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

Fetch types (LAZY vs EAGER) in Spring Boot JPA

📖 Scenario: You are building a simple Spring Boot application to manage Authors and their Books. Each author can have multiple books.In real life, sometimes you want to load all books of an author immediately when you get the author (EAGER fetching). Other times, you want to load the books only when you really need them (LAZY fetching).

🎯 Goal: Learn how to define JPA entity relationships with fetch = FetchType.LAZY and fetch = FetchType.EAGER in Spring Boot. You will create two entities, configure fetch types, and see how to set them up in code.

📋 What You'll Learn

Create two JPA entities: Author and Book

Set up a one-to-many relationship from Author to Book

Configure the fetch type for the books collection

Understand how to switch between LAZY and EAGER fetching

💡 Why This Matters

🌍 Real World

In real applications, controlling when related data loads helps improve performance and user experience. For example, loading all books eagerly might slow down the app if an author has many books.

💼 Career

Understanding fetch types is essential for backend developers working with Spring Boot and JPA to write efficient and maintainable data access code.

1

Create the Author entity with a books list

Create a JPA entity class called Author with fields id (Long, primary key), name (String), and a list of Book called books. Use @Entity and @Id annotations. Initialize books as an empty list.

Spring Boot

@Entity
public class Author {
    // Your code here
}

Need a hint?

Remember to import javax.persistence.Entity, javax.persistence.Id, and java.util.List and java.util.ArrayList.

2

Create the Book entity with a reference to Author

Create a JPA entity class called Book with fields id (Long, primary key), title (String), and a reference to Author called author. Use @Entity, @Id, and @ManyToOne annotations.

Spring Boot

@Entity
public class Author {
    @Id
    private Long id;
    private String name;
    @OneToMany(mappedBy = "author")
    private List books = new ArrayList();

    // getters and setters
}

@Entity
public class Book {
    // Your code here
}

Need a hint?

Remember the @ManyToOne annotation to link back to the author.

3

Set fetch type to LAZY on the books collection

Modify the books field in the Author class to use fetch = FetchType.LAZY in the @OneToMany annotation.

Spring Boot

@Entity
public class Author {
    @Id
    private Long id;
    private String name;
    @OneToMany(mappedBy = "author", fetch = FetchType.LAZY)
    private List books = new ArrayList();

    // getters and setters
}

// Your code here if needed

Need a hint?

Use fetch = FetchType.LAZY inside the @OneToMany annotation.

4

Change fetch type to EAGER on the books collection

Change the fetch type on the books field in Author to FetchType.EAGER in the @OneToMany annotation.

Spring Boot

import javax.persistence.FetchType;
// Change fetch type to EAGER below
@Entity
public class Author {
    @Id
    private Long id;
    private String name;
    @OneToMany(mappedBy = "author", fetch = FetchType.EAGER)
    private List books = new ArrayList();

    // getters and setters
}

// Your code here if needed

Need a hint?

Simply replace FetchType.LAZY with FetchType.EAGER in the annotation.