/**
  * Check whether a resource is valid. A valid resource must have a non-blank ID, a non-blank link
  * and a media type.
  *
  * @param book the book the resource is for
  * @param resource the resource to check
  * @return whether the resource is valid
  */
 private static boolean isValidResource(final Book book, final Resource resource) {
   if (resource == null
       || (resource.getMediaType() == MediatypeService.NCX
           && book.getSpine().getTocResource() != null)) {
     return false;
   }
   if (StringUtil.isBlank(resource.getId())) {
     LOGGER.error(
         "resource id must not be blank (href: "
             + resource.getHref()
             + ", mediatype:"
             + resource.getMediaType()
             + ")");
     return false;
   }
   if (StringUtil.isBlank(resource.getHref())) {
     LOGGER.error(
         "resource href must not be blank (id: "
             + resource.getId()
             + ", mediatype:"
             + resource.getMediaType()
             + ")");
     return false;
   }
   if (resource.getMediaType() == null) {
     LOGGER.error(
         "resource media type must be specified (id: "
             + resource.getId()
             + ", href:"
             + resource.getHref()
             + ")");
     return false;
   }
   return true;
 }
 public void setContextResource(Resource resource) {
   if (resource == null) {
     return;
   }
   if (StringUtils.isNotBlank(resource.getHref())) {
     int lastSlashPos = resource.getHref().lastIndexOf('/');
     if (lastSlashPos >= 0) {
       this.currentFolder = resource.getHref().substring(0, lastSlashPos + 1);
     }
   }
 }
  /**
   * Write a resource as an item element.
   *
   * @param serializer the XML serialiser to write the item element to
   * @param book the book to write the resource for
   * @param resource the resource to write as an item element
   * @throws IOException if an I/O error occurs
   */
  private static void writeItem(
      final XmlSerializer serializer, final Book book, final Resource resource) throws IOException {
    if (!isValidResource(book, resource)) {
      return;
    }

    serializer.startTag(NAMESPACE_OPF, OPFElements.ITEM);
    serializer.attribute(PREFIX_EMPTY, OPFAttributes.ID, resource.getId());
    serializer.attribute(PREFIX_EMPTY, OPFAttributes.HREF, resource.getHref());
    serializer.attribute(PREFIX_EMPTY, OPFAttributes.MEDIA_TYPE, resource.getMediaType().getName());
    if (StringUtil.isNotBlank(resource.getProperties())) {
      serializer.attribute(PREFIX_EMPTY, OPFAttributes.PROPERTIES, resource.getProperties());
    }
    serializer.endTag(NAMESPACE_OPF, OPFElements.ITEM);
  }