OAuthClient.java

package com.example.yourapp;

import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import com.example.yourapp.AuthenticationModel;
import retrofit.client.Header;
import retrofit.client.OkClient;
import retrofit.client.Request;
import retrofit.client.Response;

/**
 * Extends the default Retrofit Http client with a check if the request got denied because the OAuth
 * token expired. In this case
 * To be entirely sure that there will be no StackOverFlowErrors using this implementation
 * Don't let your authentication model use this as a client.
 **/
public class OAuthClient extends OkClient {

    private AuthenticationModel _authModel;

    public OAuthClient(AuthenticationModel authModel){
        _authModel = authModel;
    }

    /**
     * Replaces the access token with the new one in the request.
     * @param request Input request to modify
     * @return An other request with the same parameters, but the replaced authorization header
     */
    private Request changeTokenInRequest(Request request){
        List<Header> tempHeaders = request.getHeaders(); // this one is an unmodifiable list.
        List<Header> headers = new ArrayList<Header>();
        headers.addAll(tempHeaders); // this one is modifiable
        Iterator<Header> iter = headers.iterator();
        boolean hadAuthHeader = false;
        // we check if there was an authentication header in the original request
        while(iter.hasNext()){
            Header h = iter.next();
            if (h.getName().equals("Authorization")){
                iter.remove();
                hadAuthHeader = true;
            }
        }
        // if there was an authentication header, replace it with another one containing the new access token.
        if (hadAuthHeader){
            headers.add(new Header("Authorization", "Bearer " + _authModel.getAccessToken()));
        }
        // everything stays the same, except the headers
        return new Request(request.getMethod(), request.getUrl(), headers, request.getBody());
    }

    @Override
    public Response execute(Request request) throws IOException {
        Response response = super.execute(request);
        // 401: Forbidden, 403: Permission denied
        if (response.getStatus() == 401 || response.getStatus() == 403) {
            // the next call should be a synchronous call, otherwise it will immediately continue, and use the old token instead.
            _authModel.refreshToken();
            // the headers should be modified because the access token changed
            Request newRequest = changeTokenInRequest(request);
            return super.execute(newRequest);
        } else {
            return response;
        }
    }
}

Utils.java

// Create your RestAdapter this way
RestAdapter adapter = new Builder()
            .setEndpoint(endpoint)
            .setClient(new OAuthClient(yourAuthModel)
            ...
            .build();


// Util to convert TypedInput to Java String
 public static String stringFromTypedInput(TypedInput input) {
        try {
            InputStream istream = input.in();
            java.util.Scanner s = new java.util.Scanner(istream).useDelimiter("\\A");
            return s.hasNext() ? s.next() : "";
        } catch (IOException e) {
            return null;
        }
    }
    
// Example call
private interface OAuthInterface {
            
        @FormUrlEncoded
        @POST("/oauth2/access_token")
        public void getAccessToken(@Field("username") String username, @Field("password") String password, @Field("client_id") String clientId, @Field("client_secret") String clientSecret, @Field("grant_type") String grantType,
                Callback<TokenData> cb);

        // NOTICE: This call is synchronous! This should only be called from the OAuthClient.
        @FormUrlEncoded
        @POST("/oauth2/access_token")
        public String refreshToken(@Field("refresh_token") String refreshToken, @Field("client_id") String clientId, @Field("client_secret") String clientSecret, @Field("grant_type") String grantType);
}