GoogleTokenResponse getRefreshToken（）返回null

Question

使用OAuth2.0对具有访问令牌的用户进行身份验证但是 GoogleAuthorizationCodeTokenRequest 会返回新的访问令牌，但它会为getRefreshToken令牌返回null，以下是我的代码：

 GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(HTTP_TRANSPORT, JSON_FACTORY, CLIENT_ID, CLIENT_SECRET, SCOPES)
            .setAccessType("offline")
            .setApprovalPrompt("force").build();

          String authorizeUrl =
                flow.newAuthorizationUrl().setRedirectUri(CALLBACK_URI).build();
                System.out.println("Paste this url in your browser: \n" + authorizeUrl + '\n');

                GoogleAuthorizationCodeTokenRequest tokenRequest =
                        flow.newTokenRequest(dfaToken);
                         tokenRequest.setRedirectUri(CALLBACK_URI);
                         GoogleTokenResponse tokenResponse = tokenRequest.execute();

                GoogleCredential credential = new GoogleCredential.Builder()
                         .setTransport(new NetHttpTransport())
                         .setJsonFactory(new JacksonFactory())
                         .setClientSecrets(CLIENT_ID, CLIENT_SECRET)
                         .build();              
                credential.setFromTokenResponse(tokenResponse);
    System.out.println(credential.getAccessToken());
    System.out.println(credential.getRefreshToken());

我们需要在项目中设置任何配置吗？