jpa-lazy-loading.md

fetch = FetchType.LAZY:

This controls when the related data (Role) is loaded from the database.

There are two options:

💤 LAZY (lazy loading)

• Don’t load the Role data immediately when you load a User.
• Only load it when it is accessed (i.e., when you call user.getRole()).

This is the default for @OneToMany, but not for @ManyToOne.

⚡ EAGER (eager loading)

• Load the Role immediately along with the User.

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🧠 Why Use LAZY?

Because it:

• Avoids loading unnecessary data
• Saves memory and improves performance
• Makes queries faster when you don’t always need the related entity

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⚠️ But Be Careful

Lazy loading can lead to the N+1 problem or LazyInitializationException if you're not careful.

Example of a LazyInitializationException:

User user = userRepo.findById(1L); // role is not fetched yet
entityManager.close(); // session is closed

user.getRole().getName(); // ❌ Throws LazyInitializationException!

Because role is fetched lazily, and the session is already closed.

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✅ Best Practice

Use LAZY by default, but:

• Use JOIN FETCH or @EntityGraph when you know you’ll need the related data
• Be mindful of when the session is open

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🧾 Final Example

@Entity
public class User {

    @ManyToOne(fetch = FetchType.LAZY)
    private Role role;
}

This means:

• A user has one role
• The role will only be fetched from DB when needed