code: public Content getTagletOutput(Tag tag, TagletWriter writer) {
          FieldDoc field = getFieldDoc(
              writer.configuration(), tag, tag.text());
          if (field == null) {
              if (tag.text().isEmpty()) {
                  //Invalid use of @value
                  writer.getMsgRetriever().warning(tag.holder().position(),
                          "doclet.value_tag_invalid_use");
              } else {
                  //Reference is unknown.
                  writer.getMsgRetriever().warning(tag.holder().position(),
                          "doclet.value_tag_invalid_reference", tag.text());
              }
          } else if (field.constantValue() != null) {
              return writer.valueTagOutput(field,
                  field.constantValueExpression(),
                  ! field.equals(tag.holder()));
          } else {
              //Referenced field is not a constant.
              writer.getMsgRetriever().warning(tag.holder().position(),
                  "doclet.value_tag_invalid_constant", field.name());
          }
          return writer.getOutputInstance();
      }
docs: {@inheritDoc}