File.separator and "/" work differently in a runnable jar

Question

Context

I have detected a strange behaviour of File.separator when used in a piece of code compiled as a runnable jar.

For context, I have a database that is supported by xml. The program then interacts with that database, parsing the whole document at the start, creating the objects, modifying them, and then updating the xml file when the program ends.

The actual contents and/or file type are actually irrelevant, but I have included a simple Hello World txt as an example. Either way, the file is read as an InputStream by the database (the only difference is that I'm parsing the actual xml from the that InputStream).

MWE

Database.java

import java.io.File;
import java.io.InputStream;

public class Database {
    public InputStream getInputStreamDependentSeparator(String fileName) {
        return getClass().getResourceAsStream("/resources/" + fileName);
    }
    
    public InputStream getInputStreamIndependentSeparator(String fileName) {
        return getClass().getResourceAsStream("/resources" + File.separator + fileName);
    }
}

Main.java

import java.io.InputStream;

public class Main {
    public static void main(String[] args) {
        Database db = new Database();
        
        InputStream is1 = db.getInputStreamDependentSeparator("HelloWorld.txt");
        
        InputStream is2 = db.getInputStreamIndependentSeparator("HelloWorld.txt");
        
        System.out.println(is1.toString());
        System.out.println(is2.toString());
    }
}

HelloWorld.txt

Hello World!

Problem description

• is1 uses / to separate directories, while is2 uses File.separator
• Running through the IDE (Eclipse), neither statement results in an exception, and produce the following output:

java.io.BufferedInputStream@5305068a
java.io.BufferedInputStream@1f32e575

• Compiling the code as a runnable jar (in Eclipse, right-clicking on the project, left-clicking on Export, and then following the prompts for a runnable jar), and running from the command line results in a NullPointerException thrown by is2. This is the following output:

java -jar fileSeparator.jar
sun.net.www.protocol.jar.JarURLConnection$JarURLInputStream@85ede7b
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "Object.toString()" because "is2" is null
        at Main.main(Main.java:12)

• Note that the System.out.println(); statement is not meant to actually give the content of the HelloWorld.txt file, just to check if the InputStream is valid or trigger an exception otherwise

Questions

• Quoting directly from Oracle: EDIT: As Erwin Bolwidt pointed out, this was regarding the atg and is not applicable to jdk. Sorry for not noticing this.

When specifying values for file properties, Nucleus translates the forward slash (/) to the file separator for your platform (for example, Windows uses a backslash (\) as a file separator).

• This means that both File.separator and / should provide identical results. However, the MWE shows that the former is not working when running the jar, while the latter works both in the IDE and from the jar. Is this a bug? Or am I missing something, and this is the actual intended behaviour?
• I have been using File.separator since, according to this post, it is the safest way to ensure future platform compatibility. Whether this is a bug or an actual intended behaviour, how should I fix the issue? (I could just use the /, but then again I'd lose platform independency...)