Week 10: Validation, DTOs & Error Handling

This week focuses on improving the quality, security, and cleanliness of our Spring Boot APIs by introducing:

• Bean Validation (using annotations like @NotNull, @Size, etc.)
• DTOs (Data Transfer Objects) to separate API input/output from database entities
• Global exception handling using @ControllerAdvice
• ResponseEntity and proper use of HTTP status codes

The goal is to write professional, maintainable, and user-friendly APIs.

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📚 Learning Objectives

By the end of this week, students will be able to:

• Apply validation rules to incoming request data
• Explain the purpose of DTOs and convert between Entities ⇄ DTOs
• Implement custom exception handling using @ControllerAdvice
• Return appropriate HTTP response codes using ResponseEntity
• Provide clear and consistent error responses for the frontend

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🧠 Key Concepts

1. Bean Validation

Spring Boot integrates with Jakarta Bean Validation to validate user input.

Examples of common annotations:

Annotation	Purpose
@NotNull	Field must not be null
@NotBlank	String must not be empty or blank
@Size(min, max)	String/Collection must have defined length
@Min / @Max	Numeric constraints
@Email	Must be a valid email format

Validation is usually applied in DTOs and enforced using @Valid in Controller methods.

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2. DTOs (Data Transfer Objects)

DTOs are simple data classes used to handle input/output in API requests.

Why use DTOs?

• Protects database entities from being exposed directly
• Prevents users from modifying fields like id or timestamps
• Allows different shapes of data for create, update, and response

Common pattern:

Entity (database model)  ←→  DTO (API structure)

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3. ResponseEntity & HTTP Status Codes

ResponseEntity lets us control the HTTP response:

return ResponseEntity.status(HttpStatus.CREATED).body(data);

Examples of commonly used status codes:

Code	Meaning	Usage Example
200	OK	Successful request
201	Created	New resource created
400	Bad Request	Validation failed
404	Not Found	Resource doesn’t exist
500	Internal Server Error	Unexpected server failure

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4. Global Exception Handling with @ControllerAdvice

Instead of returning messy error messages, we create a centralized error handler.

Benefits:

• Clean controller logic
• Consistent error response format
• Easier debugging and front-end integration

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🧰 Tools & Libraries

Tool / Library	Purpose
jakarta.validation	Validation annotations and processing
ModelMapper / MapStruct (optional)	Automate conversion Entity ↔ DTO
Postman	Test API errors and success cases

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🏗️ Example Project Structure

spring-boot-advanced-api/
├── controller/           → REST controllers
├── service/              → Business logic
├── repository/           → Database layer
├── model/                → JPA entities
├── dto/                  → Data Transfer Objects
│   ├── request/          → Input DTOs
│   └── response/         → Output DTOs
├── mapper/               → MapStruct mappers
├── exception/            → Custom exceptions + Global handler
└── resources/
    └── application.properties

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✅ Expected Outcome by End of Week

Students will be able to:

• Ensure API input is safe, valid, and user-friendly
• Return consistent JSON responses for errors
• Use DTOs to decouple database design from API design
• Write cleaner, more maintainable Spring Boot applications

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