3.1 Spring MVC Architecture & Request Flow

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Spring MVC implements the Front Controller pattern via DispatcherServlet. Every inbound HTTP request travels through a refined pipeline of handlers, interceptors, and adapters before returning a serialized response to the client.

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End-to-End Request Processing Flow

[ Client Browser / Mobile App ]
              │ HTTP Request
              ▼
    [ Filter Chain (Security, CORS, Logging) ]
              │
              ▼
   ┌─────────────────────────────────────────┐
   │         DispatcherServlet               │
   │         (The Front Controller)          │
   └───────┬───────────────────▲─────────────┘
           │ 1. Find Handler   │ 2. HandlerExecutionChain
           ▼                   │    (Handler + Interceptors)
   ┌───────────────────────────┴─────────────┐
   │             HandlerMapping              │
   │ (RequestMappingHandlerMapping)          │
   └─────────────────────────────────────────┘
           │
           │ 3. Execute Handler
           ▼
   ┌─────────────────────────────────────────┐
   │             HandlerAdapter              │
   │ (RequestMappingHandlerAdapter)          │
   └───────┬───────────────────▲─────────────┘
           │ Invokes method    │ Returns Object or ModelAndView
           ▼                   │
   ┌───────────────────────────┴─────────────┐
   │            @RestController              │
   │         (Your Business Code)            │
   └─────────────────────────────────────────┘
           │
           │ 4. If REST (@ResponseBody):
           ▼
   ┌─────────────────────────────────────────┐
   │          HttpMessageConverter           │
   │ (MappingJackson2HttpMessageConverter)   │
   └─────────────────────────────────────────┘
              │ Serialized JSON
              ▼
    [ HTTP Response 200 OK to Client ]

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Core Architectural Components

1. DispatcherServlet: Central front controller registered with the servlet container. Coordinates all request flow.
2. HandlerMapping: Examines request URL, HTTP method, and headers to locate the corresponding controller method.
3. HandlerExecutionChain: Wraps the matched controller method along with all applicable HandlerInterceptor instances.
4. HandlerAdapter: Abstracts controller method execution; resolves method arguments (e.g., extracting @PathVariable, deserializing @RequestBody).
5. HttpMessageConverter: Serializes Java return objects into JSON/XML (e.g., Jackson) or deserializes inbound payloads.

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Filters vs HandlerInterceptors

A classic senior engineering interview comparison:

Characteristic	Servlet Filter (Filter)	Spring Interceptor (HandlerInterceptor)
Specification	Standard Java EE / Jakarta Servlet API	Spring MVC proprietary framework
Execution Point	Outside DispatcherServlet (lowest level)	Inside DispatcherServlet, around controller execution
Awareness	Knows only ServletRequest & ServletResponse	Knows the target handler (HandlerMethod) and model
Common Use Cases	Low-level security, CORS, GZIP compression, raw payload caching	Auth validation, request audit logs, performance timing, tenant switching

What is Statelessness in REST?

REST APIs are generally designed to be stateless.

It means:

Each request should contain the information necessary for the server to process it; the server shouldn't rely on remembering the client's previous request as conversational session state.

For example:

Request 1:
GET /customer/101
Authorization: Bearer JWT

Request 2:
GET /customer/101/orders
Authorization: Bearer JWT

Each request carries its authentication information.

This is especially useful in microservices because requests can be handled by different instances.

Rest vs Soap

REST	SOAP
Architectural style	Protocol
Commonly uses JSON	Commonly uses XML
Lightweight	More formal/heavy
Easy to consume	More strict standards
Common in modern microservices	Common in many legacy/enterprise integrations
Uses HTTP commonly	Can work over multiple transports