You can integrate Firebase Authentication with a custom authentication system by modifying your authentication server to produce custom signed tokens when a user successfully signs in. Your app receives this token and uses it to authenticate with Firebase.
Before you begin
- If you haven't already, add Firebase to your Android project.
-
In your module (app-level) Gradle file
(usually
<project>/<app-module>/build.gradle.kts
or<project>/<app-module>/build.gradle
), add the dependency for the Firebase Authentication library for Android. We recommend using the Firebase Android BoM to control library versioning.dependencies { // Import the BoM for the Firebase platform implementation(platform("com.google.firebase:firebase-bom:33.4.0")) // Add the dependency for the Firebase Authentication library // When using the BoM, you don't specify versions in Firebase library dependencies implementation("com.google.firebase:firebase-auth") }
By using the Firebase Android BoM, your app will always use compatible versions of Firebase Android libraries.
(Alternative) Add Firebase library dependencies without using the BoM
If you choose not to use the Firebase BoM, you must specify each Firebase library version in its dependency line.
Note that if you use multiple Firebase libraries in your app, we strongly recommend using the BoM to manage library versions, which ensures that all versions are compatible.
dependencies { // Add the dependency for the Firebase Authentication library // When NOT using the BoM, you must specify versions in Firebase library dependencies implementation("com.google.firebase:firebase-auth:23.0.0") }
- Get your project's server keys:
- Go to the Service Accounts page in your project's settings.
- Click Generate New Private Key at the bottom of the Firebase Admin SDK section of the Service Accounts page.
- The new service account's public/private key pair is automatically saved on your computer. Copy this file to your authentication server.
Authenticate with Firebase
- In your sign-in activity's
onCreate
method, get the shared instance of theFirebaseAuth
object:Kotlin+KTX
private lateinit var auth: FirebaseAuth // ... // Initialize Firebase Auth auth = Firebase.auth
Java
private FirebaseAuth mAuth; // ... // Initialize Firebase Auth mAuth = FirebaseAuth.getInstance();
- When initializing your Activity, check to see if the user is currently signed in:
Kotlin+KTX
public override fun onStart() { super.onStart() // Check if user is signed in (non-null) and update UI accordingly. val currentUser = auth.currentUser updateUI(currentUser) }
Java
@Override public void onStart() { super.onStart(); // Check if user is signed in (non-null) and update UI accordingly. FirebaseUser currentUser = mAuth.getCurrentUser(); updateUI(currentUser); }
- When users sign in to your app, send their sign-in credentials (for example, their username and password) to your authentication server. Your server checks the credentials and returns a custom token if they are valid.
- After you receive the custom token from your authentication server, pass
it to
signInWithCustomToken
to sign in the user:Kotlin+KTX
customToken?.let { auth.signInWithCustomToken(it) .addOnCompleteListener(this) { task -> if (task.isSuccessful) { // Sign in success, update UI with the signed-in user's information Log.d(TAG, "signInWithCustomToken:success") val user = auth.currentUser updateUI(user) } else { // If sign in fails, display a message to the user. Log.w(TAG, "signInWithCustomToken:failure", task.exception) Toast.makeText( baseContext, "Authentication failed.", Toast.LENGTH_SHORT, ).show() updateUI(null) } } }
Java
mAuth.signInWithCustomToken(mCustomToken) .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() { @Override public void onComplete(@NonNull Task<AuthResult> task) { if (task.isSuccessful()) { // Sign in success, update UI with the signed-in user's information Log.d(TAG, "signInWithCustomToken:success"); FirebaseUser user = mAuth.getCurrentUser(); updateUI(user); } else { // If sign in fails, display a message to the user. Log.w(TAG, "signInWithCustomToken:failure", task.getException()); Toast.makeText(CustomAuthActivity.this, "Authentication failed.", Toast.LENGTH_SHORT).show(); updateUI(null); } } });
AuthStateListener
you can use thegetCurrentUser
method to get the user's account data.
Next steps
After a user signs in for the first time, a new user account is created and linked to the credentials—that is, the user name and password, phone number, or auth provider information—the user signed in with. This new account is stored as part of your Firebase project, and can be used to identify a user across every app in your project, regardless of how the user signs in.
-
In your apps, you can get the user's basic profile information from the
FirebaseUser
object. See Manage Users. In your Firebase Realtime Database and Cloud Storage Security Rules, you can get the signed-in user's unique user ID from the
auth
variable, and use it to control what data a user can access.
You can allow users to sign in to your app using multiple authentication providers by linking auth provider credentials to an existing user account.
To sign out a user, call
signOut
:
Kotlin+KTX
Firebase.auth.signOut()
Java
FirebaseAuth.getInstance().signOut();