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...
Related
Assume that I have several realm objects (tables) with many connections between them, and I don't yet have all the objects pre populated.
I want to be able to create relationships by their ids (primary keys).
For example, I have JSON file with an object that has a list of another object's ids (not the actual object, although that another object is already exists in the realm db), I want to be able to populate those into objects directly without having to query them first.
How is that possible?
Realm is designed to be an object database without impedance mistach that could happen from mapping Java objects to an entry in table. Relation in Realm is specifically handled to adhere the principle above.
For example, one can describe relationship between object in following.
public class ObjectA extends RealmObject {
...
#PrimaryKey
private long id;
...
}
public class ObjectB extends RealmObject {
...
private RealmList<ObjectA> entries;
...
}
The relation you want, i.e. relation by primarykey id, seems much close to relational databases and it is not supported.
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")) })
I use EmailAlert bean as DTO to get data by means of Hibernate.
So, my class contains only fields that I have in DB.
But in some cases I need additional fields to be in EmailAlert to hold intermediate data. For example "caption" field - will be calculated on java side depends of user locale, time, etc.
So, I have some variants to solve this issue.
Add additional property (ex: caption) to EmailAlert bean, but do not map it with any field of DB table.
Drawback: In this case we have to do not use "caption" property in hashCode() and equals() because as:
It really don't have a matter - field holds only intermediate data
I am not sure it not be a cause of problem with cache and Hibernate itself.
I think it is very ugly to have a property of class but do not use it in equals() and hashCode() methods.
Someone can be confusing in the future with this logic.
Extend EmailAlert as EmailAlertExt with adding of "caption" property. And constructor that takes EmailAlert as argument.
But in this case I am not sure underwater stones in case I will store EmailAlert as EmailAlertExt bean again into DB.
Extend EmailAlert as EmailAlertExt2 with adding of "caption" property and take a refference to the original object. In this case EmailAlertExt2 will behave as original EmailAlert, but with additional property we need. In case we save EmailAlert we could call getOriginalValue() of EmailAlertExt2 that will return refference to original object.
Drawback: too many coding :)
Guys, which of these solutions is better? May be someone have other proposals?
Use '#Transient' it won't map to db hibernate will ignore this field
Extending a model object just because you want to separate mapped vs non-mapped fields is not a good idea. A good guideline would be to ask yourself the question "What is the difference between an EmailAlert and an EmailAlertX, and can I clearly define the situations where I would use one over the other?". If you cannot answer that question cleanly, or if you realize that you will always be using your subclass over the parent class, that is a sure sign that the parent class should be abstract or that you have too many classes.
In your particular case, it would make more sense to have both the mapped, and non-mapped properties on the same class, and to mark the non-mapped properties so that your ORM provider does not try to process them. You can do this by annotating these properties as being #Transient.
public class EmailAlert implements Serializable {
#Id
private Long id;
#Column(name = "recipient")
private String recipient;
#Transient
private transient String caption;
// Constructor, Getters/Setters, etc
}
Also, with respect to to your comment on hashcode/equals methods. You do not and should not include every property of a Java Bean in these methods. Only include those properties that are:
required to uniquely identify the object
are (fairly) guaranteed to have the same value over the lifecycle of the object
It sounds like the EmailAlert object you need at the moment is a business object, because of the "intermediate data" and "calculated on java side" bits.
Maybe use the EmailAlertDto object to populate the fields of the EmailAlertBusiness and store the extra caption field and the methods in the business object.
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.
I'm currently studying the official JPA 2 final specification.
Is the following statement contained anywhere in the Spec?
The Entity Manager guarantees that within a single Persistence Context,
for any particular database row, there will be only one object
instance.
Either I don't clearly understand the spec or I just can't find the part that proves that the quoted statement is part of the specification.
No, specification does not give such a guarantee. But in my opinion it is implicitly assumed.
In practice sometimes same table is mapped to the two different entities. One of them being treated as read only entity. Read only entity can for example be used for reporting purposes and as an optimization contains only subset of fields in other entity. This can be done for example as follows:
#Entity
public class EntityA {
#Id private Integer id;
#Lob
byte[] tooHeavyToLoadAlways;
}
#Entity
#Table(name="EntityA")
public class EntityALightWeight {
#Id private Integer id;
}
For JPA there is no connection between these two entities, so keeping care that only first one of them is modified and that second one is refreshed is responsibility of application. Because of that should be used only with caution, because EntityALightWeight can be refreshed from database but will never contain changes made to the EntityA in same transaction.