ClassNotFoundException for the plugin main class

Server log

[ModernPluginLoadingStrategy] Could not load plugin 'MyPlugin.jar' in folder 'plugins'
...
Caused by: java.lang.ClassNotFoundException: com.example.myplugin.MyPlugin

What it means

The server read plugin.yml and tried to load the class named in main, but that class is not in the jar.

What causes it

A typo in main, a package that was renamed without updating plugin.yml, or a jar built without the compiled classes.

How to fix it

Before

main: com.example.myplugin.Main   # the class is really MyPlugin

After

main: com.example.myplugin.MyPlugin

Drop the built jar into Permistry: it checks that the class from main is really inside the jar and tells you which Java version it was compiled for.

Check the whole file. Paste your plugin.yml or drop the plugin jar into Permistry. It finds this and about 40 other problems, and explains each one in plain words. It runs in your browser and nothing is uploaded.

Open the checker