to do it manually.
Note that the collections won't take memory unless:
You use them for the first time
Or you mark them a "load eagerly" in the POJO.
The default is lazy loading, so even if the tables are huge, you won't notice.
The question you should ask is, can you model your classes such that a many-to-many relationship can be established between them without collections?
It would be ideal to use collections and then let Hibernate use lazy-loading to populate the object graph.