This happens because either you have many JDKs installed and compiling by another and running by another or you are using the Javafx Application jar feature when creating artifacts in Intellij which is unfortunately broken. Before proceeding with the below steps make sure that you are compiling with and running with the same JDK version.
Here is you fix it:
1 - Create a Launcher class:
The Launcher class is going to call the main JavaFx class from which your appliaction runs. Choosing to make the Jar directly through the Main class is going to error out giving the following error:
Error: Could not find or load main class Main
Caused by: java.lang.ClassNotFoundException: Main
Your Launcher class should look something like this:
public class Launcher {
public static void main(String[] args) {
MainGUI.main(args);
}
}
2 - On to building the Jar
You probably still have a META-INF
folder from the previous build so delete it.
Build the project as a JAR:
File->Project Structure -> Artifacts -> "+" -> JAR-> from modules with dependancies..
Choose the Launcher class for you main and check "copy to the output directory and link via Manifest" and Press Ok
Press Apply then OK.
go to Build -> Build artifacts-> Rebuild