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Maven child module in parent pom as dependency

I'm trying to create a maven spring-boot project with multiple modules. I have created a parent module with packaging type pom and many children submodules with packaging type jar.
So my parent's pom.xml looks like:
<groupId>Creator</groupId>
<artifactId>DPAI</artifactId>
<packaging>pom</packaging>
<version>1.0-SNAPSHOT</version>
<modules>
<module>starter</module>
<module>DatabaseApi</module>
...
</modules>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.2.4.RELEASE</version>
</parent>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
</dependencies>
One of submodules: starter contains only starting class annotated with #SpringBootApplicatoion and in its pom.xml there is a section with other child artifacts like:
<parent>
<artifactId>DPAI</artifactId>
<groupId>Creator</groupId>
<version>1.0-SNAPSHOT</version>
</parent>
<artifactId>starter</artifactId>
<packaging>jar</packaging>
<dependencies>
<dependency>
<groupId>Creator</groupId>
<artifactId>DatabaseApi</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>
...
</dependencies>
So I'm trying to do some refactoring and move Main.class and all dependencies to my parent's pom, but it doesn't compile with error with message that my dependencies referencing itself.
In my opinion, the problem is that my parent pom contains section with it's own submodules. Parent of that submoduls is the same pom, where I try to add described dependencies

The parent.pom can't contain any java code, only Maven specifics e.g. See: https://howtodoinjava.com/maven/maven-parent-child-pom-example/#parent-content
Maybe tell us, what you want to achieve.

In a Maven multi module project you usually have a parent Pom (with packaging Pom) and several modules at the same level as you already set your project up.
Build the modules without dependecies on your code first, the the dependent modules: In your parent Pom change the order of the modules to
<modules>
<module>DatabaseApi</module>
<module>starter</module>
...
</modules>

So I'm trying to do some refactoring and move Main.class and all
dependencies to my parent's pom
I dont think this is possible. Your parent pom is actually of type pom, meaning you're not actually supposed to have any java code in it. Its meant to hold the versions of jars used in your child modules. You can relate this to the spring-boot-parent module. When we declare the spring-boot-parent module in a spring boot project, your adding your project as a child of the spring-boot-parent. And the parent will manage the versions of all of your dependencies.
I think the best way forward would be to maintain all your service related code in your spring-boot module. Filters, controllers,etc. The other stuff like your jdbc, integration layers can be maintained in other child modules and then referred to the spring module as jar references similar to your example.

So I'm trying to do some refactoring and move Main.class and all
dependencies to my parent's pom,
I'm not 100% sure if Maven would support something like the following in the parent POM itself:
<dependencies>
<dependency>
<groupId>${project.groupId}</groupId>
<artifactId>DatabaseApi</artifactId>
<version>${project.version}</version>
</dependency>
...
</dependencies>
But for sure it won't support Java classes in a Module with pom-packaging (such as parent modules or multi-module modules). The compiler:compile goal etc. are not bound to any phase for pom-packaging by default. In other words: Maven does not compile Java classes for pom-modules by default.
My recommendation:
Keep the SpringBootApplication in a Java-based module. For Spring MVC/ WebFlux application I usually create a "-web" module with:
SpringBootApplication
web service controllers
http/ web filters
global configs such as: security, swagger, async
application.yml
...
It's also the module where I configure the Spring Boot Plugin to create an executable JAR.

Vaadin problem in Spring Boot multi module project

I made up an example spring boot project running with Vaadin (latest version). I only have one view:
#Route
public class MainView extends VerticalLayout {
The UI was working like a charm, then I had to refactor the project in modules.
I put the SpringBootApplication in a module and Vaadin in another one. I'm getting into modules, so I don't know exactly how they interact, but I had to put the dependency in the boot pom to the vaadin pom in order to let it start.
Now it is not working, when I call localhost it says
Could not navigate to ''
Reason: Couldn't find route for ''
Available routes:
This detailed message is only shown when running in
development mode.
The spring boot application:
#SpringBootApplication
#ComponentScan(basePackages = {"my.app"})
#EntityScan(basePackages = {"my.app"})
#EnableJpaRepositories(basePackages = {"my.app"})
#EnableJpaAuditing
public class LicensemanagerApplication
boot module pom.xml dependency snippet:
<dependency>
<groupId>${project.groupId}</groupId>
<artifactId>app_frontend</artifactId>
<version>${project.version}</version>
<scope>runtime</scope>
</dependency>
app_frontend module pom.xml dependency snippet:
<dependency>
<groupId>com.vaadin</groupId>
<artifactId>vaadin-spring-boot-starter</artifactId>
</dependency>
.............
<dependencyManagement>
<dependencies>
<dependency>
<groupId>com.vaadin</groupId>
<artifactId>vaadin-bom</artifactId>
<version>13.0.0</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>

Vaadin by default only looks for #Route annotated classes within the same package that contains the #SpringBootApplication annotation. To make it look in other packages, you need to pass those as the value to the #EnableVaadin annotation, e.g. #EnableVaadin({"my.app"}).

Spring Boot BOM for non boot spring project

I'd like to use spring boot specific BOM for my non-boot spring project. Is it acceptable to include spring-boot-dependencies or platform-bom and what's better?
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-dependencies</artifactId>
<version>...</version>
<type>pom</type>
<scope>import</scope>
</dependency>
or
<dependency>
<groupId>io.spring.platform</groupId>
<artifactId>platform-bom</artifactId>
<version>...</version>
<type>pom</type>
<scope>import</scope>
</dependency>
Thanks.

Both are valid approaches.
To quote the Spring IO Platform documentation :
Spring IO Platform builds on top of Spring Boot
So, they provide what you search for a non spring boot application : a set of consistent dependencies and some specific beans provided by Spring Boot.
Now, personally, I would chose org.springframework.boot:spring-boot-dependencies as Spring Boot is mature, have an important and active community and is often updated.
See the spring-boot github.
It is not the case of the spring-io/platform.

How to determine the <parent> dependency for a set of springframework dependencies

I like to know if below is possible and how.
I was following a tutorial for spring boot and it was mentioned there we can have a parent dependency.
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.5.6.RELEASE</version>
</parent>
And then define the dependencies without the version number.
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
This will add the dependencies version 1.5.6.RELEASE of spring-boot-starter and spring-boot-starter-web in to the projects dependencies.
Just like that I want to find what is the <parent> code snippet for the following dependencies I need to add in to a new project.
Dependencies in <groupId>org.springframework</groupId>. I need to use the version 4.3.9.RELEASE.
spring-context
spring-jdbc
spring-test
Thanks!

If you are using Spring Boot then these three dependencies will be provided for you by the following starters:
spring-test will be provided by spring-boot-starter-test
spring-context will be provided by spring-boot-starter-data-jpa
spring-jdbc will be provided by spring-boot-starter-jdbc
So, with the following parent:
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.5.6.RELEASE</version>
</parent>
... if you add these dependencies:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-jdbc</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
... then you will get
spring-context
spring-jdbc
spring-test
However, Spring Boot 1.5.6.RELEASE depends on v4.3.10.RELEASE of those core Spring libraries not 4.3.9.RELEASE as suggested in your question. Typically, you would accept Spring's curation of dependencies so if Sping provides 4.3.10.RELEASE then either (a) you should use that version or (b) downgrade Spring Boot toa version which provides 4.3.9.RELEASE.
Read on for details on how to identify the correct starter for a given curated library ...
The spring-boot-starter-parent is a special starter that provides useful Maven defaults and a dependency-management section which defines numerous dependencies which you might want to use in your POM. These dependencies are often referred to as "curated" or "blessed" and since they are defined in a dependency-management section somewhere in the maven hierarchy you can refer to them in your POM without a version tag (i.e. they inherit the version from the dependency-management section entry.)
You can see the spring-boot-starter-parent POM here and peeking inside you can see that it references the spring-boot-dependencies POM here.
Looking at your question you mentioned that you can declare a dependency like so ...
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
... this is because the spring-boot-dependencies POM declares the following:
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>${revision}</version>
</dependency>
So, the parent and the starters are just a means of wrapping up dependency declarations and making them easier for application developers to use. The Spring docs summarise this as:
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 technologies 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, include the spring-boot-starter-data-jpa dependency in your project.
However, this does not mean that all dependencies must be declared via parents or starters so, if you are not using Spring Boot then you can declare a dependency without using a parent or a starter and what you have described in your question (declaring dependencies on 3 core Spring libraries) can be safely covered by simply depending on those 3 libraries explicitly. For example, just add the following to your your pom.xml:
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>4.3.9.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-jdbc</artifactId>
<version>4.3.9.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-test</artifactId>
<version>4.3.9.RELEASE</version>
<scope>test</scope>
</dependency>

Since you are going though the tutorials I'm assuming you are new to spring.
The folks at spring were nice enough to setup a site that generates projects.
It is very easy to use. I recommend trying that while learning. Download a few apps with the dependencies you want and look at how they are set up.
Once you are comfortable and want to dive deeper, read #glytching's answer again, it is very good.

Use spring-framework-bom if you don't use Spring Boot and need Spring Framework dependencies only:
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-framework-bom</artifactId>
<version>4.3.9.RELEASE</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
In such case dependency would be without version was specified:
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-jdbc</artifactId>
</dependency>
Also, yet another option exists if you use Spring Boot but you don't want to use spring-boot-starter-parent as parent artifact:
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-dependencies</artifactId>
<version>1.5.9.RELEASE</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
See Spring Boot docs for more details. An important note from the docs:
Each release of Spring Boot is associated with a base version of the Spring Framework so we highly recommend you to not specify its version on your own.
It means that you should use Spring Framework version is defined for Spring Boot.

Can I Add SpringBoot Framework to Existing Maven Project

I have a Maven Project in Eclipse and now I need to add database connectivity. My textbook did all json tutorials in Maven. Now in this chapter on JDBC they are using SpringBoot.
Can I convert the project to SpringBoot? Or start a SpringBoot and import my previous Maven classes.

Here is described how to use maven for a SpringBoot Project.
You will need to modify your existing pom.xml to add something like this to make it a SpringBoot Project:
<!-- Inherit defaults from Spring Boot -->
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.2.2.RELEASE</version>
</parent>
<!-- Add typical dependencies for a web application -->
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
</dependencies>
<!-- Package as an executable jar -->
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>

Convert Maven Project to SpringBoot
Ist option :Spring Boot Using parent POM
Add following dependencies in pom.xml file**:-
1.) Inheriting the starter parent(spring-boot-starter-parent): spring-boot-starter-parent is a special starter that provides useful Maven defaults.
2.) spring-boot-starter-web :- Starter for building web, including RESTful, applications using Spring MVC. Uses Tomcat as the default embedded container.
3.) spring-boot-maven-plugin:- Spring Boot includes a Maven plugin that can package the project as an executable jar.
here is pom.xml :-
<!-- Inherit defaults from Spring Boot -->
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.4.7.RELEASE</version>
</parent>
<!-- Add typical dependencies for a web application -->
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
</dependencies>
<!-- Package as an executable jar -->
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
</plugins>
</build>
2nd option:Using Spring Boot without the Parent POM
Not everyone likes inheriting from the spring-boot-starter-parent POM. You may have your own corporate standard parent that you need to use or you may prefer to explicitly declare all your Maven configuration.
If you do not want to use the spring-boot-starter-parent, you can still keep the benefit of the dependency management (but not the plugin management) by using a scope=import dependency, as follows:
< dependency> < !-- Import dependency management from Spring Boot -->
< groupId>org.springframework.boot< /groupId>
< artifactId>spring-boot-dependencies< /artifactId>
< version>2.0.0.BUILD-SNAPSHOT< /version> < type>pom< /type>
< scope>import< /scope> < /dependency>
Fore more details find here in Spring Boot Docs:- https://docs.spring.io/spring-boot/docs/1.4.7.RELEASE/reference/htmlsingle/#getting-started-maven-installation

1) Add Spring boot starter Parent and Web in Pom.xml file
2) Add #SpringBootApplication in the main class
3) Add SpringApplication.run(App.class, args); in main method.

Yes you can use Springboot with Maven as dependency management.
You will have to add spring boot dependencies
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.3.5.RELEASE</version>
<relativePath /> <!-- lookup parent from repository -->
</parent>
Also add #SpringBootApplication and #EnableAutoConfiguration in the main class

I will face this question in two more interviews so it can help full your self also.
Simply right-click on your project select "Configure" and choose on "convert to Maven Project" then it will create or application as maven and .