Does Hibernate return null or empty collection if table in relation is empty?

Question

I was curious about something. Let's say we have a simple relation between Employee and Phone:

@Entity
public class Employee {
  @Id
  @Column(name="EMP_ID")
  private long id;
  ...
  @OneToMany(mappedBy="owner")
  private List<Phone> phones;
  ...
}
@Entity
public class Phone {
  @Id
  private long id;
  ...
  @ManyToOne(fetch=FetchType.LAZY)
  @JoinColumn(name="OWNER_ID")
  private Employee owner;
  ...
}

Let's assume that an Employee has no phones, no entries in the Phone table. If I were to have a piece of code that gets the phones of an Employee and iterates over them for whatever reason

for (Phone phone : employee.getPhones())
{
     ...
}

Would the getter retun NULL or an empty Collection and would the getching strategy play a part.

If I remember correctly, hibernate has its own implementation of collection using proxies and for LAZY fetch, it instantiates with one of those and when needed retrieves the data from the table (correct If I am wrong). So would at the time the getter is called try to retrieve the data from the table, get an empty set as a result and return an empty collection. (This is what I think). Or should I always check if the result of the getter is NULL or not?

Very probably an empty list. Returning null instead of an empty list is a bad practice. — Tunaki
– Tunaki, Commented Oct 13, 2015 at 14:10
You can always make check in your getter and return empty list if phones is null — arados
– arados, Commented Oct 13, 2015 at 14:11
Yeah, I am going to eventually but unfortunately I don't have access right now to my laptop and wont for some time. Was discussing some things with some friends and we needed to know. I know it sounds lazy but if I could check it right now I would... Sorry — Andreas Andreou
– Andreas Andreou, Commented Oct 13, 2015 at 14:12
This seems like one of those cases where the best way to answer the question is to just try it and see what happens. — Ian McLaird
– Ian McLaird, Commented Oct 13, 2015 at 14:16
it is also good practice to initialize array as instance variable in the entity: private List<Phone> phones = new ArrayList<Phone>(); so u know that u prevent npe — HRgiger
– HRgiger, Commented Oct 13, 2015 at 14:42

Thomas · Accepted Answer · 2020-03-12 09:18:30Z

Since those collections are lazy by default employee.getPhones() should return a proxy for that collection (e.g. PersistentList or similar) which loads the list elements when you access the list.

Additionally, because Phone is the owner of the relation Hibernate won't know whether there are any phones for an employee or not so it has to assume the list exists - although it might be empty. That said it wouldn't make much sense for Hibernate to return null since:

Hibernate would need to try and load the phones first to see that there are none
to implement lazy loading of the collection getPhones() must not return null but a proxy
returning null would be bad practice anyways (the list would still exist, it's just empty)
if the list was null you could not add a phone and let Hibernate use cascading etc. to automatically persist that change (thanks to Gimby for pointing that out)

Using eager loading shouldn't change that. Hibernate would know that there is no phone for the employee but returning null instead of an empty list, which also would express that there are no phones, would still make little sense (think of allowing to add phones for a loaded employee, differences in code where you'd not need them if null was used for eager fetching, etc.).

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