How to Authenticate user, password with Google App Engine + Java?

Question

I need to authenticate user email and password with Google. I am using App Engine + Java environment. Is there any provision in Google APIs ?

currently i am using

ContactService client = new ContactService ("yourCompany-yourAppName-v1");
client.setUserCredentials("user@example.com", "pa$$word");

I need to replace this Google APIs. I need to authenticate user with Google not application.

Please suggest me how can i do this.

Answer 1

In your last comment you say that the google username and password is entered in the client and validated in the server. That is NOT possible, google has deprecated the ability of an app to capture the password and thats excellent.
The only way to do this is with oauth2, BUT addionally you need to pass a special parameter &max_auth_age=0. At this point this question is basically a duplicate of this: No prompt for re-authentication with OAUth2. Why and how to force it?
however I didnt mark duplicate because yours is a different situation.
Explaining oauth in detail is not part of this answer as its explained well in the official documentation and many guides. Do not use the built-in appengine userService. Instead configure appengine pages so they are all public and on top implement oauth2, for example using the existing google+ javascript signin libraries (thou not sure if those let you set the max_auth_age parameter. If not, you need to roll your own oauth2 "client side 3-legged" flow. In that flow, the client will see the google login page every time, and after the client-side flow finishes, the client will end up with an access token which is what it sends to the server to validate that the user entered the correct username and password. If you include the correct scope (userinfo.email scope) then the server can use that access token to ask google the username that generated the token, thus you have validated that the client did login to google and you know their email.
But you will NOT receive the password. Just enough info to authenticate the user.

Answer 2

The question isn't very clear, I'll try to respond in two way.

1) Authenticate users on appengine with custom username and password.

To do that you need the "classic" way, I suggest you to use a framework like Spring with his module "spring-security", take a look at this sample.

2) Authenticate users on appengine with Google Account.

This is the best way I think, you only need to follow this guide.