@RequestMapping(path = "requestToken")
void requestToken(HttpServletResponse response) throws IOException {
    RequestPermissionsResponse requestPermissionsResponse = requestPermissions();
    requestPermissionsResponse.getToken();
    response.sendRedirect("https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_grant-permission&request_token=" + requestPermissionsResponse.getToken());
}

@RequestMapping(method = GET, path = "callback")
void oauthCallback(String request_token, String verification_code, HttpServletResponse response) throws IOException, ClientActionRequiredException, InterruptedException, MissingCredentialException, SSLConfigurationException, OAuthException, InvalidResponseDataException, InvalidCredentialException, HttpErrorException {
      logger.info("action=oauth_callback, request_token={}, verification_code={}", request_token, verification_code);

      GetAccessTokenResponse accessToken = getAccessToken(request_token, verification_code);
      ThirdPartyAuthorization thirdPartyAuthorization = new TokenAuthorization(accessToken.getToken(), accessToken.getTokenSecret());

      // Construct a payment from a customer to 2 vendors.
      PayRequest payRequest = new PayRequest();

      List<Receiver> receivers = new ArrayList<>();
      Receiver secondaryReceiver = new Receiver();
      secondaryReceiver.setAmount(1.00);
      secondaryReceiver.setEmail("vendor_1@example.com");
      receivers.add(secondaryReceiver);

      Receiver primaryReceiver = new Receiver();
      primaryReceiver.setAmount(2.00);
      primaryReceiver.setEmail("vendor_2@example.com");
      primaryReceiver.setPrimary(true);
      receivers.add(primaryReceiver);
      ReceiverList receiverList = new ReceiverList(receivers);

      payRequest.setReceiverList(receiverList);
      payRequest.setSenderEmail("customer_1@example.com");

      RequestEnvelope requestEnvelope = new RequestEnvelope("en_US");
      payRequest.setRequestEnvelope(requestEnvelope);
      payRequest.setActionType("PAY");
      payRequest.setCancelUrl("https://devtools-paypal.com/guide/ap_chained_payment?cancel=true");
      payRequest.setReturnUrl("https://devtools-paypal.com/guide/ap_chained_payment?success=true");
      payRequest.setCurrencyCode("USD");
      payRequest.setIpnNotificationUrl("https://example.com/events");

      // Make the payment
      AdaptivePaymentsService adaptivePaymentsService = new AdaptivePaymentsService(sdkConfig);
      String username = sdkConfig.get("acct1.UserName");
      String password = sdkConfig.get("acct1.Password");
      String signature = sdkConfig.get("acct1.Signature");
      String applicationId = sdkConfig.get("acct1.AppId");

      SignatureCredential credential = new SignatureCredential(username, password, signature);
      credential.setThirdPartyAuthorization(thirdPartyAuthorization);
      credential.setApplicationId(applicationId);

      String authorizationHeader = OAuthSignature.getFullAuthString(username, password, accessToken.getToken(), accessToken.getTokenSecret(), OAuthSignature.HTTPMethod.POST, "https://api.sandbox.paypal.com/nvp", null);
      logger.info("X-PAYPAL-AUTHORIZATION={}", authorizationHeader);

      PayResponse payResponse = adaptivePaymentsService.pay(payRequest, credential);
}

2016-03-07 10:38:21.983  INFO 41378 --- [nio-8080-exec-6] com.paypal.core.APIService:
    responseEnvelope.timestamp=2016-03-07T09%3A38%3A21.955-08%3A00&
    responseEnvelope.ack=Failure&
    responseEnvelope.correlationId=50b62800b3a0f&
    responseEnvelope.build=20420247&
    error(0).errorId=520003&
    error(0).domain=PLATFORM&
    error(0).subdomain=Application&
    error(0).severity=Error&
    error(0).category=Application&
    error(0).message=Authentication+failed.+API+credentials+are+incorrect.