Unusual behavior regarding "detached entity passed to persist" exception when trying to persist a detached object?

Question

I have a simple model class like this

@Entity
public class User {

    @Id
    @GeneratedValue
    long id;

    @Column(unique = true)
    String name;
    

    public long getId() {
        return id;
    }


    public void setId(long id) {
        this.id = id;
    }


    public String getName() {
        return name;
    }


    public void setName(String name) {
        this.name = name;
    }

}

I am trying to persist instance of User class detached (when the id of the transient object is not null).

I was expecting "detached entity passed to persist" exception, however I am not getting that, for this code

User u = new User();
u.setName("u2");
u.setId(2);
repo.save(u);

Here the id is not null, I should get a detached entity passed to persist exception Can somebody explain why?

Answer 1

Spring data is not the same as JPA but a wrapper or a collection of utilities (or so) to ease the use of JPA related stuff and EntityManager .

Meaning also that Repository is not the same as EntityManager . So how are those two related?

If you take a look at the source code of SimpleJpaRepository

Default implementation of the {@link org.springframework.data.repository.CrudRepository} interface

you will see there:

@Transactional
public <S extends T> S save(S entity) {
    if (entityInformation.isNew(entity)) {
        em.persist(entity);
        return entity;
    } else {
        return em.merge(entity);
    }
}

Tracing after some more classes for entityInformation.isNew(entity) leads to AbstractPersistable and method:

@Transient
public boolean isNew() {
    return null == getId();
}

It will not try to persist entity that has an Id set but merge .