Join fetch for optimization in Spring Boot - Mini Project: Build & Apply

Join Fetch for Optimization in Spring Boot

📖 Scenario: You are building a Spring Boot application that manages authors and their books. Each author can have multiple books. You want to fetch authors along with their books efficiently to avoid performance issues.

🎯 Goal: Learn how to use @Query with JOIN FETCH in Spring Data JPA to optimize fetching related entities in one query.

📋 What You'll Learn

Create an Author entity with a list of Book entities

Create a Spring Data JPA repository for Author

Add a configuration variable to control fetching behavior

Write a repository method using JOIN FETCH to fetch authors with their books in one query

💡 Why This Matters

🌍 Real World

Fetching related data efficiently is important in real-world applications to reduce database load and improve performance.

💼 Career

Understanding join fetch queries and Spring Data JPA repositories is essential for backend developers working with Spring Boot and relational databases.

1

Create Author and Book Entities

Create two entities: Author and Book. In Author, add a field List books with @OneToMany(mappedBy = "author"). In Book, add a field Author author with @ManyToOne. Use exact class and field names.

Spring Boot

// Your code here to create Author and Book entities with the specified fields and annotations

Need a hint?

Remember to use @Entity on both classes and map the relationship correctly with @OneToMany and @ManyToOne.

2

Add Repository Interface for Author

Create an interface AuthorRepository that extends JpaRepository<Author, Long>. Add a boolean variable fetchBooks in your service or config class to control whether to fetch books eagerly or lazily.

Spring Boot

import org.springframework.data.jpa.repository.JpaRepository;

// Your code here to create AuthorRepository interface

// Your code here to add boolean fetchBooks variable in config or service

Need a hint?

Create the repository interface with the exact name AuthorRepository and extend JpaRepository<Author, Long>. Add the boolean variable fetchBooks exactly as shown.

3

Add Repository Method with Join Fetch

In AuthorRepository, add a method List<Author> findAllWithBooks() annotated with @Query that uses SELECT a FROM Author a JOIN FETCH a.books to fetch authors with their books in one query.

Spring Boot

import org.springframework.data.jpa.repository.Query;
import java.util.List;

// Your code here to add findAllWithBooks() method with @Query and JOIN FETCH

Need a hint?

Use @Query annotation with the exact JPQL query SELECT a FROM Author a JOIN FETCH a.books and method name findAllWithBooks().

4

Use the Join Fetch Method Conditionally

In your service class, write a method List<Author> getAuthors() that returns authorRepository.findAllWithBooks() if AuthorServiceConfig.fetchBooks is true, otherwise returns authorRepository.findAll(). Use exact method and variable names.

Spring Boot

import java.util.List;

// Your code here to write getAuthors() method using fetchBooks variable and repository methods

Need a hint?

Use the boolean fetchBooks to decide which repository method to call inside getAuthors().