Internal flow of spring boot/spring/spring mvc

The internal flow of Spring Boot refers to what happens behind the scenes when you run a Spring Boot application — from startup to handling requests. 

---

🔁 1. Application Startup (main() method)

@SpringBootApplication

public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);
    }
}

When you run a Spring Boot application using the entry point(SpringApplication.run(...)) then bootstraps the entire Spring Boot application

• And creates a SpringApplication object.

• And sets up:

  • Environment (like application.properties)

  • ApplicationContext (Spring Container)

  • Auto-configuration

  • Logging system

  • Banner display

  • Listeners and initializers

---

🔧 2. Component Scan & Bean Creation

Triggered by:

java
@SpringBootApplication  // = @Configuration + @ComponentScan + @EnableAutoConfiguration

Internals:

• And ComponentScan internally scans all classes under the base package and sub-packages.

• Registers beans for:

  • @Component, @Service, @Repository, @Controller, @RestController

• Spring’s BeanFactory or ApplicationContext creates and manages the lifecycle of these beans.

• Dependency injection using @Autowired, @Value, @Qualifier, etc.

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⚙️ 3. Auto Configuration

Provided by:

java
@EnableAutoConfiguration

Internals:

• Uses spring.factories to load classes annotated with @Configuration.

• Auto-configures beans (like DataSource, Kafka, MVC, etc.) based on classpath and properties.

• Uses conditional annotations like:

  • @ConditionalOnClass

  • @ConditionalOnMissingBean

  • @ConditionalOnProperty

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🛠 4. Initialization of Embedded Server

Example:

• Tomcat (default), Jetty, or Undertow

Internals:

• Spring Boot auto-configures and starts the embedded server.

• DispatcherServlet is initialized and mapped to handle all requests (/).

• Servlet context is set up.

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🌐 5. Request Handling (Web Layer)

Steps:

1. HTTP request hits the embedded server (Tomcat).

2. Routed to the DispatcherServlet.

3. DispatcherServlet uses HandlerMapping to find the correct controller method.

4. Executes:

   • HandlerInterceptors

   • Controller method (e.g., @GetMapping, @PostMapping)

5. Returns the result (ResponseEntity, JSON, View).

6. DispatcherServlet sends response back to the client.

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📦 6. Persistence Layer (If Configured)

• Auto-configures:

  • DataSource (JDBC or JPA)

  • Hibernate EntityManager

  • Repositories via @Repository and @EnableJpaRepositories

• Transaction management via @Transactional

---

📊 7. Actuator & Monitoring (Optional)

• Spring Boot Actuator auto-configures endpoints for:

  • /actuator/health

  • /actuator/info

  • /actuator/metrics

• Useful for health checks and integration with Prometheus/Grafana

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🛑 8. Shutdown Flow

• Triggers graceful shutdown:

  • Calls SmartLifecycle.stop()

  • Closes ApplicationContext

  • Releases resources (e.g., DB connections, Kafka consumers)

• Beans with @PreDestroy or implementing DisposableBean are cleaned up.

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⚙️ Internal Components Summary

Component	Role
SpringApplication	Bootstraps and configures the application
ApplicationContext	Manages beans and dependency injection
DispatcherServlet	Front controller for web requests
BeanPostProcessor	Allows custom modification of beans
@Configuration	Declares Spring configurations
@EnableAutoConfiguration	Enables Spring Boot’s intelligent auto setup
CommandLineRunner / ApplicationRunner	Executes code after startup

🔍 Spring Boot Internal Flow

✅ Visual Diagram + Call Stack Trace

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📌 A. Visual Architecture Diagram of SpringApplication.run(...)

+------------------------------------------------------+
|               Your Application Class                 |
|    @SpringBootApplication                            |
|    public class MyApp {                              |
|        public static void main(String[] args) {      |
|            SpringApplication.run(MyApp.class, args); |
|        }                                             |
|    }                                                 |
+---------------------------|--------------------------+
                            |
                            v
+----------------------------[SpringApplication.run()]-----------------------------+
|                                                                                  |
| 1. Create SpringApplication instance                                             |
|    - Set application type (Servlet, Reactive, None)                              |
|    - Load initializers and listeners (from spring.factories)                     |
|                                                                                  |
| 2. Prepare Environment                                                            |
|    - Load `application.properties`, profiles, command-line args, env vars        |
|    - Configure ConfigurableEnvironment                                            |
|                                                                                  |
| 3. Create ApplicationContext (IoC container)                                     |
|    - e.g., AnnotationConfigServletWebServerApplicationContext                    |
|                                                                                  |
| 4. Refresh ApplicationContext (Spring Core)                                      |
|    - Call AbstractApplicationContext.refresh()                                   |
|      a. Prepare BeanFactory                                                       |
|      b. Load Bean Definitions (via classpath scanning)                            |
|      c. Register BeanPostProcessors                                               |
|      d. Instantiate Singleton Beans                                               |
|      e. Inject Dependencies (`@Autowired`, etc.)                                  |
|      f. Apply Lifecycle Callbacks (`@PostConstruct`, AOP proxies, etc.)          |
|                                                                                  |
| 5. Run ApplicationRunner / CommandLineRunner                                     |
|                                                                                  |
| 6. Publish ApplicationReadyEvent                                                 |
+----------------------------------------------------------------------------------+
                            |
                            v
             🚀 Application is Ready to Handle Requests

---

📌 B. Internal Call Stack Trace

(Important Classes and Methods Behind the Scenes)

---

java
// Entry point
SpringApplication.run(MyApp.class, args)

// → Step 1: Constructor
new SpringApplication(MyApp.class)
   └── setInitializers()
   └── setListeners()
   └── deduceApplicationType()

// → Step 2: Run method
SpringApplication.run()
   └── prepareEnvironment()
        └── configurePropertySources()
        └── bindEnvironmentProperties()
   └── printBanner()
   └── createApplicationContext()
        └── AnnotationConfigServletWebServerApplicationContext
   └── prepareContext()
        └── applyInitializers()
        └── listeners.contextPrepared()
   └── refreshContext() → context.refresh()

// → Step 3: Spring Core Container (IoC)
AbstractApplicationContext.refresh()
   └── prepareBeanFactory()
   └── invokeBeanFactoryPostProcessors()
   └── registerBeanPostProcessors()
   └── initMessageSource()
   └── initApplicationEventMulticaster()
   └── onRefresh() // For web apps: create embedded Tomcat, setup dispatcher
   └── registerListeners()
   └── finishBeanFactoryInitialization()
         └── create all singleton beans
         └── apply dependency injection
   └── finishRefresh()

// → Step 4: Call ApplicationRunners
   └── callRunners(context, args)
        └── invoke all ApplicationRunner & CommandLineRunner beans

// → Step 5: Events Published
   └── ApplicationStartedEvent
   └── ApplicationReadyEvent

📦 Summary Diagram:

SpringApplication.run()
     |
     └── Create SpringApplication instance
             |
             └── Prepare Environment
                     |
                     └── Create ApplicationContext (Spring Container)
                             |
                             └── refresh()
                                 ├── Load bean definitions
                                 ├── Instantiate beans
                                 ├── Inject dependencies
                                 └── Apply lifecycle methods
                                     |
                                     └── Application is ready

---

✅ Key Classes Involved:

Component	Responsibility
SpringApplication	Entry point for bootstrapping the app
ConfigurableApplicationContext	Spring’s container holding all beans
ApplicationContextInitializer	Modify context before refresh
BeanFactoryPostProcessor	Modify BeanFactory before beans instantiated
BeanPostProcessor	Modify beans after instantiation
AnnotationConfigServletWeb...Ctx	IoC + embedded web server (Tomcat)
@SpringBootApplication	Combo of @Configuration, @ComponentScan, @EnableAutoConfiguration
ApplicationRunner, CommandLineRunner	Code to run just before app is ready

📌 Optional: View Actual SpringApplication.run() Flow in Debug Mode

Set a breakpoint in your IDE:

java
SpringApplication.run(MyApp.class, args);

And then step into to trace each class mentioned above.

SpringApplication class in Spring Boot

✅ SpringApplication is a class in Spring Boot that is responsible for bootstrapping and launching a Spring application from the main() method. It sets up the Spring ApplicationContext, performs auto-configuration, and starts the embedded server (like Tomcat).

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📌 Simple Definition:

SpringApplication is the entry point used to start a Spring Boot application.

---

🧱 Syntax:

java
SpringApplication.run(ApplicationClass.class, args);

---

🔧 Example:

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

---

🔍 What It Does Internally:

1. Creates an ApplicationContext

2. Loads beans and applies auto-configuration

3. Sets up logging, environment, and banner

4. Starts embedded web server (Tomcat/Jetty/Undertow)

5. Runs any CommandLineRunner or ApplicationRunner beans

---

🔩 Advanced Usage:

java
SpringApplication app = new SpringApplication(MyApplication.class);
app.setBannerMode(Banner.Mode.OFF);
app.run(args);

---

📚 Key Features:

• Auto-configuration

• Environment loading (application.properties/yaml)

• Command line argument parsing

• Banner control

• Listener and event publishing support

---------------------------

✅ 2. Spring Framework Internal Flow

🌱 Spring Core Container Flow

• IoC Container (Inversion of Control):

  • Reads configuration (@Configuration, @Component)

  • Instantiates beans

  • Resolves dependencies via:

    • @Autowired, @Qualifier, @Primary, XML, etc.

🔄 Bean Lifecycle:

1. Instantiation

2. Populate properties (@Autowired, setters)

3. BeanNameAware, BeanFactoryAware callbacks

4. InitializingBean.afterPropertiesSet() or @PostConstruct

5. Bean is ready to use

6. On shutdown: @PreDestroy or DisposableBean.destroy()

---

✅ 3. Spring MVC Internal Flow

🌐 HTTP Request Flow (Spring Web MVC DispatcherServlet)

text
Client -> Embedded Tomcat -> DispatcherServlet -> HandlerMapping
       -> Controller -> Service -> Repository -> DB
       <- ResponseBody/ModelAndView <- ViewResolver <- DispatcherServlet <- Client

🔁 Step-by-step Request Flow:

---

🔹 1. Client Sends HTTP Request

Example: GET /users/123

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🔹 2. DispatcherServlet (Front Controller)

• Configured by Spring Boot auto-config.

• Intercepts all incoming requests (/ mapping).

• Delegates request to appropriate components.

---

🔹 3. HandlerMapping

• Finds appropriate @Controller class & method based on URL and HTTP method.

• Ex: @GetMapping("/users/{id}")

---

🔹 4. HandlerAdapter

• Executes the controller method.

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🔹 5. Controller Method

java
@GetMapping("/users/{id}")
public ResponseEntity<User> getUser(@PathVariable int id) {
    return ResponseEntity.ok(userService.findById(id));
}

• Calls the Service layer.

• Business logic executes.

---

🔹 6. Service Layer

• Contains business logic

• Calls the Repository (DAO)

---

🔹 7. Repository Layer

• Interacts with the DB using JPA/Hibernate.

java
@Repository
public interface UserRepository extends JpaRepository<User, Integer> {}

---

🔹 8. Return Response

• Controller returns ResponseEntity or object.

• If annotated with @RestController or @ResponseBody, data is serialized (JSON/XML) by HttpMessageConverters (like Jackson).

---

🔹 9. ViewResolver (If using Thymeleaf/JSP)

• If returning a view name, ViewResolver resolves it to an actual .html or .jsp.

---

🔹 10. Response Sent to Client

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✅ Internal Objects in Spring MVC

Component	Purpose
DispatcherServlet	Central controller of Spring MVC
HandlerMapping	Finds which controller to call
HandlerAdapter	Calls the actual controller method
HttpMessageConverter	Converts object to JSON/XML
ViewResolver	Resolves view names to templates
ModelAndView	Holds model + view data
WebDataBinder	Binds request params to Java objects

🔚 Summary Diagram

SpringApplication.run()
        ↓
Creates ApplicationContext
        ↓
Scans Beans (@Component, etc.)
        ↓
Initializes DispatcherServlet
        ↓
Starts Embedded Server (Tomcat)
        ↓
Client HTTP Request
        ↓
DispatcherServlet
        ↓
HandlerMapping → Controller
        ↓
Service → Repository → DB
        ↓
Controller returns Response/ModelAndView
        ↓
HttpMessageConverter/ViewResolver
        ↓
Response back to Client
🔍 Detailed Breakdown:
✅ 1. SpringApplication.run(...)
This is the entry point to boot your Spring Boot application.

It performs:
Environment preparation
ApplicationContext creation
Auto-configuration
Running ApplicationRunner or CommandLineRunner beans

Internally:
java
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args)

✅ 2. Creates ApplicationContext
Based on the application type (web or non-web), it creates:
AnnotationConfigServletWebServerApplicationContext (for Spring MVC)
or another variant like ReactiveWebServerApplicationContext (for WebFlux)
This is the Spring container that manages all beans and lifecycle.

✅ 3. Scans Beans (@Component, @Service, @Controller, etc.)
Spring scans packages based on:
@SpringBootApplication → combines @ComponentScan
Beans annotated with:
@Component, @Service, @Repository, @Controller, @RestController
These are registered in the ApplicationContext.

✅ 4. Initializes DispatcherServlet
The DispatcherServlet is the Front Controller in Spring MVC.
It is automatically registered as a bean and mapped to /.
It handles all incoming HTTP requests.

✅ 5. Starts Embedded Server (Tomcat/Jetty/Undertow)
Spring Boot uses embedded servers (default is Tomcat).
The server is started programmatically, no need for web.xml.
Example log:
scss
Tomcat started on port(s): 8080 (http)

✅ 6. Client HTTP Request
A user (browser/API client) makes an HTTP request:

bash
GET http://localhost:8080/users
✅ 7. DispatcherServlet
DispatcherServlet intercepts the request.
It is configured via WebMvcAutoConfiguration.
It coordinates the request-handling workflow.

✅ 8. HandlerMapping → Controller
DispatcherServlet asks HandlerMapping for the correct controller.
Finds a @RequestMapping, @GetMapping, etc.
Then, calls the method in the controller class:

java
@RestController
public class UserController {
    @GetMapping("/users")
    public List<User> getUsers() { ... }
}
✅ 9. Service → Repository → DB
The controller delegates logic to the @Service layer.
The service calls the @Repository, which interacts with the database:

java
@Service
public class UserService {
    @Autowired
    private UserRepository repo;

    public List<User> getAllUsers() {
        return repo.findAll();
    }
}
✅ 10. Controller returns Response or ModelAndView
The controller returns:
ResponseEntity<?> for REST
Or ModelAndView for traditional MVC
Spring uses this return type to decide how to process the response.

✅ 11. HttpMessageConverter / ViewResolver
If REST:
Uses HttpMessageConverter (e.g., Jackson) to convert Java objects → JSON/XML

If MVC:
Uses ViewResolver to resolve .jsp or .html views

✅ 12. Response back to Client
The generated response (JSON, HTML, etc.) is sent back via HTTP to the client:

css
HTTP 200 OK
Content-Type: application/json
Body: [{"id":1,"name":"John"}, ...]
✅ Summary Flow Diagram
text
SpringApplication.run()
    ↓
ApplicationContext created
    ↓
Beans scanned and registered
    ↓
Embedded server (Tomcat) started
    ↓
↓ Client makes request
DispatcherServlet receives it
    ↓
HandlerMapping → Controller method
    ↓
Service → Repository → DB
    ↓
Controller returns response
    ↓
HttpMessageConverter or ViewResolver
    ↓
HTTP Response to client