wro4j-core/src/main/java/ro/isdc/wro/http/handler/ResourceProxyRequestHandler.java
protected boolean isResourceChanged(final HttpServletRequest request) {
    try {
      final long ifModifiedSince = request.getDateHeader(HttpHeader.IF_MODIFIED_SINCE.toString());
      return ifModifiedSince < getHeadersConfigurer().getLastModifiedTimestamp();
    } catch (final Exception e) {
      LOG.warn("Could not extract IF_MODIFIED_SINCE header for request: " + request.getRequestURI() + ". Assuming content is changed. ", e);
      return true;
    }
  }
protected boolean isResourceChanged(final HttpServletRequest request) {
    try {
      final long ifModifiedSince = request.getDateHeader(HttpHeader.IF_MODIFIED_SINCE.toString());
      return ifModifiedSince < getHeadersConfigurer().getLastModifiedTimestamp();
    } catch (final Exception e) {
      LOG.warn("Could not extract IF_MODIFIED_SINCE header for request: " + request.getRequestURI() + ". Assuming content is changed. ", e);
      return true;
    }
  }
Used to identify whether the {@link HttpServletResponse#SC_NOT_MODIFIED} or {@link HttpServletResponse#SC_OK}
should be returned. Currently a single timestamp is used to detect the change for all resources. This might be no
accurate, but at least it allows sending NOT_MODIFIED header much often resulting in less load on the server.
<p/>
Override this method if a different way detecting change is required.

@return true if the requested resource is changed on the server and the latest version should be returned.