Java Spring Boot Basics – A Complete Beginner Guide

🟩 What is Spring Boot?

Spring Boot is a framework built on top of Spring that helps you create production-ready applications quickly.

Key Features

• No XML configuration required
• Embedded Tomcat/Jetty server
• Ready-to-use starters (dependencies)
• Auto-configuration
• Production-ready metrics
• Integration with Spring ecosystem

🟩 Why Spring Boot? (Advantages)
✔ Faster Development

No need to configure servers manually.

✔ Embedded Server

Your application runs simply using:

java -jar yourapp.jar


✔ Auto-Configuration

Spring Boot automatically configures beans depending on your application classpath.

✔ Microservices Friendly

Easy to build independent and scalable services.

🟩 Spring Boot Architecture (Simple Explanation)

Spring Boot follows layered architecture:

1. Controller Layer – Handles API requests
2. Service Layer – Business logic
3. Repository Layer – Database operations (JPA/Hibernate)
4. Model Layer – Java POJO classes

This structure improves readability, scalability, and maintainability.

🟦 How to Create a Spring Boot Project (Step-by-Step)

Use Spring Initializr:
https://start.spring.io

Select:

• Project: Maven
• Language: Java
• Spring Boot Version: Latest
• Dependencies:
  • Spring Web
  • Spring Boot DevTools
  • Spring Data JPA
  • H2 / MySQL

Click Generate, download the project, and open in IntelliJ or Eclipse.

🟩 Spring Boot Project Structure Explained

src/main/java
    └── com.example.demo
          ├── DemoApplication.java   // Main class (entry point)
          ├── controller
          ├── service
          ├── repository
          └── model
src/main/resources
    ├── application.properties
    └── static / templates




🟦 Main Application Class (Entry Point)

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




What @SpringBootApplication includes?

• @EnableAutoConfiguration
• @ComponentScan
• @Configuration

🟩 Create Your First REST API in Spring Boot
1. Create Controller Class

@RestController
@RequestMapping("/api")
public class HelloController {
     @GetMapping("/hello")
    public String sayHello() {
        return "Welcome to Spring Boot!";
    }
}

Run Application → Visit

http://localhost:8080/api/hello

You will see the response.

🟩 POST API Example (JSON Input)

@PostMapping("/employee")
public Employee createEmployee(@RequestBody Employee emp) {
    return emp;
}




JSON Body:

{
  "name": "Piyush",
  "role": "Developer"
}




🟦 Spring Boot Important Annotations (Must Know)
✔ @RestController

Defines a controller that returns JSON.

✔ @RequestMapping

Base URL mapping.

✔ @GetMapping, @PostMapping, @PutMapping, @DeleteMapping

HTTP operations.

✔ @Autowired

Dependency injection.

✔ @Entity

Represents database table.

✔ @Repository, @Service

Layer identification.

🟩 Working with Database (Spring Data JPA Example)
Entity Class

@Entity public class Employee { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id; private String name; private String role; }

Repository Interface

public interface EmployeeRepo extends JpaRepository<Employee, Long> {}




Service Example

@Service public class EmployeeService { @Autowired private EmployeeRepo repo; public List<Employee> getAllEmployees() { return repo.findAll(); } }

🟦 Spring Boot application.properties Example

@Service
public class EmployeeService {
     @Autowired
    private EmployeeRepo repo;
     public List<Employee> getAllEmployees() {
        return repo.findAll();
    }
}