How Spring Boot Uses Java

What is Spring Boot?

Spring Boot is a popular Java web framework that makes building backends:

• Fast to start

• Easy to run

• Ready for production

It hides a lot of the complex Servlet setup, so you just write controllers, services, repositories, and Spring handles routing, JSON, DB, security.

How your core Java connects:

• Controllers → Java methods with @GetMapping, @PostMapping.

• Services → Your business logic classes.

• Models/Entities → Your User class with ID, name, email.

• Repositories → DB access layer, often built with Spring Data JPA.

• Exception Handling → @ExceptionHandler for clean error JSON.

Simple Example: Your CRUD as a REST API

Here’s how the same UserService idea expands to Spring Boot:

java
// User.java (model)
public class User {
    private int id;
    private String name;
    private String email;

    // getters/setters
}

// UserService.java
import org.springframework.stereotype.Service;
import java.util.ArrayList;
import java.util.List;

@Service
public class UserService {
    private List<User> users = new ArrayList<>();

    public void addUser(String name, String email) {
        users.add(new User(users.size() + 1, name, email));
    }

    public List<User> getAllUsers() {
        return users;
    }
}

// UserController.java
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
@RequestMapping("/users")
public class UserController {

    private final UserService userService;

    // Constructor Injection
    public UserController(UserService userService) {
        this.userService = userService;
    }

    @PostMapping
    public void addUser(@RequestBody User user) {
        userService.addUser(user.getName(), user.getEmail());
    }

    @GetMapping
    public List<User> getAllUsers() {
        return userService.getAllUsers();
    }
}

Key differences vs plain Java:

• No main() doing manual calls — instead, Spring Boot runs an embedded server (Tomcat) that listens for HTTP requests.

• @RestController means methods return JSON by default.

• @RequestMapping("/users") maps the URL to your controller.

• @PostMapping, @GetMapping handle different HTTP methods.

• @RequestBody binds incoming JSON to your User class.

20.3 Mini Lab: Run a Tiny Spring Boot CRUD

This is how your plain Java CRUD turns into an actual API.

1️⃣ In your IDE, create a Spring Boot project with:

• Spring Web dependency

• Spring Boot DevTools (optional, for auto-reload)

2️⃣ Place the User, UserService, and UserController in the right packages (model, service, controller).

3️⃣ Run your app (DemoApplication.java).

4️⃣ Use Postman or curl:

bash
curl -X POST http://localhost:8080/users \
  -H "Content-Type: application/json" \
  -d '{"name":"Alice","email":"alice@example.com"}'

curl http://localhost:8080/users

🚀 Congrats:
You just turned core Java → real RESTful backend → JSON → usable by React, Vue, mobile apps!