code: private static LineMap loadScriptMap(Class cl)
    {
      ClassLoader loader = cl.getClassLoader();
  
      if (loader == null)
        return new LineMap(); // null map
      
      try {
        String pathName = cl.getName().replace('.', '/') + ".class";
  
        InputStream is = loader.getResourceAsStream(pathName);
  
        if (is == null)
          return null;
        
        try {
          JavaClass jClass = new ByteCodeParser().parse(is);
  
          Attribute attr = jClass.getAttribute("SourceDebugExtension");
  
          if (attr == null) {
            int p = cl.getName().indexOf('$');
  
            if (p > 0) {
              String className = cl.getName().substring(0, p);
  
              return loadScriptMap(loader.loadClass(className));
            }
  
            return new LineMap();
          }
          else if (attr instanceof OpaqueAttribute) {
            byte []value = ((OpaqueAttribute) attr).getValue();
  
            ByteArrayInputStream bis = new ByteArrayInputStream(value);
  
            ReadStreamOld rs = VfsOld.openRead(bis);
            rs.setEncoding("UTF-8");
  
            try {
              return parseSmap(rs);
            } finally {
              rs.close();
            }
          }
          else
            throw new IllegalStateException(L.l("Expected opaque attribute at '{0}'",
                                                attr));
        } finally {
          if (is != null)
            is.close();
        }
      } catch (Throwable e) {
        log.finer(e.toString());
        log.log(Level.FINEST, e.toString(), e);
  
        return new LineMap(); // null map
      }
    }
docs: Loads the script map for a class