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Spring, Maven: Correct the classpath of your application

I am trying to create a maven version of the gradle-based treehouse 'unit-testing-a-spring-application' project used in this course. See course code here.
After not being able to build initially, I ended up adding dependency versions specified in the original gradle build file. Maven will build fine, but running the app gives this:
The following method did not exist:
org.springframework.data.repository.config.RepositoryConfigurationSource.getAttribute(Ljava/lang/String;)
Ljava/lang/String;
The method's class, org.springframework.data.repository.config.RepositoryConfigurationSource, is
available from the following locations:
jar:file:/C:/Users/home/.m2/repository/org/springframework/data/spring-data-
commons/2.1.6.RELEASE/spring-data-commons-2.1.6.RELEASE.jar!/org/springframework/data/repository/config/RepositoryConfigurationSource.class
It was loaded from the following location:
file:/C:/Users/home/.m2/repository/org/springframework/data/spring-data-commons/2.1.6.RELEASE/spring-
data-commons-2.1.6.RELEASE.jar
Action
Correct the classpath of your application so that it contains a single, compatible version of
org.springframework.data.repository.config.RepositoryConfigurationSource.
Several similar problems are solved here on S.O. by changing the version of some related dependency. I have tried various versions of the following to no avail (I stick normally with spring-boot-starter-parent v2.1.4 cause it works all the time with my projects):
spring-data-commons
spring-boot-starter-parent
I see the needed class below listed in my project Dependencies directory, but what dependency versions will work in this case? How can I solve this otherwise?
org/springframework/data/repository/config/RepositoryConfigurationSource.class

I looked at your code and found some mistakes. Since your project is a spring-boot one, you mustn't add direct spring dependencies in your pom file. Instead try to use spring-boot based dependencies. For instance, instead of having such this dependency:
<dependency>
<groupId>org.springframework.data</groupId>
<artifactId>spring-data-jpa</artifactId>
<version>1.9.2.RELEASE</version>
</dependency>
You must have the following one in your pom file.
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

Maven - "Package from transitive module dependency referenced, declare a direct dependency to fix."

I never worked with Maven before, but after having a lot of problems with mockito dependencies in my Netbeans module, I decided to create a new one using Maven and move everything to this new project. After some time doing so, I managed to get the module running just ok.
The problem is that when I use some functionalities that require the Netbeans API I get an exception.
My guess is that the problem is in the dependencies declared in pom.xml. In some of the imports that I use in my application, I get the following message:
Package from transitive module dependency referenced, declare a
direct dependency to fix.
When I clean and build or run mvn clean install in the root directory I get this error:
Project uses classes from transitive module
org.netbeans.api:org-netbeans-modules-projectapi:jar:RELEASE73 which
will not be accessible at runtime.
To fix the problem, add this module as direct dependency. For OSGi bundles that are supposed to be wrapped in NetBeans modules, use the
useOSGiDependencies=false parameter
In the org.codehaus.mojo plugin declaration (which was the only one that had the useOSGiDependencies tag) I tried to change useOSGiDependencies=true to useOSGiDependencies=false but that didn't work. I also tried to change RELEASE73 to RELEASE82, since I am using Netbeans 8.2 but that didn't work as well. I also tried a bunch of other possible solutions that I found on google, but none seemed to work for me.
These are the Netbeans api dependencies that I have in my pom.xml file:
<dependencies>
<dependency>
<groupId>org.netbeans.api</groupId>
<artifactId>org-netbeans-modules-project-libraries</artifactId>
<version>RELEASE73</version>
</dependency>
<dependency>
<groupId>org.netbeans.api</groupId>
<artifactId>org-netbeans-api-annotations-common</artifactId>
<version>RELEASE82</version>
</dependency>
<dependency>
<groupId>org.netbeans.api</groupId>
<artifactId>org-openide-util</artifactId>
<version>RELEASE82</version>
</dependency>
<dependency>
<groupId>org.netbeans.api</groupId>
<artifactId>org-openide-awt</artifactId>
<version>RELEASE82</version>
</dependency>
<dependency>
<groupId>org.netbeans.modules</groupId>
<artifactId>org-netbeans-core</artifactId>
<version>RELEASE73</version>
</dependency>
<!-- ... -->
</dependencies>
I looked everywhere but couldn't find a straightforward answer on how to declare a direct dependency.
Can anyone explain to me what am I doing wrong here?

After digging a bit more, I found that I had to add all the dependencies manually. This answer helped me a lot.
I just lack experience with maven. Basically I googled org-netbeans-modules-projectapi:jar:RELEASE73 maven entered the first link and added the needed dependency to my project.

difference between com.sun.jna and net.java.dev.jna?

I'm looking through the central maven repository and seeing a net.java.dev.jna and a com.sun.jna groupId for JNA. The github for JNA, using the com.sun.jna path as the directories in their source code, indicates that 4.1 has been pushed into the repository as part of a comment, but I'm seeing net.java.dev.jna at 4.1 and com.sun.jna at 3.0.
Obviously, I want to use JNA, but am baffled. What's going on with this package?

The correct 4.1 version to use is net.java.dev.jna:
<dependency>
<groupId>net.java.dev.jna</groupId>
<artifactId>jna</artifactId>
<version>4.1.0</version>
</dependency>
The package name still seems to be com.sun.jna, but if you look in the POM file, the Maven coordinates have become net.java.dev.jna:jna.
They seem to have been that way for quite a long time, based on the MVN respository results.

Maven: benefit of specifying dependencies's versions as properties for single module project

On various projects I've been working on, I've seen diferent ways of specifying dependencies versions. On some projects, the package version is written on the same dependency declaration:
<dependency>
<groupId>org.apache.myfaces.extensions.validator.validation-modules</groupId>
<artifactId>myfaces-extval-property-validation</artifactId>
<version>2.0.7</version>
<scope>compile</scope>
</dependency>
On others, a property is used, as in:
<dependency>
<groupId>org.apache.myfaces.extensions.validator.validation-modules</groupId>
<artifactId>myfaces-extval-property-validation</artifactId>
<version>${versions.extval}</version>
<scope>compile</scope>
</dependency>
For multimodule projects, I can see a clear benefit in declaring versions on the parent pom to avoid duplication (and the potential confusion and errors that come with it), but on single module applications, would there be a benefit to use such a level of indirection?
What would be a best practice for this and why?
Thanks a lot :)

With a version property you can override it on the command line whereas with a fixed version you cannot.
So you can recompile your project with a newer version just by specifying it on the command line.
mvn -Dversions.extval=2.0.8 clean package
Or something.
apart from that, mostly used on multi-projects, and although there you have the dependency management section as well for versions.

How to add Spring libs using Maven

I learned Spring via Spring In Action 3 few month ago. I downloaded Spring libraries from official site (list was like in SIA3(aop, asm, aspects, beans ...)), added them to my project and everything worked fine. Now I want to use Maven, but I am getting a lot of errors and sinking in searching what library to add.
I am newby, dont know all Spring dependencies(within it libs) and the question is not about my errors, but about the way to add all Spring libraries to my project via Maven. How do you usually add Spring libs using Maven?

You don't have to download the libraries themselves anymore. That is what Maven is for. (and quite some more, of course)
set up Maven properly
set up Maven in the IDE tool you have (like this)
edit the pom.xml to include what you need, adding the dependencies in the in the dependencies tag.
Maven takes care of resolving the dependencies of the specified packages. If a package depends on other packages, it will do it for you. You only have to specify the packages you directly need
For example
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring</artifactId>
<version>2.5.6</version>
</dependency>
You can easily find the packages using Google, and searching for "maven repository "
Avoiding version clashes
Also, as Bart mentioned, the common way of having Spring in the pom.xml - as it has way too many versions, and clashes can occur - is through a common property specifying the version for all Spring components. (Based on this answer)
Specify the property in the properties tag:
<properties>
<spring.version>3.0.5.RELEASE</spring.version>
</properties>
Then use it in the dependencies like this:
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>${spring.version}</version>
</dependency>
Be careful to use it for ALL components to avoid version clashes. (of course, issues mught still occur, bz having different libraries reference spring too, but that is another story in its own.)
Side note
Keep in mind note that Maven projects use specific directory layout. When I first started using maven for my own projects, first I created a new blank one, and played around with it, before I began migrating my older projects to use maven. Believe me, it pays off.

Add spring artifacts to your pom.xml file. For example
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-beans</artifactId>
<version>3.2.4.RELEASE</version>
You can find more artifact info here
http://mvnrepository.com/

HERE you can find the dependencies as per your requirement. Just click on the dependency and inside click on the latest release, scroll down there is your code inside the <dependencies> Your required dependency and version</dependencies>.
Just copy the XML code and paste it in your pom.xml file.