39. Introduction to Spring Boot

[!NOTE] Nobody in the modern corporate world builds web endpoints using raw Java Web Sockets or legacy Servlets. They use the Spring Framework—specifically, the Spring Boot extension.

Spring Boot takes the insanely verbose hundreds of XML configuration files required by older Java EE systems, and automates it via "Opinionated Convention over Configuration."

The Inversion of Control (IoC) Container

The core majesty of Spring is something called Dependency Injection.

Normally, if an AuthenticationService needs to talk to a UserRepository, you write: UserRepository repo = new UserRepository();

This is called "Tight Coupling". It makes unit testing and upgrading impossible.

In Spring, you NEVER type new. Spring creates one giant master Object (a Bean) of every class at startup. When your AuthenticationService needs a repository, you just use the @Autowired annotation, and Spring magically injects the required database connector object into the slot perfectly!

import org.springframework.stereotype.Service;
import org.springframework.beans.factory.annotation.Autowired;

// 1. We tell Spring: "You are in control of this class. Track it."
@Service
public class AuthenticationService {

    // 2. We ask Spring: "Hey, give me a master SQL connector. I refuse to make it myself!"
    @Autowired
    private UserRepository repo;

    public void login() {
        // We use the injected repo flawlessly!
        repo.findUserByUsername("admin");
    }
}

  

Building a Lightning Fast REST API

Spring Boot embeds an entire Tomcat Web Server directly inside the Jar file.

You literally write 15 lines of code, click "Run", and you instantly have a production-ready HTTP server listening on Port 8080.

import org.springframework.web.bind.annotation.*;
import java.util.List;

// Signals Spring Boot to spin up routes over the internet
@RestController
@RequestMapping("/api/v1/users")
public class UserController {

    @Autowired
    private UserService service;

    // Handles an incoming 'GET https://quizmaker.co.in/api/v1/users' request!
    @GetMapping
    public List<User> fetchAllUsers() {
        // Spring automatically converts this Java ArrayList into a JSON Array payload 
        // to send to the Firefox/Chrome frontend browser!
        return service.getAll(); 
    }
}

  

[!TIP] If you want to get hired as a Java Backend Engineer, learning Core Java is only 50% of the battle. The other 50% is demonstrating deep fluency in building Spring Boot REST APIs connected to PostgreSQL!

HTTP request
  -> Controller
  -> Service
  -> Repository
  -> Database
  

@Service
class UserService {
    private final UserRepository userRepository;

    UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
}