App Engine-specific helpers make quick work of authenticated calls to APIs, and you do not need to worry about exchanging code for tokens.

For example:

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
  AppIdentityCredential credential =
      new AppIdentityCredential(Arrays.asList(UrlshortenerScopes.URLSHORTENER));
  Urlshortener shortener =
      new Urlshortener.Builder(new UrlFetchTransport(), new JacksonFactory(), credential)
      .build();
  UrlHistory history = shortener.URL().list().execute();
  ...
}

If you are building a web app that interacts with a user's data via an OAuth 2.0-enabled API, we've created some helpers to assist you with the process. The helpers aim to:

• Simplify the process of obtaining access tokens (AuthorizationCodeFlow).
• Manage tokens, after they are obtained, by marking them as PersistenceCapable.
• Simplify the process of making authenticated calls using the access token's credential.
• Insulate you from the details of authentication when writing servlets.

1. Install the Google API Client Library for Java:
   • Follow the download instructions and put the library jar files into your war/WEB-INF/lib directory.
   • Alternatively, you can use Maven.
2. Learn about using OAuth 2.0 with the authorization code flow for Google App Engine applications.
3. Learn about using OAuth 2.0 with the Google App Engine Identity API.
4. Take a look at the Calendar App Engine sample. This sample combines our Java library and auth helpers to show you how to access end-user data from within a Google App Engine web app. The sample also uses GWT for the user interface.