Springboot starter Detail

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The Spring Boot Startup Process 🚀

When a Spring Boot application starts, it follows a well-defined sequence of steps to initialize the application, create and wire up components, and get ready to handle requests.

1. The Entry Point: main() Method

Everything begins in the standard public static void main(String[] args) method. The key instruction here is SpringApplication.run().

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MySpringBootApplication {
    public static void main(String[] args) {
        // This single line kicks off the entire process
        SpringApplication.run(MySpringBootApplication.class, args);
    }
}

The SpringApplication class bootstraps the application, creating the Spring Application Context (the IoC container) and triggering the startup sequence.

2. The Core Annotation: @SpringBootApplication

This single annotation is the key to Spring Boot’s power and is a combination of three critical annotations:

• @SpringBootConfiguration: Marks the class as a source of bean definitions and application configuration.
• @EnableAutoConfiguration: Activates Spring Boot’s auto-configuration mechanism. It intelligently configures your application based on the JAR dependencies present on the classpath.
• @ComponentScan: Tells Spring to scan the current package and its sub-packages for other components (like @Service, @Repository, @Controller, etc.) to register them as beans in the Application Context.

3. Step-by-Step Startup Sequence

The SpringApplication.run() method orchestrates the following key steps:

1. Environment Preparation: It prepares the application environment, loading configuration from application.properties or application.yml and resolving active profiles.

2. Banner Display: A startup banner (from banner.txt or a default) is printed to the console.

3. Application Context Creation: An ApplicationContext is created. For a web application, this is typically an AnnotationConfigServletWebServerApplicationContext.

4. Component Scanning & Bean Definition: The @ComponentScan process kicks in, finding all your annotated classes and creating bean definitions for them.

5. Auto-Configuration Execution: The @EnableAutoConfiguration process runs. Spring Boot inspects the classpath and your configuration. For example, if it finds the spring-boot-starter-web dependency, it automatically configures an embedded Tomcat server, Spring MVC, and other web-related beans. This process uses conditional annotations (@ConditionalOnClass, @ConditionalOnProperty, etc.) to decide which configurations to apply.

6. Application Context Refresh: This is a core Spring phase where all beans are instantiated, dependencies are injected (@Autowired), and any post-processing is performed.

7. Embedded Server Startup: If it’s a web application, the embedded server (e.g., Tomcat, Jetty) is started and is ready to accept HTTP requests.

8. Runners Execution: Finally, Spring Boot calls the run() method of any CommandLineRunner and ApplicationRunner beans in your context. This is the perfect place for executing custom logic after the application is fully initialized, such as seeding a database or printing summary logs.

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A Typical Spring Boot Project Structure 📂

A well-organized project structure is crucial for maintainability. The following layout is a widely adopted convention.

src/main/java - Java Source Code

This is where your application’s logic resides, organized into packages based on their role (layer).

• controller: Contains classes annotated with @RestController. Their role is to handle incoming HTTP requests, process them (often by calling a service), and return an HTTP response.
• service: Contains classes annotated with @Service. This layer holds the core business logic of your application. It orchestrates calls to repositories and other services.
• repository: Contains interfaces that extend Spring Data JPA interfaces (like JpaRepository). Annotated with @Repository, they handle all database interactions.
• model / entity: Contains your Plain Old Java Objects (POJOs), typically annotated with @Entity to represent database tables.
• dto (Data Transfer Object): Simple objects used to transfer data between layers (e.g., from the controller to the service) or as a response to the client, preventing exposure of your internal entity models.
• config: Contains classes annotated with @Configuration to define beans, security settings, or other application-level configurations.
• exception / advice: Holds custom exception classes and global exception handlers (using @ControllerAdvice) to manage errors cleanly across the application.

src/main/resources - Application Resources

This directory contains all non-Java files.

• application.properties / application.yml: The primary file for application configuration, such as database connection strings, server port, and custom properties.
• static: For static assets like CSS, JavaScript files, and images.
• templates: For server-side view templates, such as Thymeleaf or FreeMarker files.