在实体中覆盖等于导致 LazyInitializationException

Question

I setup a project using spring/spring-boot/spring-jpa/spring-mvc.

I created a abstract base class for all entities

@MappedSuperclass
public abstract class IdEntity implements Persistable<Long> {

    @Id
    @Column(unique = true, nullable = false)
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    public Long getId() {
        return id;
    }

    protected void setId(Long id) {
        this.id = id;
    }

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

    @Override
    public boolean equals(Object obj) {

        if (null == obj) {
            return false;
        }

        if (this == obj) {
            return true;
        }

        if (!(obj instanceof IdEntity)) {
            return false;
        }

        IdEntity that = (IdEntity) obj;

        return null != this.getId() && this.getId().equals(that.getId());
    }

    @Override
    public int hashCode() {
       int hashCode = 17;

       hashCode += null == getId() ? 0 : getId().hashCode() * 31;

       return hashCode;
    }
}

I have a 'User' and 'GroceryList' class which both extend from 'IdEntity'. The class 'GroceryList' contains a field which maps lists to users like this:

@ManyToMany
@JoinTable(name = "users_grocerylists",
           joinColumns = {@JoinColumn(name = "grocerylists_id")},
           inverseJoinColumns = {@JoinColumn(name = "users_id")})
private Set<User> users = new HashSet<>();

Within a controller I have created a route which allows to create new lists:

 @PostMapping(value = "/grocery-list/new")
 public void newGroceryList(@RequestParam("name") String name, HttpServletResponse response) throws IOException {
     UserPrincipal currentUser = getUserPrincipalOrThrow();

     GroceryList groceryList = new GroceryList();
     groceryList.setName(name);
     groceryList.getUsers().add(userRepository.getOne(currentUser.getUserId()));

     groceryListRepository.save(groceryList);

     response.sendRedirect("/grocery-lists");
 }

I'm using the 'getOne' method from the repository in order to save me a database lookup for the user object (or cache a instance somewhere in the session).

The problem: as long as I'm overriding the equals method within the User class via IdEntity, this throws

org.hibernate.LazyInitializationException: could not initialize proxy [grocery.model.User#1] - no Session
    at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:169) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:309) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor.intercept(ByteBuddyInterceptor.java:45) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at org.hibernate.proxy.ProxyConfiguration$InterceptorDispatcher.intercept(ProxyConfiguration.java:95) ~[hibernate-core-5.3.10.Final.jar:5.3.10.Final]
    at grocery.model.User$HibernateProxy$xc99OVLQ.hashCode(Unknown Source) ~[main/:na]
    at java.util.HashMap.hash(HashMap.java:339) ~[na:1.8.0_252]
    at java.util.HashMap.put(HashMap.java:612) ~[na:1.8.0_252]
    at java.util.HashSet.add(HashSet.java:220) ~[na:1.8.0_252]

when trying to save the entity. Once I remove the equals method, everything is working as expected.

Actually this makes no sense to me. I thought I could use 'getOne' in order to get a cheap placeholder for the User... is this wrong? Why is the interceptor checking whether the equals method is overriden? Additionally the exception is pretty misleading to me...

Answer 1

Why is the interceptor checking whether the equals method is overriden? Additionally the exception is pretty misleading to me

As a rule, when you override hashCode method, you must override equals method as well. When not done, you are potentially breaking equals hashcode contract. Otherwise the results can be unexpected like in this case.

So, ideally the scenarios should be with both these methods overridden or without these 2 methods overridden in the entity class.

Under the hood, Hibernate generates a Proxy object of the pojo entity. It does so as to enable Lazy loading. So, here we are dealing with Proxy User Object and the real User Object.

The getOne method is returning a proxy object. With reference to here -

https://docs.spring.io/spring-data/jpa/docs/current/api/org/springframework/data/jpa/repository/JpaRepository.html#getOne-ID-

T getOne(ID id)

Returns a reference to the entity with the given identifier. Depending on how the JPA persistence provider is implemented this is very likely to always return an instance and throw an EntityNotFoundException on first access. Some of them will reject invalid identifiers immediately.

Because of proxy object, the behaviour changes at run time -

Scenario 1 - equals and hashCode method are overridden

In this scenario, proxy delegates equals and hashCode method invocations to the real objects because they are defined.

Scenario 2 - equals and hashCode method are not overridden

The method invocations on equals / hashCode will be on Proxy Object itself. Real objects are not needed.

---

You are getting this exception in 1st scenario as there is no Hibernate session available there and before it can invoke methods on real User Object, it must first load it using Hibernate Session.

org.hibernate.LazyInitializationException: could not initialize proxy [grocery.model.User#1] - no Session

When you add a User to the HashSet, then add method invokes the hashCode method and the exception is thrown.

groceryList.getUsers().add(userRepository.getOne(currentUser.getUserId()));

I would prefer to move this method to a Service class if you have one and make that method Transactional.

Answer 2

This is because of missing transaction during initialize users in Grocery class

As Grocery have one to many relationship with users and hibernate/jpa always lazy load collection (List/Set etc) by default. Instead of fetching collection with entity jpa/hibernate create proxy for it and when we perform some operation on it hibernate will do a query for collection and this needs transaction, if there will be no transaction then it will throw a LazyInitializationException, with the help of @Transactional annotation you can achieve transaction with in your required method.

@Transactional
 @PostMapping(value = "/grocery-list/new")
 public void newGroceryList(@RequestParam("name") String name, HttpServletResponse response) throws IOException {
   //your code
 }

Also you can configure your transaction for personalized behavior check: https://www.baeldung.com/transaction-configuration-with-jpa-and-spring