How to get the path of a running JAR file?

Answer 1

Here's upgrade to other comments, that seem to me incomplete for the specifics of

using a relative "folder" outside .jar file (in the jar's same location):

String path = 
  YourMainClassName.class.getProtectionDomain().
  getCodeSource().getLocation().getPath();

path = 
  URLDecoder.decode(
    path, 
    "UTF-8");

BufferedImage img = 
  ImageIO.read(
    new File((
        new File(path).getParentFile().getPath()) +  
        File.separator + 
        "folder" + 
        File.separator + 
        "yourfile.jpg"));