Spring Boot Interview Questions and Answers

Spring Boot Interview Questions 

In this post we will look at Spring Boot Interview questions. Examples are provided with explanation. 

Q: What is Spring Boot?
A: Over the years spring has become more and more complex as new functionalities have been added. Just visit the page-https://spring.io/projects and we will see all the spring projects we can use in our application for different functionalities. If one has to start a new spring project we have to add build path or add maven dependencies, configure application server, add spring configuration . So a lot of effort is required to start a new spring project as we have to currently do everything from scratch. Spring Boot is the solution to this problem. Spring boot has been built on top of existing spring framework. Using spring boot we avoid all the boilerplate code and configurations that we had to do previously. Spring boot thus helps us use the existing Spring functionalities more robustly and with minimum efforts.
More details and miscellaneous examples 

Q: What are advantages of Spring Boot ?
A: The advantages of Spring Boot are

• Reduce Developement, Testing time and efforts.
• Use of JavaConfig helps avoid usage of XML.
• Avoid lots of maven imports and the various version conflicts.
• Provide Opinionated Development approach.
• Quick start to development by providing defaults.
• No Separate Web Server Needed.Which means that you no longer have to boot up Tomcat, Glassfish, or anything else.
• Requires less configuration-Since there is no web.xml file. Simply add classes annotated with@Configuration and then you can add methods annotated with@Bean, and Spring will automagically load up the object and manage it like it always has. You can even add @Autowired to the bean method to have Spring autowire in dependencies needed for the bean.
• Environment Based Configuration-Using these properties, you can pass into the application which environment you are using with:-Dspring.profiles.active={enviornment}. Spring will then load up the subsequent application properties file at (application-{environment}.properties) after loading up the main application properties file.

Q: Which build tool have you used to develop Spring Boot Application ?
A: Spring Boot application can be developed using Maven as well as Gradle. 
Spring Boot application using Maven 
Spring Boot application using Gradle 

Q: What is JavaConfig?
A: Spring JavaConfig is a product of the Spring community that provides a pure-Java approach to configuring the Spring IoC Container. It thus helps avoid using XML configurations. The advantages of using JavaConfig are
The advantages of JavaConfig are

• Object-oriented configuration. Because configurations are defined as classes in JavaConfig, users can take full advantage of object-oriented features in Java. One configuration class may subclass another, overriding its @Bean methods, etc.
• Reduced or eliminated XML configuration. The benefits of externalized configuration based on the principles of dependency injection have been proven. However, many developers would prefer not to switch back and forth between XML and Java. JavaConfig provides developers with a pure-Java approach to configuring the Spring container that is conceptually similar to XML configuration. It is technically possible to configure the container using only JavaConfig configuration classes, however in practice many have found it ideal to mix-and-match JavaConfig with XML.
• Type-safe and refactoring-friendly. JavaConfig provides a type-safe approach to configuring the Spring container. Thanks to Java 5.0's support for generics, it is now possible to retrieve beans by type rather than by name, free of any casting or string-based lookups.

Added security configuration without xml using java config.

Q:How to reload my changes on Spring Boot without having to restart server?
A: This can be achieved using DEV Tools. With this dependency any changes you save, the embedded tomcat will restart. Spring Boot has a Developer tools (DevTools) module which helps to improve the productivity of developers. One of the key challenge for the Java developers is to auto deploy the file changes to server and auto restart the server. Developers can reload changes on Spring Boot without having to restart my server. This will eliminates the need for manually deploying the changes every time. Spring Boot doesn’t have this feature when it has released it’s first version. This was a most requested features for the developers. The module DevTools does exactly what is needed for the developers. This module will be disabled in the production environment. It also provides H2-database console for better testing the application. The following dependency is used

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

The DevTool dependency usage for autorestart and H2 DB console is illustrated in this example

Q:What is Actuator in Spring Boot?
A: Spring boot actuator is one of the important feature in spring boot framework. Spring boot actuator helps you to access the current state of the running application in production environment. There are several metrics that has to be checked and monitored in the production environment. Even some external applications may be using those services to trigger the alert message to concerned person. Actuator module exposes set of REST endpoints that can be directly accessed as a HTTP URL to check the status.

Q:How to depoy Spring Boot application as a WAR?
A: Spring Boot WAR Deployment

Q:What is Docker? How to deploy Spring Boot Applications to Docker?
A: What is Docker
Deploying Spring Based WAR Application to Docker
Deploying Spring Based JAR Application to Docker 

Q:How to disable Actuator endpoint security in Spring Boot?
A: By default all sensitive HTTP endpoints are secured such that only users that have an ACTUATOR role may access them. Security is enforced using the standard HttpServletRequest.isUserInRole method.
We can disable security using - 
management.security.enabled=false
It is suggested to disable security only if the actuator endpoints are accessed behind firewall.

Q: How to run Spring boot application to custom port ?
A: In order to run a spring boot application on a custom port you can specify the port in application.properties.
server.port=8090 

Q: What is ELK stack?How to use it with Spring Boot?
A: The ELK Stack consists of three open-source products - Elasticsearch, Logstash, and Kibana from Elastic. 

• Elasticsearch is a NoSQL database that is based on the Lucene search engine.
• Logstash is a log pipeline tool that accepts inputs from various sources, executes different transformations, and exports the data to various targets. It is a dynamic data collection pipeline with an extensible plugin ecosystem and strong Elasticsearch synergy
• Kibana is a visualization UI layer that works on top of Elasticsearch.

These three projects are used together for log analysis in various environments. So Logstash collects and parses logs, Elastic search indexes and store this information while Kibana provides a UI layer that provide actionable insights.
Spring Boot + ELK stack 

Q: Have you written Test cases using Spring Boot ?
A: Spring Boot provides the @SpringBootTest for writing Unit Test Cases
Spring Boot Unit Test Simple Example

Q: What is YAML ?
A: YAML is a human-readable data serialization language. It is commonly used for configuration files.
Compared to properties file, YAML file is much more structured and less confusing in case we want to add complex properties in the configuration file. As can be seen YAML has hierarchical configuration data. 
Use YAML properties in Spring Boot

Q: How to implement security for Spring boot application ?
A: For Implementing security for Spring Boot we use the spring-boot-starter-security dependency and have to add the Security config. It requires very little code. Config class will have to extend WebSecurityConfigurerAdapter and override its methods.
Spring Boot Security example and explanation 

Q: Have you integrated Spring Boot and ActiveMQ ?
A: For integrating Spring Boot and ActiveMQ we use the spring-boot-starter-activemq dependency
It requires very little configuration and no boilerplate code.
Spring Boot ActiveMQ explanation 

Q: Have you integrated Spring Boot and Apache Kafka ?
A: For integrating Spring Boot and Apache Kafka we use the spring-kafka dependency.
Spring Boot + Apache Kafka Example 

Q: How to implement Pagination and Sorting with Spring Boot ?
A: Using Spring Boot achieving pagination is very simple. Using the Spring Data-JPA this is achieved passing pageable org.springframework.data.domain.Pageable to the repository methods.
Spring Boot pagination explanation

Q: What is Swagger ? Have you implemented it using Spring Boot ?
A: Swagger is widely used for visualizing APIs, and with Swagger UI it provides online sandbox for frontend developers. For the tutorial, we will use the Springfox implementation of the Swagger 2 specification. Swagger is a tool, a specification and a complete framework implementation for producing the visual representation of RESTful Web Services. It enables documentation to be updated at the same pace as the server. When properly defined via Swagger, a consumer can understand and interact with the remote service with a minimal amount of implementation logic. Thus Swagger removes the guesswork in calling the service.
Spring Boot + Swagger2 

Q: What is Spring Profiles ? How do you implement it using Spring Boot ?
A: Spring Profiles allows users to register beans depending on the profile(dev, test, prod etc). So when the application is running in DEVELOPMENT only certain beans can be loaded and when in PRODUCTION certain other beans can be loaded. Suppose our requirement is that the Swagger documentation be enabled only for the QA environment and disabled for all others. This can be done using Profiles. Spring Boot makes using Profiles very easy. 
Spring Boot + profiles 

Q: What is Spring Batch ? How do you implement it using Spring Boot ?
A: Spring Boot Batch provides reusable functions that are essential in processing large volumes of records, including logging/tracing, transaction management, job processing statistics, job restart, skip, and resource management. It also provides more advanced technical services and features that will enable extremely high-volume and high performance batch jobs though optimization and partitioning techniques.Simple as well as complex, high-volume batch jobs can leverage the framework in a highly scalable manner to process significant volumes of information. 
Spring Boot + Batch 

Q: What is FreeMarker Template? How do you implement it using Spring Boot ?
A: FreeMarker is a Java-based Template Engine, originally focusing on dynamic web page generation with MVC software architecture. The major advantage of using Freemarker is the complete separation of the Presentation layer and the Business Layer. The Programmers can work on the application code while the designers can work on the html page design. Finally using freemarker these can then be combined to give the final output page. 
Spring Boot + FreeMarker Example 

Q: How to implement Exception Handling using Spring Boot ?
A: Spring provides a very useful way to handle exceptions using ControllerAdvice. 
We will be implementing a ControlerAdvice class which will handle all exceptions thrown by the controller class.
Spring Boot Exception Handling

Q: What is caching? Have you used any caching framework with Spring Boot ?
A: A cache is an area of local memory that holds a copy of frequently accessed data that is otherwise expensive to get or compute. Have used Hazelcast for caching.
Spring Boot + Hazelcast Example

Q: Have you exposed a SOAP webservice endpoint using Spring Boot?
A: Yes. Using Spring Boot exposed a web service to be consumed. Used Contract first approach to generate the classes from wsdl. 
Spring Boot + SOAP Web Service Example 

Q: How did you perform database operations using Spring Boot ?
A: Spring Boot Tutorial-Spring Data JPA
Spring Boot JDBC Example 

Q: How to develop a full stack application using Spring Boot and Angular ?
A: In full stack application we expose the back end point to get the data. This data can then be used by any application or device as per the need. In future even if another front end device is to be used, there will not be much change and the new device will need to consume these end points. 

The project architecture we will be developing is as follows- 

Angular 7 + Spring Boot Tutorials
Q: How to upload a file using Spring ?
A: Spring Boot + File Upload Example

Q: How to implement interceptors with Spring Boot ?
A: Using Spring MVC HandlerInterceptor with Spring Boot 

Q: How to use schedulers with Spring Boot ?
A: Spring Boot Task Scheduler Example 

Q: Which all starter maven dependencies have you used ?
A: Have used different starter dependencies like spring-boot-starter-activemq dependency, spring-boot-starter-security dependency, spring-boot-starter-web dependency. 
This helps in adding less number of dependencies and also in reducing version conficts. 
Spring Boot Security example and explanation 

Q: What is CSRF attack? How to enable CSRF protection against it?
A: CSRF stands for Cross-Site Request Forgery. It is an attack that forces an end user to execute unwanted actions on a web application in which they are currently authenticated. CSRF attacks specifically target state-changing requests, not theft of data, since the attacker has no way to see the response to the forged request. 
Spring Boot Security - Enabling CSRF Protection 

Q: How to use Form Login Authentication using Spring Boot?
A: Spring Boot Form Security Login Hello World Example 

Q: What is OAuth2? How to implement it using Spring Boot?
A: Spring Boot + OAuth2 implementation 

Q: What is GZIP?How to implement it using Spring Boot?
A: gzip is a file format and a software application used for file compression and decompression.
Spring Boot + GZIP Compression

Q: Have you used any integration framework with Spring Boot?
A: Have integrated Apache Camel with Spring Boot. Made use of Apache Camel Spring Boot starter dependency.Spring Boot +Apache Camel 

Q: What is Apache Freemarker? When to use it instead of JSP? How to integrate it with Spring Boot?
A:JSP is tailor made for Web pages, Freemarker template is a more generic templating language - it can be used to generate html, plain text, emails, etc.
Spring Boot + FreeMarker Example

Q: When will you use WebSockets? How tto implement it using Spring Boot?
A:WebSocket is a computer communications protocol, providing full-duplex communication channels over a single TCP connection. 

• WebSocket are bi-directional - Using WebSocket either client or server can initiate sending a message.
• WebSocket are Full Duplex - The client and server communication is independent of each other.
• Single TCP connection - The initial connection is using HTTP, then this connection gets upgraded to a socket based connection. This single connection is then used for all the future communication
• Light - The WebSocket message data exchange is much lighter compared to http.

Spring Boot + WebSockets Example

Q: What is AOP? How to use it with Spring Boot?
A:During software development, functions that span multiple points of an application are called cross-cutting concerns. These cross-cutting concerns differ from the application’s main business logic. Hence ,separating these cross-cutting concerns from the business logic is where aspect-oriented programming (AOP) comes into picture.
Spring Boot + AOP Example 

Q: What is Apache Kafka? How to integrate it with Spring Boot?
A: Apache Kafka is a distributed publish-subscribe messaging system. It is a scalable, fault-tolerant, publish-subscribe messaging system which enables us to build distributed applications. It is an Apache Top Level project. Kafka is suitable for both offline and online message consumption.
Spring Boot + Apache Kafka Example 

Q: How can we monitor all the Spring Boot Microservices?
A:Spring Boot provides actuator endpoints to monitor metrics of individual microservices. These endpoints are very helpful for getting information about applications like if they are up, if their components like database etc are working good. But a major drawback or difficulty about using actuator enpoints is that we have to individually hit the enpoints for applications to know their status or health. Imagine microservices involving 50 applications, the admin will have to hit the actuator endpoints of all 50 applications. To help us deal with this situation, we will be using open source project located at https://github.com/codecentric/spring-boot-admin. 
Built on top of Spring Boot Actuator, it provides a web UI to enable us visualize the metrics of multiple applications.
Spring Boot Admin

Q: Have you used any Spring Cloud Components with Spring Boot?
A: Have used Spring Cloud components like Netflix Eureka for Service Registration,Ribbon for Load Balancing.
Spring Boot + Cloud Components 
Spring Cloud interview Questions

Q: How to deploy Spring Boot Application to Pivotal Cloud Foundry(PCF)?
A: Deploying Spring Boot Application to PCF 

Q: How to deploy Spring Boot + MySQL Application to Pivotal Cloud Foundry(PCF)?
A: Pivotal Cloud Foundry Tutorial - Deploying Spring Boot + MySQL Application to PCF 

Q: How to deploy Spring Boot + RabbitMQ Application to Pivotal Cloud Foundry(PCF)?
A: Pivotal Cloud Foundry Tutorial - Deploying Spring Boot + RabbitMQ Application to PCF 

Spring Boot

Spring Boot is the best Java framework for microservices. We recommend you to become an expert at Spring Boot!

Q : Spring Boot vs Spring MVC vs Spring - How do they compare?

Spring Framework

Most important feature of Spring Framework is Dependency Injection. At the core of all Spring Modules is Dependency Injection or IOC Inversion of Control.

When DI or IOC is used properly, we can develop loosely coupled applications. And loosely coupled applications can be easily unit tested.

Spring MVC

Spring MVC Framework provides decoupled way of developing web applications. With simple concepts like Dispatcher Servlet, ModelAndView and View Resolver, it makes it easy to develop web applications.

Spring Boot

The problem with Spring and Spring MVC is the amount of configuration that is needed.

<bean
        class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix">
            <value>/WEB-INF/views/</value>
        </property>
        <property name="suffix">
            <value>.jsp</value>
        </property>
  </bean>
  
  <mvc:resources mapping="/webjars/**" location="/webjars/"/>

Spring Boot solves this problem through a combination of Auto Configuration and Starter Projects. Spring Boot also provide a few non functional features to make building production ready applications faster.

For complete answer with code examples refer - Spring Boot vs Spring vs Spring MVC

Q : What is Auto Configuration?

The problem with Spring and Spring MVC is the amount of configuration that is needed.
<bean
        class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix">
            <value>/WEB-INF/views/</value>
        </property>
        <property name="suffix">
            <value>.jsp</value>
        </property>
  </bean>
  
  <mvc:resources mapping="/webjars/**" location="/webjars/"/>
Can we bring more intelligence into this? When a spring mvc jar is added into an application, can we auto configure some beans automatically?
Spring Boot looks at a) Frameworks available on the CLASSPATH b) Existing configuration for the application. Based on these, Spring Boot provides basic configuration needed to configure the application with these frameworks. This is called Auto Configuration.



For complete answer with code examples refer Auto Configuration.

Q : What are Spring Boot Starter Projects?


Starters are a set of convenient dependency descriptors that you can include in your application. You get a one-stop-shop for all the Spring and related technology that you need, without having to hunt through sample code and copy paste loads of dependency descriptors.


For example, if you want to get started using Spring and JPA for database access, just include the spring-boot-starter-data-jpa dependency in your project, and you are good to go.

Q : Can you explain more about Starters with an example?

Let’s consider an example starter - Spring Boot Starter Web.

If you want to develop a web application or an application to expose restful services, Spring Boot Start Web is the starter to pick. Lets create a quick project with Spring Boot Starter Web using Spring Initializr.

Dependency for Spring Boot Starter Web


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




Following screenshot shows the different dependencies that are added in to our application



Dependencies can be classified into:

Spring - core, beans, context, aop
Web MVC - (Spring MVC)
Jackson - for JSON Binding
Validation - Hibernate Validator, Validation API
Embedded Servlet Container - Tomcat
Logging - logback, slf4j


Any typical web application would use all these dependencies. Spring Boot Starter Web comes pre packaged with these.


As a developer, I would not need to worry about either these dependencies or their compatible versions.


Q : What are the other Starter Project Options that Spring Boot provides?

Spring Boot also provides other starter projects including the typical dependencies to develop specific type of applications

spring-boot-starter-web-services - SOAP Web Services
spring-boot-starter-web - Web & RESTful applications
spring-boot-starter-test - Unit testing and Integration Testing
spring-boot-starter-jdbc - Traditional JDBC
spring-boot-starter-hateoas - Add HATEOAS features to your services
spring-boot-starter-security - Authentication and Authorization using Spring Security
spring-boot-starter-data-jpa - Spring Data JPA with Hibernate
spring-boot-starter-data-rest - Expose Simple REST Services using Spring Data REST


Q : How does Spring enable creating production ready applications in quick time?

Spring Boot aims to enable production ready applications in quick time. Spring Boot provides a few non functional features out of the box like caching, logging, monitoring and embedded servers.

spring-boot-starter-actuator - To use advanced features like monitoring & tracing to your application out of the box
spring-boot-starter-undertow, spring-boot-starter-jetty, spring-boot-starter-tomcat - To pick your specific choice of Embedded Servlet Container
spring-boot-starter-logging - For Logging using logback
spring-boot-starter-cache - Enabling Spring Framework’s caching support


What is the minimum baseline Java Version for Spring Boot 2 and Spring 5?

Spring Boot 2.0 requires Java 8 or later. Java 6 and 7 are no longer supported.

Recommended Reading

https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.0.0-M1-Release-Notes


Q : What is the easiest approach to create a Spring Boot Project?

Spring Initializr http://start.spring.io/ is great tool to bootstrap your Spring Boot projects.



As shown in the image above, following steps have to be done

Launch Spring Initializr and choose the following

Choose com.in28minutes.springboot as Group
Choose student-services as Artifact
Choose following dependencies

Web
Actuator
DevTools




Click Generate Project.
Import the project into Eclipse. File -> Import -> Existing Maven Project.


Q : Is Spring Initializr the only way to create Spring Boot Projects?

No.

Spring Initializr makes it easy to create Spring Boot Projects. But you can setup a maven project and add the right dependencies to start off.

In our Spring course, we use 2 approaches to create projects.

The first one is start.spring.io.
The other one - setting up a project manually is used in the Section titled - “Basic Web Application”


Setting up a maven project manually

Here are the important steps:

In Eclipse, Use File -> New Maven Project to create a new project.
Add dependencies.
Add the maven plugins!
Add the Spring Boot Application class


You are ready to go!

Q : Why do we need spring-boot-maven-plugin?

spring-boot-maven-plugin provides a few commands which enable you to package the code as a jar or run the application

spring-boot:run runs your Spring Boot application.
spring-boot:repackage repackages your jar/war to be executable.
spring-boot:start and spring-boot:stop to manage the lifecycle of your Spring Boot application (i.e. for integration tests).
spring-boot:build-info generates build information that can be used by the Actuator.


Q : How can I enable auto reload of my application with Spring Boot?

Use Spring Boot Developer Tools.

Adding Spring Boot Developer Tools to your project is very simple.

Add this dependency to your Spring Boot Project pom.xml


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




Restart the application.

You are all Set.

If you would want to auto load the page as well, you can look at LiveReload

http://www.logicbig.com/tutorials/spring-framework/spring-boot/boot-live-reload/.


In my trials, we found LiveReload buggy. Do let us know if you have a better experience with it.

Q : What and Why Embedded Servers?

Think about what you would need to be able to deploy your application (typically) on a virtual machine.

Step 1 : Install Java
Step 2 : Install the Web/Application Server (Tomcat/Websphere/Weblogic etc)
Step 3 : Deploy the application war


What if we want to simplify this?

How about making the server a part of the application?


You would just need a virtual machine with Java installed and you would be able to directly deploy the application on the virtual machine. Isn’t it cool?


This idea is the genesis for Embedded Servers.

When we create an application deployable, we would embed the server (for example, tomcat) inside the deployable.


For example, for a Spring Boot Application, you can generate an application jar which contains Embedded Tomcat. You can run a web application as a normal Java application!


Embedded server is when our deployable unit contains the binaries for the server (example, tomcat.jar).

Q : How can I add custom JS code with Spring Boot?

Create a folder called static under resources folder. You can put your static content in that folder.

For your example the path to myapp.js would be resources\static\js\myapp.js

You can refer to it in jsp using


<script src="/js/myapp.js"></script>




Error : HAL browser gives me unauthorized error - Full authentication is required to access this resource. How can I fix it?


{
  "timestamp": 1488656019562,
  "status": 401,
  "error": "Unauthorized",
  "message": "Full authentication is required to access this resource.",
  "path": "/beans"
}




Two options

Option 1 : Disable security

application.properties


management.security.enabled: FALSE 




Option 2 : Search for password in the log and pass it in the request header

Q : What is Spring Data?

From http://projects.spring.io/spring-data/


Spring Data’s mission is to provide a familiar and consistent, Spring-based programming model for data access while still retaining the special traits of the underlying data store. It makes it easy to use data access technologies, relational and non-relational databases, map-reduce frameworks, and cloud-based data services.


To make it simpler, Spring Data provides Abstractions (interfaces) you can use irrespective of underlying data source.

An example is shown below


interface TodoRepository extends CrudRepository<Todo, Long> {




You can define a simple repository and use it to insert, update, delete and retrieve todo entities from the database - without writing a lot of code.

Q : What is Spring Data REST?

Spring Data REST can be used to expose HATEOAS RESTful resources around Spring Data repositories.

An example using JPA is shown below


@RepositoryRestResource(collectionResourceRel = "todos", path = "todos")
public interface TodoRepository
        extends PagingAndSortingRepository<Todo, Long> {




Without writing a lot of code, we can expose RESTful API around Spring Data Repositories.

A few example REST Services are shown below:

POST

URL : http://localhost:8080/todos
Use Header : Content-Type:application/json
Request Content



{
  "user": "Jill",
  "desc": "Learn Hibernate",
  "done": false
}




Response Content


{
  "user": "Jill",
  "desc": "Learn Hibernate",
  "done": false,
  "_links": {
    "self": {
      "href": "http://localhost:8080/todos/1"
    },
    "todo": {
      "href": "http://localhost:8080/todos/1"
    }
  }
}




The response contains the href of the newly created resource.

Q : How does path=”users”, collectionResourceRel=”users” work with Spring Data Rest?


@RepositoryRestResource(collectionResourceRel = "users", path = "users")

public interface UserRestRepository extends
PagingAndSortingRepository<User, Long>




path - The path segment under which this resource is to be exported.
collectionResourceRel - The rel value to use when generating links to the collection resource. This is used when generating HATEOAS links.


Q : What happens in the background when a Spring Boot Application is “Run as Java Application”?

If you are using Eclipse IDE, Eclipse maven plugin ensures that as soon as you add a dependency or make a change to the class file, it is compiled and ready in the target folder! And after that its just like any other Java application.

When you launch the java application, then the spring boot auto configuration magic kicks in.

It launches up tomcat when it sees that you are developing a web application!


Q : Can we use jetty instead of tomcat in spring-boot-starter-web?

Remove the existing dependency on spring-boot-starter-web and add these in.


<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>




Q : How to generate a WAR file with Spring Boot?

Recommended Reading

https://spring.io/guides/gs/convert-jar-to-war/


Here’s the direct link to spring documentation

https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#build-tool-plugins-maven-packaging


Q : How to deploy to a different server with with Spring Boot?

You would need to do 2 Steps

Generate a war from the project.
Deploy it to your favourite server (Websphere or Weblogic or Tomcat or …).


Step 1 : This getting started guide should help - https://spring.io/guides/gs/convert-jar-to-war/

Step 2 : Depends on your server

Q : What is the difference between RequestMapping and GetMapping?

RequestMapping is generic - you can use with GET, POST, PUT or any of the other request methods using the method attribute on the annotation.
GetMapping is specific to GET request method. It’s just an extension of RequestMapping to improve clarity.


Q : Why do we recommend not to use Spring Data Rest in real world applications?

We think Spring Data Rest is Good for quick prototyping! Be cautious about using this in Big applications!

With Spring Data REST you are exposing your database entitities directly as REST Services.

When you design RESTful services, Best design practices suggests that your interface should consider two important things

Your Domain Model
Your Consumers


With Spring Data REST, you are not considering either of those. You just expose entities as REST Services.

Thats why we suggest to use it for quick prototyping or the initial evolution of a project. It may not be a great idea for a fully evolved project.

Q : How do I change the package name of a project in Spring Initializer?

Good news is you can customise it. Click the link “Switch to the full version.“. You would be able to configure the package name you would want!

Q : Where can I find the complete list of properties that can be configured in application.properties?

Here’s the complete guide

https://docs.spring.io/spring-boot/docs/current/reference/html/common-application-properties.html


Q : What is the difference between JPA and Hibernate?

Short Story

JPA is a specification/Interface
Hibernate is one of JPA implementations


When we use JPA, we use the annotation and interfaces from javax.persistence package, without using the hibernate import packages.

We recommend using JPA annotations as we are not tied to Hibernate as implementation. Later (I know - <1% Chance), we can use another JPA implementation.

Q : In which layer, should the boundary of a transaction start?

We recommend managing transactions in the Service layer. Logic for business transactions is in the business/service layer and you would want to enforce transaction management at that level.

Q : What are the dependencies needed to start up a JPA Application connecting to in memory database H2 with Spring Boot?

In a Spring Boot project, you should be able to launch up H2 Console as long as you ensure the following dependencies are on the class path.

web starter
h2
data jpa starter


The exact dependencies are shown below:


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

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

<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>




A few tips:

An in-memory database is live only during the time of execution of the application. It is an efficient way to learn a framework.
This is not how you want your real world applications to behave.
We explain how to connect to a database of your choice in the answer to the question “How do we connect to a external database?”.


Q : How is Hibernate chosen as the default implementation for JPA without any configuration?

Because of Spring Boot Auto Configuration.

This is the dependency we added in


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




The Starter spring-boot-starter-data-jpa has a transitive dependency on Hibernate and JPA.

When Spring Boot sees Hibernate in the class path, it auto configures it as the default JPA Implementation.

Q : Where is the database connection info specified? How does it know to automatically connect to H2?

Thats Spring Boot Autoconfiguration magic.

From https://docs.spring.io/spring-boot/docs/current/reference/html/using-boot-auto-configuration.html


Spring Boot auto-configuration attempts to automatically configure your Spring application based on the jar dependencies that you have added. For example, If HSQLDBis on your classpath, and you have not manually configured any database connection beans, then we will auto-configure an in-memory database


More Reading

http://www.springboottutorial.com/spring-boot-auto-configuration


Q : How do we connect to a external database like MSSQL or oracle?

Let’s consider one of those as an example - MySQL

Step 1 - Add dependency for mqsql connector to pom.xml


<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
</dependency>




Step 2 - Remove H2 Dependency from pom.xml

Or atleast make its scope as test


<!--
<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>test</scope>
</dependency>
-->




Step 3 - Setup your My SQL Database

For more check out - https://github.com/in28minutes/jpa-with-hibernate#installing-and-setting-up-mysql


Step 4 - Configure your connection to My SQL Database

Configure application.properties


spring.jpa.hibernate.ddl-auto=none
spring.datasource.url=jdbc:mysql://localhost:3306/todo_example
spring.datasource.username=todouser
spring.datasource.password=YOUR_PASSWORD




Step 5 - Restart and You are ready!

That’s it

Q : What is the default h2 database name configured by Spring Boot? Why is the default database name testdb?

This is where all the default values in application.properties are listed

https://docs.spring.io/spring-boot/docs/current/reference/html/common-application-properties.html


Look for the property below


spring.datasource.name=testdb # Name of the datasource.




If you are using an H2 in-memory database, thats exactly the name that Spring Boot uses to setup your H2 database.

Q : What happens if H2 is not in the classpath?

You get this error


Cannot determine embedded database driver class for database type NONE




Add H2 to the pom.xml and Restart your server


<dependency>
    <groupId>com.h2database</groupId>
    <artifactId>h2</artifactId>
    <scope>runtime</scope>
</dependency>




Q : Can you give an example for ReadOnly as true in Transaction management?

When you read stuff from the database, user details or any other details, you wanna set read only on the transaction so that Hibernate does not need to check for changes to the entities. This is more efficient.


Q : What is best way to expose custom application configuration with Spring Boot?

The problem with @Value is that you would have your configuration values distributed through out your application. A better option would be to have a centralized approach.

You can define a configuration component using @ConfigurationProperties.


@Component
@ConfigurationProperties("basic")
public class BasicConfiguration {
    private boolean value;
    private String message;
    private int number;





The values can be configured in application.properties


basic.value: true
basic.message: Dynamic Message
basic.number: 100




Q : What is the need for Profiles?

Enterprise application development is complex. You have multiple environments

Dev
QA
Stage
Production


You want to have different application configuration in each of the environments.


Profiles help to have different application configuration for different environments.


Spring and Spring Boot provide features where you can specify

What is the configuration for various environments in different profiles?
Set the active profile for a specific environment.


Spring Boot would pick up the application configuration based on the active profile that is set in a specific environment.

Q : How can you use profiles to configure environment specific configuration with Spring Boot?

Profile is nothing but a key to identify an environment.

In this example, we will use two profiles

dev
prod


The default application configuration is present in application.properties. Let’s consider an example.

application.properties


basic.value= true
basic.message= Dynamic Message
basic.number= 100




We would want to customize the application.properties for dev profile. We would need to create a file with name application-dev.properties and override the properties that we would want to customize.

application-dev.properties


basic.message: Dynamic Message in DEV




Similarly you can configure properties for prod profile.

application-prod.properties


basic.message: Dynamic Message in Prod




Once you have profile specific configuration, you would need to set the active profile in an environment.

There are multiple ways of doing this

Using -Dspring.profiles.active=prod in VM Arguments
Use spring.profiles.active=prod in application.properties