What is the difference between #Embedded annotation technique and #OneToOne annotation technique because in Embedded the java class contain "Has a" relationship in class and with the help of #Embedded annotation we persist the has a object in database. and in OneToOne relationship we also persist the has a object in database.
#OneToOne is for mapping two DB tables that are related with a one to one relationship. For example a Customer might always have one record in a Name table.
Alternatively if those name fields are on the Customer table (not in a separate table) then you might want an #embedded. On the face of it you could just add the name fields as standard attributes to the Customer entity but it can be useful if those same columns appear on multiple tables (for example you might have the name columns on a Supplier table).
Its the difference between composition and aggregation. #Embedded objects are always managed within the lifecycle of their parents. If the parent is updated or deleted, they are updated or deleted as well. #OneToOne objects may mimic composition via the cascadeType option of their #Join annotation, but by default they are aggregated, aka their lifecycle is separate from that of their parent objects.
#Embedded is used with Value Objects (Objects which have a meaning only when attached to an Object) whereas one to one mapping is between two objects having their own existence and meaning.
For e.g.
Value Object and #Embedded: If we have a User class and this class has an address Object in it, it can be considered as a value object as the address alone does not have any significance until unless associated with a user. Here address object can be annotated with #Embedded.
One to One mapping and #OneToOne: If we have a User class and this class has a 'Father' Object or a 'Mother' object, we would want to annotate the 'Father' or 'Mother' instance as #OneToOne as 'Father' or 'Mother' have their own meaning and existence and are not Value objects to User class.
A closely related difference is between #OneToMany and #ElementCollection. Both are used to save instance variables of Collection type in Java class. The difference being, #ElementCollection is to be used when the elements of Collection being saved are Value Objects whereas #OneToMany is used when the elments and object have well defined meaning and existence.
Use #OneToOne, only if fields can be reused. Otherwise, go for #Embeddable.
A quote from Beginning Hibernte, 3rd Edition:
There is nothing intrinsically wrong with mapping a one-to-one association between two entities where one is not
a component of (i.e., embedded into) the other. The relationship is often somewhat suspect, however. You should
give some thought to using the embedded technique described previously before using the #OneToOne annotation.
#Embeddable:
If the fields in an entity (X) are contained within the same table as another entity (Y), then entity X is called "component" in hibernate terms or "embedded" in JPA terms. In any case, JPA or hibernate do not allow to use 2nd table to store such embedded entities.
Generally, we think of normalizing a table when data is being reused by more than one table. Example: A Customer (id, name, street, city, pin, landmark) can be normalized into Customer(id, name) and CustomerAddress(cust_id, street, city, pin, landmark). In this case, we can reuse CustomerAddress by linking the same using cust_id with other tables. But if this reuse is not required in your application, then we can just keep all columns in one table.
So, a thumb rule is,
If reuse -> #OneToOne,
If no reuse -> #Embeddable
#Embedded is typically to represent a composite primary key as an embeddable class:
#Entity
public class Project {
#EmbeddedId ProjectId id;
:
}
#Embeddable
Class ProjectId {
int departmentId;
long projectId;
}
The primary key fields are defined in an embeddable class. The entity contains a single primary key field that is annotated with #EmbeddedId and contains an instance of that embeddable class. When using this form a separate ID class is not defined because the embeddable class itself can represent complete primary key values.
#OneToOne is for mapping two DB tables that are related with a one to one relationship. #Id will be the primary key.
Related
Is it possible to annotate a class as #Embeddable or a property as #Embedded?
Sample code:
#Embeddable
class A{
...
}
class B{
...
}
#Entity
class Foo {
A a;
#Embedded B b;
}
When to prefer #Embedded and #Embeddable?
There are two primary uses for #Embedded/#Embeddable as far as I know:
First, and most important: Splitting up large entity classes. In the database world, a large table (one with many columns) is fine. Breaking up such a table might even make things worse, and be in collision with database design principles. In Java (or object oriented languages in general), on the other hand, a large class is a code smell. Here, we would like to split the classes, including entity classes, into smaller units. #Embedded/#Embeddable allows us to easily do this without having to split the database table.
Second, it allows for reuse of common mappings between entities. Say each table has a simple revision tracking, with two columns containing the username of the person who changed the row, and the time it happened. Then, one can make an #Embeddable entity covering these rows, and then reuse this across all entities by embedding it (rather than repeating the variables corresponding to these columns in each entity.)
If we have Person and Address that are two POJOs, You would not want to create another table for Address but you would want to embed the address within the person table. So Address is adding up value to the Person object but doesn't make any sense individually. In this case we may go with:
#Embeddable
public class Address{
}
#Entity
public class Person
{
#Embedded
private Address address;
}
You would use #Embeddable and #Embedded together. You mark your class as #Embeddable, which indicates that this class will not exist in the DB as a separate table. Now when you use #Embedded on the field itself.
The word embeddable and embedded gives you a big clue actually.
Embeddable = This class can be embedded in a class
Embedded = This class will now be embedded in your class as a field.
I guess if you annotate class as #Embeddable you don't need to annotate field as #Embedded. Also, if you annotate class as #Embeddable and you want to use it as primary key, you can use #Id only, but if it is not annotated as #Embeddable, you have to use #EmbeddedId on field to work as primary key.
In Hibernate, to specify a column for joining association, #JoinColumn annotation in used, for example like this:
#ManyToOne
#JoinColumn(name="address_id")
public Address getAddress() {
return address;
}
In most cases, name of the column is snaked-cased class name plus _id. So it is reasonable to expect from Hibernate to derive it automatically (as it is done, for example, in Django's ORM). But is such behavior implemented somehow?
It is not necessary, JPA follows convention over configuration principle which means there are allways some default values that you can override with annotations.
In case of #JoinColumn, the default column name is generated like this: <field_name>_<id_column_name>
field_name is address in your case, and id_column_name is referring to the related entity's id, which is id. Thus, you get address_id by default.
It is not necessary to have #JoinColumn annotation. You can always override it. If you won't provide it in your code then Hibernate will automatically generate one for you i.e. default name for your column.
What's the use of #Embedded and #Embeddable In Hibernate ? Because every example i found on internet is inserting data inside of a single table and to do that using two different class. My point is if I am using a single table then I can map all the columns inside of a single class then why should i use different class. and if We use two different table then there is one-to-one and one-to-many hibernate relationship.
There are two types of objects in Hibernate
1. Value Object
2. Entities
Value Objects are the objects which can not stand alone. Take Address, for example. If you say address, people will ask whose address is this. So it can not stand alone.
Entity Objects are those who can stand alone like College and Student.
So in case of value objects preferred way is to Embed them into an entity object.
To answer why we are creating two different classes: first of all, it's a OOPS concept that you should have loose coupling and high cohesion among classes. That means you should create classes for specialized purpose only. For example, your Student class should only have the info related to Student.
Second point is that by creating different classes you promote re-usability.
When we define the value object for the entity class we use #Embeddable.
When we use value type object in entity class we use #Embedded
suppose we have employee table annotated with #entity and employee has Address so here i dont want to create two tables i.e employee and address, i just want create only one table i.e employee not Address table then we need to declare Address instance in Employee and add #embedable annotation on top of Address class, so finally we get table employee with its record and address records as well in single employee table
One entity can be embedded in another entity. The attributes of an entity can be common attributes of more than one entity. In this case there can be one embeddable entity. And this embeddable entity can be embedded in more than one entity.
Let's consider an example. We have one Animal entity, which has name and location attributes. Now two different entities Lion and Elephant can have Animal attributes just by embedding the Animal entity. We can override the attributes. In Animal entity there is location attribute and in Elephant there is place attribute. So with the help of #AttributeOverrides we can do like below:
#AttributeOverrides({ #AttributeOverride(name = "location", column = #Column(name = "place")) })
Is there a possibility in JPA 2.0 to asure that an embedded object is embedded with only one object, but not several?
In my case I have an Address that I can assign to a Customer. I want every customer to use its own address object and would like to create a constraint that makes sure that no two customers share the actually same object.
My code looks like this:
#Entity
public Customer {
#Id
#GeneratedValue
private Long id;
#Embedded
private Address address;
// ..
}
#Embeddable
public Address {
private String street;
private String city;
// ..
}
Currently, if I create two customers and assign them the same Address object, then persist and read them, they again share the object with the same identity. I want to prohibit saving such customers that share addresses with other customers.
The simpliest approach in this case is to create a copy of Address object in Customer.setAddress().
Also, I'm not sure that different Customers can share Address with the same identity when retrieved from the database. Perhaps you get the same objects from the session cache because you save and read them in the same session.
It is the nature of the #Embedded mechanism that embedded instances of an embeddable class are NEVER shared among different instances of the enclosing class. If you observed this behavior in your code, it must have been because you were accessing cached data during reading from the entity manager.
So, even if you assign the same instance of an embeddable class to multiple instances of the enclosing class, then "persist()", then destroy the entity manager and EntityManagerFactories, or invalidate the caches "entityManager.getEntityManagerFactory().getCache().evictAll()", then create a new EntityManager and "find()" the enclosing objects, each of them should have their own instance of (in your case) "Address" objects, even if their content is the same.
The JPA spec says the following about embedded objects in section 2.5:
[...] Instances of these classes, unlike entity instances, do not have
persistent identity of their own. Instead, they exist only as part of
the state of the entity to which they belong. [...]
If your JPA implementation doesn't adhere to that, it isn't really JPA standards-compliant...
I have banged my self with a very particular problem. Using OpenJPA (KODO 4.1) is there a way to use more than one column as a discriminator column?
My problem is that i have a table structure (which i have limited ability to modify of course) similar to this:
Table VEHICLE EXPENSIVE_CAR CHEAP_CAR EXPENSIVE_BOAT CHEAP_BOAT
---------------------------------------------------------------------------------
HORSE_POWER LUXURY_ACC CLASIFICATION SIZE SIZE
MEDIUM EXTRAS TV_SIZE
IS_EXPENSIVE CLASIFICATION
Where medium would discriminate between boat and car and is expensive would discriminate bettwen expensive or cheap.
So, is there any way to achieve this with the inheritance capabilities provided by OpenJPA (i know hibernate can use discriminator formulas but i am trying not to switch from the default JPA provider).
As a bonus if you can tell me about the custom discriminator strategies from OpenJPA that would be great since i have a hunch that it could be a plausible solution (even though i would prefer a vendor independent one)
Thanks a lot
Let's start backwards.
Discriminator strategies define the type of the column that discriminates related entities in the hierarchy. In JPA 1.x, it can be either a string (this is the default), a char and an integer.
Example:
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
#Entity
#DiscriminatorColumn(name = "TYPE", discriminatorType = DiscriminatorType.STRING, length = 5)
#DiscriminatorValue("FOO")
public class Foo { ... }
#Entity
#DiscriminatorValue("BAR")
public class Bar extends Foo { ... }
#Entity
#DiscriminatorValue("BAZ")
public class Baz extends Baz { ... }
If you are using is single table inheritance strategy as the default, this setup means that all these entities that are in the hierarchy will be mapped to the database table of the parent class, meaning you'll have a FOO table in your db with all the attributes of classes Foo, Bar and Baz plus a discriminator column called TYPE with the type of string (most likely some varchar-variant, length 5) and for each entity type, the respective discriminator value will be inserted automatically when persisted.
When you are finding Bar or Baz entities with JPQL, JPA will be able to find the entities from the FOO table (because that is the parent entity's table), and by relying on the contents of the discriminator column, your JPA provider will be able to discriminate between creating some Bar or Baz entities.
If you'd set the discriminator type to INTEGER or CHAR, then you could write for the values 1, 2, 3 or "A", "B", "C" etc. respectively.
Now to the OpenJPA question.
AFAIK there's no way to easily specify multiple discriminator values with OpenJPA, but you can create some more complex entity hierarchies, so if you'd be able to modify the schema, you could create a Vehicle entity, a Car, a Boat and both an ExpensiveBoat and an ExpensiveCar.
If you have to stick with your schema, I guess (but FIXME) you are using the joined or the table per class inheritance strategy which means you cannot use the discriminator feature that JPA provides.