code: public static @CheckForNull Permission fromId(@Nonnull String id) {
          int idx = id.lastIndexOf('.');
          if(idx<0)   return null;
  
          try {
              // force the initialization so that it will put all its permissions into the list.
              Class cl = Class.forName(id.substring(0,idx),true, Jenkins.getInstance().getPluginManager().uberClassLoader);
              PermissionGroup g = PermissionGroup.get(cl);
              if(g ==null)  return null;
              return g.find(id.substring(idx+1));
          } catch (ClassNotFoundException e) {
              return null;
          }
      }
docs: Convert the ID representation into {@link Permission} object.
  
  @return
  null if the conversion failed.
  @see #getId()