Authenticating for Google Services, Part 2

Introduction

In the last blog entry I described how to use OAuth for access/authentication for Google's API services. Unfortunately, as I discovered a bit later, the approach I used was OAuth 1.0, which has apparently now been officially deprecated by Google in favor of version 2.0 of OAuth. Obviously, I was a bit bummed to discovered this, and promised I would create a new blog entry with instructions on how to use 2.0. The good news is that, with the 2.0 support, Google has added some additional helper classes that make things easier, especially if you are using Google App Engine, which is what I'm using for this tutorial.

The Google Developers site now has a pretty good description on how to setup OAuth 2.0. However, it still turned out to be a challenge to configure a real-life example of how it's done, so I figured I'd document what I've learned.

Tutorial Scenario

In the last tutorial, the project I created illustrated how to access a listing of a user's Google Docs files. In this tutorial, I changed things up a bit, and instead use YouTube's API to display a list of a user's favorite videos. Accessing a user's favorites does require authentication with OAuth, so this was a good test.

Getting Started

(Eclipse project for this tutorial can be found here).

The first thing you must do is follow the steps outlined in Google's official docs on using OAuth 2.0. Since I'm creating a web-app, you'll want to follow the section in those docs titled "Web Server Applications". In addition, the steps I talked about previously for setting up a Google App Engine are still relevant, so I'm going to jump right into the code and bypass these setup steps.

(NOTE: The Eclipse project can be found here -- I again elected not to use Maven in order to keep things simple for those who don't have it installed or are knowledgeable in Maven).

The application flow is very simple (assuming a first-time user):

1. When the user accesses the webapp (assuming you are running it locally at http://localhost:8888 using the GAE developer emulator), they must first login to Google using their gmail or Google domain account.
2. Once logged in, the user is redirected to a simple JSP page that has a link to their YouTube favorite videos.
3. When the click on the link, a servlet will initiate the OAuth process to acquire access to their YouTube account. The first part of this process is being redirected to a Google Page that prompts them whether they want to grant the application access.
4. Assuming the user responds affirmative,  a list of 10 favorites will be displayed with links. 
5. If they click on the link, the video will load.

Here's the depiction of the first 3 pages flow:

And here's the last two pages (assuming that the user clicks on a given link):

While this example is specific to YouTube, the same general principles apply for accessing any of the Google-based cloud services, such as Google+, Google Drive, Docs etc. They key enabler for creating such integrations is obviously OAuth, so let's look at how that process works.

OAuth 2.0 Process Flow

Using OAuth can be a bit overwhelming for the new developer first learning the technology. The main premise behind it is to allow users to selectively identify which "private" resources they want to make accessible to an external application, such as we are developing for this tutorial. By using OAuth, the user can avoid having to share their login credentials with a 3rd party, but instead can simply grant that 3rd party access to some of their information.

To achieve this capability, the user is navigated to the source where their private data resides, in this case, YouTube. They can then either allow or reject the access request. If they allow it, the source of the private data (YouTube) then returns a single-use authorization code to the 3rd party application. Since it's rather tedious for the user to have to grant access every time access is desired, there is an additional call that can be played that will 'trade-in" their single use authorization for a longer term one. The overall flow for the web application we're developing for this tutorial can be seen below.

OAuth Flow

 

The first step that takes place is to determine whether the user is already logged into Google using either their gmail or Google Domain account. While not directly tied to the OAuth process, it's very convenient to enable users to login with their Google account as opposed to requiring them to sign up with your web site. That's the first callout that is made to Google. Then, once logged in, the application determines whether the user has a local account setup with OAuth permissions granted. If they are logging in for the first time, they won't.  In that case, the OAuth process is initiated.

The first step of that process is to specify to the OAuth provider, in this case Google YouTube, what "scope" of access is being requested.  Since Google has a lot of services, they have a lot of scopes. You can determine this most easily using their OAuth 2.0 sandbox. 

When you kickoff the OAuth process, you provide them the scope(s) you want access to, along with the OAuth client credentials that Google has provided you (these steps are actually rather generic to any provider that supports OAuth).  For our purposes, we're seeking access to the user's YouTube account, so the scope provided by Google is: https://gdata.youtube.com/.

If the end-user grants access to the resource identify by the scope, Google will then post back an authorization code to the application. This is captured in a servlet. Since the returned code is only a "single-use" code, it is exchanged for a longer running access token (and related refresh token). That step is represented above by the activity/box titled "Access & Refresh Token Requested". 

Once armed with the access token, the application can then access the users' private data by placing an API call along with the token. If everything checks out, the API will return the results.

It's not a terrible complicated process -- it just involves a few steps. Let's look at some of the specific implementation details, beginning with the servlet filter that determines whether the user has already logged into Google and/or granted OAuth access.

AuthorizationFilter

Let's take a look at the first few lines of the AuthorizationFilter (to see how it's configured as a filter, see the web.xml file).

The first few lines simply cast the generic servlet request and response to their corresponding Http equivalents -- this is necessary since we want access to the HTTP session. The next step is to determine whether a CredentialStore is present in the servlet context. As we'll see, this is used to store the user's credentials, so it's convenient to have it readily available in subsequent servlets. The guts of the matter begin when we check to see whether the user is already present in the session using:

if (session.getAttribute(Constant.AUTH_USER_ID) == null) {

If not,  we get their Google login credentials using Google's UserService class. This is a helper class available to GAE users to fetch the user's Google userid, email and nickname. Once we get this info from UserService, we store some of the user's details in the session.

At this point, we haven't done anything with OAuth, but that will change in the next series of code lines:
try {
    Utils.getActiveCredential(request, credentialStore);
} catch (NoRefreshTokenException e1) {
    // if this catch block is entered, we need to perform the oauth process
    LOGGER.info("No user found - authorization URL is: " +   
                 e1.getAuthorizationUrl());
    response.sendRedirect(e1.getAuthorizationUrl());
}
A helper class called Utils is used for most of the OAuth processing. In this case, we're calling the static method getActiveCredential().  As we will see in a moment, this method will return a NoRefreshTokenException if no OAuth credentials have been previously captured for the user. As a custom exception, it will return URL value that is used for redirecting the user to Google to seek OAuth approval.

Let's take a look at the getActiveCredential() method in more detail, as that's where much of the OAuth handling is managed.


The first thing we do is fetch the Google userId from the session (they can't get this far without it being populated).  Next, we attempt to get the user's OAuth credentials (stored in the Google class with the same name) from the CredentialStore using the Utils static method getStoredCredential(). If no credentials are found for that user, the Utils method called getAuthorizationUrl() is invoked. This method, which is shown below, is used to construct the URL that the browser is redirected to which is used to prompt the user to authorize access to their private data (the URL is served up by Google, since it will ask the user for approval).


As you can see, this method is using the class (from Google) called GoogleAuthorizationCodeRequestUrl. It constructs an HTTP call using the OAuth client credentials that is provided by Google when you sign up for using OAuth (those credentials, coincidentally, are stored in a file called client_secrets.json. Other parameters include the scope of the OAuth request and the URL that the user will be redirected back to if approval is granted by the user.  That URL is the one you specified when signing up for Google's OAuth access:

Now, if the user had already granted OAuth access, the getActiveCredential()method would instead grab the credentials from the CredentialStore.

Turning back to the URL that receives the results of the OAuth credentials, in this case, http://localhost:8888/authSub, you maybe wondering, how can Google post to that internal-only address? Well, it's the user's browser that is actually posting back the results, so localhost, in this case, resolves just fine. Let's look that the servlet called OAuth2Callback that is used to process this callback (see the web.xml for how the servlet mapping for authSub is done).


The most important take-away from this class is the line:

AuthorizationCodeResponseUrl authResponse = 

       new AuthorizationCodeResponseUrl(fullUrlBuf.toString());

The AuthorizationCodeResponseUrl class is provided as a convenience  by Google to parse the results of the OAuth request. If the getError() method of that class isn't null, that means that the user rejected the request. In the event that it is null, indicating the user approved the request, the method call getCode() is used to retrieve the one-time authorization code. This code value is placed into the user's session, and when the Utils.getActiveCredential() is invoked following the redirect to the user's target URL (via the filter), it will exchange that authorization code for a longer-term access and refresh token using the call:
credential = exchangeCode((String) request.getSession().getAttribute("code"));
The Utils.exchangeCode() method is shown next:

This method also uses a Google class called GoogleAuthorizationCodeTokenRequest that is used to call Google to exchange the one-time OAuth authorization code for the longer-duration access token.

Now that we've (finally) got our access token that is needed for the YouTube API, we're ready to display to the user 10 of their video favorites.

 

Calling the YouTube API Services

With the access token in hand, we can now proceed to display the user their list of favorites. In order to do this, a servlet called FavoritesServlet is invoked. It will call the YouTube API, parse the resulting JSON-C format into some local Java classes via Jackson, and then send the results to the JSP page for processing. Here's the servlet:

Since this post is mainly about the OAuth process, I won't go into too much detail how the API call is placed, but the most important line of code is:

feed = YouTube.fetchFavs(credential.getAccessToken());

Where feed is an instance of VideoFeed. As you can see, another helper class called YouTube is used for doing the heavy-lifting. Just to wrap things up, I'll show the fetchFavs() method.


It uses the Google class called HttpRequestFactory to construct an outbound HTTP API call to YouTube. Since we're using GAE, we're limited as to which classes we can use to place such requests.  Notice the line of code:
url.access_token = accessToken;
That's where we are using the access token that was acquired through the OAuth process.

So, while it took a fair amount of code to get the OAuth stuff working correctly, once it's in place, you are ready to rock-and-roll with calling all sorts of Google API services!

Authenticating for Google Services in Google App Engine

(NOTE: This tutorial uses the older, and now deprecated Oauth 1.0 approach -- See my new blog entry for an OAuth 2.0 tutorial).

Introduction

This post will illustrate how to build a simple Google App Engine (GAE) Java application that authenticates against Google as well as leverages Google's OAuth for authorizing access to Google's API services such as Google Docs.  In addition, building on some of the examples already provided by Google, it will also illustrate how to persist data using the App Engine Datastore and Objectify.

Project Source Code

The motivation behind this post is that I struggled to previously find any examples that really tied these technologies together. Yet, these technologies really represent the building-blocks for many web applications that want to leverage the vast array of Google API services. 

To keep things simple, the demo will simply allow the user to login via a Google domain; authorize access to the user's Google Docs services; and display a list of the user's Google Docs Word and Spreadsheet documents. Throughout this tutorial I do make several assumptions about the reader's expertise, such as a pretty deep familiarity with Java.

Overview of the Flow

Before we jump right into the tutorial/demo, let's take a brief look at the navigation flow. 

While it may look rather complicated, the main flow can be summarized as:

1. User requests access to listFiles.jsp (actually any of the JSP pages can be used).
2. A check is make to see if the user is logged into Google. If not, they are re-directed to a Google login page -- once logged in, they are returned back. 
3. A check is then made to determine whether the user is stored in the local datastore. If not, the user is added along with the user's Google domain email address.
4. Next, we check to see if the user has granted OAuth credentials to the Google Docs API service. If not, the OAuth authentication process is initiated. Once the OAuth credentials are granted, they are stored in the local user table (so we don't have to ask each time the user attempts to access the services).
5. Finally, a list of Google Docs Spreadsheet or Word docs is displayed.

This same approach could be used to access other Google services, such as YouTube (you might display a list of the user's favorite videos, for example).

Environment Setup

For this tutorial, I am using the following:

• Eclipse Indigo Service Release 2 along with the Google Plugin for Eclipse (see setup instructions).
• Google GData Java SDK Eclipse plugin version 1.47.1 (see setup instructions). 
• Google App Engine release 1.6.5. Some problems exist with earlier versions, so I'd recommend making sure you are using it. It should install automatically as part of the Google Plugin for Eclipse.
• Objectify version 3.1. The required library is installed already in the project's war/WEB-INF/lib directory.

 After you have imported the project into Eclipse, your build path should resemble:

The App Engine settings should resemble:

You will need to setup your own GAE application, along with specifying your own Application ID (see the Google GAE developer docs). 

The best tutorial I've seen that describes how to use OAuth to access Google API services can be found here. One of the more confusing aspects I found was how to acquire the necessary consumer key and consumer secret values that are required when placing the OAuth request. The way I accomplished this was:

1. Create the GAE application using the GAE Admin Console. You will need to create your own Application ID (just a name for your webapp). Once you have it, you will update your Application ID in the Eclipse App Engine settings panel that is shown above.
2. Create a new Domain for the application. For example, since my Application ID was specified above as "tennis-coachrx", I configured the target URL path prefix as: http://tennis-coachrx.appspot.com/authSub. You will see how we configure that servlet to receive the credentials shortly. 
3. To complete the domain registration, Google will provide you an HTML file that you can upload. Include that file the root path under the /src/war directory and upload the application to GAE. This way, when Google runs it's check, the file will be present and it will generate the necessary consumer credentials. Here's a screenshot of what the setup looks like after it is completed:

Once you have the OAuth Consumer Key and OAuth Consumer Secret, you will then replace the following values in the com.zazarie.shared.Constant file:

final static String CONSUMER_KEY = "";

final static String CONSUMER_SECRET = "";

Whew, that seemed like a lot of work! However, it's a one-time deal, and you shouldn't have to fuss with it again.

Code Walkthrough

Now that we got that OAuth configuration/setup out of the way, we can dig into the code. Let's begin by looking at the structure of the war directory, where your web assets reside:

The listFiles.jsp is the default JSP page that is displayed when your first enter the webapp. Let's now look at the web.xml file to see how this is configured, along with the servlet filter which is central to everything.

The servlet filter called AuthorizationFilter is invoked whenever a JSP file located in the html directory is requested. The filter, as we'll look at in a moment, is responsible for ensuring that the user is logged into Google, and if so, then ensures that the OAuth credentials have been granted for that user (i.e., it will kick off the OAuth credentialing process, if required).

The servlet name of Step2 represents the servlet that is invoked by Google when the OAuth credentials have been granted -- think of it as a callback. We will look at this in more detail in a bit.

Let's take a more detailed look at the AuthorizationFilter.

AuthorizationFilter Deep Dive

The doFilter method is where the work takes place in a servlet filter.  Here's the implementation:


Besides the usual housekeeping stuff, the main logic begins with the line:

AppUser appUser = LoginService.login(request, response);

As we will see in a moment, the LoginService is responsible for logging the user into Google, and also will create the user in the local BigTable datastore. By storing the user locally, we can then store the user's OAuth credentials, eliminating the need for the user to have to grant permissions every time they access a restricted/filtered page.

After LoginService has returned the user (AppUser object), we then store that user object into the session (NOTE: to enable sessions, you must set sessions-enabled in the appengine-web.xml file):

session.setAttribute(Constant.AUTH_USER, appUser);

We then check to see whether the OAuth credentials are associated with that user:

if (appUser.getCredentials() == null) {

   session.setAttribute(Constant.TARGET_URI, request.getRequestURI());

   OAuthRequestService.requestOAuth(request, response, session);
   return;
} else 
   session.setAttribute(Constant.DOC_SESSION_ID,LoginService.docServiceFactory(appUser));

If getCredentials() returns a null, the OAuth credentials have not already been assigned for the user. This means that the OAuth process needs to be kicked off. Since this involves a two-step process of posting the request to Google and then retrieving back the results via the callback (Step2 servlet mentioned above), we need to store the destination URL so that we can later redirect the user to it once the authorization process is completed. This is done by storing the URL requested into the session using the setAttribute method.

We then kick off the OAuth process by calling the OAuthRequestService.requestOAuth() method (details discussed below).

In the event that if getCredentials() returns a non-null value, this indicates that we already have the user's OAuth credentials from their local AppUser entry in the datastore, and we simply add it to the session so that we can use it later.

LoginService Deep Dive

The LoginService class has one main method called login, followed by a bunch of JPA helper methods for saving or updating the local user in the datastore. We will focus on login(), since that is where most of the business logic resides.


The first substantive thing we do is use Google UserService class to determine whether the user is logged into Google:

UserService userService = UserServiceFactory.getUserService();

User user = userService.getCurrentUser();

If the User object returned by Google's call is null, the user isn't logged into Google, and they are redirected to a login page using:

res.sendRedirect(userService.createLoginURL(URI));

If the user is logged (i.e., not null), the next thing we do is determine whether that user exists in the local datastore. This is done by looking up the user with their logged-in Google email address with appUser = findUser(userEmail). Since JPA/Objectify isn't the primary discussion point for this tutorial, I won't go into how that method works. However, the Objectify web site has some great tutorials/documentation.

If the user doesn't exist locally, the object is populated with Google's email address and created using appUser = addUser(userEmail). If the user does exist, we simply update the login timestamp for logging purposes.

OAuthRequestService Deep Dive

As you may recall from earlier, once the user is setup locally, the AuthorizationFilter will then check to see whether the OAuth credentials have been granted by the user. If not, the OAuthRequestService.requestOAuth() method is invoked. It is shown below:


To simplify working with OAuth, Google has a set of Java helper classes that we are utilizing. The first thing we need to do is setup the consumer credentials (acquiring those was discussed earlier):

GoogleOAuthParameters oauthParameters = new GoogleOAuthParameters();
oauthParameters.setOAuthConsumerKey(Constant.CONSUMER_KEY);
oauthParameters.setOAuthConsumerSecret(Constant.CONSUMER_SECRET);

Then, we set the scope of the OAuth request using:
oauthParameters.setScope(Constant.GOOGLE_RESOURCE);

Where Constant.GOOGLE_RESOURCE resolves to https://docs.google.com/feeds/. When you make an OAuth request, you specify the scope of what resources you are attempting to gain access. In this case, we are trying to access Google Docs (the GData API's for each service have the scope URL provided).  Next, we establish where we want Google to return the reply.

oauthParameters.setOAuthCallback(Constant.OATH_CALLBACK);

This value changes whether we are running locally in dev mode, or deployed to the Google App Engine. Here's how the values are defined in the the Constant interface:

// Use for running on GAE
//final static String OATH_CALLBACK = "http://tennis-coachrx.appspot.com/authSub";

// Use for local testing
final static String OATH_CALLBACK = "http://127.0.0.1:8888/authSub";

When then sign the request using Google's helper:

GoogleOAuthHelper oauthHelper = new GoogleOAuthHelper(new OAuthHmacSha1Signer());

We then generate the URL that the user will navigate to in order to authorize access to the resource. This is generated dynamically using:

String approvalPageUrl = oauthHelper.createUserAuthorizationUrl(oauthParameters);

The last step is to provide a link to the user so that they can navigate to that URL to approve the request. This is done by constructing some simple HTML that is output using res.getWriter().print().

Once the user has granted access, Google calls back to the servlet identified by the URL parameter /authSub, which corresponds to the servlet class RequestTokenCallbackServlet. We will examine this next.

RequestTokenCallbackServlet Deep Dive

The servlet uses the Google OAuth helper classes to generate the required access token and secret access token's that will be required on subsequent calls to to the Google API docs service.  Here is the doGet method that receives the call back response from Google:

The Google GoogleOAuthHelper is used to perform the housekeeping tasks required to populate the two values we are interested in:

String accessToken = oauthHelper.getAccessToken(oauthParameters);
String accessTokenSecret = oauthParameters.getOAuthTokenSecret();

Once we have these values, we then requery the user object from the datastore, and save those values into the AppUser.OauthCredentials subclass:

appUser = LoginService.getById(appUser.getId());
appUser = LoginService.updateUserCredentials(appUser,
          new OauthCredentials(accessToken, accessTokenSecret));
req.getSession().setAttribute(Constant.DOC_SESSION_ID,
LoginService.docServiceFactory(appUser));

In addition, you'll see they are also stored into the session so we have them readily available when the API request to Google Docs is placed.

Now that we've got everything we need, we simply redirect the user back to the resource they had originally requested:

RequestDispatcher dispatcher = req.getRequestDispatcher((String) req
.getSession().getAttribute(Constant.TARGET_URI));
dispatcher.forward(req, resp);

Now, when they access the JSP page listing their documents, everything should work!

Here's a screencast demo of the final product:



Hope you enjoyed the tutorial and demo -- look forward to your comments!

Business Rules with Drools Intro

I've created an introductory presentation on the use and benefit of using a rules engine/business rule management system.  It uses JBoss Drools for the samples. I will be following up shortly with a more detailed presentation on how to use Drools integration so that you may explose your business rules as a web service API.