Full-Stack Java (Spring + Database)

So far, you have learned Spring Boot, REST APIs, Hibernate mapping, and CRUD operations. Now it is time to connect everything together and understand how a full-stack Java backend works in real applications.

This lesson focuses on how Spring Boot, REST APIs, and Hibernate work together to serve frontend applications.

What Is a Full-Stack Java Application?

A full-stack Java application usually consists of:

• A frontend (web or mobile app)
• A backend built using Spring Boot
• A database connected through Hibernate

The backend acts as the bridge between the frontend and the database.

High-Level Architecture

A typical flow looks like this:

• User interacts with frontend UI
• Frontend sends HTTP request to backend API
• Controller receives request
• Service processes business logic
• Repository communicates with database
• Response is sent back as JSON

Each layer has a clear responsibility.

Controller Layer

The controller handles incoming HTTP requests.

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

    private final UserService userService;

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

    @GetMapping
    public List getUsers() {
        return userService.getAllUsers();
    }
}

The controller does not talk to the database directly.

Service Layer

The service layer contains business logic.

@Service
public class UserService {

    private final UserRepository userRepository;

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

    public List getAllUsers() {
        return userRepository.findAll();
    }
}

This keeps business rules separate from controllers.

Repository Layer

The repository layer interacts with the database using Hibernate.

public interface UserRepository
        extends JpaRepository {
}

Spring Data JPA automatically handles database operations.

Entity Layer

Entities represent database tables.

@Entity
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    private String name;
    private String email;
}

Hibernate maps this class to a database table.

How Frontend Connects to Backend

The frontend sends HTTP requests using tools like:

• Fetch API
• Axios
• Mobile app HTTP clients

Example frontend request:

fetch("/users")
  .then(response => response.json())
  .then(data => console.log(data));

The backend responds with JSON data.

Real-World Example

Consider an employee management system:

• Frontend shows employee list
• Backend API fetches employees
• Hibernate retrieves data from database
• Data is returned as JSON

This same pattern is used in most enterprise systems.

Why This Architecture Works

• Clear separation of concerns
• Easy to maintain and scale
• Industry-standard design
• Easy integration with frontend frameworks

This structure is commonly tested in interviews.

Common Mistakes Beginners Make

Some mistakes to avoid:

• Putting business logic inside controllers
• Direct database access from controllers
• Skipping service layer

Following the layered approach keeps applications clean.

Industry Perspective

Most production Java applications follow this exact structure.

Understanding this flow prepares you for real backend roles.

What Comes Next?

In the next and final lesson, you will build a Java Capstone Project that combines everything you have learned.

This will mark the completion of the Java journey.