SecurityManager sm = securityManager;
SecurityManager sm = SecurityManager.current;
  public boolean removeShutdownHook(Thread hook)  {    SecurityManager sm = securityManager; // Be thread-safe!    if (sm != null)      sm.checkPermission(new RuntimePermission("shutdownHooks"));    synchronized (libpath)      {        if (exitSequence != null)          throw new IllegalStateException();        if (shutdownHooks != null)          return shutdownHooks.remove(hook);      }    return false;	}