java constructed relative path using "../../" is not recognized, why?

Question

I'm using intellij for java developing, under my project/src directory, I got a "my.json" file, and my java program is:

import java.io.File;
public class My {
    public static void main(String [] args){
        String s = My.class.getResource(".") + "../../src/my.json";
        File f = new File(s);
        System.out.println(s+" exists: "+f.exists());
    }
}

It prints:

file:/Users/x/java_local/target/classes/../../src/my.json exists: false

But if I "ls" under command line:

ls /Users/x/java_local/target/classes/../../src/my.json

Then I can see it.

So it seems that the java code constructed relative path is not valid. If I use absolute path in code, then it works.

I'm on mac. Any explanations on how this problem happens and how can I fix it?

Answer 1

The comparison you present in the question explains the nature of the problem clearly. Consider this code:

        File f = new File(s);
        System.out.println(s+" exists: "+f.exists());

and the corresponding output:

file:/Users/x/java_local/target/classes/../../src/my.json exists: false

Evidently s, the string you are attempting to use as a file name, is file:/Users/x/java_local/target/classes/../../src/my.json. See that "file:" at the beginning? You have a valid String representation of a URL, but File(String) expects the argument to be a plain path.

Possibly you want

    File f = new File(new URI(s));

, but note that this will fail if your class is packaged inside a Jar, because then your URI will have the jar: scheme, and File(URI) accepts only URIs having the file: scheme.

If you want to read a resource associated with your class and located relative to it, then I would suggest using Class.getResourceAsStream(relativePath) and consuming the resulting InputStream.