Spring-Data JPA: EntityExistsException: A different object with the same identifier value - jpa

Question

I have an issue, what I am close to solution or better said I come closer how to avoid the issue EntityExistsException: A different object with the same identifier value BUT without understanding what is really the problem. To keep code short and concentrated I did a small application to simulate the problem in my project.
First I have a CrudRepository what stays always the same:
public interface EntityARepository extends CrudRepository<EntityA, Long> {}
First I have to entities, one of them has a relation to the other:
#Entity
#EqualsAndHashCode(of={"name"})
#ToString(of={"name"})
#XmlRootElement
public class EntityA {
#Id
#GeneratedValue
#Setter
private Long id;
#Setter
#Column(nullable=false, unique=true)
private String name;
#Setter
#ManyToOne(fetch=EAGER, cascade={PERSIST, MERGE})
private EntityB entityB;
}
#ToString(of = { "name" })
#EqualsAndHashCode(of = { "name" })
#Entity
class EntityB {
#Id
// #GeneratedValue => produces issue!
#Setter
private Long id;
#Setter
#XmlAttribute
#Column(nullable=false, unique=true)
private String name;
}
Then I generate data and try to save them:
#Component
public class DatabaseInitializer implements InitializingBean {
#Autowired EntityARepository repository;
#Override
public void afterPropertiesSet() throws Exception {
final Set<EntityA> aEntities = createAEntities();
repository.save(aEntities);
}
private Set<EntityA> createAEntities() throws Exception {
Set<EntityA> aEntities = new HashSet<>();
aEntities.add(getFirstEntityA());
aEntities.add(getSecondEntityA());
return aEntities;
}
private EntityA getFirstEntityA(){
EntityA a = new EntityA();
// a.setId(1L);
a.setName("a-1");
a.setEntityB(getFirstEntityB());
return a;
}
private EntityA getSecondEntityA(){
EntityA a = new EntityA();
// a.setId(2L);
a.setName("a-2");
a.setEntityB(getFirstEntityB());
return a;
}
private EntityB getFirstEntityB() {
EntityB b = new EntityB();
b.setId(1l);
b.setName("b-1");
return b;
}
}
With this constallation I become a EntityExistsException:
Caused by: javax.persistence.EntityExistsException: A different object with the same identifier value was already associated with the session : [com.example.EntityB#1]

Answer 1

When firstEntityA gets saved, it also saves firstEntityB (because of cascade). So after that Session contains already entity of class EntityB with id 1.
When secondEntityA gets persisted, it also calls save on firstEntityB and then exception gets thrown.
The reason is that you can't call save on an object with id that is already present in the Session.
There are many ways to fix it. For example you can call merge instead of save.
repository.merge(aEntities);
Another way to fix this would to make both instances of EntityA have a referene to the same EntityB object:
private Set<EntityA> createAEntities() throws Exception {
Set<EntityA> aEntities = new HashSet<>();
EntityB entityB = getFirstEntityB();
aEntities.add(getFirstEntityA(entityB));
aEntities.add(getSecondEntityA(entityB));
return aEntities;
}
private EntityA getFirstEntityA(EntityB entityB){
EntityA a = new EntityA();
// a.setId(1L);
a.setName("a-1");
a.setEntityB(entityB);
return a;
}
private EntityA getSecondEntityA(EntityB entityB){
EntityA a = new EntityA();
// a.setId(2L);
a.setName("a-2");
a.setEntityB(entityB);
return a;
}
private EntityB getFirstEntityB() {
EntityB b = new EntityB();
b.setId(1l);
b.setName("b-1");
return b;
}