Master Spring Boot Interview Questions

Introduction to Spring Boot

Spring Boot is a high-level framework that simplifies the development of Java applications. First released in 2014, it is built on top of the Spring framework and is widely recognized for its emphasis on rapid development, scalability, and ease of use. Spring Boot simplifies the process of setting up and deploying applications with features like embedded servers, a powerful dependency injection mechanism, auto-configuration, and comprehensive REST API support. By adhering to the principle of convention over configuration and incorporating strong security practices, Spring Boot effectively addresses common web development issues such as dependency management, configuration complexity, and application security.

Table of Contents

• Junior-Level Spring Boot Interview Questions
• Mid-Level Spring Boot Interview Questions
• Expert-Level Spring Boot Interview Questions
• Proven Strategies and Best Practices for Spring Boot Interview

Here are some junior-level interview questions for Spring Boot:

Question 01: What is Spring Boot and what are its main features?

Answer: Spring Boot is an open-source Java-based framework used to create stand-alone, production-grade Spring-based applications. It simplifies the configuration and deployment of Spring applications by providing default configurations, embedded servers (like Tomcat), and a wide range of plugins. Its main features include:

• Spring Boot automatically configures your application based on the dependencies you add to the project. This reduces the need for manual configuration and boilerplate code.
• Spring Boot applications are standalone, meaning they do not require a separate application server. They embed Tomcat, Jetty, or Undertow directly, making deployment easier.
• It simplifies dependency management through its built-in dependency resolver. You declare dependencies and Spring Boot manages versions to avoid conflicts.
• Spring Boot Actuator provides production-ready features to help monitor and manage your application. It includes endpoints for health checks, metrics, logging configuration, etc.

Question 02: What is the role of the @SpringBootApplication annotation?

Answer: The @SpringBootApplication annotation is a convenience annotation that combines three important annotations: @EnableAutoConfiguration, @ComponentScan, and @Configuration. It is used to mark a configuration class that declares one or more @Bean methods and triggers auto-configuration and component scanning. This annotation simplifies the setup of a Spring Boot application. For example:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

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

Question 03: Explain the concept of auto-configuration in Spring Boot.

Answer: Auto-configuration is a key feature of Spring Boot that automatically configures your Spring application based on the dependencies present on the classpath. It eliminates the need for manual bean configuration in many cases by using sensible defaults for a wide range of use cases.

Spring Boot achieves auto-configuration by using the @EnableAutoConfiguration annotation, which looks for spring.factories files in META-INF directories. These files contain a list of @Configuration classes that Spring Boot should use for auto-configuration. Developers can override these defaults by providing their own bean definitions or by customizing configuration properties.

Question 04: What will be the output of the following code?

@RestController
public class MyController {

    @GetMapping("/hello")
    public String hello() {
        return "Hello, Spring Boot!";
    }
}

Answer: When a GET request is made to the /hello endpoint, the output will be:

Hello, Spring Boot!

Question 05: What is Spring Boot Actuator and what are its uses?

Answer: Spring Boot Actuator provides a set of production-ready features that help you monitor and manage your application. These features include health checks, metrics, info, environment properties, and more, all accessible via HTTP endpoints.

Actuator endpoints can be used to monitor the state of your application, track performance metrics, and gather operational data. This helps in diagnosing issues, understanding application behavior, and ensuring the application is running as expected in a production environment.

Question 06: What is the use of application.yml in a Spring Boot application?

Answer: The application.yml file in a Spring Boot application is used for configuration. It allows you to define properties that configure various aspects of the application, such as database settings, server ports, logging levels, and other application-specific parameters. For example:

server:
  port: 8080

spring:
  datasource:
    url: jdbc:mysql://localhost:3306/mydb
    username: user
    password: pass

logging:
  level:
    org.springframework: INFO

Question 07: What is the use of the @RestController annotation in Spring Boot?

Answer:The @RestController annotation in Spring Boot designates a class as a RESTful web service controller. It combines @Controller and @ResponseBody, automatically serializing return values into JSON or XML. For Example:

@RestController
public class MyController {
    
    @GetMapping("/hello")
    public String sayHello() {
        return "Hello, World!";
    }
}                        

Question 08: What is @RequestMapping annotation in Spring Boot?

Answer: The @RequestMapping annotation in Spring Boot is used to map HTTP requests to handler methods in a controller. It specifies the URL pattern, request method, and other parameters for routing requests. For example:

@RequestMapping("/greet")
public String greet() {
    return "Hello, World!";
}

Question 09: What is the error in the following Spring Boot configuration class?

@Configuration
public class AppConfig {

    @Bean
    public DataSource dataSource() {
        return new DataSource();
    }
}                        

Answer: The error is that DataSource is an interface and cannot be instantiated directly. You need to use a concrete implementation, like HikariDataSource or DriverManagerDataSource, depending on your setup:

@Bean
public DataSource dataSource() {
    return new DriverManagerDataSource();
}

Question 10: Explain the concept of dependency injection in Spring Boot.

Answer: Dependency Injection (DI) is a design pattern used to implement Inversion of Control (IoC), allowing the creation of dependent objects outside of a class and providing those objects to a class through various means. In Spring Boot, DI helps in managing object dependencies and wiring beans automatically.

DI allows you to inject the required dependencies without creating them manually. This makes the code more modular, testable, and easier to maintain. Spring Boot supports several DI techniques, including constructor injection, setter injection, and field injection using the @Autowired annotation.

Here are some mid-level interview questions for Spring Boot:

Question 01: Explain the difference between @Component, @Service, @Repository, and @Controller annotations.

Answer: In Spring, @Component is a generic stereotype annotation used to indicate that a class is a Spring-managed component. @Service is a specialization of @Component that is typically used to annotate service layer classes. @Repository is another specialization of @Component and is used to annotate DAO (Data Access Object) classes. @Controller is used to define a web controller in the MVC pattern, handling HTTP requests and responses.

Each of these annotations serves a specific role in the application context but ultimately, they are all detected by component scanning and registered as beans in the Spring container.

Question 02: How can you customize error handling in Spring Boot applications?

Answer: In Spring Boot, error handling can be customized using several approaches. One common method is to use the @ControllerAdvice annotation to create a global exception handler. This allows you to define methods to handle specific exceptions or all exceptions globally. For example:

@ControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(Exception.class)
    public ResponseEntity handleException(Exception ex) {
        return new ResponseEntity<>("An error occurred: " + ex.getMessage(), HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

Question 03: What will be the output of the following code snippet?

@Service
public class MyService {
    @Value("${my.property}")
    private String property;

    public String getProperty() {
        return property;
    }
}

Answer: The property field in the MyService class will be injected with the value of my.property from the application properties file (application.properties or application.yml). If my.property is not defined in the properties file, property will be null, and calling getProperty() will return null.

Question 04: How can you enable Actuator endpoints in a Spring Boot application?

Answer: To enable Actuator endpoints in a Spring Boot application, you need to include the spring-boot-starter-actuator dependency in your pom.xml or build.gradle file. Actuator provides production-ready features like metrics, health checks, and application information. You can configure which Actuator endpoints are enabled or disabled in the application.properties or application.yml file:

management.endpoints.web.exposure.include=health,info

Question 05: What is @Bean annotation in Spring Boot?

Answer: The @Bean annotation in Spring Boot is used to define a bean in the application context. It indicates that a method produces a bean to be managed by the Spring container, allowing for custom configuration and instantiation of beans. For example:

@Configuration
public class AppConfig {

    @Bean
    public MyService myService() {
        return new MyService();
    }
}

Question 06: How do you configure a Spring Boot application to use an external database?

Answer: To configure a Spring Boot application to use an external database, you need to provide database connection properties in the application.properties or application.yml file. Common properties include spring.datasource.url, spring.datasource.username, and spring.datasource.password. You also need to include the appropriate database driver dependency in your build file. Example configuration in application.properties:

spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=pass
spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=true

Question 07: What is the purpose of the @EnableAutoConfiguration annotation?

Answer: The @EnableAutoConfiguration annotation is used in Spring Boot to enable the auto-configuration mechanism, which automatically configures Spring application context based on the dependencies present in the classpath. It helps in setting up default configurations and beans based on the application's requirements, reducing the need for manual configuration.

This annotation is typically included in the main application class annotated with @SpringBootApplication.

Question 08: What is the role of @Qualifier annotation in Spring Boot?

Answer: The @Qualifier annotation is used to resolve ambiguity when multiple beans of the same type exist in the Spring application context. It helps to specify which bean should be injected when there are multiple candidates. For example:

@Autowired
@Qualifier("specificBean")
private MyBean myBean;

Question 09: What is the error in the following code snippet?

@RestController
public class MyController {
    @GetMapping("/data")
    public ResponseEntity getData() {
        return new ResponseEntity<>(HttpStatus.OK);
    }
}

Answer: The error is that the ResponseEntity is missing a body. The constructor used here only sets the HTTP status, so the response body will be null. To include a body, it should be:

return new ResponseEntity<>("Data", HttpStatus.OK);

Question 10: How does Spring Boot handle logging, and what are the default logging levels?

Answer: Spring Boot uses the spring-boot-starter-logging dependency, which includes Logback as the default logging framework. Logging configuration can be customized using application.properties or application.yml files.

The default logging levels are ERROR, WARN, INFO, DEBUG, and TRACE, with INFO being the default level. You can adjust the logging level for specific packages or classes by setting properties like logging.level.org.springframework.web=DEBUG.

Here are some expert-level interview questions for Spring Boot:

Question 01: How can you handle exceptions globally in a Spring Boot application?

Answer: You can handle exceptions globally in a Spring Boot application using the @ControllerAdvice annotation. This annotation allows you to define a global exception handler that can catch and process exceptions thrown by any @Controller or @RestController. For example:

@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(Exception.class)
    public ResponseEntity handleException(Exception e) {
        return new ResponseEntity<>(e.getMessage(), HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

Question 02:How does Spring Boot support embedded databases?

Answer: Spring Boot supports embedded databases like H2, Derby, and HSQLDB through its autoconfiguration mechanism. When you include a dependency for an embedded database, Spring Boot automatically configures it for you.

For instance, including the spring-boot-starter-data-jpa and h2 dependencies in your pom.xml will auto-configure an H2 database with default settings. You can then use it without additional configuration.

Question 03: How does Spring Boot support integration with messaging systems like RabbitMQ or Kafka?

Answer: Spring Boot provides integration support for messaging systems through starters and auto-configuration. For RabbitMQ, you can include spring-boot-starter-amqp, and for Kafka, you can use spring-boot-starter-kafka. These starters configure the necessary components for connecting and interacting with the messaging systems.

You can then use @RabbitListener for RabbitMQ and @KafkaListener for Kafka to handle messages from queues or topics.

Question 04: How can you customize the banner in a Spring Boot application?

Answer: You can customize the banner in a Spring Boot application by creating a banner.txt file in the src/main/resources directory. This file can contain ASCII art or any text you want to display during application startup. For example, you can add:

  _____ _                 _       
 / ____| |               | |      
| (___ | |_ __ ___  _ __ | |_ ___ 
 \___ \| | '_ ` _ \| '_ \| __/ _ \
 ____) | | | | | | | |_) | ||  __/
|_____/|_|_| |_| |_| .__/ \__\___|
                   | |              
                   |_|              

Question 05: Explain how Spring Boot supports scheduling tasks.

Answer: Spring Boot supports scheduling tasks through the @Scheduled annotation, which allows you to define tasks that should be executed at fixed intervals or according to a cron expression. To enable scheduling, you need to add @EnableScheduling to one of your configuration classes. For example:

@EnableScheduling
@Configuration
public class AppConfig {

    @Scheduled(fixedRate = 5000)
    public void scheduledTask() {
        System.out.println("Task executed every 5 seconds");
    }
}

Question 06: How can you configure Spring Boot to support multiple datasources?

Answer: To configure multiple datasources in Spring Boot, you need to define separate DataSource beans and configure each with its own properties. You can use @Primary to indicate the default datasource and use @Qualifier to specify which datasource to use in different components. For example:

@Configuration
public class DataSourceConfig {

    @Bean
    @Primary
    @ConfigurationProperties(prefix = "spring.datasource.primary")
    public DataSource primaryDataSource() {
        return DataSourceBuilder.create().build();
    }

    @Bean
    @ConfigurationProperties(prefix = "spring.datasource.secondary")
    public DataSource secondaryDataSource() {
        return DataSourceBuilder.create().build();
    }
}

Question 07: How does Spring Boot handle application configuration profiles?

Answer: Spring Boot handles application configuration profiles using the @Profile annotation and the spring.profiles.active property. By defining beans with the @Profile annotation, you can specify which beans should be loaded for different profiles. For example:

@Configuration
@Profile("dev")
public class DevConfig {
    // Beans specific to the dev profile
}

Question 08: How can you set up Spring Boot to use asynchronous methods?

Answer: To use asynchronous methods in Spring Boot, you need to enable asynchronous processing by adding @EnableAsync to a configuration class and then use the @Async annotation on methods you want to run asynchronously. For example:

@Configuration
@EnableAsync
public class AppConfig {
}
@Service
public class AsyncService {

    @Async
    public CompletableFuture asyncMethod() {
        // Perform asynchronous task
        return CompletableFuture.completedFuture("result");
    }
}

Question 09: How can you configure Spring Boot to work with a message broker like RabbitMQ?

Answer: To work with RabbitMQ, include the spring-boot-starter-amqp dependency and configure the connection details in application.properties. You can then use RabbitTemplate to send and receive messages. For example, in application.properties:

spring.rabbitmq.host=localhost
spring.rabbitmq.port=5672
                        

@Service
public class MessagingService {

    @Autowired
    private RabbitTemplate rabbitTemplate;

    public void sendMessage(String message) {
        rabbitTemplate.convertAndSend("exchange", "routingKey", message);
    }
}                        

Question 10: Explain how you can implement caching in Spring Boot.

Answer: To implement caching in Spring Boot, you need to enable caching using @EnableCaching and then use @Cacheable, @CachePut, or @CacheEvict annotations. For example:

@Configuration
@EnableCaching
public class CacheConfig {
}

@Service
public class MyService {

    @Cacheable("items")
    public String getItem(String id) {
        // Method implementation
        return "item";
    }
}