Develop Reference

Why does the addition of a dependency in Maven trigger functionality?

I have a simple question: I'm just getting started with Open API 3. For this purpose I have added the following dependency in Maven.
<dependency>
<groupId>org.springdoc</groupId>
<artifactId>springdoc-openapi-ui</artifactId>
<version>1.2.30</version>
</dependency>
With the addition of this dependency, can I access the service via localhost:8082/v3/api-docs without having previously set anything and called a function of the dependency? How can this happen? What is the concept behind this ?

Adding the OpenAPI dependency in your Maven pom.xml just adds the librar(ies) to your project. That's all.
If this were a "traditional" project (like a JSP web app, for example), you'd have to write the code to create the web service (e.g. "localhost:8082/v3/api-docs").
But it sounds like your project might be Spring Boot:
https://developer.ibm.com/technologies/java/tutorials/j-spring-boot-basics-perry/
If you let it, Spring Boot will use its #EnableAutoConfiguration
annotation to automatically configure your application.
Auto-configuration is based on the JARS in your classpath and how
you’ve defined your beans:
Spring Boot uses the JARs you have specified to be present in the CLASSPATH to form an opinion about how to configure certain automatic
behavior. For example, if you have the H2 database JAR in your
classpath and have configured no other DataSource beans, then your
application will be automatically configured with an in-memory
database.
Spring Boot uses the way you define beans to determine how to automatically configure itself. For example, if you annotate your JPA
beans with #Entity, then Spring Boot will automatically configure JPA
such that you do not need a persistence.xml file.

It is called convention over configuration.
Wiki link https://en.wikipedia.org/wiki/Convention_over_configuration

spring boot wants me to start server because I have servlet on classpath

today I faced following problem:
One of my core dependencies unfortunately pulls servlet.api to my classpath. Because of this, my spring-boot thinks I'm automatically a server, while I'm a desktop app, and does not want to start without some factories needed to web.
This is what it says:
Caused by: org.springframework.context.ApplicationContextException: Unable to start ServletWebServerApplicationContext due to missing ServletWebServerFactory bean.
Unfortunately I can't get rid of this dependency, and for them to fix their transitive dependencies will probably take some time.
Is there any hack to walk around this?
Thanks

If you're using Spring boot 2.x, you can disable the web application by setting the spring.main.web-application-type property to none:
spring.main.web-application-type=none
If you're using Spring boot 1.x, you could set the spring.main.web-environment property:
spring.main.web-environment=false
The reason this changed is because Spring boot 2.x can now be configured to be either reactive, servlet-based or none, while in Spring boot 1.x it was either servlet-based or none (so it could be just a boolean).
Alternatively, you can also use a custom SpringApplication instance as mentioned by the documentation (and in the comments):
public static void main(String[] args) {
new SpringApplicationBuilder(Application.class)
.web(WebApplicationType.NONE) // Use this for Spring boot 2.x
.web(false) // Use this for Spring boot 1.x
.run(args);
}
Not all Spring applications have to be web applications (or web services). If you want to execute some code in a main method but also bootstrap a Spring application to set up the infrastructure to use, you can use the SpringApplication features of Spring Boot. A SpringApplication changes its ApplicationContext class, depending on whether it thinks it needs a web application or not. The first thing you can do to help it is to leave server-related dependencies (e.g. servlet API) off the classpath. If you cannot do that (for example, you run two applications from the same code base) then you can explicitly call setWebApplicationType(WebApplicationType.NONE) on your SpringApplication instance or set the applicationContextClass property (through the Java API or with external properties).

You are missing some required JAR file to run it as web application. Please make sure to that you have included spring-boot-starter-web.jar file.
If we are using maven, do it as follows.
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>1.3.2.RELEASE</version>
</dependency>

Why do we need an application container when deploying a Spring Boot app to Openshift

It maybe a trivial question for experienced web application developers, but for me as a new developer, I cannot understand that why do we need an application container(like Tomcat or Wildfly) when deploying a Spring Boot web application to Openshift, Heroku, or Google App Engine, etc? My understanding is that Spring Boot already contains an embedded container (Tomcat). Can someone explain this to me? Thanks

SpringBoot is Java API that relies on an embedded Java Servlet engine to support the API calls. These dependencies are typically pulled in by Maven as dependencies. So for the end user, it just looks like a FAR JAR with a bunch dependencies included (where one of those dependencies is Embed Tomcat, Jetty or Undertow for example)
More information can be found on the main SpringBoot project page.

MVN WebApplication Template with Spring Securing + Hibernate

I am looking for a mvn project template that can be downloaded and used as a kick-off for a web application.
The requeriments are:
Spring security (for user authentication and pages accesing control)
Hibernate integration (for data persistance like users and more)
The application must run on tomcat (i use TomEE)
I have a web application already running with pages and servlets and daos, persistence.xml and more. The problem is that i cant find the way to integrate this app with spring security, and for this reason i am looking for a project template...

Give Spring Boot a shot. You can either use Spring Tool Suite or Spring Initializer site to get a secure web app running and using hibernate as ORM.
You can later choose to run the app in the embedded Tomcat/Jetty or package as WAR and deploy in container of your choice.

Using spring core functionalities in tomcat based jersey application

I want to use spring's dependency injection for now(other core functionalites later maybe) in tomcat application.
I want to set up spring 2.5.5 in tomcat7, But don't have clarity on how to do this.
Specifically I am confused because I don't know whether to use Spring MVC or use just spring in tomcat.
I found this question helpful: Tomcat with Spring, But still didn't get the whole scenario on how to setup tomcat with spring.

You can do either, using just core spring with tomcat is fine. MVC provides additional functionality.
Take a look at the spring source examples on github, and read their docs.
(BTW I thouroughy spring-MVC component - it really saves time developing webapps)