DTO validation in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a validation annotation that ensures the 'name' field is not empty.

Spring Boot

public class UserDTO {
    @[1]
    private String name;

    // getters and setters
}

Drag options to blanks, or click blank then click option'

ASize

BNotEmpty

CEmail

DMin

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Size without specifying min value

Using @Email for a name field

Using @Min which is for numbers

The @NotEmpty annotation ensures the 'name' field is not null or empty.

2fill in blank

medium

Complete the code to validate that the 'email' field contains a valid email format.

Spring Boot

public class UserDTO {
    @[1]
    private String email;

    // getters and setters
}

Drag options to blanks, or click blank then click option'

ANotNull

BPattern

CEmail

DNotBlank

Attempts:

3 left

💡 Hint

Common Mistakes

Using @NotNull which only checks for null, not format

Using @Pattern without regex

Using @NotBlank which only checks for empty strings

The @Email annotation checks that the string is a valid email format.

3fill in blank

hard

Fix the error in the code by adding the correct annotation to ensure 'age' is at least 18.

Spring Boot

public class UserDTO {
    @[1]
    private int age;

    // getters and setters
}

Drag options to blanks, or click blank then click option'

AMax(18)

BPositive

CSize(min=18)

DMin(18)

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Max instead of @Min

Using @Size which is for strings or collections

Using @Positive which only checks greater than zero

The @Min(18) annotation ensures the 'age' field value is at least 18.

4fill in blank

hard

Fill both blanks to create a DTO field 'password' that must be at least 8 characters and not blank.

Spring Boot

public class UserDTO {
    @[1]
    @[2](min = 8)
    private String password;

    // getters and setters
}

Drag options to blanks, or click blank then click option'

ANotBlank

BNotNull

CSize

DPattern

Attempts:

3 left

💡 Hint

Common Mistakes

Using @NotNull instead of @NotBlank

Using @Pattern without regex

Using @Size without min parameter

@NotBlank ensures the password is not empty or just spaces.
@Size(min = 8) ensures the password has at least 8 characters.

5fill in blank

hard

Fill all three blanks to create a DTO field 'username' that is not null, has length between 5 and 15, and matches only letters and digits.

Spring Boot

public class UserDTO {
    @[1]
    @[2](min = 5, max = 15)
    @[3](regexp = "^[a-zA-Z0-9]+$")
    private String username;

    // getters and setters
}

Drag options to blanks, or click blank then click option'

ANotNull

BSize

CPattern

DNotBlank

Attempts:

3 left

💡 Hint

Common Mistakes

Using @NotBlank instead of @NotNull

Omitting max in @Size

Using wrong regex in @Pattern

@NotNull ensures the username is not null.
@Size(min = 5, max = 15) restricts length.
@Pattern restricts characters to letters and digits.

Practice

1. What is the main purpose of using DTO validation in a Spring Boot application?

easy

A. To handle user authentication and login

B. To speed up database queries automatically

C. To generate HTML pages from data

D. To check and ensure input data meets rules before processing

Solution

1. Step 1: Understand DTO role

   A DTO (Data Transfer Object) carries data between processes and needs validation to ensure data is correct.

2. Step 2: Purpose of validation

   Validation checks input data early to prevent bad data from reaching business logic or database.

3. Final Answer:

   To check and ensure input data meets rules before processing -> Option D

4. Quick Check:

   DTO validation = input data check [OK]

Hint: Validation means checking input data early [OK]

Common Mistakes:

• Confusing validation with database optimization
• Thinking validation generates UI
• Mixing validation with authentication

2. Which annotation is used on a DTO field to require that it must not be empty or null?

easy

A. @Size(min = 1)

B. @NotEmpty

C. @NotNull

D. @Valid

Solution

1. Step 1: Understand annotations meaning

   @NotNull only checks for null, but allows empty strings. @NotEmpty checks for both null and empty strings.

2. Step 2: Choose correct annotation

   To ensure a field is neither null nor empty, @NotEmpty is the best choice.

3. Final Answer:

   @NotEmpty -> Option B

4. Quick Check:

   @NotEmpty = no null or empty [OK]

Hint: Use @NotEmpty to block null and empty strings [OK]

Common Mistakes:

• Using @NotNull but allowing empty strings
• Confusing @Valid with field validation
• Using @Size without min value

3. Given this DTO class snippet:

public class UserDTO {
  @NotNull
  private String username;

  @Min(18)
  private int age;

  // getters and setters
}

What happens if a request sends username=null and age=16 when validated with @Valid?

medium

A. Validation fails for both username and age fields

B. Validation passes because age is int and can't be null

C. Validation fails only for age field

D. Validation passes because @NotNull is ignored on String

Solution

1. Step 1: Check username validation

   @NotNull on username means null value is invalid, so username=null fails validation.

2. Step 2: Check age validation

   @Min(18) means age must be at least 18. Given age=16, this fails validation.

3. Final Answer:

   Validation fails for both username and age fields -> Option A

4. Quick Check:

   @NotNull + @Min(18) fail for null and age 16 [OK]

Hint: Check each annotation rule against input values [OK]

Common Mistakes:

• Assuming int fields can't fail validation
• Ignoring @NotNull effect on String
• Thinking validation passes if one field is valid

4. Identify the error in this controller method for validating a DTO:

@PostMapping("/users")
public ResponseEntity<String> addUser(UserDTO user) {
  // save user
  return ResponseEntity.ok("User added");
}

medium

A. Missing @Validated annotation on controller class

B. Method should return void instead of ResponseEntity

C. Missing @RequestBody annotation on UserDTO parameter

D. No error, code is correct

Solution

1. Step 1: Check parameter annotations

   To validate JSON input as DTO, @RequestBody is needed to bind request body to UserDTO.

2. Step 2: Check validation annotation

   @Valid is also needed to trigger validation, but missing @RequestBody causes binding failure first.

3. Final Answer:

   Missing @RequestBody annotation on UserDTO parameter -> Option C

4. Quick Check:

   @RequestBody needed for JSON binding [OK]

Hint: Use @RequestBody to bind JSON to DTO [OK]

Common Mistakes:

• Forgetting @RequestBody causes no binding
• Thinking @Valid alone binds JSON
• Assuming return type must be void

5. You want to validate a DTO with a nested object, where the nested object also needs validation. Which is the correct way to enable validation on the nested DTO field?

hard

A. Add @Valid annotation on the nested DTO field inside the parent DTO

B. Add @NotNull on the nested DTO field only

C. Add @Valid on the parent DTO class only

D. No annotation needed, nested DTOs are validated automatically

Solution

1. Step 1: Understand nested validation

   Spring Boot requires @Valid on nested DTO fields to trigger validation of inner objects.

2. Step 2: Why @Valid on nested field

   @NotNull only checks presence, but @Valid triggers validation of nested object's fields.

3. Final Answer:

   Add @Valid annotation on the nested DTO field inside the parent DTO -> Option A

4. Quick Check:

   @Valid on nested field triggers inner validation [OK]

Hint: Use @Valid on nested DTO fields for full validation [OK]

Common Mistakes:

• Using only @NotNull on nested DTO
• Assuming parent @Valid covers nested fields
• Skipping @Valid and expecting automatic nested validation