Spring Boot Interview Questions: Ultimate Guide 2023

Welcome to our comprehensive guide on Spring Boot interview questions! Spring Boot has gained immense popularity in the Java ecosystem due to its simplicity and productivity enhancements for developing Java applications. Whether you are a beginner exploring Spring Boot or an experienced developer preparing for an interview, this article will provide you with a curated list of commonly asked Spring Boot interview questions to help you ace your interviews.

Table of Contents

In this guide, we have categorized the Spring Boot interview questions into three levels: Basic, Intermediate, and Advanced. The questions cover various aspects of Spring Boot, including its fundamentals, core features, best practices, and advanced topics. By going through these questions, you’ll not only solidify your understanding of Spring Boot but also gain insights into its real-world application and challenges.

Basic Spring Boot Interview Questions

What is Spring Boot ?

Spring Boot is an open-source Java framework built on top of the Spring framework.Spring Boot aims to make it easier to create stand-alone, production-ready applications with minimal setup and configuration.

What are the major advantages of Spring Boot ?

Spring Boot offers several significant advantages for Java application development. Some of the key advantages include:

Simplified Configuration: Spring Boot eliminates the need for manual configuration by providing sensible defaults and auto-configuration. It reduces the boilerplate code and enables developers to focus more on business logic.
Rapid Application Development: Spring Boot provides a range of productivity-enhancing features, such as embedded servers, automatic dependency management, and hot reloading. These features accelerate development and reduce time-to-market.
Opinionated Approach: Spring Boot follows an opinionated approach, providing predefined conventions and best practices. It promotes consistency and reduces the cognitive load of making configuration decisions.
Microservices-friendly: Spring Boot seamlessly integrates with Spring Cloud, facilitating the development of microservices architectures. It offers built-in support for service discovery, distributed configuration, load balancing, and more.
Production-Ready Features: Spring Boot Actuator provides a set of production-ready features for monitoring, health checks, metrics, and security. It allows developers to gain insights into application performance and monitor system health easily.
Embedded Servers: Spring Boot comes with embedded servers like Tomcat, Jetty, and Undertow. This eliminates the need for manual server setup and configuration.
Auto-Configuration: Spring Boot’s auto-configuration feature automatically configures the application based on classpath dependencies. It simplifies the setup process and reduces the manual configuration effort.
Community Support: Spring Boot has a large and active community of developers. This means there is a wealth of resources, documentation, and community support available for troubleshooting and sharing best practices.
Ecosystem Integration: Spring Boot seamlessly integrates with other Spring projects and third-party libraries. It leverages the powerful Spring ecosystem, allowing developers to utilize a wide range of tools and libraries for various purposes.
Testability: Spring Boot provides excellent support for testing, including unit testing, integration testing, and end-to-end testing. It offers features like test slices, mock objects, and easy configuration for different testing frameworks.

What are the Key Components of Spring Boot?

Spring Boot incorporates several key components that work together to provide a streamlined and efficient development experience. The major components of Spring Boot are:

Auto-configuration: Spring Boot’s auto-configuration feature automatically configures the application based on the dependencies detected on the classpath. It eliminates the need for manual configuration and reduces boilerplate code.
Starter Dependencies: Spring Boot provides a set of starter dependencies, which are pre-packaged dependencies that facilitate the configuration of common use cases. Starters simplify dependency management and help developers get started quickly with essential features such as web applications, data access, security, testing, and more.
Embedded Servers: Spring Boot includes embedded servlet containers like Tomcat, Jetty, and Undertow. These embedded servers allow developers to package the application as an executable JAR file, simplifying deployment and making the application self-contained.
Spring Boot Actuator: Spring Boot Actuator provides production-ready features for monitoring and managing the application. It offers endpoints for health checks, metrics, logging, tracing, and more. Actuator enables easy monitoring and management of the application in a production environment.
Spring Boot CLI: The Spring Boot Command-Line Interface (CLI) allows developers to interact with Spring Boot applications using a command-line interface. It provides a convenient way to quickly prototype, develop, and test Spring Boot applications without the need for complex setup and configuration.
Spring Boot DevTools: Spring Boot DevTools is a set of tools that enhance the development experience. It includes features like automatic application restart, live reload of static resources, and enhanced error reporting. DevTools significantly improves developer productivity and speeds up the development process.
Spring Boot Testing: Spring Boot provides excellent support for testing applications. It offers various testing utilities, including test slices, mock objects, and easy configuration for different testing frameworks. Spring Boot Testing makes it easier to write and execute tests for Spring Boot applications.

What are the differences between Spring and Spring Boot?

Here’s a comparison between Spring and Spring Boot:

Feature	Spring	Spring Boot
Configuration	Requires manual configuration	Provides auto-configuration and sensible defaults
Dependency Management	Manual dependency management	Simplified dependency management with starters
XML Configuration	Relies heavily on XML configuration	Encourages the use of annotations and Java configuration
Embedded Servers	Requires manual setup and configuration	Includes embedded servers for easy deployment
Auto-configuration	Limited auto-configuration capabilities	Powerful auto-configuration for rapid development
Development Time	Longer development setup and configuration	Faster development with out-of-the-box defaults
Convention Over Configuration	Emphasizes configuration	Emphasizes convention over configuration
Microservices Support	Supports microservices architecture	Provides seamless integration with Spring Cloud
Testing Support	Strong testing support with Spring Test	Enhanced testing support with Spring Boot Test
Actuator	Actuator available as a separate module	Actuator integrated into the core framework

It’s important to note that Spring and Spring Boot are not mutually exclusive. Spring Boot is built on top of the Spring framework and provides additional capabilities to simplify and accelerate Spring application development.

What is the purpose of the `@SpringBootApplication` annotation?

The @SpringBootApplication annotation combines three other commonly used annotations: @Configuration, @EnableAutoConfiguration, and @ComponentScan. This single annotation allows for concise and streamlined configuration of the application.

The @SpringBootApplication annotation is typically placed on the main class of a Spring Boot application. It acts as an entry point for the application and bootstraps the Spring Boot runtime environment.

What are Spring Boot starters ?

Spring Boot starters are curated collections of pre-configured dependencies that simplify the setup and configuration of specific functionalities in a Spring Boot application. They provide the necessary dependencies, sensible default configurations, and auto-configuration.

For instance, the spring-boot-starter-web starter includes dependencies for web-related libraries and provides default configurations for handling web requests. Starters streamline dependency management and ensure that the required components work together seamlessly.

By including a starter in your project, you save time and effort by avoiding manual configuration and gain the benefits of Spring Boot’s opinionated approach to application development.

Provide some examples of commonly used Spring Boot Starters

Here are some examples of commonly used Spring Boot starters:

spring-boot-starter-web: This starter is used for developing web applications with Spring MVC. It includes dependencies for handling HTTP requests, managing web sessions, and serving static resources.
spring-boot-starter-data-jpa: This starter provides support for data access using Java Persistence API (JPA) with Hibernate as the default implementation. It includes dependencies for database connectivity, entity management, and transaction management.
spring-boot-starter-security: This starter is used for adding security features to a Spring Boot application. It includes dependencies for authentication, authorization, and secure communication.
spring-boot-starter-test: This starter is used for writing unit tests and integration tests in a Spring Boot application. It includes dependencies for testing frameworks like JUnit, Mockito, and Spring Test.
spring-boot-starter-actuator: This starter adds production-ready features to monitor and manage the application. It includes endpoints for health checks, metrics, logging, and more.
spring-boot-starter-data-redis: This starter is used for working with Redis, an in-memory data store. It includes dependencies for connecting to Redis server and performing data operations.
spring-boot-starter-amqp: This starter provides support for messaging with Advanced Message Queuing Protocol (AMQP). It includes dependencies for messaging components like RabbitMQ.
spring-boot-starter-mail: This starter is used for sending emails in a Spring Boot application. It includes dependencies for email-related functionalities.
spring-boot-starter-cache: This starter provides support for caching data in a Spring Boot application. It includes dependencies for caching frameworks like Ehcache and Redis.
spring-boot-starter-oauth2-client: This starter is used for implementing OAuth 2.0 client functionality in a Spring Boot application. It includes dependencies for interacting with OAuth 2.0 authorization servers.

The entire list of Spring Boot starters can be found on the official Spring Boot website. Here’s the link to the official Spring Boot Starters documentation: Spring Boot Starters.

What is the default embedded server used by Spring Boot?

The default embedded server used by Spring Boot is Apache Tomcat. Spring Boot includes Tomcat as a dependency and automatically configures it as the default embedded server when you use the spring-boot-starter-web starter or any other web-related starters.

How do you configure properties in a Spring Boot Application?

In a Spring Boot application, properties can be configured using various methods. Here are the commonly used approaches:

application.properties or application.yml file: Spring Boot allows you to define configuration properties in an application.properties file (for properties in key-value format) or an application.yml file (for properties in YAML format).
Command-line arguments: Spring Boot supports configuring properties using command-line arguments. You can pass properties as command-line arguments in the format --property=value when running the application. For example: java -jar myapp.jar --server.port=8080.
Environment variables: Spring Boot can read properties from environment variables. You can define environment variables with property names and values, and Spring Boot will automatically map them to the corresponding properties.
System properties: Spring Boot also supports configuring properties using system properties. You can pass system properties to the application using the -D flag when running the application. For example: java -jar myapp.jar -Dserver.port=8080.
@ConfigurationProperties annotation: Spring Boot provides the @ConfigurationProperties annotation, which allows you to bind external properties directly to Java objects. You can define a configuration class annotated with @ConfigurationProperties and specify the prefix of the properties to be bound. Spring Boot will automatically map the properties to the corresponding fields of the configuration class.

What is Spring Boot Auto-configuration, and how does it work?

Spring Boot Auto-configuration automatically configures the application context based on the classpath dependencies, reducing the need for manual configuration. It scans the classpath for required libraries and sets up the necessary beans and components. Auto-configuration follows predefined rules and uses annotations like @ConditionalOnClass and @ConditionalOnMissingBean to enable configurations selectively. By adding the @EnableAutoConfiguration annotation to the main application class, Spring Boot triggers the auto-configuration process. Auto-configuration classes are typically packaged in starters, which contain the necessary configuration classes and dependencies. Including a starter in your project enables Spring Boot to automatically configure relevant components.

The benefits of Spring Boot Auto-configuration include:

Reduced boilerplate code: Auto-configuration eliminates the need for manual configuration, reducing the amount of boilerplate code required to set up common functionalities.
Opinionated defaults: Auto-configuration provides sensible defaults and conventions based on best practices. This allows developers to quickly get started with Spring Boot projects without spending time on manual configuration.
Integration with third-party libraries: Auto-configuration seamlessly integrates with popular libraries and frameworks, automatically configuring the necessary beans and components required for their usage.
Conditional configuration: Auto-configuration applies configurations conditionally based on the presence or absence of specific classes or beans. This ensures that conflicting configurations are avoided and only relevant configurations are applied.

How can you create a Spring Boot application using Spring Initializer?

Creating a Spring Boot application is made easy by utilizing the Spring Initializr. The Spring Initializr is a web-based tool that generates a project template with all the necessary dependencies and configurations for a Spring Boot application.

To create a Spring Boot application easily using the Spring Initializr, you can follow these steps:

Visit the Spring Initializr Website: Go to the official Spring Initializr website at start.spring.io.
Configure Project Settings: On the Spring Initializr website, you’ll find various options to configure your project. Provide the following details:
- Project: Select the project type (Maven or Gradle).
- Language: Choose the programming language (Java or Kotlin).
- Spring Boot Version: Select the desired version of Spring Boot.
- Project Metadata: Specify the group, artifact, and package name for your project.
Add Dependencies: In the “Dependencies” section, you can search for and select the dependencies you need for your project. The Spring Initializr provides a wide range of options, such as Spring Web, Spring Data JPA, Spring Security, and more. You can also search for specific dependencies in the search bar.
Generate the Project: Once you’ve configured the project settings and added the desired dependencies, click on the “Generate” button. The Spring Initializr will generate a downloadable project archive (a ZIP file) based on your selections.
Extract the Project: Download the generated ZIP file and extract it to your desired location on your computer.
Import the Project in your IDE: Open your preferred IDE (e.g., IntelliJ IDEA, Eclipse, or Visual Studio Code) and import the extracted project as a Maven or Gradle project.
Start Developing: With the project imported, you can start developing your Spring Boot application. Add your application logic, create controllers, services, repositories, and other necessary components to implement your desired functionality.
Run the Application: Use your IDE’s run configuration or command-line tools to run the Spring Boot application. The application will start, and you can access it using the provided URLs or endpoints.

What are Spring Boot Actuators ? What are different Actuator Endpoints?

Spring Boot Actuators are a set of production-ready management and monitoring tools provided by the Spring Boot framework. They enable you to monitor and interact with your Spring Boot application at runtime, providing valuable insights into its health, metrics, and various other aspects.

Actuators expose a set of RESTful endpoints that allow you to access useful information and perform certain operations on your Spring Boot application. Some of the commonly used endpoints include:

/actuator/health: Provides information about the health of your application, indicating whether it is up and running or experiencing any issues.
/actuator/info: Displays general information about your application, which can be customized to include details such as version, description, and other relevant metadata.
/actuator/metrics: Provides metrics about various aspects of your application, such as memory usage, CPU usage, request/response times, and more. These metrics can be helpful for monitoring and performance analysis.
/actuator/env: Shows the current environment properties and their values, including configuration properties from external sources like application.properties or environment variables.
/actuator/loggers: Allows you to view and modify the logging levels of your application’s loggers dynamically. This can be useful for troubleshooting and debugging purposes.
/actuator/mappings: Displays a detailed mapping of all the endpoints exposed by your application, including the HTTP methods supported by each endpoint.
/actuator/beans: Provides a complete list of all the Spring beans in your application, including information such as their names, types, and dependencies.

Explain the concept of Spring Boot profiles and how they can be used?

Spring Boot profiles provide a way to manage application configurations for different environments or deployment scenarios. With profiles, you can define different sets of configurations for development, testing, production, and any other specific environment.

Here’s a brief explanation of how Spring Boot profiles work and how they can be used:

Defining Profiles: In a Spring Boot application, you can define profiles by creating separate properties files for each environment. For example, you can have application-dev.properties for the development environment, application-test.properties for testing, and application-prod.properties for production.
Activating Profiles: Profiles can be activated in various ways:
- By setting the spring.profiles.active property in application.properties or as a command-line argument when starting the application.
- By using the @ActiveProfiles annotation at the class level in your tests.
- By using system environment variables or JVM system properties to specify the active profiles.
Profile-Specific Configurations: Once a profile is activated, Spring Boot will load the corresponding property files and apply the configurations defined in them. For example, if the dev profile is active, Spring Boot will load the application-dev.properties file and apply the configurations defined within it.
Overriding Configurations: Profile-specific configurations can override the default configurations defined in application.properties or other property files. This allows you to customize certain settings specifically for each environment without modifying the core application code.
Bean and Component Scanning: Profiles can also be used to control the bean and component scanning process. You can annotate beans or components with @Profile to specify that they should only be created and registered when a specific profile is active.
Spring Boot’s Default Profiles: Spring Boot provides some default profiles such as default, dev, test, and prod. The default profile is always active by default, and other profiles can be activated based on the deployment environment.

What are some commonly used Annotations in Spring Boot?

Here are some commonly used Spring Boot annotations:

@SpringBootApplication: This annotation is used to mark the main class of a Spring Boot application. It combines three annotations: @Configuration, @EnableAutoConfiguration, and @ComponentScan. It enables auto-configuration and component scanning in your application.
@RestController: This annotation is used to indicate that a class is a RESTful controller. It combines the @Controller and @ResponseBody annotations. It simplifies the development of RESTful web services by eliminating the need to annotate each method with @ResponseBody.
@RequestMapping: This annotation is used to map HTTP requests to specific handler methods. It can be applied at the class level to specify a base URL for all methods within the class, or at the method level to define the URL and HTTP method for a specific handler method.
@Autowired: This annotation is used to automatically wire dependencies into your Spring managed beans. It allows Spring to automatically discover and inject the required beans without the need for explicit bean configuration.
@Value: This annotation is used to inject values from properties files or environment variables into Spring beans. It can be used to inject simple values like strings or numbers, as well as complex objects.
@Configuration: This annotation is used to indicate that a class provides configuration to the Spring application context. It is often used in conjunction with @Bean to define beans and other configuration elements.
@ComponentScan: This annotation is used to specify the base packages to scan for Spring components, such as controllers, services, and repositories. It allows Spring to automatically discover and register these components in the application context.
@EnableAutoConfiguration: This annotation enables Spring Boot’s auto-configuration mechanism, which automatically configures various components and beans based on the dependencies and the classpath.
@Conditional: This annotation is used to conditionally enable or disable beans and configurations based on certain conditions. It allows you to customize the behavior of your application based on specific conditions or environment settings.

Intermediate Level Spring Boot Interview Questions

What is the use of @configurationProperties Annotation ?

The @ConfigurationProperties annotation in Spring Boot is used to bind external configuration properties to a Java class. It provides a convenient way to map the properties defined in configuration files (such as application.properties or application.yml) to corresponding fields in a configuration class.

The benefits of using @ConfigurationProperties include:

Type Safety: The annotation ensures that the configuration properties are bound to the appropriate types defined in the configuration class, preventing type mismatches and potential runtime errors.
Property Validation: You can validate the properties using various validation annotations provided by Spring, such as @NotNull, @Min, @Max, and custom validations.
Hierarchical Property Mapping: You can define nested configuration classes to represent complex configuration structures and map them hierarchically to the corresponding properties.
Easy Integration: The annotated configuration class can be easily autowired and used throughout the application, simplifying the retrieval of configuration values in different components.

Here’s an example of using @ConfigurationProperties:

@Configuration
@ConfigurationProperties(prefix = "myapp")
public class MyAppConfiguration {
    private String name;
    private int port;

    // Getters and setters

    // Other custom methods or business logic
}

# Database Configuration
spring.datasource.url=jdbc:mysql://localhost:3306/mydatabase
spring.datasource.username=myusername
spring.datasource.password=mypassword

# Server Configuration
server.port=8080
server.servlet.context-path=/myapp

# Custom Application Properties
myapp.name=My Application
myapp.api.key=abc123

In this example, the MyAppConfiguration class is annotated with @ConfigurationProperties and specifies a prefix of “myapp”. The properties defined with the prefix “myapp” in the configuration files will be bound to the corresponding fields in this class.

How does Spring Boot support Microservices Architecture?

Spring Boot provides extensive support for building microservices-based applications. It offers a range of features and integrations that simplify the development, deployment, and management of microservices. Here’s how Spring Boot supports the microservices architecture:

Spring Cloud: Spring Boot integrates seamlessly with Spring Cloud, which is a set of tools and frameworks designed to build and operate cloud-native microservices. Spring Cloud provides capabilities such as service discovery, client-side load balancing, distributed configuration management, circuit breakers, and more.
Microservice Design Patterns: Spring Boot embraces microservice design patterns, such as the use of RESTful APIs for communication between services, stateless services for scalability, and decentralized data management. It provides a lightweight and flexible framework that enables developers to implement these patterns easily.
Service Registration and Discovery: Spring Boot integrates with service registry and discovery tools, such as Netflix Eureka and Consul. These tools allow microservices to register themselves with the registry and discover other services dynamically. This helps in achieving service resilience, load balancing, and automatic service discovery.
Externalized Configuration: Spring Boot supports externalized configuration, allowing microservices to be easily configured based on the environment or specific deployment needs. It enables the separation of configuration from the code, making it easier to manage configuration properties across multiple microservices.
Distributed Tracing and Monitoring: Spring Boot integrates with distributed tracing systems like Zipkin and Sleuth, enabling the tracing of requests across multiple microservices. It also provides integrations with monitoring tools like Prometheus and Grafana to monitor the health, performance, and resource usage of microservices.
Resilience and Fault Tolerance: Spring Boot includes support for implementing fault-tolerant microservices using features such as circuit breakers (e.g., Netflix Hystrix), which help prevent cascading failures in distributed systems. It also provides mechanisms for handling retries, timeouts, and fallbacks in microservice interactions.
Containerization and Deployment: Spring Boot applications can be easily containerized using technologies like Docker, allowing for seamless deployment and scaling of microservices using container orchestration platforms like Kubernetes.

What is Spring Data? What are different Spring Data Starters used in Spring Boot?

Spring Data is a subproject of the Spring Framework that simplifies data access by providing a unified programming model for different data storage technologies. It reduces boilerplate code and allows developers to focus on business logic. Spring Data supports relational databases, NoSQL databases, and more. It utilizes repositories to abstract data access operations, eliminating the need for manual CRUD code. Spring Data’s starters offer pre-configured dependencies and auto-configuration for specific databases, streamlining the setup process. With Spring Data, developers can easily interact with data sources and benefit from its powerful query capabilities.

Here are some examples of Spring Data starters for different types of databases:

spring-boot-starter-data-jpa: This starter provides support for Java Persistence API (JPA) and Hibernate. It includes the necessary dependencies and configurations for working with relational databases using JPA.
spring-boot-starter-data-mongodb: This starter provides support for MongoDB, a popular NoSQL database. It includes the necessary dependencies and configurations for working with MongoDB using Spring Data MongoDB.
spring-boot-starter-data-redis: This starter provides support for Redis, an in-memory data structure store. It includes the necessary dependencies and configurations for working with Redis using Spring Data Redis.
spring-boot-starter-data-cassandra: This starter provides support for Apache Cassandra, a highly scalable NoSQL database. It includes the necessary dependencies and configurations for working with Cassandra using Spring Data Cassandra.
spring-boot-starter-data-elasticsearch: This starter provides support for Elasticsearch, a distributed search and analytics engine. It includes the necessary dependencies and configurations for working with Elasticsearch using Spring Data Elasticsearch.

How can you consume RESTful web services in a Spring Boot application?

In a Spring Boot application, you can consume RESTful web services using RestTemplate or WebClient. RestTemplate provides a synchronous API for making HTTP requests, while WebClient offers a non-blocking and reactive approach. Both allow you to send GET, POST, PUT, DELETE requests, handle response data, and deserialize it into Java objects.

How can you create and run unit tests for a Spring Boot application?

In a Spring Boot application, you can create and run unit tests using the Spring Test framework. By leveraging annotations such as @RunWith(SpringRunner.class) and @SpringBootTest, you can initialize the application context and perform tests on beans and components.

Additionally, you can use Mockito or other mocking frameworks to mock dependencies and isolate the units under test. With the help of assertions from JUnit or AssertJ, you can verify expected behavior and assertions.

Finally, running the tests can be done using tools like Maven or Gradle, which execute the tests and provide reports on test results and coverage.

How to enable debugging log in the spring boot application?

To enable debugging logs in a Spring Boot application, you can set the logging.level property in the application.properties or application.yml file to “DEBUG”. This configuration will enable the logging framework to output detailed debug information. Alternatively, you can use the @Slf4j annotation on a class to enable logging for that specific class. Additionally, you can configure logging levels for specific packages or classes by setting the logging.level.{package/class} property in the configuration file.

To enable debugging logs for the entire application, use:

logging.level.<package-name>=DEBUG

To enable debugging logs for the entire application, use:

logging.level.root=DEBUG

How reactive programming is supported in Spring Boot?

Spring Boot provides support for reactive programming through its integration with the Spring WebFlux module. It allows developers to build non-blocking, event-driven applications that can handle a high volume of concurrent requests efficiently, making use of reactive streams and the reactive programming model.

How do you enable Security in Spring Boot Application ?

We can use the follow different options.

Using Spring Security: Enable security by adding the Spring Security Starter dependency to your project’s build configuration.
OAuth2 and OpenID Connect: Enable security using OAuth2 and OpenID Connect protocols for secure authentication and authorization.
LDAP Integration: Enable security by integrating with an LDAP (Lightweight Directory Access Protocol) server for user authentication and authorization.
JWT (JSON Web Token) Authentication: Enable security by implementing JWT-based authentication for stateless and scalable authentication.
Custom Authentication Providers: Enable security by creating custom authentication providers to handle authentication based on your own logic.

What is the purpose of Spring Boot DevTools? How do we enable it?

It is designed to improve productivity, streamline development workflows, and enable quick application restarts during the development phase.

Here are some key features and benefits of Spring Boot DevTools:

Automatic Restart: DevTools monitors the classpath for any changes and automatically restarts the application when it detects modifications.
Live Reload: DevTools supports live reloading of static resources such as HTML, CSS, and JavaScript files.
Remote Application Restart: DevTools provides the ability to remotely trigger a restart of the application. This can be useful when working in a distributed environment or when the application is running on a remote server.
Developer-Friendly Error Page: DevTools provides an enhanced error page that provides detailed information about exceptions and errors encountered during application development.
Configuration Properties Support: DevTools enables hot-reloading of Spring Boot configuration properties. Changes made to application.properties or application.yml files are automatically applied without restarting the application, allowing for quick configuration updates.
Database Console: DevTools includes an embedded database console that provides a web-based interface to interact with the application’s database. This allows developers to easily execute SQL queries, view and modify data, and perform other database-related tasks without requiring external tools.

To enable Spring Boot DevTools in your Spring Boot application, you need to include the appropriate dependencies and configurations. Here are the steps to enable DevTools:

Add the DevTools Dependency: In your pom.xml file (for Maven) or build.gradle file (for Gradle), add the following dependency:

<!-- Maven -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
    <optional>true</optional>
</dependency>

// Gradle
implementation 'org.springframework.boot:spring-boot-devtools'

Enable Automatic Restart: By default, DevTools is enabled for applications run using the spring-boot:run command or from an IDE. However, you can also enable it for packaged applications by adding the following configuration property to your application.properties or application.yml file:

spring.devtools.restart.enabled=true

Advanced Level Spring Boot Interview Questions for Experienced Folks

How can you enable HTTPS in a Spring Boot Application?

To enable HTTPS in a Spring Boot application, you need to configure the application’s server properties and provide the necessary SSL/TLS certificates. Here are the general steps to enable HTTPS:

Obtain SSL/TLS Certificates: Acquire the SSL/TLS certificates from a trusted certificate authority (CA) or generate a self-signed certificate for development/testing purposes.
Configure Server Properties: Update the application.properties or application.yml file with the following server configurations:

server.port=8443
server.ssl.key-store-type=PKCS12
server.ssl.key-store=classpath:keystore.p12
server.ssl.key-store-password=your_keystore_password
server.ssl.key-alias=your_alias_name

In the above example, replace keystore.p12 with the path to your keystore file, and set the appropriate password and alias values.

Provide SSL/TLS Certificates: Place the SSL/TLS certificate file (keystore.p12) in the classpath or specify the absolute file path in the server properties.
Restart the Application: Restart your Spring Boot application for the changes to take effect.
After completing these steps, your Spring Boot application will be configured to use HTTPS. You can access the application using https://localhost:8443 or the appropriate hostname and port specified in the server configuration.

How to configure external configuration in Spring Boot ?

To configure external configuration outside the project in Spring Boot, you can use one of the following approaches:

External Configuration Files: Instead of placing the application.properties or application.yml file within the project, you can specify the location of an external configuration file using the spring.config.name and spring.config.location properties. For example, you can place the configuration file in a separate directory and provide its location through the command line or environment variable.
Operating System Environment Variables: You can leverage the environment variables provided by the operating system to configure your Spring Boot application. Define the required configuration properties as environment variables and access them in your application using the @Value annotation or the Environment object.
Spring Cloud Config: If you have a more complex configuration setup or need centralized configuration management, you can use Spring Cloud Config. It provides a server-side component where you can store and manage configurations for multiple applications. Your Spring Boot application can then fetch its configuration from the Spring Cloud Config server.
Configuration Servers: Another option is to use external configuration servers like Apache ZooKeeper or HashiCorp Consul. These servers act as central repositories for configurations and can be accessed by multiple applications.

How do you create a Spring Boot application using Maven ?

To create a Spring Boot application using Maven, follow these steps:

Set Up Maven: Ensure that Maven is installed on your system. You can download Maven from the Apache Maven website and follow the installation instructions.

Create a Maven Project: Open your command line or terminal and navigate to the directory where you want to create your project. Use the following Maven command to create a new project:

mvn archetype:generate -DgroupId=com.example -DartifactId=my-spring-boot-app -DarchetypeArtifactId=maven-archetype-quickstart -DinteractiveMode=false

This command creates a new Maven project with the specified groupId and artifactId. Adjust these values according to your project’s needs.

Add Spring Boot Starter Dependency: Open the pom.xml file of your project and add the following dependency for Spring Boot:

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
        <version>2.5.2</version>
    </dependency>
</dependencies>

This dependency includes the necessary Spring Boot libraries for your application.

Create a Spring Boot Main Class: Create a new Java class in the appropriate package of your Maven project. This class will serve as the entry point for your Spring Boot application. Here’s an example:

package com.example;

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

@SpringBootApplication
public class MySpringBootApplication {

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

The @SpringBootApplication annotation combines several annotations required for a basic Spring Boot application.

Build and Run the Application: Use the following Maven command to build and run your Spring Boot application:

mvn spring-boot:run

Maven will build your project, resolve the dependencies, and start the Spring Boot application.

Once the application is running, you can access it in your web browser using http://localhost:8080 (by default) or the specified port if you have customized it.

That’s it! You have created a Spring Boot application using Maven. You can now add additional dependencies, configure your application, and develop your desired functionality.

What are different types of Conditional Annotations?

Some of the commonly used conditional annotations in Spring Boot are:

@ConditionalOnClass: This annotation configures a bean or component if a specific class is present in the classpath.
@ConditionalOnMissingClass: This annotation configures a bean or component if a specific class is not present in the classpath.
@ConditionalOnBean: This annotation configures a bean or component only if another specific bean is present in the application context.
@ConditionalOnMissingBean: This annotation configures a bean or component only if another specific bean is not present in the application context.
@ConditionalOnProperty: This annotation configures a bean or component based on the values of specific properties in the application configuration files.
@ConditionalOnExpression: This annotation configures a bean or component based on a SpEL (Spring Expression Language) expression.
@ConditionalOnWebApplication: This annotation configures a bean or component only if the application is a web application.
@ConditionalOnNotWebApplication: This annotation configures a bean or component only if the application is not a web application.

Could you provide an explanation of Spring Boot Actuator health checks and the process for creating custom health indicators?

Health checks provide valuable information about the application’s overall health, such as database connectivity, external service availability, or any other custom checks you define.

By default, Spring Boot Actuator provides a set of predefined health indicators that check the health of various components like the database, disk space, and others. However, you can also create custom health indicators to monitor specific aspects of your application.

To create a custom health indicator, you need to implement the HealthIndicator interface and override the health() method. The health() method should return an instance of Health, which represents the health status of your custom component. You can use the Health class to indicate whether the component is up, down, or in an unknown state.

Here’s an example of a custom health indicator:

import org.springframework.boot.actuate.health.Health;
import org.springframework.boot.actuate.health.HealthIndicator;
import org.springframework.stereotype.Component;

@Component
public class CustomHealthIndicator implements HealthIndicator {

    @Override
    public Health health() {
        // Perform your custom health check logic here
        boolean isHealthy = true; // Replace with your actual health check logic

        if (isHealthy) {
            return Health.up().build();
        } else {
            return Health.down().withDetail("CustomComponent", "Not Healthy").build();
        }
    }
}

n this example, the CustomHealthIndicator class implements the HealthIndicator interface and overrides the health() method. Inside the health() method, you can write your custom health check logic. If the component is healthy, you can return Health.up(). Otherwise, you can return Health.down() along with additional details using the withDetail() method.

Once you create a custom health indicator, it will be automatically detected by Spring Boot Actuator, and its health check will be exposed through the /actuator/health endpoint.

How do you create custom Actuator endpoints in Spring Boot?

To create custom Actuator endpoints in Spring Boot, you can follow these steps:

Create a Custom Endpoint: Create a new class that represents your custom endpoint. This class should be annotated with @Endpoint to indicate that it is an Actuator endpoint. You can also use additional annotations like @ReadOperation, @WriteOperation, or @DeleteOperation to define the type of operations supported by your endpoint.
Define Endpoint Operations: Inside your custom endpoint class, define the operations that your endpoint should perform. You can use annotations like @ReadOperation for read-only operations, @WriteOperation for write operations, and @DeleteOperation for delete operations. These annotations help in defining the HTTP methods and request mappings for your endpoint.
Implement Endpoint Logic: Implement the logic for each operation of your custom endpoint. This can include retrieving information, modifying application state, or performing any other desired actions. You have the flexibility to define the functionality based on your specific requirements.
(Optional) Add Security Configuration: If your custom endpoint requires security restrictions, you can configure it by adding security annotations or by modifying the security configuration of your application.

Here’s an example of creating a custom Actuator endpoint:

import org.springframework.boot.actuate.endpoint.annotation.Endpoint;
import org.springframework.boot.actuate.endpoint.annotation.ReadOperation;
import org.springframework.boot.actuate.endpoint.annotation.WriteOperation;
import org.springframework.stereotype.Component;

@Component
@Endpoint(id = "customEndpoint")
public class CustomEndpoint {

    @ReadOperation
    public String getInformation() {
        // Retrieve and return information
        return "This is custom endpoint information.";
    }

    @WriteOperation
    public void updateInformation(String newInformation) {
        // Update information
        // ...
    }
}

In this example, the CustomEndpoint class is annotated with @Endpoint to define it as an Actuator endpoint. It has two operations: getInformation() annotated with @ReadOperation for retrieving information, and updateInformation() annotated with @WriteOperation for updating information.

After creating your custom endpoint, it will be automatically registered with Spring Boot Actuator, and you can access it through the /actuator base path along with the endpoint ID. In this case, the custom endpoint can be accessed via /actuator/customEndpoint.

How can you enable CORS (Cross-Origin Resource Sharing) in a Spring Boot application?

To enable Cross-Origin Resource Sharing (CORS) in a Spring Boot application, you can follow these steps:

Add CORS Configuration: Create a configuration class and annotate it with @Configuration to define CORS configuration. Inside the class, create a bean of type CorsFilter to configure CORS settings.

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.cors.CorsConfiguration;
import org.springframework.web.cors.UrlBasedCorsConfigurationSource;
import org.springframework.web.filter.CorsFilter;

@Configuration
public class CorsConfig {

    @Bean
    public CorsFilter corsFilter() {
        UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
        CorsConfiguration config = new CorsConfiguration();

        // Allow requests from any origin
        config.addAllowedOrigin("*");
        
        // Allow specific HTTP methods (e.g., GET, POST, PUT, DELETE)
        config.addAllowedMethod("*");
        
        // Allow specific HTTP headers
        config.addAllowedHeader("*");

        source.registerCorsConfiguration("/**", config);
        return new CorsFilter(source);
    }
}

In this example, we configure CORS to allow requests from any origin (*), allow all HTTP methods (*), and allow all HTTP headers (*). You can customize these settings based on your specific requirements.

Enable Web MVC Configuration: If you haven’t done so already, make sure to enable Web MVC configuration in your Spring Boot application by either using the @EnableWebMvc annotation on a configuration class or extending the WebMvcConfigurerAdapter class.

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.EnableWebMvc;

@Configuration
@EnableWebMvc
public class WebMvcConfig extends WebMvcConfigurerAdapter {

    // Additional Web MVC configuration if needed
}

Test CORS Configuration: Once you have enabled CORS in your Spring Boot application, you can test it by making cross-origin requests to your endpoints. Ensure that the necessary CORS headers are included in the response, such as Access-Control-Allow-Origin, Access-Control-Allow-Methods, and Access-Control-Allow-Headers.

Enabling CORS allows your Spring Boot application to handle cross-origin requests and respond appropriately. It’s important to consider the security implications and configure CORS settings based on your application’s requirements and security policies.

How can you schedule tasks in a Spring Boot application?

In a Spring Boot application, you can schedule tasks using the @Scheduled annotation provided by Spring’s Task Scheduling feature. Here’s how you can schedule tasks in a Spring Boot application:

Enable Scheduling: First, make sure that task scheduling is enabled in your Spring Boot application. This can be done by either using the @EnableScheduling annotation on a configuration class or by adding the @SpringBootApplication annotation along with @EnableScheduling on your main application class.

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.scheduling.annotation.EnableScheduling;

@SpringBootApplication
@EnableScheduling
public class MyAppApplication {

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

Create Scheduled Task Method: Define a method in your application that you want to schedule. Annotate the method with @Scheduled and specify the desired scheduling expression using cron, fixed delay, or fixed rate.

import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Component;

@Component
public class MyScheduledTasks {

    @Scheduled(cron = "0 0 8 * * *") // Run at 8 AM every day
    public void executeTask() {
        // Logic for the scheduled task
        System.out.println("Scheduled task executed!");
    }
}

In this example, the executeTask() method is scheduled to run at 8 AM every day based on the cron expression provided.

Test the Scheduled Task: Once you have defined the scheduled task, you can start your Spring Boot application and observe the scheduled task executing based on the specified schedule.

The @Scheduled annotation provides several options for specifying the scheduling expression, including cron expressions, fixed delays, and fixed rates. You can choose the most appropriate option based on your scheduling requirements.

How can you enable caching in a Spring Boot application?

To enable caching in a Spring Boot application, you can follow these steps:

Add Caching Dependencies: Ensure that the necessary caching dependencies are included in your project’s dependencies. Spring Boot provides support for various caching providers such as Ehcache, Redis, and Caffeine. Add the corresponding caching dependency to your project’s pom.xml file.

For example, to use the Ehcache caching provider, add the following dependency:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-cache</artifactId>
</dependency>
<dependency>
    <groupId>net.sf.ehcache</groupId>
    <artifactId>ehcache</artifactId>
</dependency>

Enable Caching: To enable caching in your Spring Boot application, add the @EnableCaching annotation to your configuration class. This annotation enables Spring’s caching infrastructure and prepares the application for caching.

import org.springframework.cache.annotation.EnableCaching;
import org.springframework.context.annotation.Configuration;

@Configuration
@EnableCaching
public class CachingConfig {
    // Additional configuration if needed
}

Annotate Methods for Caching: Identify the methods in your application that you want to cache and annotate them with the appropriate caching annotations. Spring Boot provides annotations such as @Cacheable, @CachePut, and @CacheEvict for caching operations.

For example, suppose you have a method that retrieves data from a database and you want to cache the results. You can annotate the method with @Cacheable and specify the cache name.

import org.springframework.cache.annotation.Cacheable;
import org.springframework.stereotype.Service;

@Service
public class DataService {

    @Cacheable("dataCache")
    public Data getData(String key) {
        // Logic to fetch data from a database or external service
        return data;
    }
}

In this example, the getData() method is annotated with @Cacheable and specifies the cache name as “dataCache”. The first time this method is called with a specific key, the data will be fetched and cached. Subsequent calls with the same key will retrieve the data from the cache instead of executing the method.

Configure Cache Settings: If you need to customize the caching behavior, you can provide additional configuration properties specific to your chosen caching provider. These configuration properties can be set in the application.properties or application.yml file.

For example, if you are using Ehcache, you can configure the cache settings in the ehcache.xml file and specify the location of the file in the application.properties file.

By following these steps, you can enable caching in your Spring Boot application and leverage the benefits of caching to improve performance and reduce database or external service calls.

Conclusion

In conclusion, these Spring Boot interview questions cover a wide range of topics related to Spring Boot, its features, and best practices. By familiarizing yourself with these questions and their answers, you can better prepare for Spring Boot interviews and demonstrate your knowledge and expertise in developing Spring Boot applications.

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