Custom query methods by naming convention in Spring Boot - Mini Project: Build & Apply

Custom Query Methods by Naming Convention in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage a list of books in a library. You want to find books by their title and by their author using Spring Data JPA's custom query methods by naming convention.

🎯 Goal: Create a Spring Data JPA repository interface with custom query methods that find books by their exact title and by their author's name.

📋 What You'll Learn

Create a Book entity class with fields id, title, and author.

Create a repository interface called BookRepository that extends JpaRepository.

Add a custom query method findByTitle to find a book by its exact title.

Add a custom query method findByAuthor to find all books by a given author.

💡 Why This Matters

🌍 Real World

Finding data by specific fields is common in applications like libraries, stores, or user management systems. Spring Data JPA lets you write simple method names to get this functionality without writing SQL.

💼 Career

Knowing how to create custom query methods by naming convention is a key skill for Java backend developers working with Spring Boot and databases.

1

Create the Book entity

Create a class called Book with private fields Long id, String title, and String author. Annotate it with @Entity and mark id as the primary key with @Id and @GeneratedValue.

Spring Boot

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

Hint

Use @Entity on the class, @Id and @GeneratedValue on the id field.

2

Create the BookRepository interface

Create an interface called BookRepository that extends JpaRepository<Book, Long>.

Spring Boot

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;

@Entity
public class Book {
    @Id
    @GeneratedValue
    private Long id;
    private String title;
    private String author;

    // Getters and setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }
}

// Your code here

Hint

Extend JpaRepository<Book, Long> in the interface declaration.

3

Add custom query method to find by title

Inside the BookRepository interface, add a method declaration Book findByTitle(String title); to find a book by its exact title.

Spring Boot

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;

@Entity
public class Book {
    @Id
    @GeneratedValue
    private Long id;
    private String title;
    private String author;

    // Getters and setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }
}

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

public interface BookRepository extends JpaRepository {
    // Your code here
}

Hint

Use the method name findByTitle with a String title parameter.

4

Add custom query method to find by author

Inside the BookRepository interface, add a method declaration List<Book> findByAuthor(String author); to find all books by the given author. Import java.util.List.

Spring Boot

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;

@Entity
public class Book {
    @Id
    @GeneratedValue
    private Long id;
    private String title;
    private String author;

    // Getters and setters
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }
}

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

public interface BookRepository extends JpaRepository {
    Book findByTitle(String title);
    // Your code here
}

Hint

Use List<Book> as the return type and import java.util.List.

Practice

1. What does a Spring Data JPA method named findByLastName do?

easy

A. Counts records where the lastName matches the given value

B. Deletes records where the lastName matches the given value

C. Checks if any record exists with the given lastName

D. Finds all records where the lastName matches the given value

Solution

1. Step 1: Understand the method prefix

   The prefix findBy in Spring Data JPA means it will search and return matching records.

2. Step 2: Analyze the property name

   The method uses LastName as the property to filter by, so it finds records with that lastName.

3. Final Answer:

   Finds all records where the lastName matches the given value -> Option D

4. Quick Check:

   findBy + property = find matching records [OK]

Hint: findBy means search and return matching records [OK]

Common Mistakes:

• Confusing findBy with deleteBy or countBy
• Thinking it returns a boolean instead of records
• Ignoring the property name after findBy

2. Which of the following is the correct syntax for a method that counts users by their age in Spring Data JPA?

easy

A. findCountByAge(int age);

B. countUsersByAge(int age);

C. countByAge(int age);

D. countAgeBy(int age);

Solution

1. Step 1: Identify the correct prefix for counting

   The correct prefix to count records is countBy.

2. Step 2: Check method naming pattern

   The method should be countByAge to count users filtered by age.

3. Final Answer:

   countByAge(int age); -> Option C

4. Quick Check:

   countBy + property = count matching records [OK]

Hint: Use countBy + property to count records [OK]

Common Mistakes:

• Adding extra words like Users in method name
• Using findCountBy which is invalid
• Placing property name after count instead of after By

3. Given the method existsByEmailAndStatus(String email, String status), what will it return if a user with email "[e-mail]" and status "active" exists?

medium

A. A list of users matching the email and status

B. true

C. false

D. The count of users matching the email and status

Solution

1. Step 1: Understand the method prefix

   The prefix existsBy returns a boolean indicating if any record matches the criteria.

2. Step 2: Analyze the condition

   The method checks if a user exists with the given email and status combined with And.

3. Final Answer:

   true -> Option B

4. Quick Check:

   existsBy + conditions returns boolean [OK]

Hint: existsBy returns true if matching record exists [OK]

Common Mistakes:

• Expecting a list instead of boolean
• Confusing existsBy with findBy
• Ignoring the combined conditions with And

4. Identify the error in this Spring Data JPA method declaration: List<User> findByNameOr(int age, String name);

medium

A. The method name is missing the property after 'Or'

B. The method should use 'And' instead of 'Or' for combining conditions

C. The order of parameters does not match the method name conditions

D. The return type should be boolean for 'findBy' methods

Solution

1. Step 1: Analyze the method name structure

   The method name uses 'Or' but does not specify the property after 'Or'. It should be like 'findByNameOrAge'.

2. Step 2: Check parameter order and names

   Parameters should match the properties in the method name order, but the main error is missing property after 'Or'.

3. Final Answer:

   The method name is missing the property after 'Or' -> Option A

4. Quick Check:

   Method names must specify property after 'Or' [OK]

Hint: After 'Or' or 'And', always specify property name [OK]

Common Mistakes:

• Leaving out property name after 'Or' or 'And'
• Mixing parameter order with method name order
• Using wrong return type for findBy methods

5. You want to create a method that finds all orders where the customer's city is "New York" and the order total is greater than 100. Which method name correctly follows Spring Data JPA naming conventions?

hard

A. findByCustomerCityAndOrderTotalGreaterThan(String city, double total);

B. findOrdersByCityAndTotalGreater(String city, double total);

C. findByCityAndOrderTotalGreaterThan(String city, double total);

D. findByCustomerCityAndOrderTotalGreater(String city, double total);

Solution

1. Step 1: Identify the correct property names

   The properties are nested: customer.city and orderTotal. The method name must reflect this exactly.

2. Step 2: Use correct keywords for comparison

   For 'greater than', the keyword is 'GreaterThan' in Spring Data JPA method names.

3. Step 3: Combine conditions with 'And'

   The method name should combine both conditions with 'And' and use full property paths.

4. Final Answer:

   findByCustomerCityAndOrderTotalGreaterThan(String city, double total); -> Option A

5. Quick Check:

   Use full property names + GreaterThan + And [OK]

Hint: Use full property names and 'GreaterThan' for > comparisons [OK]

Common Mistakes:

• Using incomplete property names
• Using 'Greater' instead of 'GreaterThan'
• Omitting 'By' after 'find'