Introduction

Before building real applications with Spring Boot, you must understand the basic project structure and the core annotations that drive Spring Boot.
This lesson explains each part of a new Spring Boot project and gives you simple examples of the most important annotations.

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1. Spring Boot Project Structure Explained

When you generate a project from Spring Initializr, the structure looks like this:

src/
 └── main/
      ├── java/
      │     └── com/example/demo/
      │           ├── DemoApplication.java
      │           └── controllers/
      │                 └── HelloController.java
      └── resources/
            ├── application.properties
            ├── static/
            └── templates/

Here is what each section means:

src/main/java

This contains all your Java source code.

You typically organize it into packages such as:

• controllers
• services
• repositories
• config
• models

DemoApplication.java

This is the main class that starts your application.
It contains the main method and the @SpringBootApplication annotation.

src/main/resources

Everything here is related to configuration and static content.

It includes:

• application.properties (or application.yml)
• static/ (CSS, JS, images)
• templates/ (Thymeleaf or other server-rendered templates)

application.properties

This is where you put configuration values like:

• database connection
• server port
• logging settings

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2. Key Spring Boot Annotations for Beginners

Spring Boot relies heavily on annotations.
Here are the most important ones you must understand early on.

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@SpringBootApplication

This annotation is placed on your main class.

Example:

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

It combines three annotations:

• @Configuration
• @EnableAutoConfiguration
• @ComponentScan

This means:

• Spring Boot will auto-configure your project
• It will scan your packages for components
• It marks this class as a configuration entry point

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@RestController

Used to create a REST API controller.

@RestController
public class HelloController {

    @GetMapping("/hello")
    public String hello() {
        return "Hello Spring Boot!";
    }
}

A RestController returns JSON or plain text responses.

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@GetMapping, @PostMapping, @PutMapping, @DeleteMapping

These annotations define HTTP endpoints.

Example:

@GetMapping("/users")
public List<User> getUsers() { ... }

They map specific HTTP methods to Java methods.

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@Autowired

Spring handles dependency injection automatically with this annotation.

Example:

@Autowired
UserService userService;

It tells Spring Boot to provide the instance of UserService without you creating it manually.

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@Service

Marks a class as a business/service layer component.

@Service
public class UserService {
}

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@Repository

Used for DAO/database interaction classes.

@Repository
public class UserRepository {
}

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@Entity

Marks a class as a JPA database model.

@Entity
public class User {
    @Id
    private Long id;
}

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3. How Spring Boot Knows What to Load

Spring Boot scans your project starting from the package where your main class sits.

For example, if your main class is:

com.example.demo

Spring Boot automatically scans:

com.example.demo.*

This is why the main class should always be in the root package of your project.

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4. Summary

In this lesson, you learned:

• The structure of a Spring Boot project
• What each folder and file is used for
• The most important Spring Boot annotations
• How Spring Boot scans and loads components

You are now ready to build more complex applications because you understand the foundation.