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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>

How can I have a multi-module Maven project without specifying the version all over the place?

I am currently learning Maven for a specific project. In this project, I need to have multiple modules (in the sense of a collection of classes and resources) that depend on each-other. Specifically, I need a module to store the classes of a public-facing API (the API is a Java API, as the desktop program can load addons at runtime), another for a set of common classes that are shared between the client and server but that are not part of the API, and of course the client and server themselves need a module each.
Using the system that I used before Maven (IntelliJ Idea's build system), I would simply create different modules and setup dependencies using IntelliJ's GUI. This works well because I can then put the whole IntelliJ Idea project in a git repository, and keep everything nicely tracked together with no fuss and no problems.
When researching how to do this in Maven, however, I ran into some problems. Maven's documentation has a section that seems to explain how to accomplish something similar, but its technique seems to have two problems. When Each sub-project, with its own pom.xml, needs to specify the version of the other sub-projects that it depends on¹. This implies that I need to make many changes all over the project whenever the version changes. This will (hopefully) be quite frequent. As it is perfectly plausible that my project will grow to have hundreds of modules, this is obviously impractical.
Additionally, based on my other research, it seems like a Maven repository is involved in the documented technique. Specifically, it seems like when, for example, the API module is built to be included in the client and server modules it will first be placed in a local repository and then the client and server modules will retrieve it from there. This sounds problematic as the API module will also be published in a public repository (maybe Maven central, I haven't really thought about this too much -- but it will be public in a repo), and this repository step seems like it could end up building the client and server with a published jar rather than the local one, which is problematic for many reasons. (eg. if a developer is making local changes to the api jar, building half the project without those changes is a problem).
Is there some better way to combine multiple modules to mitigate these problems (versions all over the place, and getting wrong jars from repos)? Is it better to modify the technique in the linked documentation instead? Am I misunderstanding something and the problems don't exist?
I have checked DuckDuckGo and all relevant questions that I can find on StackOverflow, and none address either of the two aforementioned problems.
¹:
<dependencies>
<dependency>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>
</dependencies>

In your multi-module project, you usually use just one version for all modules. This means that you can define dependencies between modules with <version>${project.version}</version> where the property ${project.version}$ is resolved during the build. Maven builds all the modules in the correct order.

Regarding your first question: you can create a parent pom over your whole project. In this parent pom you can add a dependencyManagement section where you can list your own modules with their version. In your modules you then only need to specify the dependeny without the version.
Here an example:
in parent pom:
<dependencyManagement>
<dependencies>
<dependency>
<groupId>yourgroup</groupId>
<artifactId>yourmoduleA</artifactId>
<version>3.0.1-SNAPSHOT</version>
</dependency>
<dependency>
<groupId>yourgroup</groupId>
<artifactId>yourmoduleB</artifactId>
<version>3.0.1-SNAPSHOT</version>
</dependency>
</dependencies>
</dependencyManagement>
and if your module A uses module B, you can add in pom of module A
<dependencies>
<dependency>
<groupId>yourgroup</groupId>
<artifactId>yourmoduleB</artifactId>
</dependency>
</dependencies>

javax.jnlp as maven dependency

Using JNLP (javax.jnlp) in one java project, I reaized that it is not part of the normal JDK.
As it is a Maven project I would like to add it as a dependency to my POM.
The one dependency I found working is:
<dependency>
<groupId>javax.jnlp</groupId>
<artifactId>jnlp-api</artifactId>
<version>8.0</version>
<scope>system</scope>
<systemPath>${java.home}/lib/javaws.jar</systemPath>
</dependency>
But depending on a system path looks bad to me - really bad.
system is marked as deprecated here: https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html
Is there no other way?
Getting it from repositories as an reasonable up to date version (java 8)?
Or what would be the clean way?

Hot swapping in Spring Boot

I've been doing a P.O.C with Spring Boot.
So far it's been going really good and promising, but there's one major drawback: I'm using an embedded server (i.e., packaging the web app in a .jar), so when developing I have to rebuild the jar and restart the server every time I change the CSS, HTML or JS files. There's not hot-swap. This really slows down the UI development.
I can think of several quick fixes, such as loading static resources off a different domain and serving it from a local nginx, and some more variations like this, but isn't there a built-in option of some sort when working with IntelliJ/Eclipse?

There are several options. Running in an IDE (especially with debugging on) is a good way to do development (all modern IDEs allow reloading of static resources and usually also hotswapping of Java class changes). Spring Boot devtools is a cheap way to get quite a big boost (just add it to your classpath). It works by restarting your application in a hot JVM when changes are detected. It also switches off things like thymeleaf caches while it is running, so you don't have to remember to do that yourself. You can use it with an external css/js compiler process if you are writing that code with higher level tools.
Spring Loaded is no longer recommended, but probably still in use. More sophisticated agent-based tools work much better if you need hot swapping with zero delay (e.g. JRebel).
See the docs for some up to date content

but isn't there a built-in option of some sort when working with IntelliJ/Eclipse?
What helped me in IntelliJ 15.0, windows 10, was the following sequence:
STEP 1: Added the following dependency in pom (This is mentioned everywhere but this alone dint solve it), as mentioned by #jonashackt
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-devtools</artifactId>
</dependency>
STEP 2: Then from File->Settings-> Build-Execution-Deployment -> Compiler (make sure main compiler option is selected and not any of its sub-options)
enable Make Project Automatically. Click ok and close the dialog
Note : In latest version it will be Build project automatically
STEP 3: Hold Shift+Ctrl+A (on windows) you will see a search dialog with title "Enter Action or option name", type registry. Double click the first option that says "Registry..." it will open another window. Look for the following option:
compiler.automake.allow.when.app.running
and enable it, click close
STEP 4: Restart IDE
Elaborated from this source

You can get hot swapping:
for java code: using spring-loaded
for Thymeleaf templates: disabling the cache
Check this post to see more details: http://blog.netgloo.com/2014/05/21/hot-swapping-in-spring-boot-with-eclipse-sts/

I do not know how far this kind of support goes, but in case you use Eclipse IDE (or anyone reading this): starting up your Spring-Boot application via m2e in debug-mode (press the "Debug"-dropdown button and pick your maven run configuration item).
It works for me like a charm.
My maven run configuration item is configured as follows:
goal is set to "spring-boot:run"
base directory is the project directory
I am not using any further libraries (not even spring-boot-devtools).
That's it.

You can also use JRebel - it will reload all changes (better hotswap) including spring beans. It is easily integratred with both Intellij and Eclipse.

Assuming you are using gradle; use the following config in your build.gradle
apply plugin: 'java'
apply plugin: 'idea'
apply plugin: 'spring-boot'
apply plugin: 'application'
applicationDefaultJvmArgs = ["-agentlib:jdwp=transport=dt_socket,address=localhost:7000,server=y,suspend=n"]
mainClassName = "package.ApplicationRunner"
Run the application from the IDE or command line using the command gradle build run
Now the IDE can connect to the remote JVM (on port 7000) where the spring boot application runs. It also supports hot deployment of static files.
or even you can run the main class from intelliJ if the dependencies are properly managed in the IDE. The main class is the class that contains the main method which will call SpringApplication.run("classpath:/applicationContext.xml", args);

In Intellij, I can get this behavior. When the program is running in debug mode, select Run > Reload Changed Classes
Note: After Intellij completes the action, it might say Loaded classes are up to date. Nothing to reload. This is misleading, because it actually DID reload your classpath resources.
My environment/setup includes:
Intellij 13
Embedded Tomcat
Run/Debug configuration of type 'Application' (which just uses a main class)
Serving static html, css and js (no jsp)

try using this spring-boot-devtools tag in pom.xml
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-devtools</artifactId>
<optional>true</optional>
</dependency>
</dependencies>
http://mytechnologythought.blogspot.com/2017/07/how-to-run-server-of-spring-boot-auto.html

From 1.3.0. (now in Milestone 2) on, you can use the spring-boot-devtools for that as a lightweigt approach - see the docs or this blogpost.
Simply upgrade to >= 1.3.0. and add the following to your pom.xml:
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-devtools</artifactId>
</dependency>
</dependencies>
Than start your SpringBootApplication with Run As... and you´re fine.

I recommend Thymeleaf (template engine), jRebel for personal developer.
Thymeleaf template files are just HTML resources. So, they`re changed immediately after you edit template files.

If you're using maven, the spring-boot-maven-plugin in your pom.xml needs to be like this to get the hot swap:
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>springloaded</artifactId>
<version>1.2.0.RELEASE</version>
</dependency>
</dependencies>
</plugin>
and if you're using thymeleaf, add this to your application properties:
spring.thymeleaf.cache=false
But remember something: Don't use this in your production environment..

How to perform Hot Swap in Springboot Application
When using gradle include following in the dependency:
compile group: 'org.springframework.boot', name: 'spring-boot-devtools', version: '2.0.1.RELEASE' & providedRuntime('org.springframework.boot:spring-boot-starter-tomcat')
In application.properties add the property spring.devtools.restart.additional-paths=.
Build Gradle and then run application as bootRun
The application is ready to perform hot swap on modification of classes

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.