Many people who are using the maven-shade-plugin commonly set <createDependencyReducedPom> to false for reasons beyond my grasp. This is generally a bad idea, as it can lead to problems if you’re writing a library, and has absolutely no advantages.

Here’s a rule of thumbs: If your project shades dependencies, it should create a dependency-reduced-pom.xml.

The situation

Imagine you are currently writing a project called my-library. It has a few utility classes and also shades another library, which is called someones-third-party-library.

If you let maven create the dependency-reduced-pom.xml, then all is good and no problems will arise. If however you set <createDependencyReducedPom> to false in your my-library’s maven-shade-plugin configuration, the following situation will arise:

awesome-skyblock-plugin now wants to shade my-library. Maven looks at the pom.xml of my-library and sees that my-library depends on someones-third-party-library. This is called a transitive dependency.

When you now package your awesome-skyblock-plugin, maven will now first of all create a list of dependencies that require to be shaded, which will be:

1. my-awesome-library
2. All dependencies of my-awesome-library

This means that, although my-library already contains someones-third-party-library, maven will include it again (twice!) in your awesome-skyblock-plugin!

The problem

Now two things can happen:

1. You didn’t relocate anything

If you neither relocated someones-third-party-library in my-library, nor relocated my-library in awesome-skyblock-plugin, then you’ll only see a few warnings when compiling awesome-skyblock-plugin

2. You did relocate the dependency

But now imagine that you properly relocated your shaded dependencies: In my-library, you relocated com.someone.thirdpartylibrary to my.library.shaded.thirdpartylibrary.

If you now compile your awesome-skyblock-plugin, you’ll see that it contains the third-party-library twice:

1. The shaded version that’s included in your library, at my.library.shaded.thirdpartylibrary
2. At the original location, coming from the transitive dependency, at com.someone.thirdpartylibrary

The solution

You need to get rid of the transitive dependency. Maven by default creates a file called dependency-reduced-pom.xml that is essentially your pom.xml, excluding all dependencies you have already shaded. That means that the original pom.xml of my-library lists someones-third-party-library as dependency, but the dependency-reduced-pom.xml will just skip it because you already shaded it, and hence it’s not a dependency for users of my-library anymore.

This is why you should never disable the creation of dependency-reduced-pom.xml.

If you check your pom.xml and find the maven-shade-plugin inside, check if you see the following line somewhere:

<createDependencyReducedPom>false</createDependencyReducedPom> <!-- BAD! Remove this line! -->

If yes: Remove that line! It has no advantages and only leads to problems!