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.
Related
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)
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 am using hibernate for entity persistance in application along with spring mvc I have a multiple #Transient fields in an entity in a application is there a way to tell Json parser to automatically ignore all #Transient annotated fields in entity without the need to use #JsonIgnore or #JsonIgnoreProperties
#Transient is to indicate the field is not persistent. #JsonIgnore is to indicate the field is to be ignored by the serialization/deserialization processor, they have very different meanings.
One option is have Json processor base serial/deserialization from available fields (vs getter/setters), then mark your #Transient fields with additional transient keyword. If you have getters/setters, you'll also need to add appropriate visibility rules. For example, this will serialize all fields not marked with transient.
#JsonAutoDetect(fieldVisibility=Visibility.ANY, getterVisibility=Visibility.NONE, setterVisibility = Visibility.NONE)
I have found a better way instead of using #jsonignore to the fields in an entity
on child class use JsonBackreference and on parent use JsonManagedReference
For more information go through this blog post
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.
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.