@to175 your libraries are not plugins, don't drop them in the plugins folder. For Maven shading, google around a bit and you will find tutorials which explain you how to use it. If you don't use Maven (which I expect at the moment), build your jar as you usually do, and:
Open the jar with an extraction tool (7-zip, winrar, etc).
Open the META-INF folder, and open manifest.MF.
Set Class-Path to ../lib/HtmlUnit.jar (or whatever the filename is).
Save, close, perform any action required to update the manifest.MF file (so the next time you view it, your changes should be visible).
In your server root, create a folder 'lib'.
In that folder, put HtmlUnit.jar (or whatever the filename is).
Put your plugin in the plugins folder.
Restart your server.
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