Why to override hashCode and equals methods in hibernate persistence class?

Question

Why is it considered the best practice to override the hashCode() and equals() methods in a Hibernate persistence class?

Answer 1

Hibernate doc:

You have to override the equals() and hashCode() methods if you

intend to put instances of persistent classes in a Set (the recommended way to represent many-valued associations) and

intend to use reattachment of detached instances

Hibernate guarantees equivalence of persistent identity (database row) and Java identity only inside a particular session scope. So as soon as we mix instances retrieved in different sessions, we must implement equals() and hashCode() if we wish to have meaningful semantics for Sets.

see this link https://docs.jboss.org/hibernate/stable/core.old/reference/en/html/persistent-classes-equalshashcode.html

Let's consider this scenario to show some of the problems that will occurs if you are using the default implementation of equals and hashcode:

@Entity Parent{
        @Id
        @GeneratedValue
        Long id;

        @OneToMany(mappedBy = "parent",
         cascade = CascadeType.ALL, orphanRemoval = true)
        @JoinColumn(name="PARENT_ID")
        Set<Child> childrens = new HashSet<Child>();
        //get+set
        }

@Entity Child{
        @Id
        @GeneratedValue
        Long id;
        }

---

    test(){
          Parent p1 = new Parent();
          Child c1 = new Child();
          entityManager.persist(p1);
          entityManager.persist(c1);
          p1.getChilds.add(c1);
          entityManager.merge(p);
          //!!:using another instance of entitymanager just to simulate the case of detached entities.
          Parent p2 =entityManager1.find(p1.getId(),Parent.class);
          child c2 =entityManager1.find(c1.getId(),Child.class);
          boolean contains=p1.getChilds().contains(c2); // problem1: contains==false
          //Then if we add c2 to the childs set we will have 
          //a duplication inside the Set
          p2.getChilds.add(c2);//problem2:childs contains c1 and c2
          boolean remove=p2.getChilds.remove(c2);//problem3:remove==false
          entityManager1.merge(p2);//problem4: hibernate will deal with c2
          //like a new entity then an insert operation is 
          //triggered on c2 (an exception=> violation of unique id)
        }

Answer 2

There is absolutly no reason to override equals and hashCode because Hibernate do not rely on them. It compares entities themself by comparing attributes.