Request validation preview in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of request validation in Spring Boot?

Request validation ensures that the data sent by a client meets the expected rules before processing. It helps prevent errors and security issues by checking inputs early.

beginner

Which annotation is commonly used to trigger validation on a request object in Spring Boot?

The @Valid annotation is used on a method parameter to tell Spring Boot to validate the object according to its constraints.

beginner

How do you define validation rules on a request data class in Spring Boot?

You add annotations like @NotNull, @Size, @Email, etc., on the fields of the class to specify rules that must be met.

intermediate

What happens if a request fails validation in Spring Boot?

Spring Boot automatically returns a 400 Bad Request response with details about which fields failed validation.

intermediate

How can you customize the error messages returned when validation fails in Spring Boot?

You can provide custom messages in the validation annotations or use a messages.properties file to define friendly error texts.

Which annotation triggers validation on a request body in Spring Boot?

A @RequestMapping

B @Autowired

C @Component

D @Valid

The @Valid annotation tells Spring Boot to validate the request object based on its constraints.

What HTTP status code does Spring Boot return when validation fails by default?

A 404 Not Found

B 200 OK

C 400 Bad Request

D 500 Internal Server Error

Spring Boot returns 400 Bad Request when the input data does not meet validation rules.

Which annotation would you use to ensure a string field is not empty in a request object?

A @NotEmpty

B @NotNull

C @Email

D @Pattern

@NotEmpty ensures the string is not null and not empty, preventing empty strings.

Where do you place validation annotations in Spring Boot request validation?

A On controller methods

B On fields of the request data class

C On service classes

D On the main application class

Validation annotations go on the fields of the class representing the request data.

How can you customize validation error messages in Spring Boot?

A By adding messages in messages.properties or in annotation attributes

B By disabling validation

C By changing the HTTP status code

D By editing the controller code

Custom messages can be set in annotation attributes or externalized in messages.properties files.

Explain how request validation works in Spring Boot and how you set it up.

Think about annotations and automatic error handling.

Describe ways to customize validation error messages in Spring Boot.

Focus on how to make error messages clearer for users.

Practice

1. What is the main purpose of using @Valid annotation in Spring Boot request handling?

easy

A. To trigger validation of the incoming request data based on annotations in the DTO

B. To automatically log the request data for debugging

C. To convert the request body into a JSON string

D. To disable validation for the request parameters

Solution

1. Step 1: Understand the role of @Valid

   The @Valid annotation tells Spring Boot to check the incoming data against validation rules defined in the DTO class.

2. Step 2: Identify what @Valid does not do

   It does not log data, convert to JSON, or disable validation. Its sole purpose is to trigger validation.

3. Final Answer:

   To trigger validation of the incoming request data based on annotations in the DTO -> Option A

4. Quick Check:

   @Valid triggers validation [OK]

Hint: Remember: @Valid means 'check this data' [OK]

Common Mistakes:

• Thinking @Valid logs or converts data
• Confusing @Valid with JSON serialization
• Assuming @Valid disables validation

2. Which of the following is the correct way to apply validation annotations in a Spring Boot DTO class?

easy

A. public class UserDTO {
@Valid
private String name;
}

B. public class UserDTO {
@RequestBody
private String name;
}

C. public class UserDTO {
@NotNull
private String name;
}

D. public class UserDTO {
@Autowired
private String name;
}

Solution

1. Step 1: Recognize validation annotations

   Annotations like @NotNull are used on fields to define validation rules in DTO classes.

2. Step 2: Identify incorrect annotations

   @Valid is used on method parameters, not fields; @RequestBody and @Autowired are unrelated to field validation.

3. Final Answer:

   public class UserDTO { @NotNull private String name; } -> Option C

4. Quick Check:

   Use validation annotations like @NotNull on fields [OK]

Hint: Validation annotations go on fields, not on parameters [OK]

Common Mistakes:

• Using @Valid on fields instead of method parameters
• Confusing @RequestBody with validation
• Applying @Autowired to DTO fields

3. Given the following DTO and controller method, what will happen if the client sends a request with an empty email field? public class UserDTO { @NotBlank private String email; // getters and setters } @PostMapping("/users") public ResponseEntity<String> createUser(@Valid @RequestBody UserDTO user) { return ResponseEntity.ok("User created"); }

medium

A. The request will be ignored silently

B. The request will succeed and return "User created"

C. The server will throw a NullPointerException

D. The request will fail validation and return a 400 Bad Request with an error message

Solution

1. Step 1: Understand @NotBlank validation

   The @NotBlank annotation requires the field to be non-null and contain at least one non-whitespace character.

2. Step 2: Analyze the empty email input

   An empty string fails @NotBlank validation, so Spring Boot will reject the request and return a 400 error with validation details.

3. Final Answer:

   The request will fail validation and return a 400 Bad Request with an error message -> Option D

4. Quick Check:

   @NotBlank empty input causes 400 error [OK]

Hint: Empty strings fail @NotBlank, causing 400 errors [OK]

Common Mistakes:

• Assuming empty strings pass validation
• Expecting NullPointerException instead of validation error
• Thinking server ignores invalid input

4. Identify the error in this Spring Boot controller method that uses request validation: @PostMapping("/register") public ResponseEntity<String> registerUser(@Valid UserDTO user) { // method body } Assuming UserDTO has validation annotations.

medium

A. Missing @Valid annotation on the method parameter

B. Missing @RequestBody annotation on the method parameter

C. Method should return void instead of ResponseEntity

D. UserDTO should not have validation annotations

Solution

1. Step 1: Check parameter annotations for validation

   To validate request body data, the parameter must be annotated with @RequestBody so Spring can deserialize JSON into the DTO.

2. Step 2: Confirm @Valid usage

   While @Valid triggers validation, without @RequestBody the data won't be bound from the request body, so validation won't run.

3. Final Answer:

   Missing @RequestBody annotation on the method parameter -> Option B

4. Quick Check:

   Use @RequestBody to bind JSON, then @Valid to validate [OK]

Hint: Always use @RequestBody with @Valid for JSON validation [OK]

Common Mistakes:

• Forgetting @RequestBody causes no data binding
• Assuming @Valid alone binds data
• Thinking return type affects validation

5. You want to validate a user registration request with fields username (not blank), age (minimum 18), and email (valid email format). Which combination of annotations in the DTO class is correct?

hard

A. public class UserDTO {
@NotBlank
private String username;
@Min(18)
private int age;
@Email
private String email;
}

B. public class UserDTO {
@NotNull
private String username;
@Max(18)
private int age;
@Pattern(regexp = ".*@.*")
private String email;
}

C. public class UserDTO {
@NotBlank
private String username;
@Min(18)
private String age;
@Email
private String email;
}

D. public class UserDTO {
@NotEmpty
private String username;
@Min(18)
private Integer age;
@Email
private String email;
}

Solution

1. Step 1: Validate username with @NotBlank

   Username must not be null or empty, so @NotBlank is correct.

2. Step 2: Validate age with @Min(18) and correct type

   Age must be at least 18 and an integer type; int with @Min(18) fits.

3. Step 3: Validate email format with @Email

   @Email annotation checks for valid email format.

4. Final Answer:

   public class UserDTO { @NotBlank private String username; @Min(18) private int age; @Email private String email; } -> Option A

5. Quick Check:

   Use @NotBlank, @Min(18), and @Email for these fields [OK]

Hint: Match annotation to field type and validation need [OK]

Common Mistakes:

• Using @Max instead of @Min for age
• Using String type for age with @Min
• Using @NotNull instead of @NotBlank for username