code: public FluentValidator when(boolean expression) {
          if (!expression) {
              for (int i = 0; i < lastAddCount; i++) {
                  validatorElementList.getList().removeLast();
              }
          }
          return this;
      }
docs: 当满足<code>expression</code>条件时，才去使用前一个{@link Validator}或者{@link ValidatorChain}来验证
  
  @param expression 满足条件表达式
  
  @return FluentValidator