[BUG][Java][feign] NPE on OAuth2 when 'expires_in' data is not provided by authentication server

Created by: thibaudsowa

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

If OAuth server doesn't provide time of token expiration with the attribute expires_in a NPE append. For my example: Salesforce API.

openapi-generator version

Swagger Codegen 2.2.3

OpenAPI declaration file content or url

---
swagger: '2.0'
info:
  version: v1
  title: Test oauth
paths:
  "/test":
    post:
      consumes:
      - application/json
      produces:
      - application/json
      responses:
        '201':
          description: Success
securityDefinitions:
  OAuth:
    type: oauth2
    flow: password
    tokenUrl: https://test.salesforce.com/services/oauth2/token
security:
- OAuth: []

Command line used for generation

java -jar swagger-codegen-cli.jar generate \
   -i swagger.yaml \
   -l java \
   -o ./generated \
   --library feign

Steps to reproduce

Just try to request an authenticated resource with OAuth2 on an authentication server that does not provide expires_in data:

ApiClient apiClient = new ApiClient("OAuth", "clientId", "secret", "username", "password");
apiClient.setBasePath("https://test.my.salesforce.com");

DefaultApi defaultApi = apiClient.buildClient(DefaultApi.class);
defaultApi.testPost();

Suggest a fix

If expires_in is not provided by authentication server we can not know the time of expiration, so the authentication token should be asked on every request? Or at least provide a default timeout that can be customized?