/**
   * Issue access token in exchange to an Authorization Grant.
   *
   * @param tokenReqDTO <Code>OAuth2AccessTokenReqDTO</Code> representing the Access Token request
   * @return <Code>OAuth2AccessTokenRespDTO</Code> representing the Access Token response
   */
  public OAuth2AccessTokenRespDTO issueAccessToken(OAuth2AccessTokenReqDTO tokenReqDTO) {

    if (log.isDebugEnabled()) {
      log.debug(
          "Access Token request received for Client ID "
              + tokenReqDTO.getClientId()
              + ", User ID "
              + tokenReqDTO.getResourceOwnerUsername()
              + ", Scope : "
              + Arrays.toString(tokenReqDTO.getScope())
              + " and Grant Type : "
              + tokenReqDTO.getGrantType());
    }

    try {
      AccessTokenIssuer tokenIssuer = AccessTokenIssuer.getInstance();
      return tokenIssuer.issue(tokenReqDTO);
    } catch (InvalidOAuthClientException e) {
      if (log.isDebugEnabled()) {
        log.debug(
            "Error occurred while issuing access token for Client ID : "
                + tokenReqDTO.getClientId()
                + ", User ID: "
                + tokenReqDTO.getResourceOwnerUsername()
                + ", Scope : "
                + Arrays.toString(tokenReqDTO.getScope())
                + " and Grant Type : "
                + tokenReqDTO.getGrantType(),
            e);
      }
      OAuth2AccessTokenRespDTO tokenRespDTO = new OAuth2AccessTokenRespDTO();
      tokenRespDTO.setError(true);
      tokenRespDTO.setErrorCode(OAuth2ErrorCodes.INVALID_CLIENT);
      tokenRespDTO.setErrorMsg("Invalid Client");
      return tokenRespDTO;
    } catch (Exception e) { // in case of an error, consider it as a system error
      log.error(
          "Error occurred while issuing the access token for Client ID : "
              + tokenReqDTO.getClientId()
              + ", User ID "
              + tokenReqDTO.getResourceOwnerUsername()
              + ", Scope : "
              + Arrays.toString(tokenReqDTO.getScope())
              + " and Grant Type : "
              + tokenReqDTO.getGrantType(),
          e);
      OAuth2AccessTokenRespDTO tokenRespDTO = new OAuth2AccessTokenRespDTO();
      tokenRespDTO.setError(true);
      if (e.getCause().getCause() instanceof SQLIntegrityConstraintViolationException) {
        tokenRespDTO.setErrorCode("sql_error");
      } else {
        tokenRespDTO.setErrorCode(OAuth2ErrorCodes.SERVER_ERROR);
      }
      tokenRespDTO.setErrorMsg("Server Error");
      return tokenRespDTO;
    }
  }
 private OAuth2AccessTokenRespDTO handleError(
     String errorCode, String errorMsg, OAuth2AccessTokenReqDTO tokenReqDTO) {
   if (log.isDebugEnabled()) {
     log.debug(
         "OAuth-Error-Code="
             + errorCode
             + " client-id="
             + tokenReqDTO.getClientId()
             + " grant-type="
             + tokenReqDTO.getGrantType()
             + " scope="
             + OAuth2Util.buildScopeString(tokenReqDTO.getScope()));
   }
   OAuth2AccessTokenRespDTO tokenRespDTO;
   tokenRespDTO = new OAuth2AccessTokenRespDTO();
   tokenRespDTO.setError(true);
   tokenRespDTO.setErrorCode(errorCode);
   tokenRespDTO.setErrorMsg(errorMsg);
   return tokenRespDTO;
 }