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Top Spring Boot Interview Questions & Answers

These questions cover core Spring Boot concepts, annotations, and configuration as explained in the Code Decode tutorial.

1. What is Spring Boot and why should we use it?​

Spring Boot is an extension of the Spring framework that eliminates the boilerplate configurations required for setting up a Spring application. It follows an "opinionated" approach to configuration.

  • Auto-configuration: It automatically configures your application based on the dependencies in the classpath.
  • Embedded Servers: It comes with embedded servers like TomCat, Jetty, or Undertow, meaning you don't need to install an external server.
  • Starter Dependencies: Simplifies dependency management by providing aggregate dependencies (e.g., spring-boot-starter-web).

2. How to change the default port of a Spring Boot application?​

The default port is 8080. You can change it by adding the following property to your application.properties file:

server.port=8081

3. How can you change the embedded server?​

By default, Spring Boot uses Tomcat. To switch to another server like Jetty, you must exclude the Tomcat starter from the spring-boot-starter-web dependency and include the Jetty starter:

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

4. What is @SpringBootApplication?​

This is a convenience annotation that combines three other annotations:

  1. @SpringBootConfiguration: Indicates that the class provides Spring Boot application configuration.
  2. @EnableAutoConfiguration: Tells Spring Boot to start adding beans based on classpath settings.
  3. @ComponentScan: Tells Spring to look for other components, configurations, and services in the package.

5. Difference between @Controller and @RestController​

  • @Controller: Used for traditional Spring MVC controllers where the response is usually an HTML page (view). You need to use @ResponseBody on methods if you want to return data (JSON/XML).
  • @RestController: A specialized version of @Controller that includes @ResponseBody by default. It is used for RESTful web services that return data directly in the response body.

6. @RequestMapping vs @GetMapping​

  • @RequestMapping: A general-purpose annotation to map web requests. You must specify the method (e.g., method = RequestMethod.GET).
  • @GetMapping: A composed annotation that acts as a shortcut for @RequestMapping(method = RequestMethod.GET). Similarly, there are @PostMapping, @PutMapping, and @DeleteMapping.

7. What are Spring Boot Profiles?​

Profiles allow you to segregate parts of your application configuration and make it only available in certain environments (e.g., Dev, Test, Prod).

  • You can create files like application-dev.properties and application-prod.properties.
  • To activate a profile, use: spring.profiles.active=dev.