Microsoft007

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.

⚙️ 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

🛠 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.

🌐 5. Request Handling (Web Layer)

Steps:

HTTP request hits the embedded server (Tomcat).
Routed to the DispatcherServlet.
DispatcherServlet uses HandlerMapping to find the correct controller method.
Executes:
- HandlerInterceptors
- Controller method (e.g., @GetMapping, @PostMapping)
Returns the result (ResponseEntity, JSON, View).
DispatcherServlet sends response back to the client.

📦 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

🛑 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.

⚙️ 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

📌 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).

📌 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:

Creates an ApplicationContext
Loads beans and applies auto-configuration
Sets up logging, environment, and banner
Starts embedded web server (Tomcat/Jetty/Undertow)
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:

Instantiation
Populate properties (@Autowired, setters)
BeanNameAware, BeanFactoryAware callbacks
InitializingBean.afterPropertiesSet() or @PostConstruct
Bean is ready to use
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

🔹 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.

🔹 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

✅ 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

Wednesday, 25 June 2025

Internal flow of spring boot/spring/spring mvc

🔁 1. Application Startup (main() method)

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

🔧 2. Component Scan & Bean Creation

Triggered by:

Internals:

⚙️ 3. Auto Configuration

Provided by:

Internals:

🛠 4. Initialization of Embedded Server

Example:

Internals:

🌐 5. Request Handling (Web Layer)

Steps:

📦 6. Persistence Layer (If Configured)

📊 7. Actuator & Monitoring (Optional)

🛑 8. Shutdown Flow

⚙️ Internal Components Summary

🔍 Spring Boot Internal Flow

✅ Visual Diagram + Call Stack Trace

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

📌 B. Internal Call Stack Trace

📦 Summary Diagram:

✅ Key Classes Involved:

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

SpringApplication class in Spring Boot

📌 Simple Definition:

🧱 Syntax:

🔧 Example:

🔍 What It Does Internally:

🔩 Advanced Usage:

📚 Key Features:

✅ 2. Spring Framework Internal Flow

🌱 Spring Core Container Flow

🔄 Bean Lifecycle:

✅ 3. Spring MVC Internal Flow

🌐 HTTP Request Flow (Spring Web MVC DispatcherServlet)

🔁 Step-by-step Request Flow:

🔹 1. Client Sends HTTP Request

🔹 2. DispatcherServlet (Front Controller)

🔹 3. HandlerMapping

🔹 4. HandlerAdapter

🔹 5. Controller Method

🔹 6. Service Layer

🔹 7. Repository Layer

🔹 8. Return Response

🔹 9. ViewResolver (If using Thymeleaf/JSP)

🔹 10. Response Sent to Client

✅ Internal Objects in Spring MVC

🔚 Summary Diagram

🔍 Detailed Breakdown:

🔁 1. Application Startup (`main()` method)

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

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

`🔍 Detailed Breakdown:`