Java Backend Architecture

Why Java for Backend?

Java is one of the most widely used backend languages, especially in enterprise systems:

• Strongly typed — catches errors at compile time
• Mature ecosystem — decades of libraries and frameworks
• JVM performance — JIT compilation, garbage collection
• Spring Boot — industry-standard framework for building APIs
• Scalability — handles millions of requests per second

Core Components of a Java Backend

Client Request (HTTP/JSON)
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        v
   [Controller]          -- Receives and validates the request
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        v
   [Service]             -- Contains business logic
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        v
   [Repository]          -- Handles database access
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        v
   [Database]            -- Stores data (PostgreSQL, MySQL)

Spring Boot — The Standard Framework

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

@SpringBootApplication is a combination of:

• @Configuration — marks the class as a source of bean definitions
• @EnableAutoConfiguration — auto-configures Spring components
• @ComponentScan — scans for components in the package

What Happens When You Run a Spring Boot App

1. JVM starts
2. Application.main() is called
3. Spring context initializes
4. Beans are created and injected
5. Embedded Tomcat starts on configured port
6. Application is ready to accept requests

Example REST Endpoint

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

    @GetMapping("/{id}")
    public ResponseEntity<User> getUser(@PathVariable Long id) {
        User user = userService.findById(id);
        return ResponseEntity.ok(user);
    }
}

This endpoint handles GET /api/users/{id} and returns a JSON response.

Package Structure

com.example.app
├── controller/     -- REST controllers
├── service/        -- Business logic
├── repository/     -- Data access
├── model/          -- Entities and DTOs
├── config/         -- Configuration classes
└── Application.java

Organizing code this way makes it maintainable and scalable.

Common Interview Questions

Q: Why Java over Python/Go for backend?

• Strong typing catches bugs early
• JVM provides excellent performance via JIT
• Mature ecosystem (Spring, Hibernate, etc.)
• Better suited for large enterprise systems

Q: What is Spring Boot and why is it popular?

• Opinionated framework that reduces boilerplate
• Embedded server (Tomcat/Jetty) — no external deployment needed
• Auto-configuration eliminates XML setup
• Production-ready features (health checks, metrics, security)

Q: How does a Spring Boot request flow?

HTTP Request
    → DispatcherServlet (front controller)
    → HandlerMapping (finds controller method)
    → Controller (processes request)
    → Service (business logic)
    → Repository (database access)
    → Response (JSON back to client)

Real-World Java Backend at Amazon

Amazon uses Java extensively:

• Retail services — product catalog, recommendations, checkout
• Fulfillment — warehouse management, shipping
• AWS services — many AWS services are Java-based

Understanding Java backend architecture is essential for SDE-1 roles.