I am new in spring and hibernate and I got these two annotations #Entity and #Repository used for DAO class. As both the annotations are used for the DAO class. Then, what is the difference and when to use one of them.
The #Entity class is the model and the #Repository is the layer that helps you to extract the data from database. For example :
#Entity
public class Student {
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
#Column(unique=true)
private String name;
//getters - setters
}
And the repository:
#Repository
public interface StudentRepository extends CrudRepository<Student,Long> {
public Student findByName(String name);
}
The basic CRUD operations are already provided by CrudRepository interface so there is no need to implement them again. You can use them in a Service class like this:
#Service
public class StudentServiceImpl implements StudentService {
#Autowired
public StudentRepository studentRepository;
#Override
public List<Student> findAll()
{
return studentRepository.findAll():
}
#Override
public Student findByName(String name)
{
return studentRepository.findByName(name);
}
}
And in case you want to make custom queries like get a student by name, jpa hibernate is very smart and helps you to only define the method in the #Repository annotated interface and there is no need of any implementations. BUT there is a rule here if you want to make it work. Hibernate will look after method name like this : public Student findByName(String name); the find and Student return type tells hibernate that it have to look for a Student, byName will tell that it have to query the database for a Student with a specific name. (The Name keyword is actually the entity attribute with capital letter ! )
But of course, if you need some more complex queries, there is the #Query annotation that will help you with that :) .
#Entity annotation defines that a class can be mapped to a table, it is just a marker, like for example Serializable interface.
Entity is an object representing (usually) a row in a db.
#Repository annotation defines CRUD operation on table.
It is very like DAO pattern to fetch and save entities from/to storage - it represents db table.
#Entity
Let's say we have a POJO called Student which represents the data of a student and we would like to store it in the database.
public class Student {
// fields, getters and setters
}
In order to do this, we should define an entity so that JPA is aware of it.
So let's define it by making use of the #Entity annotation. We must specify this annotation at the class level.
#Entity
public class Student {
// fields, getters and setters
}
In most typical applications, we have distinct layers like data access, presentation, service, business, etc.
And, in each layer, we have various beans. Simply put, to detect them automatically, Spring uses classpath scanning annotations.
#Repository
#Repository annotates classes at the persistence layer, which will act as a database repository. #Repository’s job is to catch persistence specific exceptions and rethrow them as one of Spring’s unified unchecked exception.
to sum up #Entity is part of JPA Java Persistence API specification used mapping between a java POJO and an entity in relational database world and #Repository is a Spring stereotype used to annotate POJO beans than their jobs is database manipulation operations
Related
I have Table User in SQL Server DB with close to 30 columns. I have also created an JPA Entity for corresponding DB Table.
The JPA entity looks something like this:
#Entity
#Table(name="USER")
#Setter
#Getter
public class User{
#Id
#Column(name="ID",nullable=false)
private Integer id;
#Column(name="NAME")
private String name;
#Column(name="ADDRESS")
private String address;
#Column(name="FATHER_NAME")
private String fathersName;
}
Now I am trying to create a native query which will only fetch id and name from the User Table.
The Repository class looks something like this.
#Repository
public interface UserDao extends JpaRepository<User,Integer>{
#Query(value=" SELECT u.ID,u.NAME from USER u", nativeQuery=true)
List<User> fetchUser();
}
Now when this is being executed, it is throwing an exception as below:
com.microsoft.sqlserver.jdbc.SqlServerException: The column name address is not valid
When I add the address column to my query this exception goes but then it come for another field.
com.microsoft.sqlserver.jdbc.SqlServerException: The column name father_name is not valid
So, now I want to modify the JPA Entity in such a way that this fields became optional.
I have tried using
#Transient but if some other query is using this address field then it will come as null over there.
I have also tried all the below annotations:
#Basic
#NotFound(action=NotFoundAction.IGNORE)
#Column(nullable="true")
#JsonProperty
But none of this annotations work for me.
You can define additional constructor for the User entity:
#Entity
#Table(name="USER")
public class User {
public User(){
}
public User(Integer id, String name){
this.id = id;
this.name = name;
}
// ...
}
and then write the following query:
#Query("select new your.package.User(u.id, u.name) from User u")
List<User> fetchUser();
Note that this is not native, but jpql query.
Please also note that according to the hibernate documentation:
The projection class must be fully qualified in the entity query, and it must define a matching constructor.
The class here need not be mapped. It can be a DTO class.
If it does represent an entity, the resulting instances are returned in the NEW state (not managed!).
EDIT
You can also try to use spring data jpa projections in the following way:
Define an interface:
interface UserIdWithName {
Integer getId();
String getName();
}
The important thing here is that the properties defined here exactly match properties in the aggregate entity.
Use this interface in the query method:
#Query("select u from User u")
List<UserIdWithName> fetchUserIdWithName();
The query execution engine creates proxy instances of that interface at runtime for each element returned and forwards calls to the exposed methods to the target object. Please note that you can not select only some fields in the query here.
You can use
#Query(value=" SELECT u.ID,u.NAME, '' as address, '' as father_name from USER u", nativeQuery=true)
List<User> fetchUser();
It will set those fields to empty string.
I think this is a perfect use case for Blaze-Persistence Entity Views.
I created the library to allow easy mapping between JPA models and custom interface or abstract class defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure(domain model) the way you like and map attributes(getters) via JPQL expressions to the entity model.
A DTO model for your use case could look like the following with Blaze-Persistence Entity-Views:
#EntityView(User.class)
public interface UserDto {
#IdMapping
Integer getId();
String getName();
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
UserDto a = entityViewManager.find(entityManager, UserDto.class, id);
The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features
Page<UserDto> findAll(Pageable pageable);
The best part is, it will only fetch the state that is actually necessary!
I know how to persist entity with #ManyToMany relation in pure Hibernate. But how to do it correctly in Spring?
In hibernate that would be:
EntityManager em = getEntityManager();
em.getTransaction.begin();
Book book = new Book("title", "isbn");
Category category = new Category("horror");
category.addBook(book);
em.persist(book);
em.getTransaction().commit();
em.getTransaction().close();
something like this. But what about Spring, let's say that I have #Service which receive Book or BookDto from #RestController.
public void saveBook(Book book) {
//what now
}
What I should do here, is bookRepository.save(book); would be enough to save this #ManyToMany relation? Don't I need additional methods like addBook, addCategory, removeBook etc.?
Thank you in advance.
Spring's repository is doing nothing than just wrapping entityManager and executing it's persist method. You can have a look at default implementation
That's why whatever possible in "pure hibernate" is possible with spring's repositories with a couple of notes
Note 1 With spring you most probably will replace transaction boiler-plate with #Transactional annotations, that's why you need to be careful about entities passed between methods.
Note 2 The code you referred as "pure hibernate" is actually "pure JPA", there was no hibernate mentioned. Probably hibernate was your JPA implementation.
The standard recipe to handle state management of your entities with the use of Spring Boot is:
Create entity classess annotated with #Entity (which you have already done, I presume) - for example a Book.class entity;
Create repository interface that extends JPARepository<T,ID>, where T is your entity class (create one for Book and another one for Category), and ID is the Primary Key of your entity class. This will give you the advantage of having a default implementation of many useful repository methods, like save(), findOneById(), etc;
public interface BookRepository extends JPARepository<Book, Long> {}
assuming that you have set Long as a primary key of your book entity;
Inject the repository in the service class;
Now you can use your service class and method like so:
#RequiredArgsConstructor
#Service
public class BookService {
private final BookRepository bookRepository;
#Transactional
public void saveBook() {
// just copying the logic from your question
// but normally You would pass it as an argument to the method
Book book = new Book("title", "isbn");
Category category = new Category("horror");
category.addBook(book);
bookRepository.save(book);
}
}
Answering Your 2nd question - yes, you have to take care of keeping both entities relationships in sync, there is no get-away from that here. So in #ManyToMany relationship, you have to add category to book categories collection and vice versa.
So I've been using Spring Data Repositories most of the time. But I've reached a use-case where I cannot use my Spring Repository to retrieve the entity that I need to return to the client.
So I have my class ResourceEntity which is a Spring Data Entity. And I'd like to return this entity as a ResourceProjectioninterface.
#Getter
#Setter
#NoArgsConstructor
#Entity
public class ResourceEntity{
private Long id;
private String name;
private String anotherFieldThatIsNotInTheProjection;
}
public interface ResourceProjection {
Long getId();
String getName();
}
Usually with a Spring Repository, I'd define something like that :
public interface ResourceRepository extends PagingAndSortingRepository<ResourceEntity, Long> {
Optional<ResourceProjection> getById(Long id);
}
In this case I can't use the "automatic proxy" generated by Spring Data to automatically implement my projection with the entity's data.
So my question is : Is there a way to "manually" convert the entity to the projection ?
Another solution I thought of is returning the entity and using Jackson annotations like #JsonIgnore to prevent some of my data to be returned, but that is not optimal with the way my code was written.
Otherwise I can always create a DTO class that will fill up with the data from the Entity. But as I have already created my projection for other purposes, I would like avoid creating a second "DTO".
You can do the projection programmatically in this way:
import org.springframework.data.projection.ProjectionFactory;
import org.springframework.data.projection.SpelAwareProxyProjectionFactory;
//...
resourceEntity = //find resource
ProjectionFactory pf = new SpelAwareProxyProjectionFactory();
ResourceProjection rp = pf.createProjection(ResourceProjection.class, resourceEntity)
This is a perfect use case for Blaze-Persistence Entity Views.
I created the library to allow easy mapping between JPA models and custom interface defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure the way you like and map attributes(getters) via JPQL expressions to the entity model. Since the attribute name is used as default mapping, you mostly don't need explicit mappings as 80% of the use cases is to have DTOs that are a subset of the entity model.
A mapping for your model could look as simple as the following
#EntityView(ResourceEntity.class)
interface ResourceProjection {
#IdMapping
Long getId();
String getName();
}
Querying is a matter of applying the entity view to a query, the simplest being just a query by id.
ResourceProjection dto = entityViewManager.find(entityManager, ResourceProjection.class, id);
But the Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features
Implementing interface projection using native sql query
1. OurResourceEntity.java class
#Getter
#Setter
#NoArgsConstructor
#Entity
public class ResourceEntity{
private Long id;
private String name;
}
2. Creating projection Interface name ProjectedResource.java, which maps data collected by the SQL query from repository layer method
public interface ProjectedResource {
Long getId();
String getName();
String getAnotherProperty();
}
3. Creating Repository layer method: getProjectedResources()
We are considering the database table name is resource.
We are only fetching id and name here.But using interface projection we can change the properties name according to our desire.
#Query(name="select id, name, anotherProperty from resource", nativeQuery=true)
List<ProjectedResource> getProjectedResources();
Hope the issue will be resolved!
In my project based on spring-data-rest I have some classes annotated with the #Entity annotation.
#Entity
#Table(name = "my_table)
#Getter
#Setter
#NoArgsConstructor
#AllArgsConstructor
public class MyTable extends BaseTableEntity {
private String name;
private String description;
}
Now I am not quite sure as to how Spring handles creation of new instances of classes annotated with that annotation. I needed to create an object of such an Entity class making sure that no database operations takes place.
To make it clear, say I do MyTable table = new MyTable() in a POJO class. Can I be sure that Spring with the help of Hibernate/JPA won't create records in my database. What if I do the same in a #Component class instead of a POJO.
Creating a new entity with new keyword does not make any insert into db. The entity is in detached state. You would have to pass this entity to reporitory.save or entityManager.persist or entityManager.merge... But as long persist is not invoked (directly or indirectly), it's a regular pojo without a representation in database.
I am working on a Maven Java web project based on Spring, JPA and Hibernate.
One part of this project shall be reused in another very similar project and gets therefore extracted as Maven module. This service and the corresponding entity are annotated like this:
#Service
public class MessageService
#Entity
public class Message
Both projects have similar but slightly different UserEntities.
#Entity
public class TypeAUser
#Entity
public class TypeBUser
The Message Entity has #OneToMany relationship to one of the UserEntities in each project.
I thought about a generic UserEntity but want to avoid creating additional tables as well as tables with fields of the "other" project.
Any ideas are appreciated.
Best regards,
Stefan
If you don't want to create additional tables, then you might want to consider using SINGLE_TABLE strategy.
#Entity
#Inheritance(strategy=InheritanceType.SINGLE_TABLE)
public class UserEntity {
...
}
#Entity
public class TypeAUser extends UserEntity {
...
}
#Entity
public class TypeBUser extends UserEntity {
...
}
Well, when talking about Inheritance in Hibernate, you have three options:
Table per concrete class with unions
Table per class hierarchy(Single Table Strategy)
Table per subclass
Since you want to achieve it with one table, I suggest using option 2.
You need just a User table, with a USER_TYPE column to discriminate the user types ( Can be a number, varchar2, etc)
Then you need to create a class User with the following annotations:
You can specify a DiscriminatorColumn if you want, otherwise Hibernate will default to '{className}_TYPE
#Inheritance(strategy=InheritanceType.SINGLE_TABLE)
#DiscriminatorColumn(name="USER_TYPE", discriminatorType=DiscriminatorType.STRING)
public class User { .... }
In your concrete class implementations you can specify a Discriminator value (or if you don't, hibernate will default it to the class name ('TypeAUser'/ 'TypeBUser')
#Entity
#DiscriminatorValue('A')
public class TypeAUser
#Entity
#DiscriminatorValue('B')
public class TypeBUser