Spring Boot is one of the most popular Java frameworks used to build enterprise applications, REST APIs and microservices. It simplifies application development by reducing configuration effort and providing production-ready features out of the box.
Understanding Spring Boot Architecture is important because it is frequently asked in interviews and helps developers understand what happens internally when a request reaches an application.
What is Spring Boot?
Spring Boot is an extension of the Spring Framework that simplifies application development through auto-configuration, starter dependencies and embedded servers.
Before Spring Boot, developers had to spend significant time configuring XML files, servlet containers and application settings. Spring Boot eliminates most of this manual configuration.
Why Was Spring Boot Introduced?
Traditional Spring applications required extensive configuration before development could even begin.
Spring Boot was introduced to solve common problems such as:
- Complex XML configuration
- Manual server deployment
- Dependency management issues
- Lengthy project setup
- Boilerplate configuration code
Spring Framework vs Spring Boot
| Feature | Spring Framework | Spring Boot |
|---|---|---|
| Configuration | More Manual | Minimal |
| Server Setup | External Server | Embedded Server |
| Startup Time | Slower Setup | Faster Setup |
| Production Features | Additional Setup | Built In |
Spring Boot Architecture Overview
A typical Spring Boot application follows a layered architecture.
Client
|
v
DispatcherServlet
|
v
Controller
|
v
Service
|
v
Repository
|
v
Database
Each layer has a specific responsibility, making applications easier to maintain and scale.
Spring Boot Main Components
- SpringApplication
- Auto Configuration
- Spring IoC Container
- Embedded Server
- DispatcherServlet
- Controllers
- Services
- Repositories
SpringApplication Class
The SpringApplication class is the entry point of every Spring Boot application.
@SpringBootApplication
public class EmployeeApplication {
public static void main(String[] args) {
SpringApplication.run(
EmployeeApplication.class,
args
);
}
}
When SpringApplication.run() executes, it starts the Spring Container, scans components, creates beans and launches the embedded web server.
What Happens During Application Startup?
Internally Spring Boot performs several operations during startup.
- Creates Application Context
- Scans Spring Beans
- Creates Bean Instances
- Injects Dependencies
- Applies Auto Configuration
- Starts Embedded Tomcat
- Makes Application Ready
What is Auto Configuration?
Auto Configuration is one of the most powerful Spring Boot features. It automatically configures application components based on dependencies available in the project's classpath.
For example, when you add:
spring-boot-starter-web
Spring Boot automatically configures:
- DispatcherServlet
- Embedded Tomcat
- Jackson JSON Support
- Validation Framework
- Error Handling Components
@SpringBootApplication Annotation
The most important Spring Boot annotation is:
@SpringBootApplication
Internally it combines three annotations.
@Configuration
@EnableAutoConfiguration
@ComponentScan
This is another very common interview question.
Embedded Server
One of the biggest advantages of Spring Boot is the embedded server feature.
Traditional Spring applications required deploying WAR files into external servers. Spring Boot includes an embedded web server inside the application.
By default, Spring Boot uses:
Apache Tomcat
Other supported servers include:
- Jetty
- Undertow
Controller Layer
The Controller layer receives HTTP requests from clients.
@RestController
@RequestMapping("/employees")
public class EmployeeController {
}
Controllers should contain request handling logic only. Business logic should not be placed here.
Service Layer
The Service layer contains business logic and application rules.
@Service
public class EmployeeService {
}
This layer acts as a bridge between controllers and repositories.
Repository Layer
Repositories handle interactions with the database.
@Repository
public interface EmployeeRepository
extends JpaRepository<
Employee,
Long> {
}
Repositories simplify data access through Spring Data JPA.
Request Lifecycle in Spring Boot
Understanding request flow is extremely important for interviews.
Browser
|
v
DispatcherServlet
|
v
Controller
|
v
Service
|
v
Repository
|
v
Database
Step 1: Client Sends Request
The browser or API client sends an HTTP request to the application.
GET /employees
Step 2: DispatcherServlet Receives Request
DispatcherServlet is the front controller of Spring MVC.
Every incoming request first reaches DispatcherServlet.
Step 3: Controller Processes Request
@GetMapping("/employees")
public List<Employee> getEmployees() {
}
The Controller receives the request and delegates processing to the Service layer.
Step 4: Service Executes Business Logic
@Service
public class EmployeeService {
}
Business rules and validations are executed here.
Step 5: Repository Accesses Database
Repository fetches data from the database.
Step 6: Response Returns to Client
Spring Boot converts Java objects into JSON and returns them to the client.
[
{
"id":1,
"name":"John"
}
]
Complete Example
Entity
@Entity
public class Employee {
@Id
private Long id;
private String name;
}
Repository
@Repository
public interface EmployeeRepository
extends JpaRepository<
Employee,
Long> {
}
Service
@Service
public class EmployeeService {
private final EmployeeRepository repository;
public EmployeeService(
EmployeeRepository repository) {
this.repository = repository;
}
}
Controller
@RestController
@RequestMapping("/employees")
public class EmployeeController {
private final EmployeeService service;
public EmployeeController(
EmployeeService service) {
this.service = service;
}
}
Why Spring Boot Became So Popular
- Auto Configuration
- Starter Dependencies
- Embedded Servers
- Production Ready Features
- Microservices Support
- Less Boilerplate Code
- Fast Development Cycle
Common Interview Questions
What is Spring Boot?
Spring Boot is an opinionated framework built on top of Spring that simplifies application development.
What does SpringApplication.run() do?
It starts the Spring application, creates beans and launches the embedded server.
What is DispatcherServlet?
DispatcherServlet is the front controller that receives all incoming requests.
What is Auto Configuration?
Auto Configuration automatically configures application components based on project dependencies.
What is the difference between Spring and Spring Boot?
Spring Boot reduces configuration effort and provides production-ready features on top of the Spring Framework.
Best Practices
- Keep Controllers lightweight.
- Place business logic in Services.
- Use Constructor Injection.
- Keep Repository classes focused on data access.
- Follow layered architecture principles.
Frequently Asked Questions (FAQ)
Can Spring Boot work without Tomcat?
Yes. Spring Boot supports Jetty and Undertow as alternatives.
Can we deploy Spring Boot as a WAR file?
Yes. Although embedded servers are common, WAR deployment is still supported.
Does Spring Boot require XML configuration?
No. Most applications can be configured using annotations and properties files.
Conclusion
Spring Boot simplifies enterprise application development by combining auto-configuration, embedded servers, dependency injection and production-ready features. Understanding its architecture, component interactions and request lifecycle helps developers build scalable applications and answer Spring Boot interview questions confidently.
Keywords
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