JAR包命令行运行正常，双击启动无法加载数据问题求助

我开发了一款Java桌面应用，需读取文本文件、显示JPEG图片。该应用在NetBeans中运行正常，将JAR包移至非NetBeans目录后，通过命令行执行java -jar <myfile.jar>也能正常运行，但双击JAR包图标时，仅能打开GUI界面，本该显示数据的字段为空，图片也无法加载。

我曾按其他帖子建议，将代码从FileReaders、FileWriters改为使用IOStreams实现，但问题依旧：NetBeans及命令行运行均正常，双击JAR仅显示无数据的GUI。

我查阅过类路径（classpath）相关资料，但疑惑若为类路径问题，命令行运行也应出现异常。

简化后的代码 #

package com.steve;

import javax.swing.*;
import java.awt.*;
import java.io.File;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.charset.StandardCharsets;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.io.IOException;

public class Testing extends JFrame{

   public Testing(){
      super("My Test App");
      EntryPanel panel = new EntryPanel();
      this.getContentPane().add(panel);
      this.setSize(800, 600);
      this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      this.setVisible(true);
   }
   
   public class EntryPanel extends JPanel{

   String strTextToDisplay;

    public EntryPanel(){
         super();
         try{
           this.strTextToDisplay = getTextFromFile();
         }
         catch(IOException ioe){
            System.out.println(ioe.getMessage());
         }
    }
    
    private String getTextFromFile() throws IOException{
        String fileName = "testfile.txt";
        var filePath = Paths.get(fileName);
        String myStr = "Here it is:\n";
        try(BufferedReader br = Files.newBufferedReader(filePath, StandardCharsets.UTF_8)){
          String line;
          while((line = br.readLine()) != null){
             myStr = myStr.concat(line).concat("\n");
          }
        }
        System.out.println(myStr);
        return myStr;
    }
    
    @Override
    public void paintComponent(Graphics g){
      g.setColor(new Color(0, 0, 255));
      g.fillRect(0,0, this.getWidth(), this.getHeight());
      g.setColor(new Color(255, 100, 100));
      g.setFont(new Font("Serif", Font.BOLD, 15));
      g.drawString(strTextToDisplay, 20, 20);
   }
}
  
   
   public static void main(String[] args){
      System.out.println("In main");
      Testing myTest = new Testing();
   }
   
}

manifest.txt文件内容 #

Manifest-Version: 1.0
Class-Path: com.resources/testfile.txt
Main-Class: com.steve.Testing
Created-By: 21.0.10 (Ubuntu)

内容的提问来源于stack exchange，提问作者Steve Baker