I am new to the Spring ecosystem and I wonder if the #Entity class should always have getters and setters and that all the properties should be private? Is it correct that the JPA(or Hibernate) will require those methods (getters/setters) to fetch the values or set them in the database ?
From the JakartaEE JPA 3.0 specification, §2.2.: Persistent Fields and Properties:
2.2. Persistent Fields and Properties
The persistent state of an entity is accessed by the persistence provider runtime[1] either via JavaBeans style property accessors (“property access”) or via instance variables (“field access”). ...
...
The instance variables of a class must be private, protected, or package visibility independent of whether field access or property access is used. When property access is used, the property accessor methods must be public or protected.
....
This means that accessors are not strictly required, but highly recommended. The alternative (field access) is regarded as a code smell.
yes you should use #entity, it also helps for others when they read your code, it documents it :)
, you use private and #Data annotation from Lombok (to auto generate Getter/setter)
Related
As entity class field can be mark as private when #Entity access mode is "Field" access , just wanted to understand how provider will able to access entity state in this case as fields are marked as Private and will not be visible outside class ?
Edit #1 - As you know , for field access , getter and setter method are optional. So I wanted to understand how provider will access the field when no getter/setter method are provided. Hope this clarify my question .
Referring to the official JPA specification (final version, JPA 2.1) in Section 2.2 (page 24) we find:
The persistent state of an entity is accessed by the persistence provider runtime either via JavaBeans
style property accessors (“property access”) or via instance variables (“field access”). Whether persistent
properties or persistent fields or a combination of the two is used for the provider’s access to a
given class or entity hierarchy is determined as described in Section 2.3, “Access Type”.
In Section 2.3.1 (page 27) this definition is made more concrete - with respect to your question:
By default, a single access type (field or property access) applies to an entity hierarchy. The default
access type of an entity hierarchy is determined by the placement of mapping annotations on the
attributes of the entity classes and mapped superclasses of the entity hierarchy that do not explicitly
specify an access type. [...]
• When field-based access is used, the object/relational mapping annotations for the entity class
annotate the instance variables, and the persistence provider runtime accesses instance variables
directly. All non-transient instance variables that are not annotated with the Transient
annotation are persistent.
• When property-based access is used, the object/relational mapping annotations for the entity
class annotate the getter property accessors, and the persistence provider runtime accesses persistent state via the property accessor methods. All properties not annotated with the Transient annotation are persistent.
The term directly refers to an access strategy which allows the manipulation of an object's field (value) without the need to use getter/setter methods. In Java and for most OR-mappers (at least the ones I know of) this is achieved via Introspection - using the Java Reflection API. This way, classes' fields can be inspected for and manipulated to hold/represent data values from the (relational) database entries (i.e., their respective columns).
For instance, the provider Hibernate gives the following explanation in their User Guide:
2.5.9. Access strategies
As a JPA provider, Hibernate can introspect both the entity attributes
(instance fields) or the accessors (instance properties). By default,
the placement of the #Id annotation gives the default access strategy.
Important note:
Be careful when experimenting with different access strategies! The following requirement must hold (JPA specification, p. 28):
All such classes in the entity hierarchy whose access type is defaulted in this way must be consistent in
their placement of annotations on either fields or properties, such that a single, consistent default access
type applies within the hierarchy.
Hope it helps.
The provider can use reflection to access a private field on a class instance.
I saw this #javax.persistence.Access(javax.persistence.AccessType.FIELD) for a Entity.
What does this mean? Is it really required to declare #Access this for a entity.
No, it's not required, but can be useful. #Access is used to specify how JPA must access (get and set) mapped properties of the entity. If access type is set to FIELD, the values will directly be read/set on the field, bypassing getters and setters. If set to PROPERTY, the getters and setters are used to access the field value.
By default (at least with Hibernate), FIELD is used if the #Id annotation is on a field, and PROPERTY is used if the #Id annotation is on a getter.
why the jpa annotations are applied on field or on getter methods. if i try to apply the annotations on setter method then compiler generate the error. because compiler ignore the annotation on setter method. what is the reason behind them?
This is is how it's specified. Per JPA Specification:
When field-based access is used, the object/relational mapping annotations for the entity class annotate the instance variables, and the persistence provider runtime accesses instance variables directly. All non-transient instance variables that are not annotated with the Transient annotation are persistent.
When property-based access is used, the object/relational mapping annotations for the entity class annotate the getter property accessors[7], and the persistence provider runtime accesses persistent state via the property accessor methods. All properties not annotated with the Transient annotation are persistent.
Mapping annotations must not be applied to fields or properties that are transient or Transient.
You have two options. Either use field level annotation or property (getter method) annotation. There is no third option.
Because for an Object, there r only two ways to access the properties, fields directly or getter indirectly.
for entity bean, annotation specifys how to map properties to the columns, and JPA needs to access these status of entity, so I guess this is the most intuitional way to put annotations on fields directly or getter.
When we put annotations on getters, JPA access properties via getters.There is no need to put annotations on setters.
Assuming a JPA entity with (for example) an long id generated automatically:
#Entity
#AutoProperty
public class SomeItem {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
private long Id;
...
}
Is there any reason to not generate setter and getters for this Id? One might be tempted to not generate a setter for example, since it is the responsibility of JPA to generate the ID.
I see that other comments has misguided you so I feel myself obliged to elaborate on this issue a bit, even though I can't give you a scientific and complete answer. #vcetinick wrote the current accepted answer:
You may find that you may be able to get away [..] from the persistence side of things.
This quote in particular is wrong. All depends on where you put your #Id annotation. The specification says:
If the entity has field-based access, the persistence provider runtime
accesses instance variables directly.
Thus you are not required in any way to provide a setter or getter. Because you annotated your field and not a getter method (annotating the setter method will be ignored and have no bearing).
However, if you write a getter method, and annotated this method with your #Id annotation instead of your field, then we would tell our persistence provider to access our field through the accessor (getter) and mutator (setter) methods and not use reflection. In that case, both a getter and a setter should be present. The book Pro JPA 2: Mastering the Java™ Persistence API writes on page 71 (bold markup by me!):
When property access mode is used, the same contract as for JavaBeans applies, and there must be getter and setter methods for the persistent properties. The type of property is determined by the return type of the getter method and must be the same as the type of the single parameter passed into the setter method. Both methods must be either public or protected visibility.
Therefore, I usually annotate my id field, and write both a setter and getter method, but the setter method I give protected access. I just don't want any other pieces of code to have easy write access to such an important field. I don't know if this would cause problems in other domains. I'm no expert. But I don't find any rationale either as to why not setup an id attribute in this way. See also the Netbeans forums.
You may find that you may be able to get away without putting a getter/setter on the JPA Entity from the persistence side of things. But if you start dealing with entities that are serialized from other sources, even from your view in some cases, you will need a way to set the ID of the entity to let JPA know that it is dealing with an existing entity, if you cant set the id, then the persistence layer will just treat it as a new Object.
Id is your primary key without it you will never able to insert records in the database.
In your case #GeneratedValue(strategy=GenerationType.AUTO) it ensures that id will be generated for each persist but then also you will need a method to access it since it is primary identification of entity you should provide access to it .
Its like you asking some person his name and he doesn't provide it to you and you would thing he is just being rude .
I am using hibernate annotations. How to add methods to POJO object? For example i have "getChildNodes" method, associated with database, but i want also add recursive method "getAllChildNodes". I get "org.hibernate.PropertyNotFoundException: Could not find a setter for property progress in class" exception when i do it.
If I interpret this as "how do I add a method that is NOT related to persistence" then you need to use the #Transient annotation on the getAllChildNodes() method
There are two ways of defining the structure of your entity.
using annotations on the instance variables of your entity or
using annotations on the getter methods of your entity
When using the annotations on getter methods, Hibernate assumes that every getXxx (and isXxx for boolean types) represents definition of a persistent property. And this holds even if that particular getter does not contain any annotations, as happens in your case.
Hibernate also expects to find a matching setter method for each persistent property. And in your case that is what's missing and causes the exception.
You can solve this problem by declaring your custom getter as #Transient that says this getter does not represent a persistent property. Another way would be to convert the entity to use annotations on the instance variables. The latter would be my personal choice.
Open up the .java file and write a method named getAllChildNodes().
Hibernate doesn't write code for you, it maps fields in your database to your code. That's all. If you want to have extra logic in your domain/model classes besides the normal getters and setters for your properties, you'll have to add them yourself.