Building a RESTful API with Spring Boot and H2 Database

Learn how to create a production-ready REST API using Spring Boot, JPA, and H2 Database with complete CRUD operations and proper error handling.

🎯 Project Overview

We'll build a Customer Management API that includes:

• CRUD operations for customer data
• Input validation
• Exception handling
• Swagger/OpenAPI documentation
• In-memory H2 database
• Sample data generation

🛠️ Project Setup

First, let's set up our project with the required dependencies in pom.xml:

pom.xml

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
        <groupId>com.h2database</groupId>
        <artifactId>h2</artifactId>
        <scope>runtime</scope>
    </dependency>
</dependencies>

📦 Domain Model

Create the Customer entity with validation:

Customer.java

@Entity
@Data
@NoArgsConstructor
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @NotBlank(message = "First name is required")
    private String firstName;

    @Email(message = "Email should be valid")
    private String email;

    @Min(value = 18, message = "Age should be at least 18")
    private Integer age;

    private Boolean active = true;
}

🔄 REST Endpoints

Method	Endpoint	Description
GET	/api/customers	Get all customers
GET	/api/customers/{id}	Get customer by ID
POST	/api/customers	Create new customer
PUT	/api/customers/{id}	Update customer
DELETE	/api/customers/{id}	Delete customer
PATCH	/api/customers/{id}/deactivate	Deactivate customer

🎯 Controller Implementation

CustomerController.java

@RestController
@RequestMapping("/api/customers")
@RequiredArgsConstructor
public class CustomerController {
    private final CustomerService customerService;

    @GetMapping
    public ResponseEntity> getAllCustomers(
            @RequestParam(required = false) Boolean activeOnly) {
        List customers = activeOnly != null && activeOnly 
            ? customerService.getActiveCustomers()
            : customerService.getAllCustomers();
        return ResponseEntity.ok(customers);
    }

    @PostMapping
    public ResponseEntity createCustomer(
            @Valid @RequestBody Customer customer) {
        return new ResponseEntity<>(
            customerService.createCustomer(customer), 
            HttpStatus.CREATED
        );
    }
}

⚡ Exception Handling

Global exception handler for consistent error responses:

GlobalExceptionHandler.java

@RestControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(CustomerNotFoundException.class)
    public ResponseEntity handleCustomerNotFoundException(
            CustomerNotFoundException ex) {
        ErrorResponse error = new ErrorResponse(
            HttpStatus.NOT_FOUND.value(),
            ex.getMessage(),
            LocalDateTime.now()
        );
        return new ResponseEntity<>(error, HttpStatus.NOT_FOUND);
    }
}

📝 Sample Data

The application automatically generates sample customer data on startup:

Sample Data

{
    "firstName": "John",
    "lastName": "Doe",
    "email": "john.doe@example.com",
    "phone": "+1234567890",
    "age": 30
}

🚀 Running the Application

1. Clone the repository
2. Run mvn clean install
3. Start the application
4. Access Swagger UI at http://localhost:8080/swagger-ui.html
5. Access H2 Console at http://localhost:8080/h2-console

Testing the API:

You can use tools like Postman or curl to test the endpoints. For example:

curl http://localhost:8080/api/customers