Fetch types (LAZY vs EAGER) in Spring Boot - When to Use Which

The Big Idea

Discover how smart data loading can make your app lightning fast and efficient!

The Scenario

Imagine you have a list of books, and each book has many authors. You want to show just the book titles on a page. If you manually load all authors for every book upfront, it feels like carrying a heavy backpack full of unnecessary stuff.

The Problem

Loading all related data at once makes your app slow and uses too much memory. If you only need some data, fetching everything wastes time and resources. Manually controlling this is tricky and error-prone.

The Solution

Fetch types like LAZY and EAGER let Spring Boot decide when to load related data. LAZY loads data only when needed, like unpacking your backpack only when you want something. EAGER loads everything immediately, useful when you know you need all data.

Before vs After

✗ Before

List<Book> books = bookRepository.findAll(); // manually fetch authors for each book later

✓ After

@OneToMany(fetch = FetchType.LAZY)
List<Author> authors; // authors load only when accessed

What It Enables

This lets your app run faster and use less memory by loading data smartly, improving user experience and scalability.

Real Life Example

When showing a list of blog posts, you load just the posts first (LAZY). Only when a user clicks a post, you load comments and details, saving bandwidth and speeding up the page.

Key Takeaways

Manual data loading can slow apps and waste resources.

LAZY fetch loads related data only when needed.

EAGER fetch loads all related data immediately.

Practice

1. What does the LAZY fetch type do in Spring Boot JPA?

easy

A. It disables loading of related entities completely.

B. It loads all related entities immediately with the main entity.

C. It delays loading related entities until they are accessed.

D. It loads related entities only during application startup.

Solution

1. Step 1: Understand fetch types in JPA

   Fetch types control when related data is loaded from the database.

2. Step 2: Define LAZY fetch behavior

   LAZY means related entities are loaded only when you access them, not immediately.

3. Final Answer:

   It delays loading related entities until they are accessed. -> Option C

4. Quick Check:

   LAZY = delayed loading [OK]

Hint: LAZY means wait to load until needed [OK]

Common Mistakes:

• Confusing LAZY with EAGER loading
• Thinking LAZY loads data immediately
• Assuming LAZY disables loading

2. Which is the correct way to specify EAGER fetching on a JPA relationship in Spring Boot?

easy

A. @ManyToOne(fetch = FetchType.EAGER)

B. @OneToMany(fetch = FetchType.LAZY)

C. @OneToOne(fetch = FetchType.LAZY)

D. @ManyToMany(fetch = FetchType.LAZY)

Solution

1. Step 1: Identify correct annotation and fetch type

   EAGER fetch type is set using fetch = FetchType.EAGER inside relationship annotations.

2. Step 2: Match correct relationship and fetch type

   @ManyToOne(fetch = FetchType.EAGER) uses @ManyToOne(fetch = FetchType.EAGER), which is valid and correct.

3. Final Answer:

   @ManyToOne(fetch = FetchType.EAGER) -> Option A

4. Quick Check:

   EAGER fetch uses FetchType.EAGER [OK]

Hint: EAGER fetch uses FetchType.EAGER in annotation [OK]

Common Mistakes:

• Using LAZY instead of EAGER for eager loading
• Mixing relationship types with wrong fetch types
• Forgetting to specify fetch attribute

3. Given the following code snippet, what will happen when order.getItems() is called if items is marked with fetch = FetchType.LAZY?

@Entity
class Order {
  @OneToMany(fetch = FetchType.LAZY)
  private List<Item> items;
}
Order order = entityManager.find(Order.class, 1L);
// No call to getItems() yet

medium

A. The items list is loaded immediately when the order is fetched.

B. The items list is never loaded, causing a null pointer exception.

C. The items list is loaded during application startup.

D. The items list is loaded only when getItems() is called.

Solution

1. Step 1: Understand LAZY fetch behavior on collections

   With LAZY fetch, related collections like items are not loaded immediately.

2. Step 2: When is data loaded?

   The data loads only when the getter getItems() is called, triggering the fetch.

3. Final Answer:

   The items list is loaded only when getItems() is called. -> Option D

4. Quick Check:

   LAZY fetch loads on access [OK]

Hint: LAZY loads collections only on getter call [OK]

Common Mistakes:

• Assuming collections load immediately with LAZY
• Expecting null instead of a proxy collection
• Confusing LAZY with EAGER behavior

4. You have a @OneToMany(fetch = FetchType.LAZY) relationship, but you get a LazyInitializationException when accessing the collection outside a transaction. What is the likely cause?

medium

A. The fetch type should be LAZY, so this exception is unexpected.

B. The collection was accessed after the session was closed.

C. The entity was not annotated with @Entity.

D. The database connection is lost.

Solution

1. Step 1: Understand LazyInitializationException cause

   This exception happens when a LAZY collection is accessed outside an open session or transaction.

2. Step 2: Identify session state during access

   If the session is closed before accessing the collection, the proxy cannot load data, causing the exception.

3. Final Answer:

   The collection was accessed after the session was closed. -> Option B

4. Quick Check:

   LazyInitializationException = access after session close [OK]

Hint: Access LAZY data only inside open session/transaction [OK]

Common Mistakes:

• Thinking fetch type alone prevents exceptions
• Ignoring session lifecycle in JPA
• Blaming database connection instead

5. You have an entity Author with a @OneToMany(fetch = FetchType.LAZY) relationship to Book. You want to display all authors with their books immediately to avoid multiple queries later. Which approach is best?

hard

A. Keep LAZY fetch and use a JPQL query with JOIN FETCH to load books eagerly.

B. Use native SQL queries only to load all data at once.

C. Load authors and books separately in two queries and merge in Java code.

D. Change the fetch type to EAGER on the relationship.

Solution

1. Step 1: Understand pros and cons of fetch types

   Changing to EAGER can cause performance issues if not always needed.

2. Step 2: Use JOIN FETCH for selective eager loading

   JPQL with JOIN FETCH loads related entities eagerly only when needed, avoiding multiple queries.

3. Step 3: Evaluate other options

   Loading separately or using native queries is less efficient or more complex.

4. Final Answer:

   Keep LAZY fetch and use a JPQL query with JOIN FETCH to load books eagerly. -> Option A

5. Quick Check:

   JOIN FETCH = selective eager loading [OK]

Hint: Use JOIN FETCH query to eagerly load LAZY relations [OK]

Common Mistakes:

• Switching to EAGER globally causing overhead
• Ignoring JPQL JOIN FETCH option
• Overcomplicating with native queries