Java 2 Ada

Using the Facebook API

By stephane.carrez

Through this article you will learn how to use the OAuth 2.0 framework to let an application access service provider APIs such as Facebook API, Google+ API and others. Althought this article uses Ada as programming language and Facebook as service provider, most part also applies to other programming languages and other service providers.

Overview

OAuth 2.0 is an open standard for authorization. It is used by service providers as authorization mechanism for most of their APIs. The authorization workflow is depicted below:

  • [1], first a user goes in the application which displays a link to the OAuth API provider asking the user to grant access to his data for the application,
  • [2], the user clicks on the authenticate link and grants access to the application,
  • [3.1], The OAuth server redirects the user to a callback URL and it provides an application grant code,
  • [3.3], The application ask the API provider to transform the grant code to an access token,
  • [4] The application invokes the API provider with the access token

OAuth Workflow

Registering the application

The first step is to register the application in the service provider (Facebook, Google+, Twitter, ...). This registration process is specific to the provider. From this registration, several elements will be defined:

  • An application id is allocated, This identifier is public. This is the client_id parameter in OAuth 2.0.
  • An application secret is defined. It must be kept private to the application. This is the secret parameter in OAuth 2.0.
  • A callback URL or domain name is registered in the service provider. As far as I'm concerned, I had to register the domain demo.vacs.fr.

Facebook OAuth

For the OAuth authorization process, we will use the Ada Security library and its Application type. We will extend that type to expose some EL variables and an EL method that will be used in the authorization process. The Ada Server Faces Application Example part 3: the action bean explains how to do that and many details will no be covered by this article.

type Facebook_Auth is new Security.OAuth.Clients.Application
     and Util.Beans.Basic.Readonly_Bean
     and Util.Beans.Methods.Method_Bean with private;

FB_Auth      : aliased Facebook.Facebook_Auth;

Before anything we have to initialize the Application type to setup the application identifier, the application secret, the provider URL and a callback URL.

FB_Auth.Set_Application_Identifier ("116337738505130");
FB_Auth.Set_Application_Secret ("xxxxxxxxxxx");
FB_Auth.Set_Application_Callback ("http://demo.vacs.fr/oauth/localhost:8080/demo/oauth_callback.html");

The first step in the OAuth process is to build the authorization page with the link that redirects the user to the service provider OAuth authorization process. The link must be created by using the Get_State and Get_Auth_Params functions provided by the Application type. The first one generates a secure unique key that will be returned back by the service provider. The second one builds a list of request parameters that are necessary for the service provider to identify the application and redirect the user back to the application once the authentication is done.

Id : constant String := "...";
State  : constant String := FB_Auth.Get_State (Id);
Params : constant String := FB_Auth.Get_Auth_Params (State, "read_stream");

For a Facebook authorization process, the URI would be created as follows:

URI : constant String := "https://www.facebook.com/dialog/oauth?" & Params;

For another service provider, the process is similar but the URL is different.

OAuth callback

When the user has granted access to his data, he will be redirected to the callback defined by the application. Most service providers will require that the OAuth callback be a public URL. If you want to run you application on localhost (which is the case when you are developing), you will need a second redirection. If you are using the Apache server, you can easily setup a rewrite rule:

RewriteRule ^/oauth/localhost:([0-9]+)/(.*) http://localhost:$1/$2 [L,R=302]

With the above rewrite rule, the callback given to the OAuth provider would look like:

http://demo.vacs.fr/oauth/localhost:8080/demo/oauth_callback.html

The OAuth provider will first redirect to the public internet site which will redirect again to localhost and port 8080.

Getting the OAuth access token

The next step is to receive the code parameter from the callback which grants the application access to the service provider API. For this, we will use an XHTML view file and a view action that will be executed when the page is displayed. When this happens, the EL method authenticate will be called on the facebook bean (ie, our FB_Auth instance).

<f:view xmlns:f="http://java.sun.com/jsf/core">
    <f:metadata>
        <f:viewAction action="#{facebook.authenticate}"/>
    </f:metadata>
</f:view>

The Authenticate procedure extracts from the request the OAuth state and code parameters. It verifies that the state parameter is a valid key that we submitted and it makes a HTTP POST request on the OAuth service provider to transform the code into an access token. This step is handled by the Ada Security library through the Request_Access_Token operation.

procedure Authenticate (From    : in out Facebook_Auth;
                        Outcome : in out Ada.Strings.Unbounded.Unbounded_String) is
   use type Security.OAuth.Clients.Access_Token_Access;

   F       : constant ASF.Contexts.Faces.Faces_Context_Access := ASF.Contexts.Faces.Current;
   State : constant String := F.Get_Parameter (Security.OAuth.State);
   Code  : constant String := F.Get_Parameter (Security.OAuth.Code);
   Session : ASF.Sessions.Session := F.Get_Session;
begin
   Log.Info ("Auth code {0} for state {1}", Code, State);
   if Session.Is_Valid then
      if From.Is_Valid_State (Session.Get_Id, State) then
         declare
            Acc : Security.OAuth.Clients.Access_Token_Access
              := From.Request_Access_Token (Code);
         begin
            if Acc /= null then
               Log.Info ("Access token is {0}", Acc.Get_Name);
               Session.Set_Attribute ("access_token",
                                      Util.Beans.Objects.To_Object (Acc.Get_Name));
            end if;
         end;
      end if;
   end if;
end Authenticate;

The access token must be saved in the user session or another per-user safe storage so that it can be retrieved later on. The access token can expire and if this happens a fresh new access token must be obtained.

Getting the Facebook friends

Until now we have dealt with the authorization process. Let's look at using the service provider API and see how the Ada Utility Library will help in this task.

Defining the Ada beans

To represent the API result, we will use an Ada bean object that can easily be used from a presentation page. For the Facebook friend, a name and an identifier are necessary:

type Friend_Info is new Util.Beans.Basic.Readonly_Bean with record
   Name : Util.Beans.Objects.Object;
   Id   : Util.Beans.Objects.Object;
end record;
type Friend_Info_Access is access all Friend_Info;

Having a bean type to represent each friend, we will get a list of friends by instantiating the Ada bean Lists package:

package Friend_List is new Util.Beans.Basic.Lists (Element_Type => Friend_Info);

Mapping JSON or XML to Ada

The Ada Utility library provides a mechanism that parses JSON or XML and map the result in Ada objects. To be able to read the Facebook friend definition, we have to define an enum and implement a Set_Member procedure. This procedure will be called by the JSON/XML parser when a given data field is recognized and extracted.

type Friend_Field_Type is (FIELD_NAME, FIELD_ID);

procedure Set_Member (Into : in out Friend_Info;
                      Field : in Friend_Field_Type;
                      Value : in Util.Beans.Objects.Object);

The Set_Member procedure is rather simple as it just populates the data record with the value.

procedure Set_Member (Into : in out Friend_Info;
                      Field : in Friend_Field_Type;
                      Value : in Util.Beans.Objects.Object) is
begin
   case Field is
      when FIELD_ID =>
         Into.Id := Value;

      when FIELD_NAME =>
         Into.Name := Value;

   end case;
end Set_Member;

The mapper is a package that defines and controls how to map the JSON/XML data fields into the Ada record by using the Set_Member operation. We just have to instantiate the package. The Record_Mapper generic package will map JSON/XML into the Ada record and the Vector_Mapper will map a list of JSON/XML elements following a given structure into an Ada vector.

package Friend_Mapper is
  new Util.Serialize.Mappers.Record_Mapper (Element_Type        => Friend_Info,
                                            Element_Type_Access => Friend_Info_Access,
                                            Fields              => Friend_Field_Type,
                                            Set_Member          => Set_Member);

package Friend_Vector_Mapper is
   new Util.Serialize.Mappers.Vector_Mapper (Vectors        => Friend_List.Vectors,
                                             Element_Mapper => Friend_Mapper);

Now we need to control how the JSON/XML fields are mapped to our Ada fields. For this we have to setup the mapping. The Facebook JSON structure is so simple that we can use the default mapping provided by the mapper. For this we use the Add_Default_Mapping procedure. We also have to tell what is the JSON mapping used by the friend vector mapper.

Friend_Map        : aliased Friend_Mapper.Mapper;
Friend_Vector_Map : aliased Friend_Vector_Mapper.Mapper;
...
   Friend_Map.Add_Default_Mapping;
   Friend_Vector_Map.Set_Mapping (Friend_Map'Access);

Creating the REST client

Now it would be nice if we could get an operation that invokes the service provider API through an HTTP GET operation and put the result in our Ada object. The Facebook friends API returns a list of friends which correspond to our Friend_List.Vectors. To get our operation, we just have to instantiate the Rest_Get_Vector operation with our vector mapper (the generic parameter is a package name).

procedure Get_Friends is
  new Util.Http.Rest.Rest_Get_Vector (Vector_Mapper => Friend_Vector_Mapper);

Calling the REST client

Invoking the service provider API is now as simple as calling a procedure. The URI must include the access token as parameter. The HTTP GET operation must be made using SSL/TLS since this is part of OAuth 2.0.

 List  : Friend_List.List_Bean;
...
   Get_Friends ("https://graph.facebook.com/me/friends?access_token="
                      & Token,
                      Friend_Vector_Map'Access,
                      "/data",
                      List.List'Access);

Now you are ready to use and access the user's data as easily as other information...

References

facebook.ads

facebook.adb

Facebook API

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