Every Spring MVC application revolves around a single, powerful component called the DispatcherServlet.
It acts as the front controller, receiving every HTTP request and deciding how it should be handled.

Understanding DispatcherServlet and the request flow is critical for debugging, performance tuning, and designing clean APIs.

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1. What Is DispatcherServlet?

DispatcherServlet is the central component of Spring MVC responsible for:

• Receiving incoming HTTP requests
• Delegating requests to the correct controller
• Handling request mapping
• Coordinating view resolution
• Returning the final response

In simple terms:

All HTTP requests go through DispatcherServlet first.

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2. Front Controller Pattern

Spring MVC follows the Front Controller Design Pattern.

Instead of:

• Multiple servlets handling different requests

Spring uses:

• One front controller (DispatcherServlet)
• Centralized request handling

This provides:

• Consistent request processing
• Centralized security
• Centralized logging
• Easier maintenance

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3. DispatcherServlet in Spring Boot

In traditional Spring MVC, DispatcherServlet had to be configured manually.

Spring Boot does this automatically:

• Registers DispatcherServlet as a bean
• Maps it to /
• Configures required components

This happens via:

spring-boot-starter-web

You never need to write XML configuration.

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4. Spring MVC Request Flow (Step-by-Step)

Let’s walk through the complete request lifecycle.

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Step 1: Client Sends HTTP Request

Example:

GET /api/users/1

The request reaches the embedded server (Tomcat).

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Step 2: DispatcherServlet Receives Request

DispatcherServlet intercepts the request and becomes responsible for handling it.

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Step 3: HandlerMapping Finds the Controller

Spring uses HandlerMapping to find which controller method matches the request.

Example:

@GetMapping("/api/users/{id}")
public User getUser(@PathVariable Long id) { }

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Step 4: HandlerAdapter Invokes the Method

HandlerAdapter:

• Calls the controller method
• Passes request parameters
• Handles argument binding

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Step 5: Business Logic Execution

Controller delegates work to:

• Service layer
• Repository layer

Data is fetched, processed, or modified.

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Step 6: Response Generation

Controller returns:

• Java object
• String
• ResponseEntity

Spring converts the return value using HttpMessageConverters (JSON, XML, etc.).

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Step 7: Response Sent to Client

DispatcherServlet sends the response back to the client.

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5. Visual Request Flow Summary

Client
  ↓
DispatcherServlet
  ↓
HandlerMapping
  ↓
HandlerAdapter
  ↓
Controller
  ↓
Service
  ↓
Repository
  ↓
Response

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6. DispatcherServlet & REST APIs

In REST APIs:

• Views are NOT involved
• Responses are returned as JSON

This is enabled by:

• @RestController
• @ResponseBody
• Jackson (JSON converter)

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7. Common Issues Related to DispatcherServlet

❌ 404 Not Found

• No matching controller mapping

❌ 405 Method Not Allowed

• Wrong HTTP method used

❌ 500 Internal Server Error

• Exception thrown inside controller or service

Understanding request flow helps debug these quickly.

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8. Customizing DispatcherServlet (Advanced)

You can customize it if needed:

@Bean
public DispatcherServlet dispatcherServlet() {
    return new DispatcherServlet();
}

Most Spring Boot apps never need this.

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9. Summary

• DispatcherServlet is the heart of Spring MVC.
• It implements the Front Controller pattern.
• All HTTP requests pass through it.
• Spring Boot auto-configures DispatcherServlet.
• Understanding request flow helps in debugging and optimization.

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What's Next?

Next post:

16 - Spring IoC Container Deep Dive

We’ll explore how Spring internally manages beans, contexts, and dependency resolution.