code: public void clean() {
          if (!remove(this))
              return;
          try {
              thunk.run();
          } catch (final Throwable x) {
              AccessController.doPrivileged(new PrivilegedAction<Void>() {
                      public Void run() {
                          if (System.err != null)
                              new Error("Cleaner terminated abnormally", x)
                                  .printStackTrace();
                          System.exit(1);
                          return null;
                      }});
          }
      }
docs: Runs this cleaner, if it has not been run before.