Nested DTOs in Spring Boot - Performance & Optimization

Performance: Nested DTOs

MEDIUM IMPACT

Nested DTOs affect the data serialization and deserialization time during API calls, impacting response time and client rendering speed.

Sending complex data structures in API responses

Spring Boot

public class OrderDTO {
  private String customerName;
  private String customerCity;
  private List<String> productNames;
  // getters and setters
}

Flattening DTOs reduces nesting, lowers payload size, and simplifies serialization.

📈 Performance GainSingle serialization pass, reduces payload size by 30-50%, improves LCP and client render speed.

Sending complex data structures in API responses

Spring Boot

public class OrderDTO {
  private CustomerDTO customer;
  private List<ProductDTO> products;
  // getters and setters
}

public class CustomerDTO {
  private AddressDTO address;
  // getters and setters
}

public class AddressDTO {
  private String street;
  private String city;
  private CountryDTO country;
  // getters and setters
}

public class CountryDTO {
  private String name;
  private String code;
  // getters and setters
}

Deep nesting causes large JSON payloads and complex serialization, increasing response time and client parsing cost.

📉 Performance CostTriggers multiple serialization passes and increases payload size by 30-50%, blocking rendering longer.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Deeply Nested DTOs	High due to complex data binding	Multiple reflows possible if UI updates per nested data	High paint cost from large DOM updates	[X] Bad
Flattened DTOs	Lower DOM operations	Single reflow after data binding	Lower paint cost with simpler DOM	[OK] Good

Rendering Pipeline

Nested DTOs increase JSON size and complexity, causing longer parsing and rendering time on the client side.

→Network Transfer

→JSON Parsing

→DOM Update

⚠️ BottleneckJSON Parsing and DOM Update due to large payload and complex data structure

Core Web Vital Affected

LCP

Nested DTOs affect the data serialization and deserialization time during API calls, impacting response time and client rendering speed.

Optimization Tips

1Avoid deep nesting in DTOs to reduce JSON payload size.

2Flatten DTOs to simplify serialization and speed up client parsing.

3Monitor API payload size and parsing time to optimize LCP.

Performance Quiz - 3 Questions

How do deeply nested DTOs affect API response performance?

A Increase payload size and serialization time

B Reduce payload size and speed up serialization

C Have no impact on performance

D Only affect server CPU usage, not client

DevTools: Network

How to check: Open DevTools, go to Network tab, inspect API response payload size and timing for JSON data.

What to look for: Look for large JSON payload size and long transfer or parsing times indicating nested DTO overhead.

Practice

1. What is the main purpose of using Nested DTOs in Spring Boot applications?

easy

A. To handle HTTP requests without controllers

B. To group related data inside other data objects for better structure

C. To replace entity classes with simpler objects

D. To improve database query performance automatically

Solution

1. Step 1: Understand DTO role

   DTOs (Data Transfer Objects) are used to carry data between processes or layers.

2. Step 2: Identify Nested DTO purpose

   Nested DTOs group related data inside other DTOs to represent complex data structures clearly.

3. Final Answer:

   To group related data inside other data objects for better structure -> Option B

4. Quick Check:

   Nested DTOs = Group related data [OK]

Hint: Nested DTOs organize data inside other objects [OK]

Common Mistakes:

• Thinking nested DTOs improve database speed
• Confusing DTOs with entities
• Assuming nested DTOs replace controllers

2. Which of the following is the correct way to declare a nested DTO class inside a parent DTO in Spring Boot?

easy

A. public class ParentDTO { private class ChildDTO { private String name; } }

B. public class ParentDTO { class ChildDTO { private String name; } }

C. public class ParentDTO { public static class ChildDTO { private String name; } }

D. public class ParentDTO { static class ChildDTO { public String name; } }

Solution

1. Step 1: Check nested class modifiers

   Static nested classes are recommended for DTOs to avoid implicit reference to outer class.

2. Step 2: Validate access modifiers

   Public static nested class with private fields and getters/setters is standard practice.

3. Final Answer:

   public class ParentDTO { public static class ChildDTO { private String name; } } -> Option C

4. Quick Check:

   Static nested class with public modifier = public class ParentDTO { public static class ChildDTO { private String name; } } [OK]

Hint: Use public static nested class for nested DTOs [OK]

Common Mistakes:

• Using non-static nested classes causing memory leaks
• Declaring nested class as private making it inaccessible
• Using public fields instead of private with getters/setters

3. Given the following nested DTO classes, what will be the output of System.out.println(order.getCustomer().getName()); if order is initialized as below?

public class OrderDTO {
  private CustomerDTO customer;
  public CustomerDTO getCustomer() { return customer; }
  public void setCustomer(CustomerDTO customer) { this.customer = customer; }
  public static class CustomerDTO {
    private String name;
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
  }
}

OrderDTO order = new OrderDTO();
OrderDTO.CustomerDTO cust = new OrderDTO.CustomerDTO();
cust.setName("Alice");
order.setCustomer(cust);

medium

A. Alice

B. null

C. Compilation error

D. Runtime NullPointerException

Solution

1. Step 1: Analyze object initialization

   The customer object is created and its name is set to "Alice" before being assigned to order.

2. Step 2: Check method calls

   Calling order.getCustomer().getName() returns the name "Alice" as set previously.

3. Final Answer:

   Alice -> Option A

4. Quick Check:

   Nested DTO getter returns set value = Alice [OK]

Hint: Set nested DTO fields before accessing getters [OK]

Common Mistakes:

• Forgetting to set nested DTO before calling getter
• Confusing null with empty string
• Assuming compilation error due to nested class

4. Identify the error in the following nested DTO code snippet:

public class UserDTO {
  private AddressDTO address;
  public static class AddressDTO {
    private String city;
    public String getCity() { return city; }
    public void setCity(String city) { this.city = city; }
  }

  public AddressDTO getAddress() { return address; }
  public void setAddress(AddressDTO address) { this.address = address; }
}

UserDTO user = new UserDTO();
user.getAddress().setCity("Paris");

medium

A. Compilation error due to missing constructor

B. No error, code runs correctly

C. IllegalAccessError on accessing city field

D. NullPointerException because address is not initialized

Solution

1. Step 1: Check object initialization

   The address field in UserDTO is never initialized, so it is null by default.

2. Step 2: Analyze method call

   Calling user.getAddress().setCity("Paris") tries to call setCity on null, causing NullPointerException.

3. Final Answer:

   NullPointerException because address is not initialized -> Option D

4. Quick Check:

   Uninitialized nested DTO causes NullPointerException [OK]

Hint: Always initialize nested DTO before calling its methods [OK]

Common Mistakes:

• Assuming default constructor initializes nested DTO
• Thinking compilation error occurs
• Ignoring possibility of NullPointerException

5. You have a nested DTO structure where OrderDTO contains a list of ItemDTO objects. You want to convert this nested DTO into a flat list of item names using Java streams. Which code snippet correctly achieves this?

public class OrderDTO {
  private List<ItemDTO> items;
  public List<ItemDTO> getItems() { return items; }
  public void setItems(List<ItemDTO> items) { this.items = items; }
  public static class ItemDTO {
    private String name;
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
  }
}

OrderDTO order = ...; // initialized with items

hard

A. List<String> names = order.getItems().stream().map(OrderDTO.ItemDTO::getName).toList();

B. List<String> names = order.getItems().stream().flatMap(ItemDTO::getName).collect(Collectors.toList());

C. List<String> names = order.getItems().map(ItemDTO::getName).collect(Collectors.toList());

D. List<String> names = order.getItems().stream().map(item -> item.name).collect(Collectors.toList());

Solution

1. Step 1: Understand stream mapping

   To get a list of names, map each ItemDTO to its name using map(OrderDTO.ItemDTO::getName).

2. Step 2: Collect results

   Use toList() (Java 16+) or collect(Collectors.toList()) to gather results into a list.

3. Final Answer:

   List<String> names = order.getItems().stream().map(OrderDTO.ItemDTO::getName).toList(); -> Option A

4. Quick Check:

   Stream map + toList() = List<String> names = order.getItems().stream().map(OrderDTO.ItemDTO::getName).toList(); [OK]

Hint: Use stream().map(...).toList() to extract nested DTO fields [OK]

Common Mistakes:

• Using flatMap instead of map for simple field extraction
• Calling map on List directly without stream()
• Accessing private fields without getter