Problem: org.hibernate.LazyInitializationException

Why?

The LazyInitializationException occurs when attempting to access un-fetched data (represented by a proxy) from the database in a detached manner (outside the Hibernate session scope). This is tied to Hibernate’s default fetching policies for entity relationships.

Default Fetching Policies in JPA/Hibernate

• One-to-One: EAGER (fetches associated entity immediately).
• One-to-Many: LAZY (defers fetching the collection until accessed).
• Many-to-One: EAGER (fetches associated entity immediately).
• Many-to-Many: LAZY (defers fetching the collection until accessed).

One-to-Many: LAZY

• Meaning: When fetching one side (e.g., Category or Restaurant), Hibernate does not automatically fetch the many side (e.g., BlogPost or FoodItem).
  • Example: A SELECT query is fired only on the categories or restaurants table, not on blog_posts or food_items.
• Reason: Lazy loading improves performance by avoiding unnecessary data retrieval.

When Does LazyInitializationException Occur?

• Condition: Accessing un-fetched data (e.g., BlogPost list in Category or FoodItem list in Restaurant) outside the session scope.
• Triggers:
  • One-to-Many: Accessing a lazily-loaded collection (e.g., category.getBlogPosts()).
  • Many-to-Many: Accessing a lazily-loaded collection.
  • Session’s load (or getReference): Returns a proxy that triggers the exception if accessed outside the session.
• Mechanism: Hibernate uses a third-party library (ByteBuddy JAR) to generate dynamic proxies for un-fetched data. A proxy (e.g., for List<BlogPost> or List<FoodItem>) substitutes un-fetched data, and accessing it outside the session throws the exception.

Solutions

1. Change Fetching Policy to EAGER

• Configuration:

  @OneToMany(mappedBy = "chosenCategory", cascade = CascadeType.ALL, orphanRemoval = true, fetch = FetchType.EAGER)
  private List<BlogPost> posts = new ArrayList<>();
  
  

• Effect: Hibernate fetches the BlogPost list whenever a Category is loaded.