Using OAuth 2.0 to Access Google APIs | How to generate oauth token ?

All requests to the Google API must be authorized by an authenticated user. To access any of the google product you need oAuth token.

Authorizing requests with OAuth 2.0:

Setup your appliacation:

Go to below link and setup your application, Once application is create get your OAuth client ID

https://console.developers.google.com/apis/dashboard

Get access Code:

To get a code, you need to run a server. Install tomcat and run a simple dynamic application with only homepage.
Let think application is running on http://localhost/

Create below url and open it on chrome: 

https://accounts.google.com/o/oauth2/auth?scope=https://www.googleapis.com/auth/blogger&response_type=code&access_type=offline&redirect_uri=http://localhost&client_id=YourClientID

Login With valid google account, Once you are Login successfully. It will redirect to localhost with code.

http://localhost/?code=4/2AGswerwyro2KlmBXgXV8A73877482hsjdfhsj7AAtTERUCut7VD_Ty2gVlIlGMyD5xTxBFcYudifu7847&scope=https://www.googleapis.com/auth/blogger

Copy the code and Open postman and any rest api.

Do rest call with below parameters.

Host: oauth2.googleapis.com
Content-Type: application/x-www-form-urlencoded

code=4/P7q7W91a-oMsCeLvIaQm6bTrgtp7&
client_id=your_client_id&
client_secret=your_client_secret&
redirect_uri=urn%3Aietf%3Awg%3Aoauth%3A2.0%3Aoob%3Aauto&
grant_type=authorization_code

It will response :
{
  "access_token": "1/fFAGRNJru1FTz70BzhT3Zg",
  "expires_in": 3920,
  "token_type": "Bearer",
  "refresh_token": "1//xEoDL4iW3cxlI7yDbSRFYNG01kVKM2C-259HOF2aQbI"
}

access_token is your Authorization token. Use it for google api call.

GET https://www.googleapis.com/blogger/v3/users/self/blogs
Authorization: /* OAuth 2.0 token here */

Example:

GET https://www.googleapis.com/blogger/v3/users/self/blogs
Authorization: Bearer 1/fFAGRNJru1FTz70BzhT3Zg

Java APi call: Sending GET request with Authentication headers

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON);
headers.set("Authorization", "Bearer "+accessToken);

HttpEntity<String> entity = new HttpEntity<String>(requestJson,headers);
String result = restTemplate.postForObject(url, entity, String.class);

You can use it for all google products like blogger, Drive, YouTube, Gmail, Map, news ect.