code: public static JoinTable getJoinTable(Class<?> clazz) {
  		Class<?> curClass = clazz;
  		while (curClass != null) {
  			JoinTable joinTable = curClass.getAnnotation(JoinTable.class);
  			if(joinTable != null) {
  				return joinTable;
  			}
  			curClass = curClass.getSuperclass();
  		}
  		
  		return null;
  	}
docs: 获得clazz上注解的JoinTable，如果没有则返回null
  @param clazz
  @return 如果没有则返回null