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What do I need to add to my pom.xml to run this specific Java file?

I'm not a Java dev and am unfamiliar with the packaging and building of Java programs. I'm trying to run this file: https://github.com/CodinGame/SpringChallenge2020/blob/master/src/test/java/Spring2020Main.java
by doing
mvn clean install
java -jar .\target\spring-2020-1.0-SNAPSHOT.jar
but I get this error:
no main manifest attribute, in .\target\spring-2020-1.0-SNAPSHOT.jar
I can't figure out for the life of me what I need to add to the pom.xml or whatever else I need to do to get this to work.
Any help will be appreciated.

A few things to understand about Java:
1) If you have a Maven project like this, code is divided between src/main/ and src/test/ directories. src/test/ is intended for unit tests. In your case, Spring2020Main is not actually set up as a unit test, so I'm not sure what the author intended here.
2) When you compile using mvn clean install, a jar (library) is built, but nothing from src/test will be included in the output.
Generally, tests are executed during build. And this one would have been, except it's not set up as a real junit test, so it didn't run during build.
3) You can move the file from src/test/java to src/main/java and it will be built into your resulting jar.
4) In this case, when you run the JVM, you need to specify a classpath. This is a list of all libraries to include when the application starts. You also need to specify the (fully qualified) name of the class to run:
java -cp target/spring-2020-1.0-SNAPSHOT.jar Spring2020Main
...the above won't work directly since there are more unsatisfied dependencies (the top level pom.xml brings in at least 3 other deps you'd also need to provide on the classpath).
As others pointed out, a solution could be to build a self-executing jar, but simplest for you would be to run this from an IDE:
Run this from IntelliJ. If you haven't installed it, install it.
1) File > New From Existing Sources, find the directory where this is coned to.
2) When asked, Import Project from Existing Model (Maven)
3) When the Project view is available (alt-1), or View > Tool Windows > Project, you can expand the structure till you find Spring2020Main in the test directory.
4) Right-click it and select Run.
For me, it exposed a web server running at http://localhost:8888/test.html

You can follow the steps below:
Move Spring2020Main.java to src/main/java/com/codingame directory
Add the following to your pom.xml after the </dependencies>:
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>3.2.3</version>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>shade</goal>
</goals>
<configuration>
<transformers>
<transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
<mainClass>com.codingame.Spring2020Main</mainClass>
</transformer>
</transformers>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
Run maven build using mvn clean install
Execute the program using java -jar target/spring-2020-1.0-SNAPSHOT.jar
Info: Apache Maven Shade Plugin helps in building what is called an uber-jar or a fat-jar. This means that all the dependencies are packaged as part of the resultant jar file without the need for any 'libraries' that you'd need to add in the classpath when executing the jar file. As part of the final jar, we need to specify which file needs to be treated as the main file to be executed. This is typically done using META-INF/MANIFEST.MF file inside the uber-jar. That's what the transformer specified inside the configuration of the plugin does for us.

The project you've linked has only a basic setup for compilation (that would be enough to run it from IDE though).
What you need is an executable jar. Check this thread.
As others mentioned (and I failed to notice) the class you linked to is a test class, so it may not be included in a jar by default. Run it through IDE or set it up in a proper source directory.

Strange Maven nullpointer

I have modified this question since initially asking it. Please refer to sections UPDATE 1 and, specifically, UPDATE 2 at the end.
I am building a large JavaFX application with a lot of dependencies. I am using IntelliJ to do the job and am at the point of deployment. I am using IntelliJ's own tutorial on artifacts to build an executable jar. The tutorial can be seen in the "Working with artifacts" tutorial on jetbrains.
I have built my executable jar and it is working as it should, with one caveat, however:
I have to manually mimic the directory structure of my IntelliJ project for the executable jar file to find the resources necessary for the program to function properly.
This is where my question comes in: shouldn't IntelliJ include these files in the artifact, so it can run in and on its own?
My directory structure in IntelliJ looks like this:
Project root
.idea
.out
.src
.main
.java
.com
.myCompany
.package-with-classes1
.class1 ... N
.package-with-classes2
.class1 ... N
.package-with-files
.file1.someExtension
.file2.someExtension
.other-package-classes
.and-so-on
When I build the artifact under Project Structure - Artifacts - Output Layout, I then manually add the directory structure as can be seen above, and then place the files where they belong.
As per my question above, I would expect these files to be automatically included with the executable jar file.
UPDATE 1: Added Maven to project
Due to Andrey's comment I have now implemented Maven in my project. I have a lot of external dependencies which I have added to my pom.xml file like so:
<dependency>
<groupId>some.group.id</groupId>
<artifactId>some-artifact-id</artifactId>
<scope>system</scope>
<version>1.0.0</version>
<systemPath>${basedir}\path\to\jar\jarfile.jar</systemPath
</dependency>
I then do:
mvn clean
mvn compile
mvn package
All runs with no errors.
It places 2 jar files in my \target folder: (1) name-of-jar.jar and (2) name-of-jar-with-dependencies.jar.
Running (1) throws the error: no main manifest attribute. Running (2) throws ClassNotFoundException and NoClassDefFoundError errors. Why is this? The classes throwing the errors are included as dependencies using the above approach.
UPDATE 2: Progress with Maven, but...
I solved the issue in section UPDATE 1 by installing all my third party jar libraries to my local machine's Maven repository at C:\Users\$USER$.m2\repository. However, getting a null pointer...
I changed my dependency declarations in my pom.xml to the following:
<dependency>
<groupId>some.group.id</groupId>
<artifactId>some-artifact-id</artifactId>
<version>some.version.number</version>
</dependency>
I am currently building my fat jar using the maven assembly plugin (I have also tried using the shade plugin but am having the same issue). Here's the excerpt of the assembly plugin from my pom.xml:
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
<configuration>
<archive>
<manifest>
<mainClass>com.myCompany.myMainClass</mainClass>
</manifest>
</archive>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
</configuration>
<executions>
<execution>
<id>make-assembly</id>
<phase>package</phase>
<goals>
<goal>single</goal>
</goals>
</execution>
</executions>
</plugin>
This produces the same two jar files in the \target directory as described in section UPDATE 1.
Now, if I run jar -tf name-of-jar-with-dependencies.jar I can see from the directory contents of this jar that the jar in fact does contain all the third party jar libraries that was missing; and running the jar file using java -jar name-of-jar-with-dependencies.jar does not throw the errors as described in section UPDATE 1 any longer. So far, so good.
However, it does throw a NullPointerException which puzzles me. Specifically, it is complaining that the a certain class is missing. This seems a little strange to me since this class is part of a third party jar library which I did add as a dependency in my pom.xml. The fact that this class is indeed included in the final jar was confirmed by the approach above, printing out the contents of the name-of-jar-with-dependencies.jar, which - among a lot of other files - includes this very jar file.
Any thoughts?

Using Maven for non-Java projects (overriding clean/compile/install goals)

I have a good(ish) understanding of using Maven for Java/WebApp projects but only to the point of following the default goals/lifecycle.
However I now have a Backup project which is not a Java project at all. I was thinking of configuring it with Maven to keep some consistency but am not sure how I override the main Maven goals/phases for my bespoke processing.
The Backup project needs to do the following:
'build' - initially, the backup outputs will be a mysql database dump file and a zip exported from an existing WebApp. But I want it to be flexible so calling an ant file to do the actual work (creating the dump, calling the WebApp, or doing whatever in the future) seems sensible. The output files could then be copied into the target directory.
'install' - publish the output files to a local repository, preferably providing a datetimestamp version number instead of the usual 1.0.0-SNAPSHOT version. I'd like to think that Maven can cope with an artefact being a collection of files, rather than a single jar/war, but not sure on this.
My pom.xml declares the packaging as 'pom' as 'jar' and 'war' dont seem appropriate here.
I then want other projects to be able to have a dependency on this Backup project so they can get the lastest backup artefacts if required.
1) how do I override the maven 'compile' goal to call an ant build file?
2) how do I override the maven 'install' goal to publish all files in the target directory but as a single artefact?
Any help/guidance appreciated.

you can use maven antrun plugin for achieving this.
one example usage:
<plugin>
<artifactId>maven-antrun-plugin</artifactId>
<executions>
<execution>
<id>bundle-virgo</id>
<phase>package</phase>
<configuration>
<tasks>
<ant antfile="<path to build.xml>" target="compile"/>
</tasks>
</configuration>
<goals>
<goal>run</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
you can change the phase parameter to different maven phases to run it at different phases like package, compile etc.

Adding folders in maven project

let us say I have a standard maven project with the standard four Directories
src/main/java
src/main/resources
src/test/java
src/test/resources
Now let us suppose, I create a subdirectory named "clojure" under "src/main".
Are then the source files under "src/main/clojure" automatically compiled when a build is run or do I somehow have to tell to maven, via configuration of some plugin (e.g. build-helper-maven-plugin), that it also has to compile the sources under "src/main/clojure"?
In others words, does the creation of any folder that is not ".../java" or ".../resources" require an explicit configuration in the pom.xml so that the sources there are taken into account by maven??
Any help would be appreciated.
Regards,
Horace

A Maven project is usually built with a single compiler, which looks for all its source files in those folders known as source folders to Maven. Depending on the project, such source folders may be added automatically, e.g. src/main/java. If a different compiler is used, additional folders may automatically be added, e.g. src/main/groovy.
Sometimes Maven integrations in IDEs (like Eclipse or IntelliJ) do not pick up folders for non-Java projects, even though the correct Maven plugins are in the POM, say e.g. for building a Groovy project.
So even though a build on the command line may run nicely with files in src/main/groovy, the folder may not be detected as a source folder when importing the project in an IDE. In such cases you may have to add the additional source folders, e.g.
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>build-helper-maven-plugin</artifactId>
<version>1.5</version>
<executions>
<execution>
<id>add-source</id>
<phase>generate-sources</phase>
<goals>
<goal>add-source</goal>
</goals>
<configuration>
<sources>
<source>src/main/groovy</source>
</sources>
</configuration>
</execution>
</executions>
</plugin>

Yes, maven needs to "know" what those directories mean, though a clojure build plugin may use that directory by convention - see for example: https://github.com/talios/clojure-maven-plugin

Apache maven has a Standard Directory Layout which it understands out of the box.
To make maven understand any other structure than the above, you'll have to override these settings in pom.xml.
Look at this section of POM reference.

dependency not found at runtime maven

I'm new to maven and somewhat new to java. Tried google and related sources, but I didn't find one which resembled my situation.
Right now, I have maven project X and Y. X can be seen as a shared library with some utilities, Y is a simple JFrame with a "hello world" printed and a call to a static method in X.
I do a "run as maven install" on project X, I get a "build successful". I add project X as dependency in project Y (using the pom-editor in Eclipse, browsing the repository and locating it). I do a "run as maven package" on project Y, I get a "build successful". Upon running project Y either via java -jar or inspect the produced jar, project X is missing everywhere and I get a fancy class not found exception. Eclipse finds it and there are no compile errors in the source editor.
Why is it only working in the Eclipse editor and not as jar?
POM:
<dependency>
<groupId>com.company.deployment.shared</groupId>
<artifactId>com.company.deployment.shared</artifactId>
<version>0.0.1-SNAPSHOT</version>
<scope>compile</scope>
</dependency>

Maven doesn't produce a combined JAR file for you. What Eclipse is doing is looking at the Maven configuration and adding all the required classes / jars to your classpath for you when it runs.
If you want to run your program from the command-line, you will need to add all the JARs manually to your classpath.
Alternatively, you could run your program directly from Maven which should set up all your dependencies. There are a number of options depending on what you want to do, i.e. if it's an application which is meant to be run by an end-user you could look into the one-jar Maven plugin.

I recommend that you take a look at the Maven shade plugin. This produces an "uber-jar" comprising your project and all of its dependencies. It can also do other things such as setting the entry point class to make your JAR file an executable JAR.

You may also find exec-maven-plugin helpful
mvn exec:java -Dexec.mainClass="com.example.Main" [-Dexec.args="argument1"] ...
mvn exec:exec -Dexec.executable="maven" [-Dexec.workingdir="/tmp"] -Dexec.args="-X myproject:dist"
If your client can not download dependencies from maven m2 repo on the fly like behind firewall or no internet connection, then you also need to package the dependencies using maven-dependency-plugin to copy all dependencies and maven-assembly-plugin to assemble dependencies

It doesn't work because Maven resolves dependencies when building your project, but doesn't put all the dependencies magically in your jar. You're supposed to run your app with all its dependencies in the classpath:
java -classpath X.jar;Y.jar com.foo.bar.Main
Or you have to customize the maven jar plugin in order to create an executable jar, as described here. And you may also use the maven assemby plugin to copy all your Y project's dependencies to the target directory, next to the generated Y.jar.

The artifact produced in project Y contains only build results in project Y only, not including its dependencies.
If you want to build a JAR in Y, which u can execute directly, you can consider using assembly plugin.
For example, the easiest way to build a uber-jar for project Y:
<project>
...
<build>
...
<plugins>
<plugin>
<artifactId>maven-assembly-plugin</artifactId>
<version>2.2.1</version>
<configuration>
<descriptorRefs>
<descriptorRef>jar-with-dependencies</descriptorRef>
</descriptorRefs>
</configuration>
<executions>
<execution>
<id>make-all-in-one-jar</id>
<phase>package</phase>
<goals>
<goal>single</goal>
</goals>
</execution>
</executions>
</plugin>
...
</project>
Apart from a normal artifact, an assembly which contains classes etc from dependencies will be created, which is suitable to be executed by java -jar
visit http://maven.apache.org/plugins/maven-assembly-plugin/ for more sophisticated usage.

Phil Sacre already explained the basic problem well (there basically is just no information on where to find the X.jar embedded in your Y.jar).
Additionally you can also look at the appassembler-maven-plugin (which can e.g. generate launch scripts for your Y project that already have the right classpath set) and/or the exec-maven-plugin (which you can use to e.g. directly launch Y with the right classpath using maven).