Java Notes

Validation in Spring Boot

Complete guide to input validation in Spring Boot - Jakarta Bean Validation, custom validators, group validation, and validation error handling.

Setup

xml example

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

Validation Annotations Reference

Annotation	Applies To	Description
`@NotNull`	Any	Must not be null
`@NotBlank`	String	Not null, not empty, not whitespace
`@NotEmpty`	String/Collection	Not null and not empty
`@Size(min, max)`	String/Collection	Length/size bounds
`@Min(value)`	Number	Minimum value
`@Max(value)`	Number	Maximum value
`@Positive`	Number	> 0
`@PositiveOrZero`	Number	>= 0
`@Negative`	Number	< 0
`@Email`	String	Valid email format
`@Pattern(regexp)`	String	Matches regex
`@Past`	Date/Time	Must be in past
`@Future`	Date/Time	Must be in future
`@Digits(integer, fraction)`	Number	Digit constraints
`@DecimalMin`	Number	Minimum decimal value
`@DecimalMax`	Number	Maximum decimal value

Applying Validation in Controller

java example

@RestController
@RequestMapping("/api/users")
public class UserController {

    @PostMapping
    public ResponseEntity<?> createUser(@RequestBody @Valid UserRegistrationRequest request) {
        // If validation fails, MethodArgumentNotValidException is thrown
        // before this code executes
        User user = userService.register(request);
        return ResponseEntity.status(HttpStatus.CREATED).body(user);
    }

    // Validate path variable
    @GetMapping("/{id}")
    public ResponseEntity<?> getUser(@PathVariable @Min(1) Long id) {
        return ResponseEntity.ok(userService.getById(id));
    }

    // Validate request params
    @GetMapping
    public ResponseEntity<?> searchUsers(
            @RequestParam @NotBlank String query,
            @RequestParam @Min(0) int page,
            @RequestParam @Min(1) @Max(100) int size) {
        return ResponseEntity.ok(userService.search(query, page, size));
    }
}

Important: Add @Validated on the class for path variable / request param validation:

java example

@RestController
@Validated  // Required for method-level validation
@RequestMapping("/api/users")
public class UserController { }

Custom Validator

Step 1: Create Annotation

java example

@Documented
@Constraint(validatedBy = UniqueEmailValidator.class)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface UniqueEmail {
    String message() default "Email already registered";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

Step 2: Create Validator Class

java example

@Component
public class UniqueEmailValidator implements ConstraintValidator<UniqueEmail, String> {

    @Autowired
    private UserRepository userRepository;

    @Override
    public boolean isValid(String email, ConstraintValidatorContext context) {
        if (email == null) return true; // Let @NotNull handle null check
        return !userRepository.existsByEmail(email);
    }
}

Step 3: Use It

java example

@Data
public class UserRequest {
    @NotBlank
    @Email
    @UniqueEmail  // Custom validation!
    private String email;
}

Cross-Field Validation

java example

// Class-level validator
@Documented
@Constraint(validatedBy = PasswordMatchValidator.class)
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface PasswordMatch {
    String message() default "Passwords do not match";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

public class PasswordMatchValidator implements ConstraintValidator<PasswordMatch, Object> {

    @Override
    public boolean isValid(Object obj, ConstraintValidatorContext context) {
        if (obj instanceof ChangePasswordRequest req) {
            return req.getNewPassword().equals(req.getConfirmPassword());
        }
        return true;
    }
}

@Data
@PasswordMatch
public class ChangePasswordRequest {
    @NotBlank
    private String currentPassword;
    @NotBlank @Size(min = 8)
    private String newPassword;
    @NotBlank
    private String confirmPassword;
}

Nested Object Validation

Example

@Data public class OrderRequest { @NotBlank private String customerName; @Valid // Triggers validation on nested object! @NotNull private AddressDTO shippingAddress; @Valid @NotEmpty private List<OrderItemDTO> items; } @Data public class AddressDTO { @NotBlank private String street; @NotBlank private String city; @NotBlank @Pattern(regexp = "^[0-9]{6}$") private String pincode; } @Data public class OrderItemDTO { @NotNull @Positive private Long productId; @NotNull @Min(1) private Integer quantity; }

Handling Validation Errors

java example

@RestControllerAdvice
public class ValidationExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<Map<String, Object>> handleValidationErrors(
            MethodArgumentNotValidException ex) {

        Map<String, String> fieldErrors = new LinkedHashMap<>();
        ex.getBindingResult().getFieldErrors().forEach(error ->
                fieldErrors.put(error.getField(), error.getDefaultMessage()));

        Map<String, Object> response = new LinkedHashMap<>();
        response.put("success", false);
        response.put("message", "Validation failed");
        response.put("errors", fieldErrors);
        response.put("timestamp", LocalDateTime.now());

        return ResponseEntity.badRequest().body(response);
    }

    // For @Validated on path/query params
    @ExceptionHandler(ConstraintViolationException.class)
    public ResponseEntity<Map<String, Object>> handleConstraintViolation(
            ConstraintViolationException ex) {

        Map<String, String> errors = new HashMap<>();
        ex.getConstraintViolations().forEach(v ->
                errors.put(v.getPropertyPath().toString(), v.getMessage()));

        Map<String, Object> response = Map.of(
                "success", false,
                "message", "Constraint violation",
                "errors", errors
        );

        return ResponseEntity.badRequest().body(response);
    }
}

Interview Key Points

@Valid on @RequestBody triggers validation before controller method runs
@Validated on class enables method-level validation (params, path vars)
Custom validators implement ConstraintValidator<Annotation, Type>
Use @Valid on nested objects to trigger their validation
Validation errors throw MethodArgumentNotValidException
@NotNull vs @NotBlank vs @NotEmpty: Null → Empty → Whitespace
Custom class-level annotations enable cross-field validation
Always return field-specific error messages for good UX

Summary

In this chapter, we learned about Validation in Spring Boot in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Validation in Spring Boot.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, boot

Continue learning this concept

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Java Notes

Validation in Spring Boot

Complete guide to input validation in Spring Boot - Jakarta Bean Validation, custom validators, group validation, and validation error handling.

Setup

xml example

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

Validation Annotations Reference

Annotation	Applies To	Description
`@NotNull`	Any	Must not be null
`@NotBlank`	String	Not null, not empty, not whitespace
`@NotEmpty`	String/Collection	Not null and not empty
`@Size(min, max)`	String/Collection	Length/size bounds
`@Min(value)`	Number	Minimum value
`@Max(value)`	Number	Maximum value
`@Positive`	Number	> 0
`@PositiveOrZero`	Number	>= 0
`@Negative`	Number	< 0
`@Email`	String	Valid email format
`@Pattern(regexp)`	String	Matches regex
`@Past`	Date/Time	Must be in past
`@Future`	Date/Time	Must be in future
`@Digits(integer, fraction)`	Number	Digit constraints
`@DecimalMin`	Number	Minimum decimal value
`@DecimalMax`	Number	Maximum decimal value

Applying Validation in Controller

java example

@RestController
@RequestMapping("/api/users")
public class UserController {

    @PostMapping
    public ResponseEntity<?> createUser(@RequestBody @Valid UserRegistrationRequest request) {
        // If validation fails, MethodArgumentNotValidException is thrown
        // before this code executes
        User user = userService.register(request);
        return ResponseEntity.status(HttpStatus.CREATED).body(user);
    }

    // Validate path variable
    @GetMapping("/{id}")
    public ResponseEntity<?> getUser(@PathVariable @Min(1) Long id) {
        return ResponseEntity.ok(userService.getById(id));
    }

    // Validate request params
    @GetMapping
    public ResponseEntity<?> searchUsers(
            @RequestParam @NotBlank String query,
            @RequestParam @Min(0) int page,
            @RequestParam @Min(1) @Max(100) int size) {
        return ResponseEntity.ok(userService.search(query, page, size));
    }
}

Important: Add @Validated on the class for path variable / request param validation:

java example

@RestController
@Validated  // Required for method-level validation
@RequestMapping("/api/users")
public class UserController { }

Custom Validator

Step 1: Create Annotation

java example

@Documented
@Constraint(validatedBy = UniqueEmailValidator.class)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface UniqueEmail {
    String message() default "Email already registered";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

Step 2: Create Validator Class

java example

@Component
public class UniqueEmailValidator implements ConstraintValidator<UniqueEmail, String> {

    @Autowired
    private UserRepository userRepository;

    @Override
    public boolean isValid(String email, ConstraintValidatorContext context) {
        if (email == null) return true; // Let @NotNull handle null check
        return !userRepository.existsByEmail(email);
    }
}

Step 3: Use It

java example

@Data
public class UserRequest {
    @NotBlank
    @Email
    @UniqueEmail  // Custom validation!
    private String email;
}

Cross-Field Validation

java example

// Class-level validator
@Documented
@Constraint(validatedBy = PasswordMatchValidator.class)
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface PasswordMatch {
    String message() default "Passwords do not match";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

public class PasswordMatchValidator implements ConstraintValidator<PasswordMatch, Object> {

    @Override
    public boolean isValid(Object obj, ConstraintValidatorContext context) {
        if (obj instanceof ChangePasswordRequest req) {
            return req.getNewPassword().equals(req.getConfirmPassword());
        }
        return true;
    }
}

@Data
@PasswordMatch
public class ChangePasswordRequest {
    @NotBlank
    private String currentPassword;
    @NotBlank @Size(min = 8)
    private String newPassword;
    @NotBlank
    private String confirmPassword;
}

Nested Object Validation

Example

Handling Validation Errors

java example

@RestControllerAdvice
public class ValidationExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<Map<String, Object>> handleValidationErrors(
            MethodArgumentNotValidException ex) {

        Map<String, String> fieldErrors = new LinkedHashMap<>();
        ex.getBindingResult().getFieldErrors().forEach(error ->
                fieldErrors.put(error.getField(), error.getDefaultMessage()));

        Map<String, Object> response = new LinkedHashMap<>();
        response.put("success", false);
        response.put("message", "Validation failed");
        response.put("errors", fieldErrors);
        response.put("timestamp", LocalDateTime.now());

        return ResponseEntity.badRequest().body(response);
    }

    // For @Validated on path/query params
    @ExceptionHandler(ConstraintViolationException.class)
    public ResponseEntity<Map<String, Object>> handleConstraintViolation(
            ConstraintViolationException ex) {

        Map<String, String> errors = new HashMap<>();
        ex.getConstraintViolations().forEach(v ->
                errors.put(v.getPropertyPath().toString(), v.getMessage()));

        Map<String, Object> response = Map.of(
                "success", false,
                "message", "Constraint violation",
                "errors", errors
        );

        return ResponseEntity.badRequest().body(response);
    }
}

Interview Key Points

@Valid on @RequestBody triggers validation before controller method runs
@Validated on class enables method-level validation (params, path vars)
Custom validators implement ConstraintValidator<Annotation, Type>
Use @Valid on nested objects to trigger their validation
Validation errors throw MethodArgumentNotValidException
@NotNull vs @NotBlank vs @NotEmpty: Null → Empty → Whitespace
Custom class-level annotations enable cross-field validation
Always return field-specific error messages for good UX

Summary

In this chapter, we learned about Validation in Spring Boot in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Validation in Spring Boot.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, spring, framework, boot

Continue learning this concept

PreviousSpring Data JPA NextSpring Core Container