How do I fix org.hibernate.LazyInitializationException?

When you have a parent entity (Category) with associated entities (Gifs), there are two ways for Hibernate to deal with the relationship:

• Eager loading (every time you retrieve a category, hibernate also retrieves ALL associated gifs from the database)
• Lazy loading (by default Hibernate does not retrieve the associated gifs when a category is retrieved. You must do it explicitly when you need the gif entities)

Opting for eager loading is the easiest way to solve this. But in a real app it's terrible. Imagine you display a list of all categories: with eager loading, Hibernate also loads every single gif in the database associated with one of those categories. Even though you don't need the gifs right now. It doesn't matter in a treehouse project, you don't have much data anyway. In real life it leads to a slow, slow app.

So it's better practice to use lazy loading and initialize the relationship when needed. For example in your CategoryDao you would load a single category with its associated gifs like that:

    @Override
    public Category findById(Long id) {
        Session session = sessionFactory.openSession();
        Category category = session.get(Category.class,id);
        //with this line, you load the gif entities when you find a category by id
        Hibernate.initialize(category.getGifs());  
        session.close();
        return category;
    }

To learn more about this:

• http://www.baeldung.com/hibernate-lazy-eager-loading
• https://vladmihalcea.com/the-best-way-to-handle-the-lazyinitializationexception/