Firebase user authentication for java application (Not Android)

Question

I want to create a user authentication form for my java application using firebase. The dependencies for connection to the realtime database is available and the use of Firebase Admin is well documented here.

But presently Firebase Admin supports user authentication only for Node.js and it is documented here.

Here is my test code.

public class Login {
    private JPanel jPanel;

    public static void main(String[] args) {
//        Show My Form
        JFrame jFrame = new JFrame("Login");
        jFrame.setContentPane(new Login().jPanel);
        jFrame.pack();
        jFrame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        jFrame.setVisible(true);

//        Firebase
        FirebaseOptions options = null;
        try {
            options = new FirebaseOptions.Builder()
                    .setServiceAccount(new FileInputStream("xxx.json"))
                    .setDatabaseUrl("https://xxx.firebaseio.com/")
                    .build();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

        if (options != null) {
            FirebaseApp.initializeApp(options);
            DatabaseReference ref = FirebaseDatabase.getInstance().getReference("Clip");
            ref.addValueEventListener(new ValueEventListener() {
                public void onDataChange(DataSnapshot dataSnapshot) {
                    System.out.println("ClipText = [" + dataSnapshot.getValue() + "]");
                }

                public void onCancelled(DatabaseError databaseError) {

                }
            });
        }
    }
}

Can anyone please guide me how can I create user authentication (e.g. Create user using email & password, Sign in) for my java application?

Note: I'm using Gradle.

Answer 1

Basing on the REST API documentation here: https://firebase.google.com/docs/reference/rest/auth/ i created a singleton like that to get a valid token from email and password and to validate it using the getAccountInfo method:

import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;

import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParser;

public class FireBaseAuth {

    private static final String BASE_URL = "https://www.googleapis.com/identitytoolkit/v3/relyingparty/";
    private static final String OPERATION_AUTH = "verifyPassword";
    private static final String OPERATION_REFRESH_TOKEN = "token";
    private static final String OPERATION_ACCOUNT_INFO = "getAccountInfo";

    private String firebaseKey;

    private static FireBaseAuth instance = null;

    protected FireBaseAuth() {
       firebaseKey = "YOUR FIREBASE KEY HERE";
    }

    public static FireBaseAuth getInstance() {
      if(instance == null) {
         instance = new FireBaseAuth();
      }
      return instance;
    }

    public String auth(String username, String password) throws Exception { 

        HttpURLConnection urlRequest = null;
        String token = null;

        try {
            URL url = new URL(BASE_URL+OPERATION_AUTH+"?key="+firebaseKey);
            urlRequest = (HttpURLConnection) url.openConnection();
            urlRequest.setDoOutput(true);
            urlRequest.setRequestProperty("Content-Type", "application/json; charset=UTF-8");
            OutputStream os = urlRequest.getOutputStream();
            OutputStreamWriter osw = new OutputStreamWriter(os, "UTF-8");
            osw.write("{\"email\":\""+username+"\",\"password\":\""+password+"\",\"returnSecureToken\":true}");
            osw.flush();
            osw.close();

            urlRequest.connect();

            JsonParser jp = new JsonParser(); //from gson
            JsonElement root = jp.parse(new InputStreamReader((InputStream) urlRequest.getContent())); //Convert the input stream to a json element
            JsonObject rootobj = root.getAsJsonObject(); //May be an array, may be an object. 

            token = rootobj.get("idToken").getAsString();

        } catch (Exception e) {
            return null;
        } finally {
            urlRequest.disconnect();
        }

        return token;
    }

    public String getAccountInfo(String token) throws Exception {

        HttpURLConnection urlRequest = null;
        String email = null;

        try {
            URL url = new URL(BASE_URL+OPERATION_ACCOUNT_INFO+"?key="+firebaseKey);
            urlRequest = (HttpURLConnection) url.openConnection();
            urlRequest.setDoOutput(true);
            urlRequest.setRequestProperty("Content-Type", "application/json; charset=UTF-8");
            OutputStream os = urlRequest.getOutputStream();
            OutputStreamWriter osw = new OutputStreamWriter(os, "UTF-8");
            osw.write("{\"idToken\":\""+token+"\"}");
            osw.flush();
            osw.close();
            urlRequest.connect();

            JsonParser jp = new JsonParser(); //from gson
            JsonElement root = jp.parse(new InputStreamReader((InputStream) urlRequest.getContent())); //Convert the input stream to a json element
            JsonObject rootobj = root.getAsJsonObject(); //May be an array, may be an object. 

            email = rootobj.get("users").getAsJsonArray().get(0).getAsJsonObject().get("email").getAsString();

        } catch (Exception e) {
            return null;
        } finally {
            urlRequest.disconnect();
        }

        return email;

    }

}

This not allow you to create users dinamically but suppose to have them already created on Firebase