Spring MVC Auto-Configuration at Startup — What Spring Boot Does

Direct Answer

When a servlet web application has Spring MVC on the classpath and no conflicting user configuration, Spring Boot auto-configures the DispatcherServlet and MVC infrastructure. Spring itself detects @Controller beans and builds request mappings. Message converters and view resolvers are added conditionally from the libraries and beans present; Jackson or Thymeleaf is not guaranteed unless its support is on the classpath.

Code Example

@SpringBootApplication
public class OrderServiceApplication {
    public static void main(String[] args) {
        SpringApplication.run(OrderServiceApplication.class, args);
        // On startup:
        // 1. WebMvcAutoConfiguration registers DispatcherServlet at /
        // 2. RequestMappingHandlerMapping scans OrderController
        // 3. Jackson HttpMessageConverter registered for JSON
        // 4. Embedded Tomcat starts on port 8080
    }
}

@RestController
@RequestMapping("/api/orders")
class OrderController {
    
    @GetMapping("/{id}")
    public Order getOrder(@PathVariable Long id) {
        // Auto-configured Jackson converts Order record to JSON
        return new Order(id, "Laptop", 1299.99);
    }
}

record Order(Long id, String product, Double price) {}

// No web.xml, no DispatcherServlet config, no ObjectMapper bean
// Spring Boot auto-configuration did it all

Common Mistakes & Pitfalls

• Not understanding why it works: Freshers add spring-boot-starter-web and write a controller — it works, but they don't know DispatcherServlet, HandlerMapping, and Jackson were auto-configured. This bites them when troubleshooting.
• Adding @EnableWebMvc accidentally: This annotation disables Spring Boot's auto-configuration, forcing you to manually configure everything. Freshers copy it from old Spring tutorials and wonder why their app breaks.
• Assuming manual beans override auto-config: If you define your own ObjectMapper bean, Spring Boot's auto-configured one backs off. Freshers don't realize this and create duplicate beans without understanding the conditional logic.
• Not checking auto-configuration report: Running with --debug flag shows which auto-configurations applied — freshers never look at this when debugging configuration issues.