Java: URL.getPath() of current "main" class returns "null", why?

Question

I'm trying to print the current path of class that executes main(), as below. I found the statements below from googling, but doesn't work in my computer(windows+intellij)

public class core3 {
    public static void main(String[] args) {
        try {
            URL url = new core3().getClass().getClassLoader().getResource("");
            System.out.println(url);
            System.out.println(System.getProperty(url.getPath()));//print null
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Actually it prints

file:/D:/Documents/myproject/test01/target/classes/
null

Why the second print returns null? How to fix it?

Thanks

Answer 1

System.getProperty() is for getting system properties. For example, the "java.home" system property typically points to the location of your Java installation. System properties are described here.

Your code is looking up a system property with key equal to your URL. You do not have such a system property defined, so System.getProperty() returns null.

If you just want to print the path, you should simply omit the System.getProperty() call:

System.out.println(url.getPath());