How to Fix Hibernate\'s \'failed to lazily initialize a collection of role\' Exception?

Eliminating the "failed to lazily initialize a collection of role" Hibernate Exception

Hibernate's "failed to lazily initialize a collection of role" exception can arise when attempting to access a lazily-loaded collection outside of a session context. To resolve this issue, let's delve into the specific scenario:

The provided code defines an @OneToMany relationship in the Topic model, mapped by the Comment model. In your controller, you retrieve a Topic instance and its comments. The JSP view then iterates over the comments to display them. However, this triggers the exception because the comments collection is lazy-loaded by default.

To rectify this, you have two options:

1. Eager Loading:

Update the comments field mapping in Topic to use eager loading:

@OneToMany(fetch = FetchType.EAGER, mappedBy = "topic", cascade = CascadeType.ALL)
private Collection<Comment> comments = new LinkedHashSet<>();

Eager loading ensures that the comments collection is initialized when the Topic is loaded, preventing the exception.

2. Lazy Loading with Open Session:

Alternatively, you can keep the comments collection as lazy-loaded, but ensure that the Hibernate session is open when accessing the comments:

In your controller:

Topic topicById = service.findTopicByID(id);

// Start a new Hibernate session
Session session = sessionFactory.getCurrentSession();

Collection<Comment> commentList = topicById.getComments();

// Close the session once done
session.close();

// Pass the commentList to the view
//...

By opening a new session within the controller, Hibernate has the necessary context to initialize the comments collection. Remember to close the session explicitly when finished.

Remember that eager loading may have performance implications on larger collections, so choose the approach that best suits your application's needs.

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