Spring Boot

Spring Boot simplifies Spring application development with auto-configuration, embedded servers, and opinionated defaults. It eliminates boilerplate XML configuration. Key features: starter dependencies, application.properties/application.yml, actuator for monitoring, and the @SpringBootApplication annotation (combines @Configuration, @EnableAutoConfiguration, @ComponentScan).

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@SpringBootApplication

@SpringBootApplication combines three annotations: @Configuration (marks as config class), @EnableAutoConfiguration (auto-configures beans based on classpath), @ComponentScan (scans current package + sub-packages). The main() method calls SpringApplication.run() which bootstraps the embedded server, creates the application context, and starts the app.

Deep Dive: What It Replaces

@SpringBootApplication  // Equivalent to:
// @Configuration
// @EnableAutoConfiguration
// @ComponentScan
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

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Auto-Configuration

Spring Boot automatically configures beans based on what's on the classpath. It reads META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports (Boot 3), and each auto-config class uses @Conditional* annotations to create beans only if conditions are met. Example: adding spring-boot-starter-web auto-configures Tomcat, DispatcherServlet, and Jackson. Exclude with @SpringBootApplication(exclude = {...}).

Deep Dive: How It Works

1. @EnableAutoConfiguration triggers configuration imports
2. Each auto-config class has @Conditional* annotations
3. Beans are created only if conditions are met

Example: spring-boot-starter-web on classpath → Tomcat + DispatcherServlet + error handling + Jackson ObjectMapper

// Exclude specific auto-config
@SpringBootApplication(exclude = {DataSourceAutoConfiguration.class})
public class MyApplication { }

Debug auto-config: debug=true in properties → prints conditions report on startup.

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Starter Dependencies

Starters are curated dependency bundles — add one starter and it pulls in everything needed. No version management required (managed by parent POM). Examples: spring-boot-starter-web (Tomcat + MVC + Jackson), spring-boot-starter-data-jpa (Hibernate + HikariCP), spring-boot-starter-test (JUnit 5 + Mockito + AssertJ).

Deep Dive: Common Starters

Starter	What It Brings
spring-boot-starter-web	Tomcat, Spring MVC, Jackson
spring-boot-starter-data-jpa	Hibernate, Spring Data JPA, HikariCP
spring-boot-starter-security	Spring Security, auto-config
spring-boot-starter-test	JUnit 5, Mockito, AssertJ, MockMvc
spring-boot-starter-actuator	Health checks, metrics, monitoring
spring-boot-starter-validation	Hibernate Validator

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Configuration Properties

Externalize config via application.yml or application.properties. Use @ConfigurationProperties(prefix = "app") for type-safe binding to POJOs. Resolution order (highest priority first): command-line args → SPRING_APPLICATION_JSON → application-{profile}.yml → application.yml → @PropertySource → defaults. Use profiles (application-dev.yml) for environment-specific config.

Deep Dive: Type-Safe Config & Profiles

# application.yml
app:
  name: MyApp
  max-retry: 3
  allowed-origins:
    - http://localhost:3000
    - https://myapp.com

@ConfigurationProperties(prefix = "app")
@Component
public class AppProperties {
    private String name;
    private int maxRetry;
    private List<String> allowedOrigins;
    // getters and setters
}

Profile-specific:

# application-dev.yml
spring.datasource.url: jdbc:h2:mem:testdb

# application-prod.yml
spring.datasource.url: jdbc:postgresql://prod-db:5432/app

Activate: spring.profiles.active=dev

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Embedded Server

Spring Boot embeds a web server (Tomcat by default) — no WAR deployment needed. Run as a self-contained JAR. Switch to Jetty or Undertow by excluding spring-boot-starter-tomcat and adding the alternative. Customize via server.port, server.servlet.context-path, server.tomcat.max-threads.

Deep Dive: Switching Servers

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-jetty</artifactId>
</dependency>

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Actuator

Spring Boot Actuator provides production-ready features: health checks (/actuator/health), metrics (/actuator/metrics), environment info, bean listings, request mappings. Expose endpoints via management.endpoints.web.exposure.include. Create custom health indicators by implementing HealthIndicator.

Deep Dive: Endpoints & Custom Health

Endpoint	Description
/actuator/health	Application health status
/actuator/info	Application info
/actuator/metrics	JVM, HTTP, custom metrics
/actuator/env	Environment properties
/actuator/beans	All registered beans
/actuator/mappings	All request mappings

@Component
public class DatabaseHealthIndicator implements HealthIndicator {
    @Override
    public Health health() {
        if (isDatabaseHealthy()) {
            return Health.up().withDetail("database", "reachable").build();
        }
        return Health.down().withDetail("database", "unreachable").build();
    }
}

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Common Interview Questions

Common Interview Questions

• What is Spring Boot? How is it different from Spring?
• What does @SpringBootApplication do?
• How does auto-configuration work?
• How do you disable a specific auto-configuration?
• What are starter dependencies?
• How does Spring Boot externalize configuration?
• What is the property resolution order?
• How do you switch from Tomcat to Jetty?
• What is Spring Boot Actuator? Name some endpoints.
• How do you create a custom health indicator?
• How do you use profiles in Spring Boot?