了解/设置休眠配置

Question

I'm trying to understand an existing application using hibernate which I have a few knowledge about. In this application, a database schema has been created, and classes have been automatically generated from the DB schema.

Let's take an example of 2 tables in the DB:

   PERSON(id, firstname, surname)
   OBJECT(id, owner_id, description)

In the table OBJECT , owner_id would be a foreign key to PERSON.id .

The generated classes would look approximately like that:

class Person {
  // ok for the following
  int id; // with getters and setters
  String firstname; // with getters and setters
  String surname; // with getters and setters

  // dunno why the following ?
  Set objects = new HashSet(0);
  Set getObjects() { return this.objects; }
  void setObjects(Set objects)  this.objects = objects; }
}

class Object {

   int id; // with getters and setters 
   Person owner; // with getters and setters
   String description;  // with getters and setters
}

First, I dont know why is there a reference to the set of objects owned in the generated class Person . Second, if I get an Object or a List<Object> through an HQL query, is the Person member of this Object fully loaded? How should I set up hibernate to ensure that it is?

I'm asking because in my application, I have the hibernate session running as a separate executable, and another, deported application which communicate with the hibernate app and asks it to send some persistent objects. I can get an Object but if on this object I make object.getPerson().getFirstname() , I run into an org.hibernate.LazyInitializationException: could not initialize proxy - no Session

Answer 1

You're getting the LazyInitializationException because, in your client application, the values for your 'Person' field in your 'Object' object are not initialized (they are without assigned values). You are trying to access a not initialized field of a Hibernate entity without being a Hibernate Session open and therefore Hibernate is not able to get that value.

As you said, you have to make sure you receive 'Object' instances with all their fields initialized. There are several possibilities depending on the way your Hibernate entities are mapped (It would be a good idea you provide us with the mappings to be able to answer you in a more specific way). For instance, you could

1. Mark the relationship as eager so when you get the Object you get as well the Person (the way to do this depends on the way the mappings are declared)

2. Use a explicit join when you query the Object. Something like this in HQL (I don't know if you're using HQL either)

   select o from Object o join fetch o.owner where o.id = :id

Again if you provide us with more details, we could give a more accurate solution