Authenticate with Firebase Anonymously on Android

You can use Firebase Authentication to create and use temporary anonymous accounts to authenticate with Firebase. These temporary anonymous accounts can be used to allow users who haven't yet signed up to your app to work with data protected by security rules. If an anonymous user decides to sign up to your app, you can link their sign-in credentials to the anonymous account so that they can continue to work with their protected data in future sessions.

Before you begin

1. If you haven't already, add Firebase to your Android project.
2. Add the dependency for the Firebase Authentication Android library to your module (app-level) Gradle file (usually app/build.gradle):

   implementation 'com.google.firebase:firebase-auth:16.2.1'

3. If you haven't yet connected your app to your Firebase project, do so from the Firebase console.
4. Enable anonymous auth:
   1. In the Firebase console, open the Auth section.
   2. On the Sign-in Methods page, enable the Anonymous sign-in method.

Authenticate with Firebase anonymously

When a signed-out user uses an app feature that requires authentication with Firebase, sign in the user anonymously by completing the following steps:

1. In your activity's onCreate method, get the shared instance of the FirebaseAuth object:

   Java
   Android

   private FirebaseAuth mAuth;
   // ...
   // Initialize Firebase Auth
   mAuth = FirebaseAuth.getInstance();
   AnonymousAuthActivity.java

   Kotlin
   Android

   private lateinit var auth: FirebaseAuth
   // ...
   // Initialize Firebase Auth
   auth = FirebaseAuth.getInstance()
   AnonymousAuthActivity.kt

2. When initializing your Activity, check to see if the user is currently signed in:

   Java
   Android

   @Override
   public void onStart() {
       super.onStart();
       // Check if user is signed in (non-null) and update UI accordingly.
       FirebaseUser currentUser = mAuth.getCurrentUser();
       updateUI(currentUser);
   }
   AnonymousAuthActivity.java

   Kotlin
   Android

   public override fun onStart() {
       super.onStart()
       // Check if user is signed in (non-null) and update UI accordingly.
       val currentUser = auth.currentUser
       updateUI(currentUser)
   }
   AnonymousAuthActivity.kt

3. Finally, call signInAnonymously to sign in as an anonymous user:

   Java
   Android

   mAuth.signInAnonymously()
           .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, "signInAnonymously:success");
                       FirebaseUser user = mAuth.getCurrentUser();
                       updateUI(user);
                   } else {
                       // If sign in fails, display a message to the user.
                       Log.w(TAG, "signInAnonymously:failure", task.getException());
                       Toast.makeText(AnonymousAuthActivity.this, "Authentication failed.",
                               Toast.LENGTH_SHORT).show();
                       updateUI(null);
                   }
   
                   // ...
               }
           });
   AnonymousAuthActivity.java

   Kotlin
   Android

   auth.signInAnonymously()
           .addOnCompleteListener(this) { task ->
               if (task.isSuccessful) {
                   // Sign in success, update UI with the signed-in user's information
                   Log.d(TAG, "signInAnonymously:success")
                   val user = auth.currentUser
                   updateUI(user)
               } else {
                   // If sign in fails, display a message to the user.
                   Log.w(TAG, "signInAnonymously:failure", task.exception)
                   Toast.makeText(baseContext, "Authentication failed.",
                           Toast.LENGTH_SHORT).show()
                   updateUI(null)
               }
   
               // ...
           }
   AnonymousAuthActivity.kt

   If sign-in succeeds you can use the getCurrentUser method to get the user's account data.

Convert an anonymous account to a permanent account

When an anonymous user signs up to your app, you might want to allow them to continue their work with their new account—for example, you might want to make the items the user added to their shopping cart before they signed up available in their new account's shopping cart. To do so, complete the following steps:

1. When the user signs up, complete the sign-in flow for the user's authentication provider up to, but not including, calling one of the FirebaseAuth.signInWith methods. For example, get the user's Google ID token, Facebook access token, or email address and password.

2. Get an AuthCredential for the new authentication provider:

   Google Sign-In

   Java
   Android

   AuthCredential credential = GoogleAuthProvider.getCredential(googleIdToken, null);
   MainActivity.java

   Kotlin
   Android

   val credential = GoogleAuthProvider.getCredential(googleIdToken, null)
   MainActivity.kt

   Facebook Login

   Java
   Android

   AuthCredential credential = FacebookAuthProvider.getCredential(token.getToken());
   MainActivity.java

   Kotlin
   Android

   val credential = FacebookAuthProvider.getCredential(token.token)
   MainActivity.kt

   Email-password sign-in

   Java
   Android

   AuthCredential credential = EmailAuthProvider.getCredential(email, password);
   MainActivity.java

   Kotlin
   Android

   val credential = EmailAuthProvider.getCredential(email, password)
   MainActivity.kt

3. Pass the AuthCredential object to the sign-in user's linkWithCredential method:

   Java
   Android

   mAuth.getCurrentUser().linkWithCredential(credential)
           .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
               @Override
               public void onComplete(@NonNull Task<AuthResult> task) {
                   if (task.isSuccessful()) {
                       Log.d(TAG, "linkWithCredential:success");
                       FirebaseUser user = task.getResult().getUser();
                       updateUI(user);
                   } else {
                       Log.w(TAG, "linkWithCredential:failure", task.getException());
                       Toast.makeText(AnonymousAuthActivity.this, "Authentication failed.",
                               Toast.LENGTH_SHORT).show();
                       updateUI(null);
                   }
   
                   // ...
               }
           });
   AnonymousAuthActivity.java

   Kotlin
   Android

   auth.currentUser?.linkWithCredential(credential)
           ?.addOnCompleteListener(this) { task ->
               if (task.isSuccessful) {
                   Log.d(TAG, "linkWithCredential:success")
                   val user = task.result?.user
                   updateUI(user)
               } else {
                   Log.w(TAG, "linkWithCredential:failure", task.exception)
                   Toast.makeText(baseContext, "Authentication failed.",
                           Toast.LENGTH_SHORT).show()
                   updateUI(null)
               }
   
               // ...
           }
   AnonymousAuthActivity.kt

If the call to linkWithCredential succeeds, the user's new account can access the anonymous account's Firebase data.

Next steps

Now that users can authenticate with Firebase, you can control their access to data in your Firebase database using Firebase rules.