Why DTOs matter in Spring Boot - See It in Action

Why DTOs Matter in Spring Boot

📖 Scenario: You are building a simple Spring Boot application to manage books in a library. You want to send only necessary book information to users, not the entire database entity. This helps keep your app safe and efficient.

🎯 Goal: Build a Spring Boot project that uses a Book entity and a BookDTO (Data Transfer Object) to send limited book data to the client.

📋 What You'll Learn

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

Create a BookDTO class with only title and author fields

Write a method to convert a Book entity to a BookDTO

Use the BookDTO in a controller method to return book data

💡 Why This Matters

🌍 Real World

In real apps, DTOs help control what data is sent over the network, improving security and performance.

💼 Career

Understanding DTOs is essential for backend developers working with APIs and data transfer in Spring Boot applications.

1

Create the Book entity class

Create a class called Book with private fields Long id, String title, String author, and double price. Include public getters and setters for each field.

Spring Boot

// Your code here to create the Book class with fields and getters/setters

Hint

Think of the Book class as a container holding all details about a book. Use private fields and public getters/setters to follow Java best practices.

2

Create the BookDTO class

Create a class called BookDTO with private fields String title and String author. Include public getters and setters for these fields.

Spring Boot

public class Book {
    private Long id;
    private String title;
    private String author;
    private double price;

    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;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }
}

// Your code here to create the BookDTO class with title and author fields and getters/setters

Hint

The BookDTO class holds only the data you want to share. It is like a smaller package with just the title and author.

3

Add a method to convert Book to BookDTO

Inside the BookDTO class, add a public static method called fromBook that takes a Book object as a parameter and returns a new BookDTO with the title and author copied from the Book.

Spring Boot

public class Book {
    private Long id;
    private String title;
    private String author;
    private double price;

    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;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }
}

public class BookDTO {
    private String title;
    private String author;

    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;
    }

    // Add the fromBook method below
    // Your code here

Hint

This method helps convert a full Book into a smaller BookDTO. Think of it as packing only what you want to send.

4

Use BookDTO in a controller method

Create a Spring Boot controller class called BookController with a method getBookDTO that returns a BookDTO. Inside the method, create a Book object with title "Spring Boot Guide" and author "Jane Doe". Use the BookDTO.fromBook method to convert and return the DTO.

Spring Boot

public class Book {
    private Long id;
    private String title;
    private String author;
    private double price;

    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;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }
}

public class BookDTO {
    private String title;
    private String author;

    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;
    }

    public static BookDTO fromBook(Book book) {
        BookDTO dto = new BookDTO();
        dto.setTitle(book.getTitle());
        dto.setAuthor(book.getAuthor());
        return dto;
    }
}

// Your code here to create BookController with getBookDTO method

Hint

The controller sends only the BookDTO to the client. This keeps your app safe by hiding the price and id.

Practice

1. Why do we use DTOs (Data Transfer Objects) in a Spring Boot application?

easy

A. To make the application slower by adding extra layers

B. To increase the size of data sent over the network

C. To replace the database entities completely

D. To carry only the data we want to share and hide internal details

Solution

1. Step 1: Understand the purpose of DTOs

   DTOs are designed to carry only the necessary data between processes or layers, avoiding exposure of internal details.

2. Step 2: Analyze the options

   To carry only the data we want to share and hide internal details correctly states the purpose of DTOs. The other options describe incorrect or harmful uses.

3. Final Answer:

   To carry only the data we want to share and hide internal details -> Option D

4. Quick Check:

   DTOs protect and simplify data transfer = D [OK]

Hint: DTOs share only needed data, hiding internals [OK]

Common Mistakes:

• Thinking DTOs replace entities fully
• Assuming DTOs increase data size
• Believing DTOs slow down the app

2. Which of the following is the correct way to define a simple DTO class in Spring Boot?

easy

A. public class UserDTO { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } }

B. @Entity public class UserDTO { private String name; private int age; }

C. public interface UserDTO { String getName(); int getAge(); }

D. @Repository public class UserDTO { private String name; private int age; }

Solution

1. Step 1: Identify correct DTO structure

   A DTO is a simple class with private fields and public getters/setters, without annotations like @Entity or @Repository.

2. Step 2: Evaluate each option

   public class UserDTO { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } } correctly defines a DTO class with fields and accessors. @Entity public class UserDTO { private String name; private int age; } wrongly uses @Entity, which is for database entities. public interface UserDTO { String getName(); int getAge(); } defines an interface, not a DTO class. @Repository public class UserDTO { private String name; private int age; } wrongly uses @Repository, which is for data access layers.

3. Final Answer:

   public class UserDTO { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } } -> Option A

4. Quick Check:

   DTO = simple class with getters/setters = B [OK]

Hint: DTOs are plain classes with getters/setters, no @Entity [OK]

Common Mistakes:

• Adding @Entity annotation to DTO
• Using interfaces instead of classes for DTO
• Marking DTO as @Repository

3. Given this Spring Boot controller method, what will be the output JSON when calling /user?

@GetMapping("/user")
public UserDTO getUser() {
    UserDTO dto = new UserDTO();
    dto.setName("Alice");
    dto.setAge(30);
    return dto;
}

medium

A. {"name":"Alice"}

B. {"UserDTO":{"name":"Alice","age":30}}

C. {"name":"Alice","age":30}

D. Error: Cannot serialize UserDTO

Solution

1. Step 1: Understand default JSON serialization in Spring Boot

   Spring Boot uses Jackson to convert returned objects to JSON, serializing all public getters by default.

2. Step 2: Analyze the returned UserDTO object

   UserDTO has name and age fields with getters, so JSON will include both as simple key-value pairs.

3. Final Answer:

   {"name":"Alice","age":30} -> Option C

4. Quick Check:

   DTO fields serialize as JSON keys = A [OK]

Hint: Returned DTO converts to JSON with all getters [OK]

Common Mistakes:

• Expecting nested JSON with class name
• Assuming partial fields serialize
• Thinking serialization causes error

4. What is wrong with this DTO class that causes a runtime error when Spring Boot tries to deserialize it?

public class ProductDTO {
    private String name;
    private int price;
    public ProductDTO(String name, int price) {
        this.name = name;
        this.price = price;
    }
}

medium

A. Fields should be public, not private

B. Missing default no-argument constructor

C. Constructor parameters should be annotated with @Autowired

D. Class should be annotated with @Entity

Solution

1. Step 1: Identify deserialization requirements

   Jackson requires a default no-argument constructor to create an instance before setting fields via setters or reflection.

2. Step 2: Check the DTO class

   This class only has a parameterized constructor and no default constructor, causing Jackson to fail during deserialization.

3. Final Answer:

   Missing default no-argument constructor -> Option B

4. Quick Check:

   Jackson needs no-arg constructor = A [OK]

Hint: DTOs need no-arg constructor for JSON serialization [OK]

Common Mistakes:

• Thinking fields must be public
• Adding @Autowired to constructor parameters
• Confusing DTO with entity needing @Entity

5. You have an entity class User with many fields, but you want to expose only id and email in your API response. How should you use a DTO to achieve this cleanly?

hard

A. Create a UserDTO with only id and email fields, map User to UserDTO before returning

B. Return the User entity directly and ignore unwanted fields in the frontend

C. Add @JsonIgnore to all unwanted fields in the User entity

D. Use the User entity but rename unwanted fields to empty strings

Solution

1. Step 1: Understand data exposure risks

   Returning the full User entity exposes all fields, risking sensitive data leaks.

2. Step 2: Use DTO to control data

   Creating a UserDTO with only id and email fields and mapping User to UserDTO ensures only desired data is sent.

3. Step 3: Evaluate other options

   Return the User entity directly and ignore unwanted fields in the frontend risks exposing all data. Add @JsonIgnore to all unwanted fields in the User entity mixes entity with serialization concerns and can be error-prone. Use the User entity but rename unwanted fields to empty strings is a bad practice and confusing.

4. Final Answer:

   Create a UserDTO with only id and email fields, map User to UserDTO before returning -> Option A

5. Quick Check:

   DTOs control exposed data = C [OK]

Hint: Use DTO to expose only needed fields safely [OK]

Common Mistakes:

• Returning full entity directly
• Using @JsonIgnore on entity fields
• Modifying entity fields to hide data