The difference between #Entity and #Embeddable annotation when each one is added before class declaration?
the first create class as an entity, second insert column from another table?
the first create class as an table, while second is embedded in another class?
the first sets standard as a class, second define table type
the first create table for that class, second embed something into different class
the first define table property, second create union of two tables
#Entity annotation over a class defines that, it has a distinct separate existence. Thus we can run DB queries, without being dependent on any other class. #Embeddable annotation over a class defines that, it does not have independent existence. Thus we cannot run DB queries, without depending on other class. Here is an example to understand it better:
#Entity
User
-- long id
-- String name
-- String email
#Embedded
-- UserDetails userDetail
#Embeddable
UserDetails
-- Date dateOfBirth
-- String sex
-- String address
-- String maritalStatus
Here you can see without having a User, UserDetails is useless.
Generally, in OOP, we first design the classes and then we design database entities. For some classes (like UserDetails class in the above example), we do not want to have separate tables in DB, where their independent existence is meaningless. In those cases, we mark the class as embeddable.
Typically, embeddable classes share the same table as the Entity in which they are embedded
Entities have an identity and can be queried for. Embeddables have no identity of their own and can only be queried for using the owning entities.
If you open an entity class, you will always find the #Id annotation - it is mandatory. If you open an embeddable class, you will never find an #Id annotation - it is forbidden.
EDIT: It is not entirely correct that embeddables can only be stored as a part of the parent, i.e. in the same table. This is only true for one-to-one relationships. You can have Collections and Maps of embeddable objects in the parent entity and they will be mapped to own collection tables.
An entity class is an ordinary user defined Java class whose instances can be stored in the database.
#Entity
#Table(name="dog")
public class Dog{
#Id
#Column(name = "id")
private int id;
#Embedded
private Animal animal;
public Dog(int id,Animal animal){
this.id=id;
this.animal=animal;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public Animal getAnimal() {
return animal;
}
public void setAnimal(Animal animal) {
this.animal = animal;
}
}
Embeddable classes are user defined persistable classes that function as value types. As with other non entity types, instances of an embeddable class can only be stored in the database as embedded objects, i.e. as part of a containing entity object.
#Embeddable
public class Animal {
#Column(name = "name")
private String name;
#Column(name = "location")
private String location;
public Animal(){
}
public Animal(String name,String location){
this.name=name;
this.location=location;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getLocation() {
return location;
}
public void setLocation(String location) {
this.location = location;
}
}
It is an old topic but I would like to add my answer, which is more from theoretical point of view. In DDD (domain driven design) we usually have Entity and Value Objects. The first ones are identifiable only by a an identity that they have. The second ones are not defined by an identity, which means that if all the components that make that particular objects are the same, than the 2 value objects are the same.
The analogy is that in this case, if we were to apply DDD, the Entity is the class annotated with #Entity and the Value Object is the one with #Embeddable. A demonstration of this is the fact that the embeddable object is added as addditional information to an existing record, which already has its own identity defined externally to the embedded object.
Well #Entity signifies that the entity object has significance all by itself it doesn't require any further association with any other object. Where as #Embeddable object doesn't carry any significance all by itself, it needs association with some other object.
Lets take an example of say i have a Employee Object and it has a collection of Address Object as its member variable. Now when when speak of any address we need to tell whose address it is, which employees address it is. If we just talk about the address it doesn't make any sense. Hope this gives you the difference between the two.
Related
In the Hibernate docs, ยง 5.1.6.2 - Joined subclass strategy it states:
Each subclass must, however, declare a table column holding the object identifier.
The example it gives is as follows:
#Entity #Table(name="CATS")
#Inheritance(strategy=InheritanceType.JOINED)
public class Cat implements Serializable {
#Id #GeneratedValue(generator="cat-uuid")
#GenericGenerator(name="cat-uuid", strategy="uuid")
String getId() { return id; }
...
}
#Entity #Table(name="DOMESTIC_CATS")
#PrimaryKeyJoinColumn(name="CAT")
public class DomesticCat extends Cat {
public String getName() { return name; }
}
I am guessing that name is not the object identifier, so what in the DomesticCat class is the "table column" that holds the "Object identifier"? What would I usually have to do to add this particular required column to my subtypes?
This is probably referring to the "id" field (not shown, but implied by the getId() method). This field is the unique identifier for the row in the "CATS" table containing the information you are looking for. It looks like it is setting the Type of id as String and is expecting a UUID. The sub-types will inherit this field due to their extension of the main class.
I have two Entity classes which are both written into a separate table. In my database there is no connection between the two tables via key, but I want to implement a logical association between the entities.
#Entity
public class One{
#ID
long id;
String name;
List<Two> aListOfTwos;;
public List<Two> getAListOfTwos(){
//return some Twos based on some custom query, p.e. "All Twos created last week"
}
}
#Entity
public class Two{
#ID
long id;
Date createdAt;
String name;
}
Of course I could just write an appropriate query in the getAListOfTwos method, but the I would need to have access to the Session Object which I don't want. Is there an other way to specify such an association in Hibernate?
Many Thanks
Paul
You can go for one to many mapping using join table. And if your association is manytomany, it anyway needs a third table.
I am trying to use an inner class as embeddable to represent some complicated properties of the outer class. When I store this, there is no information from the inner class in the database schema generated by eclipselink.
Does what I'm trying to do seem like a good idea? Why doesn't eclipselink seem to recognize them #Basic attribute on the getRate() in Attributes?
Some other info: Measure must be instantiated using a factory which is provided to the constructor of Person, so I don't even know how I'm going to be able to use this at all. It seems more and more likely that I'll have to make a separate class just to store the state of Person in simple terms (like doubles, not Measures) and use those to create the real Person-type objects, but that has very sad implications for the rest of my application.
#Entity
public static class Person {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private int id;
#Transient
public Measure<Double, CleanupRate> rate;
#Embedded
private Attributes attributes;
#Embeddable
public static class Attributes {
#Transient
private Person person;
public Attributes() {
}
public Attributes(Person person) {
this.person = person;
}
#Basic
public double getRate() {
return person.rate.getInternalValue();
}
public void setRate(double value) {
person.rate.setInternalValue(value);
}
}
public Person() {
rate = udm.getMeasureFactory().createMeasure(0.0, CleanupRate.class);
attributes = new Attributes(this);
}
public void setRate(double rate) {
this.rate.setValue(rate);
}
}
Edit:
In order to inject the measure dependency into my objects when they are retrieved from storage, I've added an interface which injects the dependency and used it in my DAO. Since the DAO can be injected, I can propagate the dependency down to the retrieved objects. I got the idea from a blog.
private <T extends UomInjectable> List<T> //
getListOfUomInjectableType(final Class<T> klass) {
List<T> result = getListOfType(klass);
for (UomInjectable injectable : result) {
injectable.injectUomFactory(udm);
}
return result;
}
It is using the access type from the Person class, which is set to field, and so not seeing the annotation at the property level.
You will need to change the access type using Access(PROPERTY) on the embeddable class, and should remove the #Transient annotation on the person attribute.
I think in general you're going to be in trouble having Entities (Embeddable or otherwise) that need constructors with arguments. I'm not sure how that might be related to your schema generation issue, but I think this will be a problem trying to persist/retrieve these objects.
As you hinted, JPA requires all entity types to have a no-argument constructor. While your Attributes class has one, it leaves the 'person' field as null which will fairly quickly result in NPE's. Same with the Person constructor (maybe you left out the one that passes in 'udm' from the sample code?).
The set the Person for the Attributes, just use property access in Person and set it in your setAttributes method.
See,
http://en.wikibooks.org/wiki/Java_Persistence/Embeddables#Relationships
My data model represents legal entities, such as a Business or a Person. Both are tax-paying entities, and both have a TaxID, a collection of phone numbers, and a collection of mailing addresses.
I have a Java model with two concrete classes that extend an abstract class. The abstract class has properties and collections that are common to both concrete classes.
AbstractLegalEntity ConcreteBusinessEntity ConcretePersonEntity
------------------- ---------------------- --------------------
Set<Phone> phones String name String first
Set<Address> addresses BusinessType type String last
String taxId String middle
Address Phone
------- -----
AbsractLegalEntity owner AbstractLegalEntity owner
String street1 String number
String street2
String city
String state
String zip
I'm using Hibernate JPA Annotations on a MySQL database, with classes like this:
#MappedSuperclass
public abstract class AbstractLegalEntity {
private Long id; // Getter annotated with #Id #Generated
private Set<Phone> phones = new HashSet<Phone>(); // #OneToMany
private Set<Address> address = new HashSet<Address>(); // #OneToMany
private String taxId;
}
#Entity
public class ConcretePersonEntity extends AbstractLegalEntity {
private String first;
private String last;
private String middle;
}
#Entity
public class Phone {
private AbstractLegalEntity owner; // Getter annotated #ManyToOne #JoinColumn
private Long id;
private String number;
}
The problem is that Phone and Address objects need to refer to their owner, which is an AbstractLegalEntity. Hibernate complains:
#OneToOne or #ManyToOne on Phone references an unknown
entity: AbstractLegalEntity
It seems like this would be a fairly common Java inheritance scenario, so I hope that Hibernate would support it. I've tried changing the mapping for AbstractLegalEntity based on a Hibernate forum question, no longer using #MappedSuperclass:
#Entity
#Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
However, now I get the following error. When reading up on this inheritance mapping type, it looks like I have to use SEQUENCE not IDENTITY, and MySQL doesn't support SEQUENCE.
Cannot use identity column key generation with <union-subclass>
mapping for: ConcreteBusinessEntity
I'm making more progress toward getting things working when I use the following mapping.
#Entity
#Inheritance(strategy = InheritanceType.SINGLE_TABLE)
#DiscriminatorColumn(
name="entitytype",
discriminatorType=DiscriminatorType.STRING
)
I'm thinking I should continue down this path. My concern is that I'm mapping it as an #Entity when I really don't ever want an instance of AbstractLegalEntity to ever exist. I'd like to know if this is the right approach. What is the correct approach I should be taking for this situation?
Use:
#Entity
#Inheritance(strategy = InheritanceType.JOINED)
AbstractLegalEntity
In the database you will have one table for AbstractLegalEntity, and tables for classes, which extend AbstractLegalEntity class. You won't have instances of AbstractLegalEntity if it's abstract. Polymorphism can be used here.
When you use:
#MappedSuperclass
AbstractLegalEntity
#Entity
ConcretePersonEntity extends AbstractLegalEntity
This will create only one table in your database called ConcretePersonEntity, containing columns from both classes.
Add #Entity annotation to AbstractLegalEntity. Instance of AbstractLegalEntity will never exist - hibernate will load appropriate extending instances - ConcreteBusinessEntity or ConcretePersonEntity according to Id field.
You have to declare AbstracLegalEntity as an #Entity. Even with the #Entity annotation, your class remains abstract. consequently, you will only have instance of concrete subclasses.
I have two hibernate classes: a base class, and an extended class that has additional fields. (These fields are mapped by other tables.)
For example, I have:
#Entity
#Table(name="Book")
public class A {
private String ID;
private String Name;
// ...
}
#Entity
#Table(name="Book")
public class B extends A {
public String node_ID;
// ...
}
public class Node {
public String ID; // maps to B.node_ID
// ...
}
How do I map this in Hibernate? The hibernate documentation states three types of inheritence configurations: one table per class, one table with a type column, and a join table -- none of which apply here.
The reason I need to do this is because class A is from generic framework that's reused over multiple projects, and class B (and Node) are extensions specific to one project -- they won't be used again. In the future, I may have perhaps a class C with a house_ID or some other field.
Edit: If I try the above pseudo-code configuration (two entities mapped to the same table) I get an error that the DTYPE column doesn't exist. The HQL has a "where DTYPE="A" appended.
This is possible by mapping the #DiscriminatorColumn and #DiscriminatorValue to the same values for both classes; this can be from any column you use that has the same data regardless of which type (not sure if it works with null values).
The classes should look like so:
#Entity
#Table(name="Book")
#Inheritance(strategy=InheritanceType.SINGLE_TABLE)
#DiscriminatorColumn(name="published")
#DiscriminatorValue(value="true")
public class A {
private String ID;
private String Name;
// ...
}
#Entity
#Table(name="Book")
#DiscriminatorValue(value="true")
public class B extends A {
public String node_ID;
// ...
}
For anyone who got here like me and does not want to have the dtype column but instead want to use the same table for more than one entity as is I would recommend using this
Basically you can create a Base like this
#MappedSuperclass
public abstract class BaseBook<T extends BaseBook> {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "id", nullable = false)
private Long id;
... any other variables, getters + setters
}
#Entity
#Table(name= "book")
public class BookA extends BaseBook<BookA>{
//Default class no need to specify any variables or getters/setters
}
#Entity
#Table(name= "book")
public class BookB extends BaseBook<BookB>{
#Column(name = "other_field")
private String otherFieldInTableButNotMapedInBase
... Any other fields, getter/setter
}
From the above we have created base super class which does not have any entity or table mapping. We then create BookA to be default with the Entity + Table mapping. From there we can create other Entities all extending from BaseBook but pointing to one table