Hibernate延迟加载-在列表中获取Listitems

Question

I am currently experimenting with hibernate 5 in my java application. I have the following problem:

I have two pojos/models:

@Entity(name="Categories")
public class Category {
    @javax.persistence.Id
    private int Id;
    private String Name;
    @OneToMany(mappedBy = "category",cascade=CascadeType.ALL, fetch=FetchType.LAZY)
    private List<Link> links = new ArrayList<>();
    // ... getters and setters..
}

@Entity(name="Links")
public class Link {
    @javax.persistence.Id
    private int Id;
    private String Name;
    private String Url;
    @ManyToOne
    @JoinColumn(name="CategoryId")
    private Category category;
    // ... getters and setters..
}

Now i want to get all categories with links in it. My dao for that is the following:

public Iterable<Category> getCategories() {
     SessionFactory factory = HibernateUtil.getSessionFactory();
     Session session = factory.getCurrentSession();
     session.getTransaction().begin();

     String hsql = "from " + Category.class.getName() + " e";

     Query<Category> query = session.createQuery(hsql);
     List<Category> cats = query.getResultList();

     cats.forEach(item -> Hibernate.initialize(item.getLinks()));

     session.getTransaction().commit();
     return cats;
    }

This is working but i think this is a bad solution for getting all Links from all categories. I already experimented with "JOIN FETCH".

For example if i use:

String hsql =  "from " + Category.class.getName() + " e JOIN FETCH e.links l

I will get all results twice... What is the best practise to get all Items from the list "links" from all Categories of my List?

I hope this question is not too easy but i cant find anything helpful regarding this.

Answer 1

you could try using this make sure to use this inside the transcation boundary

List<Link> totalLinks = cats.stream().map(cat -> cat.getLinks()).
            flatMap(l -> l.stream()).collect(Collectors.toList());

Answer 2

Instead of fetch=FetchType.LAZY you could use fetch=FetchType.EAGER and hibernate will resolve all the links directly on load.

Depends on the use case if you want to use this. For small data sets that is absolutely ok.