Spring Framework Introduction

Spring is the most popular framework for building Java applications. Banks, e-commerce sites, Netflix, and most Java back-end jobs in India use it. Spring Boot (next course) is built on top of it, so understanding Spring’s core ideas makes everything in Spring Boot click.

The problem Spring solves#

Imagine an OrderService that needs a repository, a payment gateway and an email sender:

public class OrderService {
    private final OrderRepository repo = new MySqlOrderRepository(new HikariDataSource(config));
    private final PaymentGateway payments = new RazorpayGateway("key", "secret");
    private final EmailSender email = new SmtpEmailSender("smtp.gmail.com", 587);

    public void placeOrder(Order order) { ... }
}

Problems:

• Tight coupling: OrderService decides which database, which payment provider and which email server. Changing any of them means editing this class.
• Hard to test: you can’t test placeOrder without a real database, a real payment account and a real mail server.
• Duplicated setup: every class that needs a database creates its own connection pool.
• Plumbing everywhere: transactions, security checks and logging get copied into every method.

Spring’s answer#

1. Inversion of Control (IoC) and Dependency Injection (DI): your classes declare what they need; Spring creates and connects them.

@Service
public class OrderService {
    private final OrderRepository repo;
    private final PaymentGateway payments;
    private final EmailSender email;

    public OrderService(OrderRepository repo, PaymentGateway payments, EmailSender email) {
        this.repo = repo;            // Spring passes these in
        this.payments = payments;
        this.email = email;
    }
}

Now OrderService depends only on interfaces. Swap Razorpay for Paytm, or pass fakes in tests, without touching it.

1. AOP (Aspect-Oriented Programming): add transactions, security and logging around methods with annotations like @Transactional, instead of copying code.

1. Ready-made integrations: database access, web MVC, REST, security, messaging, caching, scheduling, testing.

The Spring ecosystem#

Project	What it gives you
Spring Framework (core)	IoC container, DI, AOP, transactions, MVC, JDBC support
Spring Boot	auto-configuration, embedded server, starters: run apps with almost no setup
Spring Data JPA	repositories with no implementation code
Spring Security	authentication, authorisation, JWT/OAuth2
Spring Cloud	microservices: config server, service discovery, gateways
Spring Batch	large batch jobs
Spring WebFlux	reactive, non-blocking web apps

Spring vs Spring Boot#

	Spring Framework	Spring Boot
What	the core framework	an opinionated way to use Spring
Configuration	you configure everything (XML or Java config)	auto-configured with sensible defaults
Server	deploy a WAR to Tomcat	embedded Tomcat: java -jar app.jar
Dependencies	pick compatible versions yourself	“starters” with tested versions
Use today	understanding the foundations, older projects	almost every new project

Spring Boot is Spring: same annotations, same container, same concepts. Boot just removes the setup work.

Key vocabulary#

• Bean: an object created and managed by Spring.
• ApplicationContext: the Spring container that creates, wires and manages beans.
• Dependency injection: Spring supplies a bean’s dependencies (usually through the constructor).
• Component scanning: Spring finds classes annotated with @Component, @Service, @Repository, @Controller.
• Configuration class: a class with @Configuration and @Bean methods that define beans.
• Proxy: a wrapper Spring puts around a bean to add behaviour (transactions, security).

Exercise#

1. List the dependencies of a CourseService that saves courses, sends a welcome email and charges a fee.
2. Rewrite its constructor so it receives those dependencies instead of creating them.
3. Explain why the rewritten class is easier to test.

Hint for 3

In a test you can pass a fake EmailSender that just records the messages, and a fake payment gateway that always succeeds or always fails, with no network calls.

Interview corner#

• What is Spring? A comprehensive Java framework providing an IoC container, DI, AOP and integrations for building enterprise applications.
• What is IoC? Inversion of Control: the framework, not your code, creates and wires objects.
• Spring vs Spring Boot? Boot is Spring with auto-configuration, starters and an embedded server.
• What is a bean? An object instantiated, assembled and managed by the Spring container.

Summary#

• Spring removes tight coupling and plumbing with IoC/DI and AOP.
• The ecosystem covers web, data, security, cloud and batch.
• Spring Boot is the fast way to use Spring; the core concepts are the same.

Next: IoC and dependency injection, animated.