code: public static synchronized Context getInstance( Hashtable<?, ?> environment ) {
          if (SINGLETON == null) SINGLETON = new SingletonInitialContext(environment);
          return SINGLETON;
      }
docs: Return the {@link Context} singleton instance. If no such context has been configured, this method will configured the
  singletone using the supplied environment.
  
  @param environment the environment for the naming context; may be null or empty
  @return the singleton context; never null
  @see #getInitialContext(Hashtable)
  @see #getInstance(Hashtable)