Spring Framework

The easiest way to understand what the Spring Framework actually does is to stop thinking of it as “magic” and start thinking of it as infrastructure code that your application delegates to.

At a high level:

Spring creates objects, wires them together, manages their lifecycle, and adds common behavior around them so you do not have to write that plumbing yourself.

1. The Core Problem Spring Solves

Without Spring, your application code often has to manually do things like:

UserRepository repository = new JdbcUserRepository(dataSource);
EmailService emailService = new SmtpEmailService(config);
UserService userService = new UserService(repository, emailService);

As the application grows, this becomes harder to manage:

Who creates each object?
In what order?
Which implementation should be used?
How are shared dependencies reused?
How do you add transactions, security, logging, validation, configuration, etc.?

Spring’s answer is:

Declare your application components, and I will create, connect, configure, and enhance them.

2. Spring Is Mainly an Application Container

The heart of Spring is the IoC container.

IoC means Inversion of Control.

Instead of your code controlling object creation:

UserService service = new UserService(new UserRepository());

Spring controls it:

@Service
public class UserService {

    private final UserRepository userRepository;

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

Then Spring sees:

@Repository
public class UserRepository {
}

And automatically creates:

a UserRepository
a UserService
injects the repository into the service

This is called Dependency Injection.

So Spring’s first major job is:

Managing your application objects, called beans.

3. What Is a Bean?

A Spring bean is simply an object managed by Spring.

For example:

@Service
public class PaymentService {
}

That class becomes a Spring-managed object.

Spring can:

create it
inject its dependencies
configure it
call lifecycle methods
wrap it with proxies
destroy it when the application shuts down

The object itself is ordinary Java. What changes is who manages it.

4. Spring Reads Metadata About Your App

Spring needs to know what objects to manage.

You give it metadata using annotations such as:

@Component
@Service
@Repository
@Controller
@Configuration
@Bean
@Autowired

Example:

@Configuration
public class AppConfig {

    @Bean
    public Clock clock() {
        return Clock.systemUTC();
    }
}

This tells Spring:

“When something needs a Clock, use this object.”

Spring scans your code, reads these annotations, builds a registry of beans, and creates an application context.

5. The ApplicationContext Is the Running Spring Container

The ApplicationContext is basically Spring’s runtime environment.

It contains all managed beans.

Conceptually:

ApplicationContext context = ...;

UserService userService = context.getBean(UserService.class);

In most Spring applications, you do not usually call getBean() yourself. Spring injects dependencies automatically.

The container knows:

which beans exist
how to create them
what dependencies they need
what order to initialize them in
what configuration values they require

6. Spring Adds Behavior Using Proxies

A lot of Spring’s “magic” comes from proxies.

For example, when you write:

@Transactional
public void transferMoney(Account from, Account to, BigDecimal amount) {
    withdraw(from, amount);
    deposit(to, amount);
}

Spring does not rewrite your method.

Instead, it may create a wrapper object around your service.

Conceptually:

beginTransaction();

try {
    transferMoney(...);
    commitTransaction();
} catch (Exception ex) {
    rollbackTransaction();
    throw ex;
}

That wrapper is a proxy.

Spring uses proxies for features like:

transactions
security
caching
async methods
method validation
aspect-oriented programming

So another major thing Spring does is:

It intercepts calls to your objects and adds infrastructure behavior around them.

7. Spring MVC Handles Web Requests

If you use Spring MVC, Spring also acts as a web framework.

You write:

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

    @GetMapping("/{id}")
    public UserDto getUser(@PathVariable Long id) {
        return new UserDto(id, "Alice");
    }
}

Spring MVC handles:

receiving the HTTP request
matching /users/{id} to the method
converting path variables
calling your controller
converting the return value to JSON
writing the HTTP response

You focus on:

public UserDto getUser(Long id)

Spring handles the web plumbing.

8. Spring Data JPA Creates Repository Implementations

With Spring Data JPA, you can write:

public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
}

You do not manually implement this interface.

Spring Data creates an implementation at runtime.

It understands method names like:

findByEmail
findByStatus
findByCreatedAtAfter

And turns them into database queries.

So Spring Data JPA does:

repository implementation generation
query method parsing
transaction integration
JPA EntityManager management

9. Spring Boot vs Spring Framework

This is an important distinction.

The Spring Framework provides the core capabilities:

dependency injection
bean lifecycle
transactions
Spring MVC
validation integration
resource loading
AOP
event system

Spring Boot

Spring Boot sits on top of Spring Framework.

It adds:

auto-configuration
embedded servers
starter dependencies
production features
simplified project setup

Spring Boot’s job is mostly:

“Based on what dependencies and settings you have, I will configure Spring automatically.”

For example, if Spring Boot sees Spring MVC on the classpath, it configures a web server and MVC infrastructure.

If it sees Spring Data JPA and a database driver, it configures JPA-related beans.

10. A Mental Model

Think of a Spring application like this:

Your code:
  Controllers
  Services
  Repositories
  Entities
  Configuration

Spring:
  Creates objects
  Injects dependencies
  Applies configuration
  Opens transactions
  Handles HTTP requests
  Converts JSON
  Manages validation
  Integrates with databases
  Publishes events
  Handles lifecycle

You write the business logic.

Spring handles the surrounding infrastructure.

11. What Happens at Startup?

Simplified startup flow:

1. Application starts
2. Spring creates an ApplicationContext
3. Spring scans classes and configuration
4. Spring discovers bean definitions
5. Spring creates beans
6. Spring injects dependencies
7. Spring applies post-processors
8. Spring creates proxies where needed
9. Web server starts, if this is a web app
10. App is ready to receive requests

For example:

@SpringBootApplication
public class MyApplication {

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

That one line starts a large amount of framework infrastructure.

12. A Practical Way to Learn Spring

To understand Spring deeply, learn it in this order:

Plain Java object creation
Dependency Injection
Beans and ApplicationContext
Configuration with @Configuration and @Bean
Component scanning with @Component, @Service, @Repository
Bean lifecycle
Spring MVC request handling
Transactions with @Transactional
Spring Data repositories
Spring Boot auto-configuration

Avoid starting with everything at once. Spring feels magical when you learn MVC, JPA, transactions, Boot, security, and annotations simultaneously.

13. Tiny Example

Your code:

@Service
public class GreetingService {

    public String greet(String name) {
        return "Hello, " + name;
    }
}

@RestController
public class GreetingController {

    private final GreetingService greetingService;

    public GreetingController(GreetingService greetingService) {
        this.greetingService = greetingService;
    }

    @GetMapping("/hello")
    public String hello() {
        return greetingService.greet("World");
    }
}

What Spring does:

1. Finds GreetingService
2. Creates a GreetingService object
3. Finds GreetingController
4. Sees that it needs GreetingService
5. Injects GreetingService into GreetingController
6. Maps GET /hello to hello()
7. Calls hello() when an HTTP request arrives
8. Sends "Hello, World" as the response

That is Spring in miniature.

Bottom Line

Spring Framework mainly does four things:

Object management
It creates and manages your application objects.
Dependency injection
It wires objects together automatically.
Infrastructure integration
It provides transactions, web handling, validation, database integration, events, configuration, etc.
Behavior wrapping
It uses proxies to add behavior such as transactions, security, caching, and async execution.

The shortest explanation is:

Spring is a container and infrastructure framework that lets you write business code while it handles object creation, wiring, lifecycle, and common enterprise concerns.

Tag Archives: Spring Framework

How do I understand what the Spring Framework actually does?