DispatcherServlet in springboot / spring framework with annotations

✅ What is DispatcherServlet?

DispatcherServlet is the Front Controller in the Spring MVC architecture. It is a subclass of HttpServlet and serves as the entry point for all HTTP requests in a Spring web application.

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📌 Responsibilities of DispatcherServlet

It intercepts all incoming HTTP requests and delegates them to the appropriate components for further processing, using annotations such as:

• @Controller

• @RequestMapping

• @GetMapping, @PostMapping, etc.

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⚙️ Internal Delegation Flow

DispatcherServlet coordinates the entire web request handling by delegating to various components:

1. HandlerMapping

• Purpose: Determines which @Controller and method should handle the request.

• Examples:

  • RequestMappingHandlerMapping

  • BeanNameUrlHandlerMapping

2. HandlerAdapter

• Purpose: Invokes the matched controller method with appropriate arguments.

• Examples:

  • RequestMappingHandlerAdapter

3. ViewResolver

• Purpose: Resolves the logical view name (like "home") to a physical view (like /WEB-INF/views/home.jsp).

• Examples:

  • InternalResourceViewResolver

  • ThymeleafViewResolver

4. HandlerExceptionResolver

• Purpose: Handles any exceptions thrown during controller execution and maps them to error views or responses.

• Examples:

  • DefaultHandlerExceptionResolver

  • ResponseStatusExceptionResolver

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🚦 DispatcherServlet Request Flow with Annotations

text
Browser Request
     ↓
[ DispatcherServlet ] ← Auto-configured servlet
     ↓
HandlerMapping → Matches @RequestMapping, @GetMapping, etc.
     ↓
@Controller / @RestController method
     ↓
ResponseBody / ViewResolver → JSON, HTML, JSP, Thymeleaf
     ↓
HTTP Response

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✅ DispatcherServlet with Spring Annotations

1. @Controller or @RestController

Used to define a controller class that handles web requests.

java
@RestController
@RequestMapping("/api")
public class HelloController {

    @GetMapping("/hello")
    public String sayHello() {
        return "Hello from Spring!";
    }
}

• @RestController = @Controller + @ResponseBody

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2. @RequestMapping, @GetMapping, @PostMapping, etc.

Used to map HTTP requests to handler methods.

java
@GetMapping("/hello")   // Shortcut for @RequestMapping(method = GET)
public String sayHello() { ... }

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3. @ResponseBody

Binds the return value directly to the HTTP response body.

java
@Controller
public class MyController {

    @ResponseBody
    @GetMapping("/data")
    public String jsonResponse() {
        return "{\"message\": \"OK\"}";
    }
}

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4. @RequestParam, @PathVariable, @RequestBody

These help extract data from the request.

java
@GetMapping("/user/{id}")
public String getUser(@PathVariable int id) { ... }

@PostMapping("/user")
public String saveUser(@RequestBody User user) { ... }

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5. @ExceptionHandler

Used to handle exceptions in the controller layer.

java
@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(Exception.class)
    public ResponseEntity<String> handleEx(Exception ex) {
        return ResponseEntity.status(500).body("Error: " + ex.getMessage());
    }
}

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✅ How DispatcherServlet is Registered (Spring Boot)

In Spring Boot, DispatcherServlet functions exactly the same as in traditional Spring MVC, but Spring Boot auto-configures and registers it for you, simplifying setup.

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🔧 Spring Boot Auto-Configuration

When you create a Spring Boot web application (with spring-boot-starter-web), Spring Boot:

• Automatically registers DispatcherServlet as a bean.

• Automatically maps it to the root path (/) using the DispatcherServletRegistrationBean.

• Sets up default HandlerMapping, HandlerAdapter, ViewResolver, and ExceptionResolvers.

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📁 Default DispatcherServlet URL Mapping

properties
# application.properties
spring.mvc.servlet.path=/     # Default mapping for DispatcherServlet

You can change it:

properties
spring.mvc.servlet.path=/api/*

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🧵 Customizing DispatcherServlet in Spring Boot

If needed, you can override the default DispatcherServlet bean:

java
@Bean
public DispatcherServlet dispatcherServlet() {
    DispatcherServlet servlet = new DispatcherServlet();
    servlet.setThrowExceptionIfNoHandlerFound(true); // Customize behavior
    return servlet;
}

To register it manually (optional):

java
@Bean
public ServletRegistrationBean<DispatcherServlet> registration(DispatcherServlet dispatcherServlet) {
    ServletRegistrationBean<DispatcherServlet> registration = new ServletRegistrationBean<>(dispatcherServlet);
    registration.setLoadOnStartup(1);
    registration.addUrlMappings("/custom/*");
    return registration;
}

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🔧 Properties (optional in application.properties)

properties
spring.mvc.servlet.path=/api   # DispatcherServlet mapped to /api/*
spring.mvc.view.prefix=/WEB-INF/views/
spring.mvc.view.suffix=.jsp

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✅ Summary of Key Annotations Involved with DispatcherServlet

Annotation	Purpose
@Controller	Marks a class as a web controller
@RestController	Same as @Controller but returns response body directly
@RequestMapping	Maps request URL to method
@GetMapping, @PostMapping	Specialized shortcuts for specific HTTP methods
@ResponseBody	Returns data directly to HTTP response
@RequestParam	Gets query parameters
@PathVariable	Extracts values from the URL
@RequestBody	Binds JSON body to method param
@ExceptionHandler	Handles exceptions at controller level

🔁 DispatcherServlet with Annotations - Simple Spring Boot Example

java
@SpringBootApplication
public class App {
    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }
}

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

    @GetMapping("/greet")
    public String greet(@RequestParam String name) {
        return "Hello, " + name;
    }
}