Success in Spring Boot Interview: Top 15 Spring Boot Questions and Answers

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Question 1. What are stereotype annotations in Spring Boot?

In the context of Spring Boot, stereotypes are special annotations that provide metadata about the role and responsibility of a class within the application. These annotations are used by the Spring container to automatically detect and instantiate components during the application startup and component scanning process.

The main stereotype annotations in Spring Boot are:

1. @Component: This is a generic stereotype annotation that marks a class as a Spring-managed component. Any class annotated with @Component will be automatically detected and instantiated by the Spring container.
   
2. @Service: This stereotype indicates that the annotated class represents a service layer component, typically containing business logic or use case implementations.
   
3. @Repository: Classes annotated with @Repository are marked as repository components, responsible for encapsulating data access logic and interacting with databases or other data sources.
   
4. @Controller: This stereotype is used to mark a class as a web controller component, responsible for handling HTTP requests and preparing a model for rendering views in a web application.
   
5. @RestController: This annotation is a specialization of @Controller and is used to mark a class as a RESTful web service controller, typically used for building REST APIs where the response is in a data format like JSON or XML.
   
6. @Configuration: Classes annotated with @Configuration are considered sources of bean definitions and other configuration metadata. These classes can contain @Bean methods for instantiating and configuring beans.

The main purposes of using stereotypes in Spring Boot are:

1. Semantic Metadata: Stereotypes provide metadata about the role and responsibility of the annotated class, making the code more self-documenting and easier to understand.
   
2. Component Scanning: Spring’s component scanning mechanism uses these stereotypes to automatically detect and instantiate components during application startup. Classes annotated with these stereotypes are automatically registered as beans in the Spring container.

By using stereotypes, Spring Boot developers can leverage the built-in conventions and auto-configuration features, reducing the amount of boilerplate code and configuration required. Additionally, these stereotypes can be used to apply specific behaviours or configurations based on the component’s role within the application.

Question 2. Difference between @Controller and @RestController annotations in a Spring Boot application.

The main difference between @Controller and @RestController annotations in a Spring Boot application lie in their intended use and the way they handle HTTP requests and responses.

@Controller:

• @Controller is used for building traditional server-rendered web applications, where the view is rendered on the server side, and the response is typically HTML.
  
• Controllers annotated with @Controller are responsible for handling HTTP requests, processing user input, and preparing a model for rendering views (typically HTML templates).
  
• Methods in @Controller classes typically return a String representing the logical view name or a ModelAndView object.
• Example:

@Controller
public class HomeController {
    @GetMapping("/")
    public String home(Model model) {
        model.addAttribute("message", "Hello, Spring MVC!");
        return "index";
    }
}

@RestController:

• @RestController is used for building RESTful web services, where the response is typically in a data format like JSON or XML.
  
• Controllers annotated with @RestController are responsible for handling HTTP requests and returning the response data directly, without rendering any views.
  
• Methods in @RestController classes return domain objects or collections, which are automatically serialized into the response body as JSON or XML (depending on the configured HTTP message converters).
  
• @RestController is a convenience annotation that combines @Controller and @ResponseBody, eliminating the need to annotate every request-handling method with @ResponseBody.
  
• Example:

@RestController
public class UserController {
    @GetMapping("/users")
    public List<User> getUsers() {
        return userService.getAllUsers();
    }
}

Key Differences:

1. Response Type: @Controller typically returns a logical view name or ModelAndView, while @RestController returns domain objects or collections that are directly mapped to the response body.
   
   
2. View Rendering: @Controller interacts with view templates (e.g., Thymeleaf, JSP) to render HTML pages, while @RestController does not involve view rendering and returns data in a specific format.
   
   
3. Typical Usage: @Controller is commonly used in traditional server-rendered web applications, while @RestController is used in RESTful API development and modern Single Page Applications (SPAs).

In summary, @Controller is used for server-side rendering of views, while @RestController is used for building RESTful web services that return data in a specific format, without the need for view rendering. The choice between @Controller and @RestController depends on whether you are building a traditional web application or a RESTful API.

Question 3: What is the process of an HTTP request in a Spring Boot application?

The flow of an HTTP request in a Spring Boot application. Here’s a step-by-step breakdown:

1. Client Sends Request: The client (e.g., a web browser) sends an HTTP request to the server where the Spring Boot application is running.
   
2. Request Received by Server: The server, often an embedded server like Apache Tomcat, receives the request. Apache Tomcat is an open-source Java servlet container that implements key Java enterprise standards.
   
3. Request Handled by DispatcherServlet: The request is then passed to the DispatcherServlet, which is the front controller in the Spring MVC application. This servlet is initialized by the server and it uses one of its worker threads to handle the request.
   
4. Request Mapping: The DispatcherServlet checks the request mapping to determine which controller should handle the request. It does this by matching the request URL with the specified URL patterns in the controllers.
   
5. Controller Handles Request: The appropriate controller method is invoked to handle the request. The method performs the necessary processing, which may involve calling services, accessing the database, etc.
   
6. Response Creation: The controller method returns a model and view. The model contains the data to be displayed, and the view is the presentation of the data.
   
7. View Resolution: The DispatcherServlet passes the model and views to the ViewResolver. The ViewResolver determines the actual view to be used, such as a JSP or Thymeleaf template.
   
8. Response Sent to Client: The selected view is rendered and sent back to the client as the HTTP response.

This is a simplified explanation of the process. The actual flow can be more complex and involves additional components depending on the specific configuration and functionality of the application. For example, you might have interceptors for pre-processing requests, or you might use RestTemplate for making HTTP requests.

Question 4: How would you set the environment properties in your spring boot application?

In a Spring Boot application, environment properties can be set using various methods. Here are some common ways to set environment properties:

1. Application Properties File:
   • Create an application.properties (or application.yml for YAML) file in the src/main/resources directory.
     
   • In this file, you can define properties using a key=value format.
     
   • Example: server.port=8081
     
2. Environment-Specific Properties Files:
   • You can create environment-specific properties files, such as application-dev.properties or application-prod.properties.
     
   • These files override the properties defined in the application.properties file.
     
   • To activate a specific profile, you can set the spring.profiles.active property or use the --spring.profiles.active=dev the command-line argument when running the application.
     
     application.properties
     
     spring.application.name=budget-management-service
     spring.datasource.driver-class-name=org.postgresql.Driver
     spring.jpa.hibernate.ddl-auto=update
     spring.jpa.show-sql=true
     spring.jpa.properties.hibernate.format_sql=true
     logging.level.org.springframework.data=DEBUG
     logging.level.org.hibernate=DEBUG
     logging.file.name=myapplication.log
     spring.profiles.active=default # you can set dev or prod


application-dev.properties
     
     spring.datasource.url=jdbc:postgresql://localhost:5433/budget_db_dev
     spring.datasource.username=dev_username
     spring.datasource.password=dev_password
     
     application-prod.properties
     
     spring.datasource.url=jdbc:postgresql://localhost:5433/budget_db_prod
     spring.datasource.username=dev_username
     spring.datasource.password=dev_password
     
     The environment variable defined in application-dev.properties or application-prod.properties will always take precedence over the value in the application.properties file
     
     When you run your Spring Boot application, it will automatically pick up the value of the SPRING_PROFILES_ACTIVE environment variable and activate the corresponding profile. This means it will use the properties defined in the application-{profile}.properties file.
     
     Here’s an example of how you can set the environment variable in a Unix-based system (like Linux or Mac OS):
     export SPRING_PROFILES_ACTIVE=dev

For Windows:
     set SPRING_PROFILES_ACTIVE=dev
     
     You don’t necessarily need to set the spring.profiles.active property in your application.properties file if you’re using the SPRING_PROFILES_ACTIVE environment variable.
     
     When you set the SPRING_PROFILES_ACTIVE environment variable, Spring Boot will automatically use its value to determine which profile is currently active. This means it will load the properties from the corresponding application-{profile}.properties file.
     
     
     
3. Command-Line Arguments:
   • You can set properties as command-line arguments when running the application.
   • Example: java -jar myapp.jar --server.port=8082
     java -jar .\target\budget-management-service-0.0.1-SNAPSHOT.jar –server.port=8082
     
     
4. System Environment Variables:
   • You can set properties using system environment variables.
   • Spring Boot will automatically pick up environment variables with the SPRING_ prefix or in the ${...} syntax.
   • Example (Linux/macOS): export SPRING_SERVER_PORT=8083
   • Example (Windows): set SPRING_SERVER_PORT=8083
     
     
5. Java System Properties:
   • You can set properties as Java system properties.
   • Example: java -Dserver.port=8084 -jar myapp.jar
     
6. Custom PropertySource:
   • You can create a custom PropertySource and register it in the SpringApplication or using the @PropertySource annotation.
   • This allows you to load properties from various sources, such as databases, remote servers, or other custom sources.

The order of precedence for these methods is as follows (from highest to lowest):

1. Command-line arguments
2. Java system properties (-D arguments)
3. Operating system environment variables
4. application.properties (or application.yml) file
5. @PropertySource annotated properties
6. Custom PropertySource instances
   You can read on ProperySource https://codetechsummit.com/propertysources-in-spring-boot/

Keep in mind that you can also use placeholders and expressions in your properties files, and Spring Boot provides additional features like property validation, property encryption, and more.

Question 5: Explain spring-boot-starter in detail.

Understanding Spring Boot Starters

Spring Boot Starters are a key feature of the Spring Boot framework that simplifies the development process by providing pre-configured dependencies and auto-configuration for specific features or functionalities. These starters act as a convenient way to include a set of related dependencies in a Spring Boot application, saving developers from the hassle of manually managing and configuring each dependency.

What are Spring Boot Starters?

Spring Boot Starters are essentially a collection of dependencies that are bundled together and designed to work out-of-the-box for a particular use case or feature. Each starter provides a pre-configured set of dependencies, along with sensible default configurations based on best practices and conventions.

For example, the spring-boot-starter-web starter includes all the necessary dependencies and configurations required for building web applications with Spring MVC, such as the embedded Tomcat server, Jackson JSON library, and validation support.

Dependency Management

One of the primary benefits of using Spring Boot Starters is that they handle dependency management for you. When you include a starter in your project, it automatically resolves and includes all the required transitive dependencies, ensuring compatibility and avoiding version conflicts.

This means that you don’t need to specify the version numbers of individual dependencies, as the starter will manage that for you. Additionally, Spring Boot Starters follow the “opinionated defaults” approach, providing sensible default configurations for the included dependencies, which you can override or customize as needed.

Transitive Dependencies

One of the key advantages of using Spring Boot Starters is that they handle transitive dependencies automatically. Transitive dependencies are dependencies that are required by the dependencies you include in your project.
Without Spring Boot Starters, you would need to manually manage and include these transitive dependencies, which can be a tedious and error-prone process.

Modular Structure

Spring Boot Starters are organized into different modules, each focused on a specific feature or functionality. This modular structure allows developers to include only the starters they need for their application, keeping the project lean and efficient.

Here are some popular Spring Boot Starters:

• spring-boot-starter-web: For building web applications with Spring MVC
• spring-boot-starter-data-jpa: For data access with JPA and Hibernate
• spring-boot-starter-security: For implementing security features
• spring-boot-starter-actuator: For monitoring and managing applications
• spring-boot-starter-test: For writing tests in Spring Boot applications

Excluding Dependencies

While Spring Boot Starters provide a convenient way to include related dependencies, developers can still exclude specific dependencies if they don’t need them in their application. This can be done by using the exclude configuration in your build tool (e.g., Maven or Gradle).

Getting Started with Spring Boot Starters

To include a Spring Boot Starter in your project, simply add the corresponding dependency to your build file (pom.xml for Maven or build.gradle for Gradle). For example, to include the spring-boot-starter-web dependency, you would add the following:

<!-- Maven -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

// Gradle
implementation 'org.springframework.boot:spring-boot-starter-web'

By including the spring-boot-starter-web dependency, you automatically get all the required dependencies for building web applications with Spring MVC, including embedded Tomcat, Jackson JSON library, and validation support.

Spring Boot Starters simplify the development process by reducing the amount of configuration and dependency management required. They follow the “opinionated defaults” approach, providing sensible defaults for common use cases while still allowing developers to override or customize these defaults as needed.

By leveraging Spring Boot Starters, developers can focus more on writing application code and less on managing dependencies and configurations.

Question 6: Annotations in JUnit Test cases?

Yes, JUnit provides several annotations to help write and configure test cases. Here are some commonly used annotations in JUnit:

1. @Test: This annotation is used to mark a method as a test case. JUnit will run all methods annotated with @Test during test execution.
2. @Before (JUnit 4) or @BeforeEach (JUnit 5): This annotation is used to mark a method that should be executed before each test case. It is typically used for setting up the test environment or initializing objects required for the tests.
3. @After (JUnit 4) or @AfterEach (JUnit 5): This annotation is used to mark a method that should be executed after each test case. It is typically used for cleaning up resources or resetting the test environment.
4. @BeforeClass (JUnit 4) or @BeforeAll (JUnit 5): This annotation is used to mark a method that should be executed once before all the test cases in a class. It is typically used for performing expensive setup operations that are shared across all test cases in the class.
5. @AfterClass (JUnit 4) or @AfterAll (JUnit 5): This annotation is used to mark a method that should be executed once after all the test cases in a class. It is typically used for cleaning up resources or performing teardown operations that are shared across all test cases in the class.
6. @Ignore (JUnit 4) or @Disabled (JUnit 5): This annotation is used to mark a test case or a test class that should be skipped during test execution.
7. @Test(expected = Exception.class): This annotation is used to test for expected exceptions. If the test case throws the specified exception, the test passes; otherwise, it fails.
8. @Test(timeout = X): This annotation is used to set a timeout for a test case. If the test case takes longer than the specified time (in milliseconds), it will be marked as a failure.
9. @ParameterizedTest (JUnit 5): This annotation is used to define a parameterized test case, which allows running the same test with different sets of input data.
10. @DisplayName (JUnit 5): This annotation is used to provide a custom display name for a test class or test method, making the test reports more readable and descriptive.

These are just a few of the commonly used annotations in JUnit. JUnit also provides several other annotations for advanced use cases, such as parallel test execution, test suite configuration, and test lifecycle hooks.

Question 6: Do you know the Actuator in Spring Boot, Do you name some endpoints provided by the Spring Boot Actuator, can we customize the Actuator?

Spring Boot Actuator is a sub-project of Spring Boot that provides production-ready features to help you monitor and manage your application when it’s pushed to production.

Setting up Spring Boot Actuator in your application is straightforward. Here are the steps:

Add the Actuator Dependency: You need to add the spring-boot-starter-actuator dependency to your Maven pom.xml file:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

Configure Actuator Endpoints: By default, only a few endpoints are exposed over HTTP. You can configure which endpoints to expose by adding properties to your application.properties or application.yml file.
For example, to expose all endpoints, you can add the following property: management.endpoints.web.exposure.include=*

Access Actuator Endpoints: Once you’ve added the Actuator dependency and configured the endpoints, you can access them via HTTP. For example, you can access the health endpoint at http://localhost:8080/actuator/health, assuming your application is running on port 8080.

Remember, exposing certain Actuator endpoints might have security implications, so it’s recommended to secure sensitive endpoints in a production environment. 😊

The Spring Boot Actuator provides several built-in endpoints that expose operational information about a running application. Here are some of the commonly used endpoints provided by the Actuator:

1. /health: Provides information about the health of the application, including any potential issues.
   
2. /info: Provides general information about the application, such as the application name, version, and other custom details.
   
3. /metrics: Exposes several metrics about the application, such as memory usage, CPU utilization, and HTTP request metrics.
   
4. /loggers: Allows you to view and configure the logging levels of your application at runtime.
5. /env: Exposes the current environment properties of the application.
   
6. /threaddump: Generates a thread dump from the running application, which can be useful for analyzing issues related to hung threads or performance problems.
   
7. /heapdump: Generates a heap dump from the running application, which can be useful for analyzing memory usage and potential memory leaks.
   
8. /shutdown: Allows you to gracefully shut down the running application.

You can customize the Actuator in several ways:

1. Enabling/Disabling Endpoints: You can enable or disable specific endpoints by configuring the management.endpoints.web.exposure.include or management.endpoints.web.exposure.exclude properties in your application.properties or application.yml file.
   
2. Customizing Endpoint Paths: You can change the root path for the Actuator endpoints by setting the management.endpoints.web.base-path property.
   
3. Exposing Endpoints via JMX: You can expose Actuator endpoints over JMX by setting the management.endpoints.jmx.exposure.include property.
   
4. Creating Custom Endpoints: You can create your custom endpoints by implementing the EndpointWebExtension interface or extending the AbstractEndpoint class.
   
5. Securing Actuator Endpoints: You can secure the Actuator endpoints by configuring Spring Security and restricting access to specific endpoints based on roles or authentication mechanisms.
   
6. Customizing Health Indicators: You can create custom health indicators by implementing the HealthIndicator interface and register them as beans in your application context.
   
7. Customizing Info Contributors: You can contribute custom information to the /info endpoint by implementing the InfoContributor interface and registering it as a bean.

By default, the Actuator is disabled in Spring Boot applications, and you need to explicitly enable it by adding the spring-boot-starter-actuator dependency to your project. You can then customize the Actuator’s behaviour and exposed endpoints according to your application’s requirements.

Overall, the Spring Boot Actuator provides a powerful set of features and endpoints to monitor and manage your application in production environments, and it can be customized to suit your specific needs.

you can control the number of threads running in the Spring Boot Actuator using the management.server.threads.max configuration property. This property sets the maximum number of threads that the Actuator’s management server will allow.

Here’s how you can include this configuration in your Spring Boot application:

application.properties:

propertiesCopy codemanagement.server.threads.max=4

application.yml:

management:
  server:
    threads:
      max: 4
      

In the above examples, we’ve set the management.server.threads.max property to 4, which means that the Actuator’s management server will allow a maximum of 4 threads to process incoming requests.

You can adjust this value based on your application’s requirements and the expected load on the Actuator endpoints. A higher value will allow more concurrent requests to be processed, but it may also consume more system resources. A lower value can help limit resource consumption but may cause requests to be queued or rejected if the maximum number of threads is reached.

Question 7: How to optimize the project in terms of latency?

To optimize a Spring Boot project in terms of latency, you can follow these strategies:

1. Caching:
   • Implement caching mechanisms like Spring’s @Cacheable annotation to cache frequently accessed or computationally expensive data.
   • Use caching providers like Redis or Memcached for distributed caching.
2. Database Optimization:
   • Ensure that your database queries are optimized by adding appropriate indexes and avoiding unnecessary joins or queries.
   • Use database connection pooling to reuse existing connections and reduce the overhead of creating new connections.
   • Consider using read-replicas or sharding for high-traffic applications.
3. Asynchronous Processing:
   • Use Spring’s @Async annotation to offload time-consuming tasks to separate threads, freeing up the main request thread.
   • Implement message queues (like RabbitMQ or Apache Kafka) for asynchronous processing of background tasks.
4. Content Delivery Network (CDN):
   • Serve static assets (CSS, JS, images) from a CDN to reduce latency for clients located far from the application server.
5. Compression:
   • Enable HTTP compression (e.g., Gzip) to reduce the size of the response payloads and improve transfer speeds.
6. Monitoring and Profiling:
   • Use tools like Spring Boot Actuator, Micrometer, and Prometheus to monitor and profile your application’s performance.
   • Identify and address performance bottlenecks based on the collected metrics.
7. Load Balancing and Scaling:
   • Use load balancers like Nginx or AWS Elastic Load Balancing to distribute traffic across multiple application instances.
   • Implement auto-scaling strategies to dynamically adjust the number of application instances based on load.
8. JVM Tuning:
   • Tune the JVM settings (e.g., heap size, garbage collection) based on your application’s memory and CPU requirements.
   • Enable JIT compilation for improved performance.
9. Code Optimization:
   • Optimize your application code by following best practices, such as avoiding unnecessary computations, minimizing object creation, and using appropriate data structures.
   • Use profiling tools like JProfiler or YourKit to identify and fix performance bottlenecks in your code.
10. Containerization and Orchestration:
    • Containerize your application using Docker to ensure consistent deployment and efficient resource utilization.
    • Use container orchestration platforms like Kubernetes for seamless scaling, load balancing, and self-healing capabilities.
11. Disk and Network Optimization:
    • If your application is disk or network-bound, consider optimizations like using solid-state drives (SSDs) or tuning network configurations.
12. Third-Party Libraries and Dependencies:
    • Review and update third-party libraries and dependencies to their latest stable versions, as they may include performance improvements or bug fixes.

It’s important to note that optimization strategies may vary depending on your application’s specific requirements, architecture, and performance bottlenecks. It’s recommended to measure and profile your application before and after applying optimizations to ensure their effectiveness.

Question 8: In the Project how to configuration file structures like Dev configuration, and product environment configuration?

In a Spring Boot project, you can manage configuration files for different environments (e.g., development, production) using Spring Profiles. Spring Profiles allow you to define configuration properties specific to a particular environment or deployment scenario.

Here’s a typical file structure for managing configuration files in a Spring Boot project:

src/
  main/
    resources/
      application.properties (or application.yml)
      application-dev.properties (or application-dev.yml)
      application-prod.properties (or application-prod.yml)

• application.properties (or application.yml): This is the main configuration file that contains properties shared across all environments.
• application-dev.properties (or application-dev.yml): This file contains configuration properties specific to the development environment.
• application-prod.properties (or application-prod.yml): This file contains configuration properties specific to the production environment.

You can have multiple profile-specific configuration files like application-staging.properties for staging, application-test.properties for testing, and so on.

To activate a specific profile, you can use the spring.profiles.active property or pass a command-line argument when running the application.

Using the spring.profiles.active property:

In application.properties (or application.yml), you can set the active profile:

spring.profiles.active=dev

This will activate the dev profile and Spring Boot will load the application.properties and application-dev.properties files.

Using command-line arguments:

You can also activate a profile when running the application by passing a command-line argument:

java -jar -Dspring.profiles.active=prod your-app.jar

This will activate the prod profile, and Spring Boot will load the application.properties and application-prod.properties files.

Within your configuration files, you can define environment-specific properties, such as database credentials, API keys, or any other settings that vary across environments.

For example, in application-dev.properties:

server.port=8080
spring.datasource.url=jdbc:postgresql://localhost:5432/dev_db

And in application-prod.properties:

server.port=8443
spring.datasource.url=jdbc:postgresql://prod-db.example.com:5432/prod_db

By separating configuration files based on profiles, you can manage environment-specific configurations more easily and avoid hard-coding sensitive information or deployment-specific settings in your codebase.

Additionally, you can use the @Profile annotation in your Spring components to conditionally load or exclude beans based on the active profile.

The @Profile annotation in Spring can be used to conditionally register beans depending on the active profiles. Here are a couple of examples:

Using @Profile on a @Component:

@Component
@Profile("dev")
public class DevDataSourceConfig {
    // This bean will only be loaded when the 'dev' profile is active
}

In this example, the DevDataSourceConfig bean will only be loaded when the ‘dev’ profile is active.

Using @Profile on a @Configuration: @Configuration @Profile("prod")

@Configuration
@Profile("prod")
public class ProdConfiguration {
    @Bean
    public MyService myService() {
        // This bean will only be loaded when the 'prod' profile is active
        return new MyServiceImpl();
    }
}

In this example, the myService bean will only be loaded when the ‘prod’ profile is active.

Using @Profile on a @Bean: @Configuration public class AppConfig

@Configuration
public class AppConfig {
    @Bean
    @Profile("dev")
    public MyService devMyService() {
        // This bean will only be loaded when the 'dev' profile is active
        return new DevMyServiceImpl();
    }

    @Bean
    @Profile("prod")
    public MyService prodMyService() {
        // This bean will only be loaded when the 'prod' profile is active
        return new ProdMyServiceImpl();
    }
}

In this example, the devMyService bean will only be loaded when the ‘dev’ profile is active, and the prodMyService bean will only be loaded when the ‘prod’ profile is active.

Remember, you can set the active profiles by using the spring.profiles.active property in your application.properties or application.yml file, or by using the SPRING_PROFILES_ACTIVE environment variable.

Question 9: Can we deploy the spring boot application as a traditional war file to an extended server?

Yes, you can deploy a Spring Boot application as a traditional WAR (Web Application Archive) file to an external server like Apache Tomcat, IBM WebSphere, or Oracle WebLogic. By default, Spring Boot applications are packaged as executable JAR files that include an embedded web server (e.g., Tomcat, Jetty, or Undertow). However, Spring Boot also provides support for deploying applications as WAR files to external servlet containers.

To deploy a Spring Boot application as a WAR file, you need to make a few changes to your project configuration:

1. Update the packaging type: In your pom.xml file (for Maven) or build.gradle file (for Gradle), change the packaging type from jar to war. For Maven:

   <packaging>war</packaging>

For Gradle:

   apply plugin: 'war'

2. Extend the SpringBootServletInitializer class: Create a class that extends the SpringBootServletInitializer class. This class is responsible for bootstrapping the Spring Boot application in the servlet container.

   import org.springframework.boot.builder.SpringApplicationBuilder;
   import org.springframework.boot.web.servlet.support.SpringBootServletInitializer;

   public class ServletInitializer extends SpringBootServletInitializer {

       @Override
       protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
           return application.sources(YourApplication.class);
       }

   }

3. Update the web.xml file (optional): If you need to configure the servlet container or register any servlet filters or listeners, you can include a web.xml file in the src/main/webapp/WEB-INF directory.
   
4. Build the project: Build your Spring Boot project, which will generate a WAR file (e.g., your-app.war) in the target directory (for Maven) or build/libs directory (for Gradle).

After building the project, you can deploy the generated WAR file to an external servlet container like Apache Tomcat by copying it to the webapps directory of the servlet container.

It’s important to note that when deploying a Spring Boot application as a WAR file, you lose the benefits of the embedded server, such as automatic application restart and other developer-friendly features provided by the Spring Boot DevTools. Additionally, you may need to configure the servlet container manually for tasks like data source configuration, logging, and other external dependencies.

While deploying as a WAR file is possible, it’s generally recommended to use the executable JAR file deployment approach for Spring Boot applications, as it simplifies the deployment process and takes advantage of the embedded server features provided by Spring Boot.

Question 10: How Spring boot handle the application.configuration?

Spring Boot handles application configuration in a very flexible and convenient way. It follows a specific order and hierarchy to load configuration properties from various sources. Here’s how Spring Boot handles application configuration:

1. Bootstrap Properties: Spring Boot first loads the bootstrap.properties or bootstrap.yml file (if present) from the root of the classpath. These properties are used for configuring the application before the application.properties or application.yml files are loaded. This is useful for configuring properties related to the external configuration source itself, such as Spring Cloud Configuration Server.
2. Application Properties: After loading the bootstrap properties, Spring Boot loads the application.properties or application.yml file from the root of the classpath. This file typically contains the main configuration properties for the application.
3. Profile-specific Properties: If a specific profile is active, Spring Boot will load the profile-specific properties file, such as application-{profile}.properties or application-{profile}.yml. For example, if the dev profile is active, it will load the application-dev.properties or application-dev.yml file.
4. Command-line Arguments: Spring Boot allows you to provide configuration properties as command-line arguments when running the application. These properties take precedence over the properties defined in the configuration files.
5. Environment Variables: Spring Boot can also load configuration properties from environment variables. By default, it will map environment variables with the SPRING_ prefix to Spring properties.
6. System Properties: Configuration properties can also be provided as Java system properties, and Spring Boot will load them.
7. Random Value Properties: Spring Boot can generate random values for properties marked with ${random.*} expressions. This is useful for generating temporary values or test data.
8. Other Sources: Spring Boot also supports loading configuration properties from other sources, such as JNDI, Servlet Context Parameters, and more, through the use of PropertySource implementations.

Spring Boot follows a specific order when loading configuration properties, with properties from higher precedence sources overriding those from lower precedence sources. The order of precedence is:

1. Command-line arguments
2. Java system properties
3. Operating system environment variables
4. Random values
5. Profile-specific application properties (from application-{profile}.properties or application-{profile}.yml)
6. Application properties (from application.properties or application.yml)
7. @Configuration classes
8. Other sources (e.g., JNDI, Servlet Context Parameters)

This configuration loading hierarchy allows for a great deal of flexibility and enables you to override and customize configuration properties based on your deployment environment or specific requirements.

Additionally, Spring Boot supports binding configuration properties to strongly-typed Java classes using the @ConfigurationProperties annotation, which can simplify the management and validation of configuration data within your application.

Question 11: What are the order of precedence in configuration?

The order of precedence for configuration sources in Spring Boot is as follows:

1. Command-line Arguments: Command-line arguments have the highest precedence and will override any other configuration sources.
2. Java System Properties: Properties set as Java system properties have the second-highest precedence.
3. Operating System Environment Variables: Environment variables from the operating system have the third-highest precedence.
4. Random Values: Random values injected using the ${random.*} syntax have the fourth-highest precedence.
5. Profile-specific Application Properties: Properties from the application-{profile}.properties or application-{profile}.yml files for the active profile have the fifth-highest precedence.
6. Application Properties: Properties from the application.properties or application.yml file have the sixth-highest precedence.
7. Additional Configuration Files: Properties from additional configuration files imported using the @ImportResource annotation or the spring.config.import property have the seventh-highest precedence.
8. @Configuration Classes: Properties defined in @Configuration classes have the eighth-highest precedence.
9. Default Properties: Default properties packaged with Spring Boot have the lowest precedence.

So, to summarize the order of precedence:

1. Command-line Arguments
2. Java System Properties
3. Operating System Environment Variables
4. Random Values
5. Profile-specific Application Properties
6. Application Properties
7. Additional Configuration Files
8. @Configuration Classes
9. Default Properties

This order of precedence allows you to easily override configuration properties for different environments or scenarios. For example, you can use command-line arguments or environment variables to override properties during deployment, or use profile-specific properties for different environments (e.g., development, staging, production).

It’s important to note that properties from higher precedence sources will always override those from lower precedence sources. So, if a property is defined in both the application.properties file and through a command-line argument, the value from the command-line argument will be used.

Question 12: How do you secure the Spring Boot Application?

There are several ways to secure a Spring Boot application. Here are some common approaches:

1. Spring Security: Spring Security is the de-facto standard for securing Spring-based applications. It provides comprehensive authentication and authorization capabilities. You can integrate Spring Security into your Spring Boot application by adding the spring-boot-starter-security dependency.
2. Authentication and Authorization:

• Authentication: Spring Security supports various authentication mechanisms, including form-based authentication, basic authentication, JWT (JSON Web Tokens), OAuth2, and more.
• Authorization: Spring Security provides role-based access control (RBAC) and method-level security using annotations like @PreAuthorize, @PostAuthorize, @Secured, and others.

1. Secure HTTP Headers: Spring Security can help secure your application by adding security-related HTTP headers, such as X-XSS-Protection, X-Frame-Options, Strict-Transport-Security, and others. These headers can help mitigate common web vulnerabilities like XSS, Clickjacking, and insecure communication.
2. HTTPS and TLS: You can configure your Spring Boot application to use HTTPS and TLS (Transport Layer Security) for secure communication. This can be done by configuring an embedded server like Tomcat or by deploying your application behind a reverse proxy like Nginx or Apache.
3. CSRF Protection: Spring Security provides built-in protection against Cross-Site Request Forgery (CSRF) attacks by using a synchronizer token pattern.
4. Secure Headers: Spring Security can help secure your application by adding security-related HTTP headers, such as X-XSS-Protection, X-Frame-Options, Strict-Transport-Security, and others. These headers can help mitigate common web vulnerabilities like XSS, Clickjacking, and insecure communication.
5. Data Encryption: Spring Boot provides utilities for encrypting and decrypting data using symmetric and asymmetric encryption algorithms. You can use the jasypt-spring-boot library to encrypt sensitive properties in your configuration files.
6. Secure Coding Practices: Implement secure coding practices, such as input validation, output encoding, protection against injection attacks (SQL, NoSQL, LDAP, etc.), and other security best practices.
7. Third-Party Libraries: Leverage third-party security libraries and tools, such as OWASP Dependency-Check, to identify and mitigate vulnerabilities in your application’s dependencies.
8. Security Monitoring and Logging: Implement security monitoring and logging mechanisms to detect and respond to potential security incidents or attacks.
9. Regular Security Updates: Keep your Spring Boot application, dependencies, and third-party libraries up-to-date with the latest security patches and updates.
10. Security Testing: Perform regular security testing, such as penetration testing and vulnerability scanning, to identify and address potential security vulnerabilities in your application.
11. Secure Deployment: Follow secure deployment practices, such as restricting access to production environments, using secure protocols for communication, and implementing security best practices for your hosting environment (e.g., cloud, on-premises).

Security is an ongoing process, and it’s essential to adopt a defense-in-depth approach by implementing multiple security measures and regularly reviewing and updating your security practices to stay ahead of emerging threats.

Also Read: https://developer.okta.com/blog/2018/07/30/10-ways-to-secure-spring-boot

Question 13: What are the Advantages of using Spring Boot over Traditional Spring Application?

Using Spring Boot offers several advantages over traditional Spring applications:

1. Opinionated Setup and Auto-Configuration:

• Spring Boot comes with a set of sensible default configurations that work well for most applications, reducing the amount of boilerplate code and configuration required.
• It automatically configures and wires up many components, such as embedded servers (Tomcat, Jetty, or Undertow), logging, and common database connections, based on the dependencies present in your project.

1. Embedded Server:

• Spring Boot comes with an embedded server (e.g., Tomcat, Jetty, or Undertow), allowing you to run your application as a standalone JAR file without the need for a separate web server installation.
• This simplifies deployment and makes it easier to create microservices or self-contained applications.

1. Starter Dependencies:

• Spring Boot provides a set of “Starter” dependencies that bundle related libraries together, reducing the need to manually manage transitive dependencies.
• These starters simplify the process of adding new functionality to your project by including the required dependencies automatically.

1. Production-Ready Features:

• Spring Boot includes many production-ready features out of the box, such as metrics, health checks, externalized configuration, and more.
• These features help in monitoring, managing, and scaling your applications more effectively.

1. Simplified Development and Deployment:

• Spring Boot simplifies the development and deployment process by providing a consistent way to package your application as an executable JAR or WAR file.
• This eliminates the need for complex build and deployment configurations, making it easier to develop, test, and deploy your applications.

1. Flexible and Extensible:

• Despite its opinionated defaults, Spring Boot remains flexible and extensible, allowing you to override or customize any part of the framework as needed.
• You can easily add or replace components with your own implementations.

1. Developer Productivity:

• Spring Boot aims to increase developer productivity by reducing boilerplate code, providing sensible defaults, and simplifying common tasks.
• This allows developers to focus more on writing the core business logic of their applications.

1. Large and Active Community:

• Spring Boot has a large and active community, with extensive documentation, tutorials, and third-party libraries and tools available.
• This community support can be invaluable when facing challenges or seeking best practices.

Overall, Spring Boot simplifies the development and deployment of Spring-based applications, reducing configuration overhead and providing a productive and opinionated approach to building applications quickly and efficiently.

Question 14: What is the default server in Spring Boot, can we change it?

The default embedded server in Spring Boot is Tomcat. However, you can easily change it to use a different server if needed.

By default, when you create a new Spring Boot project with a web dependency (e.g., spring-boot-starter-web), the Tomcat server is included and configured automatically. This embedded Tomcat server is suitable for most use cases and provides good performance and flexibility.

However, if you want to use a different server, such as Jetty or Undertow, you can exclude the default Tomcat dependency and include the dependency for the desired server instead. Here’s how you can do it:

1. For Jetty:

• Exclude the Tomcat dependency by adding the following line to your pom.xml file under the <dependencies> section:
  xml <exclusions> <exclusion> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-tomcat</artifactId> </exclusion> </exclusions>
• Add the Jetty starter dependency:
  xml <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-jetty</artifactId> </dependency>

1. For Undertow:

• Exclude the Tomcat dependency (same as above).
• Add the Undertow starter dependency:
  xml <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-undertow</artifactId> </dependency>

By making these changes, Spring Boot will automatically configure and use the specified server (Jetty or Undertow) instead of Tomcat.

You can also configure various properties of the embedded server, such as port, SSL, and other settings, using the application.properties or application.yml file in your Spring Boot project.

It’s worth noting that while you can change the embedded server, Tomcat is generally the most widely used and well-supported option for most applications. However, Jetty or Undertow might be preferred in certain scenarios, such as better performance characteristics or specific requirements.

Question 15: What is Auto Configuration in Spring Boot?