Wsrtjtyk

I was trying to learn Spring Framework and ran into a problem with saving entities into CRUD Repository. I had few Entities with automatic numeric ID generation and they work just fine, but then I tried to make a class with String being a primary key just like this:

@Entity

@Table(name = "USERS")

@Builder

public class User {

    @Id

    @Column(name = "USER_NAME", nullable = false)

    @Getter @Setter

    private String name;



    @Column(name = "USER_PASS", nullable = false)

    @Getter @Setter

    private String pass;

}

First I was getting exceptions about this class not having a default constructor:

org.springframework.orm.jpa.JpaSystemException: No default constructor for entity:  : com.company.Model.User; nested exception is org.hibernate.InstantiationException: No default constructor for entity:  : com.company.Model.User

Already weird, but still I decided to change @Builder annotation into 2 constructors, one with both arguments and second with none. I tried to save the entity instance into CRUD Repository userDAO (which is nothing more than interface extending CRUDRepository) by the typical test:

User admin = new User("admin", "6aDcZ72k");



...



@Test

public void saveUserAndFindById() {

    admin = userDAO.save(admin);

    assertThat(userDAO.findById(admin.getName())).isEqualTo(admin);

}

The result was assertion failed because the saved entity had "Optional" type:

org.junit.ComparisonFailure: 

Expected :com.company.Model.User@2c06b113

Actual   :Optional[com.company.Model.User@2c06b113]

I know I'm doing something really wrong but can't figure this out. Or maybe there is a way to just prevent making it optional? There must be few other entities with the reference on this class, but these references obviously don't work because of the above issue.

First of all,jpa require the entity has a No Arguments Constructor cause it will create a instance first and then populate it.The easiest way is to add @NoArgumentsConstructor that offered by lombok on the entity class.

And then,Optional is used by spring data jpa in order to avoid NullPointException and in fact it be is useful actually.If you want to use the interface that Spring-data-jpa offered,then you have to use Optional too.You cloud look here for more info about Optional:link1,link2

By the way,I usually use it like:

ExampleEntity example=exampleRepository.findById(id).orElseThrow(()->new ExampleNotFoundException());

In this way,you dont need to deal with Optional or think about NullPointException.

or:

ExampleEntity example=exampleRepository.findById(id).orElse(null);

In this way if you cant find the target entity,then it will be null.So dont forget to check if the entity is null.

Hope it could help~~~

It is not your save(...) that is returning Optional but userDAO.findById(admin.getName(). According to the documentation, CrudReposiotry provides a findById() whose return type is Optional<T>.

public interface CrudRepository<T, ID extends Serializable>

  extends Repository<T, ID> {

    Optional<T> findById(ID primaryKey);

}

If you do not want Optional as return type, You will need to provide your own method to do that. For example:

public interface PeronRepository extends CrudRepository<Person, String> {

    Person findById(String personId);

}