@Service
public class GoogleConnectionService implements GoogleConnection {
    private static final String CLIENT_SECRETS = "/client_secrets.json";
    // ..
    @Override
    public GoogleClientSecrets getClientSecrets() {
        if (clientSecrets == null) {
            try {
                // load client secrets
                InputStreamReader clientSecretsReader = new InputStreamReader(getSecretFile());
                clientSecrets = GoogleClientSecrets.load(Global.JSON_FACTORY, clientSecretsReader);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return clientSecrets;
    }
    @Override
    public Credential getCredentials() {
        return credential;
    }
  // ..
}

@Service
public class GoogleSheetsService implements GoogleSheets {
    private Sheets sheetsService = null;
 
    @Override
    public List<List<Object>> readTable(GoogleConnection connection) throws IOException {
        Sheets service = getSheetsService(connection);
        return readTable(service, spreadsheetId, sheetName);
    }
 
    private Sheets getSheetsService(GoogleConnection gc) throws IOException {
        if (this.sheetsService == null) {
            this.sheetsService = new Sheets.Builder(Global.HTTP_TRANSPORT, Global.JSON_FACTORY, gc.getCredentials())
                    .setApplicationName(appName).build();
        }
        return this.sheetsService;
    }
}

@RestController
public class GoogleAuthorizationController {
    @Autowired
    private GoogleConnectionService connection;
 
    @RequestMapping(value = "/ask", method = RequestMethod.GET)
    public void ask(HttpServletResponse response) throws IOException {
        // Step 1: Authorize --> ask for auth code
        String url = new GoogleAuthorizationCodeRequestUrl(connection.getClientSecrets(), connection.getRedirectUrl(), Global.SCOPES).setApprovalPrompt("force").build();
        response.sendRedirect(url);
    }
}

@RestController
public class GoogleCallbackController {
    @Autowired
    private GoogleConnectionService connection;
 
    @RequestMapping(value = "/oauth2callback", method = RequestMethod.GET)
    public void callback(@RequestParam("code") String code, HttpServletResponse response) throws IOException {
        // Step 2: Exchange code --> access tocken
        if (connection.exchangeCode(code)) {
            response.sendRedirect(connection.getSourceUrl());
        } else {
            response.sendRedirect("/error");
        }
    }
}

@RestController
public class GoogleSheetController {
    @Autowired
    private GoogleConnection connection;
    @Autowired
    private GoogleSheets sheetsService;
 
    @RequestMapping(value = "/api/sheet", method = RequestMethod.GET)
    public ResponseEntity<List<List<Object>>> read(HttpServletResponse response) throws IOException {
        List<List<Object>> responseBody = sheetsService.readTable(connection);
        return new ResponseEntity<List<List<Object>>>(responseBody, HttpStatus.OK);
    }
}

public class GoogleSheetsInterceptor implements HandlerInterceptor {
        // ..
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object obj) throws Exception {
        if (connection.getCredentials() == null) {
            connection.setSourceUrl(request.getRequestURI());
            response.sendRedirect("/ask");
            return false;
        }
        return true;
    }
}

@Service
public class GoogleConnectionService implements GoogleConnection {
    @Autowired
    private OAuth2ClientContext oAuth2ClientContext;
    private GoogleCredential googleCredentials = null;
        // ..
    @Override
    public Credential getCredentials() {
        if (googleCredentials == null) {
            googleCredentials = new GoogleCredential.Builder()
                    .setTransport(Global.HTTP_TRANSPORT)
                    .setJsonFactory(Global.JSON_FACTORY)
                    .setClientSecrets(clientId, clientSecret)
                    .build()
                    .setAccessToken(response.getAccessToken())
                    .setFromTokenResponse(oAuth2ClientContext
                        .getAccessToken().getValue());
        }
        return googleCredentials;
    }
}