code: public <T extends Annotation> T getAnnotation(Class<T> annotationClass) {
          T result = null;
          if (field != null) {
              result = field.getAnnotation(annotationClass);
          }
          if (result == null && setter != null) {
              result = setter.getAnnotation(annotationClass);
          }
          if (result == null && getter != null) {
              result = getter.getAnnotation(annotationClass);
          }
          return result;
      }
docs: 获取指定的注解
  
  @param annotationClass
  @param <T>
  @return