@Valid annotation on request body in Spring Boot

Introduction

The @Valid annotation checks if the data sent to your app is correct before using it. It helps catch mistakes early.

When you want to make sure user input in a form is complete and correct.

When receiving data from an API client and you need to check it follows rules.

When saving data to a database and want to avoid bad or missing values.

When you want to give clear error messages if input is wrong.

When you want to keep your app safe from invalid or harmful data.

Syntax

Spring Boot

@PostMapping("/example")
public ResponseEntity<?> exampleMethod(@Valid @RequestBody ExampleDto dto) {
    // method body
}

The @Valid annotation is placed before the @RequestBody parameter.

The class of the request body (like ExampleDto) must have validation annotations on its fields.

Examples

Validates the UserDto object sent in the request body before processing.

Spring Boot

@PostMapping("/user")
public ResponseEntity<?> createUser(@Valid @RequestBody UserDto user) {
    // save user
}

Example of a data class with validation rules: name cannot be blank, quantity must be positive.

Spring Boot

public record ProductDto(@NotBlank String name, @Positive int quantity) {}

Validates the product data sent in the request body using the rules in ProductDto.

Spring Boot

@PostMapping("/product")
public ResponseEntity<?> addProduct(@Valid @RequestBody ProductDto product) {
    // add product
}

Sample Program

This Spring Boot controller has a POST endpoint /api/users. It expects a JSON body matching UserDto with rules: name not blank, email valid, age at least 18. If validation passes, it returns a success message.

Spring Boot

import jakarta.validation.Valid;
import jakarta.validation.constraints.*;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;

@RestController
@RequestMapping("/api")
public class SampleController {

    public record UserDto(
        @NotBlank(message = "Name is required") String name,
        @Email(message = "Email must be valid") String email,
        @Min(value = 18, message = "Age must be at least 18") int age
    ) {}

    @PostMapping("/users")
    public ResponseEntity<String> createUser(@Valid @RequestBody UserDto user) {
        return ResponseEntity.ok("User " + user.name() + " created successfully.");
    }
}

Important Notes

Validation errors automatically return a 400 Bad Request with error details if you have Spring Boot's default error handling.

Use annotations like @NotBlank, @Email, @Min on fields inside your DTO class to define rules.

Make sure to include spring-boot-starter-validation dependency in your project to enable validation.

Summary

@Valid checks request body data before your method uses it.

Put @Valid before @RequestBody parameter in controller methods.

Define rules inside the data class with validation annotations.

Practice

1. What is the main purpose of using the @Valid annotation on a @RequestBody parameter in a Spring Boot controller?

easy

A. To automatically check if the incoming request data meets the validation rules defined in the data class.

B. To convert the request body into a JSON string.

C. To log the request body content for debugging.

D. To cache the request body for faster access.

Solution

1. Step 1: Understand the role of @Valid

   The @Valid annotation triggers validation of the request body object based on constraints defined in its class.

2. Step 2: Connect validation to request handling

   When placed before @RequestBody, it ensures the data is checked before the controller method uses it.

3. Final Answer:

   To automatically check if the incoming request data meets the validation rules defined in the data class. -> Option A

4. Quick Check:

   @Valid validates input data [OK]

Hint: Remember: @Valid checks data correctness before method runs [OK]

Common Mistakes:

• Thinking @Valid converts or logs data
• Placing @Valid after @RequestBody
• Assuming @Valid caches data

2. Which of the following is the correct way to use @Valid in a Spring Boot controller method parameter?

easy

A. public ResponseEntity create(@RequestBody User user @Valid) { ... }

B. public ResponseEntity create(@Valid User user @RequestBody) { ... }

C. public ResponseEntity create(@Valid @RequestBody User user) { ... }

D. public ResponseEntity create(User user @Valid @RequestBody) { ... }

Solution

1. Step 1: Recall annotation order for parameters

   In Spring Boot, @Valid must come before @RequestBody on the same parameter.

2. Step 2: Check each option's syntax

   Only public ResponseEntity create(@Valid @RequestBody User user) { ... } correctly places @Valid before @RequestBody on the parameter.

3. Final Answer:

   public ResponseEntity create(@Valid @RequestBody User user) { ... } -> Option C

4. Quick Check:

   Correct annotation order = public ResponseEntity create(@Valid @RequestBody User user) { ... } [OK]

Hint: Put @Valid before @RequestBody on the same parameter [OK]

Common Mistakes:

• Placing @Valid after @RequestBody
• Separating annotations incorrectly
• Using @Valid without @RequestBody

3. Given the following controller method and data class, what happens when a POST request sends an empty JSON object {}?

public record User(@NotBlank String name, @Min(18) int age) {}

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

medium

A. The request succeeds but 'age' defaults to 0.

B. The request succeeds and returns 'User added'.

C. The request fails because @Valid is ignored on records.

D. The request fails validation because 'name' is blank and 'age' is missing.

Solution

1. Step 1: Analyze validation constraints on User record

   @NotBlank requires 'name' to be non-empty; @Min(18) requires 'age' to be at least 18.

2. Step 2: Consider empty JSON input effects

   Empty JSON means 'name' and 'age' are missing, so validation fails for both fields.

3. Final Answer:

   The request fails validation because 'name' is blank and 'age' is missing. -> Option D

4. Quick Check:

   Missing required fields cause validation failure [OK]

Hint: Empty JSON fails @NotBlank and @Min validations [OK]

Common Mistakes:

• Assuming missing fields pass validation
• Thinking @Valid ignores records
• Assuming default values apply automatically

4. You have this controller method:

@PostMapping("/register")
public ResponseEntity<String> registerUser(@RequestBody @Valid User user) {
    return ResponseEntity.ok("Registered");
}

But when you send invalid data, no validation errors occur and the method runs normally. What is the most likely cause?

medium

A. Missing @Validated annotation on the controller class or configuration.

B. The User class does not have any validation annotations.

C. The @Valid annotation is placed after @RequestBody.

D. Spring Boot does not support validation on request bodies.

Solution

1. Step 1: Check if User class has validation rules

   If the User class lacks validation annotations like @NotNull, @NotBlank, etc., @Valid has nothing to check.

2. Step 2: Understand validation behavior

   Without constraints, validation passes silently, so the method runs normally even with invalid data.

3. Final Answer:

   The User class does not have any validation annotations. -> Option B

4. Quick Check:

   No constraints = no validation errors [OK]

Hint: Validation needs annotations in the data class [OK]

Common Mistakes:

• Assuming @Validated is always required
• Thinking annotation order causes no validation
• Believing Spring Boot disables validation by default

5. You want to create a Spring Boot API that accepts a JSON request body for a Product with fields name (required, not blank) and price (required, positive number). Which of the following code snippets correctly applies @Valid and validation annotations to ensure invalid data is rejected automatically?

hard

A. public record Product(@NotBlank String name, @Positive double price) {}

@PostMapping("/products")
public ResponseEntity<String> addProduct(@Valid @RequestBody Product product) {
return ResponseEntity.ok("Product added");
}

B. public class Product {
public String name;
public double price;
}

@PostMapping("/products")
public ResponseEntity<String> addProduct(@RequestBody Product product) {
return ResponseEntity.ok("Product added");
}

C. public record Product(String name, double price) {}

@PostMapping("/products")
public ResponseEntity<String> addProduct(@RequestBody @Valid Product product) {
return ResponseEntity.ok("Product added");
}

D. public record Product(@NotNull String name, @Min(1) double price) {}

@PostMapping("/products")
public ResponseEntity<String> addProduct(@RequestBody Product product) {
return ResponseEntity.ok("Product added");
}

Solution

1. Step 1: Identify correct validation annotations

   @NotBlank ensures 'name' is not empty; @Positive ensures 'price' is greater than zero.

2. Step 2: Check usage of @Valid and @RequestBody

   public record Product(@NotBlank String name, @Positive double price) {} @PostMapping("/products") public ResponseEntity<String> addProduct(@Valid @RequestBody Product product) { return ResponseEntity.ok("Product added"); } correctly places @Valid before @RequestBody in the controller method parameter.

3. Step 3: Verify other options

   public class Product { public String name; public double price; } @PostMapping("/products") public ResponseEntity<String> addProduct(@RequestBody Product product) { return ResponseEntity.ok("Product added"); } lacks validation annotations and @Valid. public record Product(String name, double price) {} @PostMapping("/products") public ResponseEntity<String> addProduct(@RequestBody @Valid Product product) { return ResponseEntity.ok("Product added"); } misses validation annotations. public record Product(@NotNull String name, @Min(1) double price) {} @PostMapping("/products") public ResponseEntity<String> addProduct(@RequestBody Product product) { return ResponseEntity.ok("Product added"); } uses @Min(1) on a double (should use @Positive) and misses @Valid.

4. Final Answer:

   public record Product(@NotBlank String name, @Positive double price) {} @PostMapping("/products") public ResponseEntity<String> addProduct(@Valid @RequestBody Product product) { return ResponseEntity.ok("Product added"); } -> Option A

5. Quick Check:

   Use @Valid + proper annotations = public record Product(@NotBlank String name, @Positive double price) {} @PostMapping("/products") public ResponseEntity<String> addProduct(@Valid @RequestBody Product product) { return ResponseEntity.ok("Product added"); } [OK]

Hint: Use @Valid + @NotBlank and @Positive for required fields [OK]

Common Mistakes:

• Omitting @Valid in controller method
• Using wrong validation annotations for types
• Not annotating fields in data class