Fetch types (LAZY vs EAGER) in Spring Boot - Performance Comparison

Performance: Fetch types (LAZY vs EAGER)

HIGH IMPACT

This affects how and when related data is loaded from the database, impacting page load speed and responsiveness.

Loading related entities in a Spring Boot application

Spring Boot

class User {
  @OneToMany(fetch = FetchType.LAZY)
  Set<Order> orders;
}

// Orders load only when accessed explicitly

LAZY fetch delays loading related data until accessed, reducing initial query size and speeding up page load.

📈 Performance GainReduces initial DB load and memory use; improves LCP by loading only needed data.

Loading related entities in a Spring Boot application

Spring Boot

class User {
  @OneToMany(fetch = FetchType.EAGER)
  Set<Order> orders;
}

// Fetching User loads all Orders immediately

EAGER fetch loads all related entities immediately, even if not needed, causing slower initial queries and higher memory use.

📉 Performance CostBlocks rendering longer due to heavy DB queries; increases LCP by loading unnecessary data upfront.

Performance Comparison

Pattern	DB Queries	Memory Usage	Initial Load Time	Verdict
EAGER Fetch	Loads all related data immediately	High due to all data in memory	Slower initial load	[X] Bad
LAZY Fetch	Loads related data on demand	Lower memory footprint	Faster initial load	[OK] Good

Rendering Pipeline

Fetch type choice affects when data is retrieved from the database, influencing the backend response time and how fast the frontend can render content.

→Data Fetching

→Backend Processing

→Frontend Rendering

⚠️ BottleneckData Fetching stage due to large or unnecessary queries with EAGER fetch.

Core Web Vital Affected

LCP

This affects how and when related data is loaded from the database, impacting page load speed and responsiveness.

Optimization Tips

1Use LAZY fetch to load related data only when needed.

2Avoid EAGER fetch for large or deeply nested relationships.

3Monitor backend response times to detect heavy eager loading.

Performance Quiz - 3 Questions

What is the main performance drawback of using EAGER fetch in Spring Boot?

A It delays loading related data until accessed.

B It loads all related data immediately, increasing initial load time.

C It reduces memory usage by loading less data.

D It improves backend response time by batching queries.

DevTools: Network and Performance panels

How to check: Use Network panel to observe backend API response times; use Performance panel to see how long rendering waits for data.

What to look for: Long backend response times or large payloads indicate EAGER fetch; faster responses with smaller payloads indicate LAZY fetch.

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