Spring boot interview questions

In this article, we will discuss some top 10 interview questions in Spring Boot. These questions are a bit tricky and trending heavily, nowadays, in the job market.

1) What does the @SpringBootApplication annotation do internally?

As per the Spring Boot doc, the @SpringBootApplication annotation is equivalent to using @Configuration, @EnableAutoConfiguration, and @ComponentScan with their default attributes. Spring Boot enables the developer to use a single annotation instead of using multiple. But, as we know, Spring provided loosely coupled features that we can use for each individual annotation as per our project needs.

2) How to exclude any package without using the basePackages filter?

There are different ways you can filter any package. But Spring Boot provides a trickier option for achieving this without touching the component scan. You can use the exclude attribute while using the annotation @SpringBootApplication. See the following code snippet:1

@SpringBootApplication(exclude= {Employee.class})

public class FooAppConfiguration {}

3) How to disable a specific auto-configuration class?

You can use the exclude attribute of@EnableAutoConfiguration, if you find any specific auto-configuration classes that you do not want are being applied.1

//By using "exclude"

@EnableAutoConfiguration(exclude={DataSourceAutoConfiguration.class})

On the other foot, if the class is not on the classpath, you can use the excludeName attribute of the annotation and specify the fully qualified name instead.1

//By using "excludeName"

@EnableAutoConfiguration(excludeName={Foo.class})

Also, Spring Boot provides the facility to control the list of auto-configuration classes to exclude by using the spring.autoconfigure.exclude property. You can add into the application.properties. And you can add multiple classes with comma separated.1

//By using property file

spring.autoconfigure.exclude=org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration

4) What is Spring Actuator? What are its advantages?

This is one of the most common interview questions in Spring Boot. As per the Spring doc:

“An actuator is a manufacturing term that refers to a mechanical device for moving or controlling something. Actuators can generate a large amount of motion from a small change.”

As we know, Spring Boot provides lots of auto-configuration features that help developers quickly develop production components. But if you think about debugging and how to debug, if something goes wrong, we always need to analyze the logs and dig through the data flow of our application to check to see what’s going on. So, the Spring Actuator provides easy access to those kinds of features. It provides many features, i.e. what beans are created, the mapping in the controller, the CPU usage, etc. Automatically gathering and auditing health and metrics can then be applied to your application.

It provides a very easy way to access the few production-ready REST endpoints and fetch all kinds of information from the web. But by using these endpoints, you can do many things to see here the endpoint docs. There is no need to worry about security; if Spring Security is present, then these endpoints are secured by default using Spring Security’s content-negotiation strategy. Or else, we can configure custom security by the help of RequestMatcher.

5) How to enable/disable the Actuator?

Enabling/disabling the actuator is easy; the simplest way is to enable features to add the dependency (Maven/Gradle) to the spring-boot-starter-actuator, i.e. Starter. If you don’t want the actuator to be enabled, then don’t add the dependency.

spring-boot-starter-actuator

6) What is the Spring Initializer?

This may not be a difficult question, but the interviewer always checks the subject knowledge of the candidate. It’s often that you can’t always expect questions that you have prepared. However, this is a very common question asked almost all of the time.

The Spring Initializer is a web application that generates a Spring Boot project with everything you need to start it quickly. As always, we need a good skeleton of the project; it helps you to create a project structure/skeleton properly. You can learn more about the Initializer here.

7) What is a shutdown in the actuator?

Shutdown is an endpoint that allows the application to be gracefully shutdown. This feature is not enabled by default. You can enable this by using management.endpoint.shutdown.enabled=true in your application.properties file. But be careful about this if you are using this.

8) Is this possible to change the port of Embedded Tomcat server in Spring boot?

Yes, it’s possible to change the port. You can use the application.properties file to change the port. But you need to mention “server.port” (i.e. server.port=8081). Make sure you have application.properties in your project classpath; REST Spring framework will take care of the rest. If you mention server.port=0 , then it will automatically assign any available port.

9) Can we override or replace the Embedded Tomcat server in Spring Boot?

Yes, we can replace the Embedded Tomcat with any other servers by using the Starter dependencies. You can use spring-boot-starter-jetty or spring-boot-starter-undertow as a dependency for each project as you need.

10) Can we disable the default web server in the Spring Boot application?

The major strong point in Spring is to provide flexibility to build your application loosely coupled. Spring provides features to disable the web server in a quick configuration. Yes, we can use the application.properties to configure the web application type, i.e. spring.main.web-application-type=none.

What is Spring Boot and What Are Its Main Features?

Spring Boot is essentially a framework for rapid application development built on top of the Spring Framework. With its auto-configuration and embedded application server support, combined with the extensive documentation and community support it enjoys, Spring Boot is one of the most popular technologies in the Java ecosystem as of date.

Here are a few salient features:

Starters – a set of dependency descriptors to include relevant dependencies at a go
Auto-configuration – a way to automatically configure an application based on the dependencies present on the classpath
Actuator – to get production-ready features such as monitoring
Security
Logging

What Are the Differences Between Spring and Spring Boot?

The Spring Framework provides multiple features that make the development of web applications easier. These features include dependency injection, data binding, aspect-oriented programming, data access, and many more.

Over the years, Spring has been growing more and more complex, and the amount of configuration such application requires can be intimidating. This is where Spring Boot comes in handy – it makes configuring a Spring application a breeze.

Essentially, while Spring is unopinionated, Spring Boot takes an opinionated view of the platform and libraries, letting us get started quickly.

Here are two of the most important benefits Spring Boot brings in:

Auto-configure applications based on the artifacts it finds on the classpath
Provide non-functional features common to applications in production, such as security or health checks

How to Disable a Specific Auto-Configuration?

If we want to disable a specific auto-configuration, we can indicate it using the exclude attribute of the @EnableAutoConfiguration annotation. For instance, this code snippet neutralizes DataSourceAutoConfiguration:

// other annotations@EnableAutoConfiguration(exclude = DataSourceAutoConfiguration.class)public class MyConfiguration { }

If we enabled auto-configuration with the @SpringBootApplication annotation — which has @EnableAutoConfiguration as a meta-annotation — we could disable auto-configuration with an attribute of the same name:

How to Register a Custom Auto-Configuration?

To register an auto-configuration class, we must have its fully-qualified name listed under the EnableAutoConfiguration key in the META-INF/spring.factories file:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.baeldung.autoconfigure.CustomAutoConfiguration

If we build a project with Maven, that file should be placed in the resources/META-INF directory, which will end up in the mentioned location during the package phase.

How to Tell an Auto-Configuration to Back Away When a Bean Exists?

To instruct an auto-configuration class to back off when a bean is already existent, we can use the @ConditionalOnMissingBean annotation. The most noticeable attributes of this annotation are:

value: The types of beans to be checked
name: The names of beans to be checked

When placed on a method adorned with @Bean, the target type defaults to the method’s return type:

@Configurationpublic class CustomConfiguration {    @Bean    @ConditionalOnMissingBean    public CustomService service() { ... }}

How to Deploy Spring Boot Web Applications as Jar and War Files?

Traditionally, we package a web application as a WAR file, then deploy it into an external server. Doing this allows us to arrange multiple applications on the same server. During the time that CPU and memory were scarce, this was a great way to save resources.

However, things have changed. Computer hardware is fairly cheap now, and the attention has turned to server configuration. A small mistake in configuring the server during deployment may lead to catastrophic consequences.

Spring tackles this problem by providing a plugin, namely spring-boot-maven-plugin, to package a web application as an executable JAR. To include this plugin, just add a plugin element to pom.xml:

<plugin>    <groupId>org.springframework.boot</groupId>    <artifactId>spring-boot-maven-plugin</artifactId></plugin>

With this plugin in place, we’ll get a fat JAR after executing the package phase. This JAR contains all the necessary dependencies, including an embedded server. Thus, we no longer need to worry about configuring an external server.

We can then run the application just like we would an ordinary executable JAR.

Notice that the packaging element in the pom.xml file must be set to jar to build a JAR file:

<packaging>jar</packaging>

If we don’t include this element, it also defaults to jar.

In case we want to build a WAR file, change the packaging element to war:

<packaging>war</packaging>

And leave the container dependency off the packaged file:

<dependency>    <groupId>org.springframework.boot</groupId>    <artifactId>spring-boot-starter-tomcat</artifactId>    <scope>provided</scope></dependency>

After executing the Maven package phase, we’ll have a deployable WAR file.

How to Use Spring Boot for Command Line Applications?

Just like any other Java program, a Spring Boot command line application must have a main method. This method serves as an entry point, which invokes the SpringApplication#run method to bootstrap the application:

@SpringBootApplicationpublic class MyApplication {    public static void main(String[] args) {        SpringApplication.run(MyApplication.class);        // other statements    }}

The SpringApplication class then fires up a Spring container and auto-configures beans.

Notice we must pass a configuration class to the run method to work as the primary configuration source. By convention, this argument is the entry class itself.

After calling the run method, we can execute other statements as in a regular program.

What Are Possible Sources of External Configuration?

Spring Boot provides support for external configuration, allowing us to run the same application in various environments. We can use properties files, YAML files, environment variables, system properties, and command-line option arguments to specify configuration properties.

We can then gain access to those properties using the @Value annotation, a bound object via the @ConfigurationProperties annotation, or the Environment abstraction.

What Does it Mean that Spring Boot Supports Relaxed Binding?

Relaxed binding in Spring Boot is applicable to the type-safe binding of configuration properties.

With relaxed binding, the key of a property doesn’t need to be an exact match of a property name. Such an environment property can be written in camelCase, kebab-case, snake_case, or in uppercase with words separated by underscores.

For example, if a property in a bean class with the @ConfigurationProperties annotation is named myProp, it can be bound to any of these environment properties: myProp, my-prop, my_prop, or MY_PROP.

What is Spring Boot Devtools Used For?

Spring Boot Developer Tools, or DevTools, is a set of tools making the development process easier. To include these development-time features, we just need to add a dependency to the pom.xml file:

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

The spring-boot-devtools module is automatically disabled if the application runs in production. The repackaging of archives also excludes this module by default. Hence, it won’t bring any overhead to our final product.

By default, DevTools applies properties suitable to a development environment. These properties disable template caching, enable debug logging for the web group, and so on. As a result, we have this sensible development-time configuration without setting any properties.

Applications using DevTools restart whenever a file on the classpath changes. This is a very helpful feature in development, as it gives quick feedback for modifications.

By default, static resources, including view templates, don’t set off a restart. Instead, a resource change triggers a browser refresh. Notice this can only happen if the LiveReload extension is installed in the browser to interact with the embedded LiveReload server that DevTools contains.

How to Write Integration Tests?

When running integration tests for a Spring application, we must have an ApplicationContext.

To make our life easier, Spring Boot provides a special annotation for testing – @SpringBootTest. This annotation creates an ApplicationContext from configuration classes indicated by its classes attribute.

In case the classes attribute isn’t set, Spring Boot searches for the primary configuration class. The search starts from the package containing the test up until it finds a class annotated with @SpringBootApplication or @SpringBootConfiguration.

What Is Spring Boot Actuator Used For?

Essentially, Actuator brings Spring Boot applications to life by enabling production-ready features. These features allow us to monitor and manage applications when they’re running in production.

Integrating Spring Boot Actuator into a project is very simple. All we need to do is to include the spring-boot-starter-actuator starter in the pom.xml file:

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

Spring Boot Actuator can expose operational information using either HTTP or JMX endpoints. Most applications go for HTTP, though, where the identity of an endpoint and the /actuator prefix form a URL path.

Here are some of the most common built-in endpoints Actuator provides:

env: Exposes environment properties
health: Shows application health information
httptrace: Displays HTTP trace information
info: Displays arbitrary application information
metrics: Shows metrics information
loggers: Shows and modifies the configuration of loggers in the application
mappings: Displays a list of all @RequestMapping paths

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