All I could gather from Google is that:
Hibernate uses a proxy object to implement lazy loading.
When we request to load the Object from the database, and the fetched Object has a reference to another concrete object, Hibernate returns a proxy instead of the concrete associated object.
Hibernate creates a proxy object using bytecode instrumentation (provided by javassist). Hibernate creates a subclass of our entity class at runtime using the code generation library and replaces the actual object with the newly created proxy.
So, what exactly does the Proxy object contain?
Does it contain a skeleton object reference object with only the id field set? Others field will be set when we call the get method?
Does the Proxy object contain the JDBC statement to fetch all data required to fully populate the referenced object.
Is there something else I might be missing?
I am not asking for spoon feeding but if you can provide any link with information that would be great.
Any correction to above description will also be welcomed.
Example.
class Address {
String city;
String country;
}
class Person{
int id;
String name;
Address address;
}
When we try to load the Person object, Hibernate will subclass Person class like:
class ProxyPerson extends Person {
int id;
String name;
Address proxyCGLIBObject;
}
and return a ProxyPerson object. Object of ProxyPerson will have a value for id and name but proxy for Address.
Am I correct?
What can I expect from adding a toString() method on the proxy object?
The Hibernate Proxy is used to substitute an actual entity POJO (Plain Old Java Object).
The Proxy class is generated at runtime and it extends the original entity class.
Hibernate uses Proxy objects for entities is for to allow [lazy loading][1].
When accessing basic properties on the Proxy, it simply delegates the call to the original entity.
Every List, Set, Map type in the entity class is substituted by a PersistentList, PersistentSet, PersistentMap. These classes are responsible for intercepting a call to an uninitialized collection.
The Proxy doesn't issue any SQL statement. It simply triggers an InitializeCollectionEvent, which is handled by the associated listener, that knows which initialization query to issue (depends on the configured fetch plan).
Related
I am in process of migrating an application to the latest spring boot version (using gradle spring-boot-dependencies with version 2.5.4).
I have an entity called Customer which is annotated with #Document(indexName = "customer"); and another entity Address which is annotated with #Document(indexName = "address"). Customer class has private Set<Address> addresses = new HashSet<>().
Getting the below error, while calling List hits = elasticsearchTemplate
.search(nativeSearchQuery, Customer.class) from the repository.
org.springframework.data.mapping.model.MappingInstantiationException:
Failed to instantiate java.util.Set using constructor NO_CONSTRUCTOR with arguments.
It is working if I follow similar MONGO suggestion Failed to instantiate java.util.Set using constructor NO_CONSTRUCTOR with arguments and make the declaration private HashSet<Address> addresses = new HashSet<>(). I can't follow this as we have hundreds of entities and their JPA criteria mappings built with JPA Metamodel are failing.
please note that I have already reviewed the below 2 threads and feel it's different:
Indexing problem with Spring Data Elastic migration from 3.x to 4.x
Spring data elasticsearch: Using #Document annotation on POJO interface class not working
if you are looking for a sample repository: https://gitlab.com/mvallapuneni/spring-es-sample
Any help is appreciated...
java.util.Set is an interface not a class.
Interfaces do not have constructors, they are joined to a class that "implements" (has the operating code methods of) the interface.
All method signatures in an interface are abstract , they have no code, only the signature of methods supplied by the implementing class.
e.g. HashSet is a class that implements java.util.Set
See the API documentation at the top of its page for "all known implementing classes"
Your HashSet is private it must use private methods to call on it e.g. set or get e.t.c.
To hazard a guess the List must become a Set but List is instantiated and Set is a variable waiting.
Somewhere in the code in private methods this goes like (figuratively)...
private Set<Address> addresses = (Set<Address>)(Collection<Address>)elasticsearchTemplate.search(nativeSearchQuery, Customer.class);
Note that in that line they are all interfaces but to have an instantiated interface means there is a class attached that implements the interface List , however the class should also be able to use interface Set.
I just presume the List returned is actually in a HashSet and the instantiation is simply reserving space in memory like "C" language, just a helper that is not required to have any class instantiated at that time.
A HashSet takes a Collection in its constructor and a Collection can be either a Set or a List.
Don't know how to properly formulate the title, but here is the problem.
Let's say I have a class Person and a class Organization and their association type is Many to One. In Java code Person object has a reference to Organization object and Organization has a Collection of Person objects. Both Person and Organization objects are Serializable. In addition to that I have programmed Person in such a way that it is affinity collocated with Organization.
If I put an instance of Person object into the cache (grid node) I would like to serialize only identifier of associated Organization, but not the whole object that is being referenced.
Is there any solution out of the box (some kind of proxy) that will skip serialization of associated object and will lazy load the associated object by identifier on a deserialized instance of Person?
I have already seen a custom solution for Oracle Coherence grid that is based on decorators, but the implementation is extremely complex.
How about storing an ID of an organization instead of the whole Organization object? This is the proper way to collocate people with their organizations.
If you need to store an actual Organization object inside of a Person instance, you may make a transient field and update it lazily, when you access it. Something like this:
class Person {
#AffinityKeyMapped
private int orgId;
private transient Organization org;
// Other fields, constructors, getter, setters.
}
// ...
// Accessor method in DAO logic.
Organization getOrganization(Person person, IgniteCache<Integer, Organization> orgCache) {
Organization org = person.getOrganization();
if (org == null) {
org = orgCache.get(person.getOrganizationId());
person.setOrganization(org);
}
return org;
}
This way org field won't be serialized, when Person is put into Ignite cache. Only orgId will be. And Organization will be requested from cache only once for a Person object. Subsequent calls to getOrganization() will use a saved value.
I have a form that should be bind to a complex object that wrap a lot of children, every time before loading this form I have to initialize all children object in a method that only have a lot of new statements and calling a setter method, I have to repeat this scenario for a lot of forms and other complex objects
Is there a better strategy than the initializeEmployee method?
For example:
#Entity
public class Employee {
Integer Id;
Contract contract;
Name name;
List<Certificate> list;
// getter and setters
}
#Entity
public class Contract {
String telephoneNum;
String email;
Address address;
// getter and setters
}
#Entity
public class Address {
String streetName;
String streetNum;
String city;
}
public class Name {
String fName;
String mName;
String lName;
// getter and setters
}
// And another class for certificates
public initializeEmployee() {
Employee emplyee = new Employee();
Name name = new Name();
employee.setName(name);
Contract contract = new Contract();
Address address = new Address();
contract.setAddress(address);
employee.setContract(contract);
// set all other employee inner objects,
}
EDIT:
According to below answers, it seems that there is no optimal answer. However, I could use the Entity constructor or a Factory Design Pattern.
But both solutions don't solve my other problem in initializing all fields strategy with Required and Optional fields.
For example:
If I have Name as required (i.e. the Employee entity will not persisted if Name object attributes are empty, on the other side the Contract entity is an optional. and I cannot persist an empty Contract object to the database, so I have to make it null first before persistence, then reinitialize it after persistence like the following
// Set Contract to null if its attributes are empty
Contract contract = employee.getContract()
if(contract.getTelephoneNum().isEmpty && contract.getEmail().isEmpty() && contract.getAddress().isEmpty()){
empolyee.setContract(null);
}
employeeDAO.persist(employee);
// reinitialize the object so it could binded if the the user edit the fields.
employee.setContract(new Contract());
You can add constructors (it is their role after all) to your entities to instanciate these fields if having a null value has no meaning for your case.
Another way, if you don't like adding contructors, is to add a static factory method to instanciate your bean which will look like initializeEmployee() but with potential parameters and returning an Employee object. http://en.wikipedia.org/wiki/Factory_method_pattern
Similarly, you can instanciate your collections too, as there is probably no meaning for a null collection (but there is one for an empty collection).
You can add behaviour to your entities, don't be locked in Anemic Domain Model which is considered an anti-pattern by Martin Fowler http://www.martinfowler.com/bliki/AnemicDomainModel.html
EDIT
I see you are using dao.persist(entity): you are probably using JPA. If so, maybe it is best to not modify your object graph (on the front side) and add an EntityListener (in the persistence layer) for Employee: here is a link for Hibernate EntityListener (it is a JPA feature, so if you are using another framework don't worry) http://docs.jboss.org/hibernate/entitymanager/3.5/reference/en/html/listeners.html
With an EntityListener, you can add small "aop like" actions before persistence and after. This will allow you to not deal with null values on the domain and front layers and will ensure that every entity fits in any case (better validation).
In PrePersist: you'all add your code to check null values (possibly with custom methods "isEmpty()" on the domain classes) and nullify fields if needed. In PostPersist you add your new object.
I couldn't get what you really need, but I think you could try this way:
#Entity
public class Employee {
Integer Id;
Contract contract = new Contract();
Name name = new Name();
List<Certificate> list;
// getter and setters
}
#Entity
public class Contract {
String telephoneNum;
String email;
Address address = new Address();
// getter and setters
}
I'm not sure it reduces the verbosity at all but since this is a UI issue, you could initialize the editable objects in your flow.xml and then put it all together in an Employee instance prior to saving to the DB.
<on-start>
<evaluate expression="new foo.bar.Name()" result="flowScope.employeeName" />
<evaluate expression="new foo.bar.Contract()" result="flowScope.contract" />
<evaluate expression="new foo.bar.Address()" result="flowScope.address" />
</on-start>
Actually I would advise against using Hibernate Entities directly in GUI. In many cases (I assume in yours too, but I'm missing some details on your use-case) it is useful to use a Data Transfer Object pattern instead. You can create such DTO that is GUI specific, has only those fields that you need, and the structure is only as complex as needed.
After specific user action (like save e.g.) use those DTOs (on event handling) to create your Entities that will be persisted.
Unless your case is that just entering the GUI screen causes Entities creation, then I would recommend Factory pattern.
Also note that in many cases initialization of component objects that are making up the main object (Employee in your example) are better to be initialized in constructor of main object, eg. if you expect that Contract cannot be null - initialize it in constructor. The same for the list of Certificates and others.
I'm working on a JPA project. I have an ExportProfile object:
#Entity
public class ExportProfile{
#Id
#GeneratedValue
private int id;
private String name;
private ExtractionType type;
//...
}
ExtractionType is an interface implemented by several classes, each for a different extraction type, these classes are singletons.
So type is a reference to a singleton object. I don't have an ExtractionType table in my DB, but i have to persist the extraction type of my export profile.
How can I persist the ExportProfile object using JPA, saving the reference to type object?
NOTE: The number of ExtractionType implementations is not defined, because new implementation can be added anytime. I'm also using Spring, can this help?
Here's an idea: make an ExtractionTypeEnum, an enumeration with one element for each of the possible singletons that implement ExtractionType, and store it as a field in your entity, instead of ExtractionType . Later on, if you need to to retrieve the singleton corresponding to a ExtractionTypeEnum value, you can implement a factory that returns the correct singleton for each case:
public ExtractionType getType(ExportProfile profile) {
switch (profile.getExtractionTypeEnum()) {
case ExtractionTypeEnum.TYPE1:
return ConcreteExtractionType1.getInstance();
case ExtractionTypeEnum.TYPE2:
return ConcreteExtractionType2.getInstance();
}
}
In the above, I'm assuming that both ConcreteExtractionType1 and ConcreteExtractionType2 implement ExtractionType.
I'm observing a very strange behaviour with an entity class and loading an object of this class whith JPA (hibernate entitymanager 3.3.1.ga). The Class has a (embedded) field, that is initialized in the declaration. The setter of the field implements a null check (i.e. would throw an exception when a null value is set).
...
#Entity
public class Participant extends BaseEntity implements Comparable<Participant> {
...
#Embedded
private AmsData amsData = new AmsData();
public void setAmsData(AmsData amsData) {
Checks.verifyArgNotNull(amsData, "amsdata");
this.amsData = amsData;
}
...
}
When I get this object with JPA, the field is null, if there is no data in the db for the fields specified in the embedded object.
...
public class ParticipantJpaDao implements ParticipantDao {
#PersistenceContext
private EntityManager em;
#Override
public Participant getParticipant(Long id) {
return em.find(Participant.class, id);
}
...
}
I debugged the process with a watchpoint on the field (should halt when the field is accessed or modified), and I see one modification when the field is initialized, but when I get the result from the find call, the field is null.
Can anybody explain, why this is so? How can I ensure, that the field is not null, also when there is no data for the embedded object's fields in the db (besides from setting it manually after the find call).
The JPA specification doesn't explicitly say how to handle a set of columns representing an embeddable object which are all empty. It could signal a null reference, or an object instance with all null fields. Hibernate chooses a null reference in this case, though other JPA implementations may pick the later.
The reason why your setter is never called is because Hibernate is accessing your field via reflection, bypassing the setter you implemented. It's doing this because you utilize field-based access rather than property-based access.
Chad's answer would provide the functionality you're looking for, but there is a caveat (see below).
"...The persistent state of an entity
is accessed by the persistence
provider runtime[1] either via
JavaBeans style property accessors or
via instance variables. A single
access type (field or property access)
applies to an entity hierarchy. When
annotations are used, the placement of
the mapping annotations on either the
persistent fields or persistent
properties of the entity class
specifies the access type as being
either field- or property-based access
respectively..." [ejb3 persistence
spec]
so by moving the annotations down to the setter, you are telling JPA that you want to used property-based access instead of field-based access. You should know, however, that field-based access - as you currently implement it - is preferred over property-based access. There are a couple reasons why property-based access is discouraged, but one is that they you're forced to add getters and setters for all of your persistent entity fields, but you may not want those same fields susceptible to mutation by external clients. In other words, using JPA's property-based access forces you to weaken your entity's encapsulation.
The answer is (thanks to rcampell), if all data of an embedded object is null (in the db), the embedded object will also be null, although when it is initialized in the declaration. The only solution seems to be, setting the object manually.
#Override
public Participant getParticipant(Long id) {
Participant participant = em.find(Participant.class, id);
if(participant != null && participant.getAmsData() == null)
{
participant.setAmsData(new AmsData());
}
return participant;
}
Still feels strange to me ...
Well, it's possible that your object could be getting constructed twice behind the scenes. JPA implementations will usually set those fields directly.
I think you need to put the annotations on the Getters and setters themselves if you want them to be used. See this answer:
Empty constructors and setters on JPA Entites
It's 2018 now and I had the same problem in a similiar situation.
Using your code as example, I solved the problem like this:
#Entity
public class Participant extends BaseEntity implements Comparable<Participant> {
...
#Embedded
private AmsData amsData = new AmsData();
public void getAmsData(AmsData amsData) {
Checks.verifyArgNotNull(amsData, "amsdata");
this.amsData = amsData;
}
public AmsData getAmsData(){
if(amsData == null){
amsData = new AmsData();
}
return amsData;
}
...
}
I was having the same problem , I just added getters and setters using #Getter and #setter lombok annotations and it started working