code: public AttributeSet validateResponsePOST(final String _authnResponse) throws SAMLException {
          final byte[] decoded = DatatypeConverter.parseBase64Binary(_authnResponse);
          final String authnResponse;
          try {
              authnResponse = new String(decoded, "UTF-8");
              if (LOG.isTraceEnabled()) {
                  LOG.trace(authnResponse);
              }
          } catch (UnsupportedEncodingException e) {
              throw new SAMLException("UTF-8 is missing, oh well.", e);
          }
          
          final Response response = parseResponse(authnResponse);
          
          try {
              validatePOST(response);
          } catch (ValidationException e) {
              throw new SAMLException(e);
          }
          
          return getAttributeSet(response);
      }
docs: Check an authnResponse and return the subject if validation succeeds. The
  NameID from the subject in the first valid assertion is returned along
  with the attributes.
  
  @param _authnResponse a base64-encoded AuthnResponse from the SP
  @throws SAMLException if validation failed.
  @return the authenticated subject/attributes as an AttributeSet