In my current project, i needed to integrate Google+ in the application using server-side API. Google uses OAuth2.0 protocol for authorization when our application tries to access the data. All we require is an access token to fetch data from Google using REST calls which serves data in JSON format.

I implemented it using Web Server Applications API and thought it worth sharing.

There are basically 3 steps to fetch data from Google.

1. Register an application.

We need to register an application at Google API Console. Go to the Google console page using the link provided and create an aplication.

Steps involve in registering an application are:-

a.) Create project by providing name to the project.

b.) Turn ON the service required, in our case it is Google Plus API.

c.) Create a Oauth 2.0 Client ID.

d.) Create the OAuth 2.0 ClientId and provide the callback URL where google will send the authorization token.

e.) Note down Client ID and Client Secret as generated in the above step.

2. Obtain an Access Token from the Google Authorization Server.

Obtaining an access token involves 2 steps.

a.) Request for Authorization Code.

In this step, we will request the Google server for authorization code by providing registered application client ID in the URL to Google server.

I created an action to redirect to Goolge, when someone want to connect to google plus.

String CLIENT_ID =client_id_obtained from registered app
String CALLBACK_URL = callback_url_as_mentioned in the registered app.
String GOOGLE_PLUS_SCOPE='https://www.googleapis.com/auth/plus.me'    // scope is the permissions we are requesting.

Action code is as:

def registerOnGooglePlus = {
String authorizeUrl = "https://accounts.google.com/o/oauth2/auth?scope=${GOOGLE_PLUS_SCOPE}&
redirect_uri=${CALLBACK_URL}&response_type=code&client_id=${CLIENT_ID}&access_type=offline"
URL urlForGooglePlus = new URL(authorizeUrl)
redirect(url: urlForGooglePlus)
}

The Redirect will take user to permissions page, if the user is already logged-in or will take to login page and then permissions page.

After approving the required permissions, user will redirect back to the application’s registered Callback URL with the authorization code.

b.) Request for Access Token with the authorization code obtained from the above action.

Access Token can be received by a POST request using the Client Secret and authorization code received.

The POST call requires 5 properties to be send in the body of the request in the encoded form.

	code : The authorization code returned from the initial request
	client_id : The client_id obtained during application registration
	client_secret : The client secret obtained during application registration
	redirect_uri : The URI registered with the application
	grant_type : authorization_code

// Sample action to receive authorization code

def callBack={
StringBuilder sb = new StringBuilder("code=");
sb.append(URLEncoder.encode(code, "UTF-8"));
sb.append("&client_id=");
sb.append(URLEncoder.encode(clientId, "UTF-8"));
sb.append("&client_secret=");
sb.append(URLEncoder.encode(clientSecret, "UTF-8"));
sb.append("&redirect_uri=");
sb.append(URLEncoder.encode(callbackUrl, "UTF-8"));
sb.append("&grant_type=");
sb.append(URLEncoder.encode('authorization_code', "UTF-8"));

String URL_TO_REQUEST_TOKEN= 'https://accounts.google.com/o/oauth2/token'

URL url = new URL(URL_TO_REQUEST_TOKEN);
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
try {
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
connection.setRequestProperty("Content-Length", "" + sb.toString().length());
connection.setRequestProperty("Host", "accounts.google.com");
OutputStreamWriter outputStreamWriter = new OutputStreamWriter(connection.getOutputStream());
outputStreamWriter.write(sb.toString());
outputStreamWriter.flush();
log.debug("Response code ${connection.responseCode} , Message : ${connection.responseMessage}")
String resultData = connection.content.text
def responseJson = JSON.parse(resultData)
String ACCESS_TOKEN = responseJson?.access_token
}
catch (Exception e) {
e.printStackTrace()
}
}

3. Calling Google API.

Now, with the help of access token, we can call google API to fetch Data by appending the access token in the GET request.

Example:

To fetch person’s profile data

GET :  https://www.googleapis.com/oauth2/v1/userinfo?access_token=ACCESS_TOKEN

To get list of profile acitivties

 GET :  https://www.googleapis.com/plus/v1/people//activities/public?access_token=ACCESS_TOKEN

References:-
http://code.google.com/apis/accounts/docs/OAuth2WebServer.html

http://code.google.com/apis/accounts/docs/OAuth2.html

Hope this helps..!!!

Vishal Sahu
vishal[at]intelligrape[dot]com
www.intelligrape.com