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 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. Add Firebase to your Android project.
2. Add the dependency for Firebase Authentication to your app-level build.gradle file:

   compile 'com.google.firebase:firebase-auth:9.6.1'

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:

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

2. Set up an AuthStateListener that responds to changes in the user's sign-in state:

   private FirebaseAuth.AuthStateListener mAuthListener;
   
   // ...
   
   @Override
   protected void onCreate(Bundle savedInstanceState) {
       // ...
       mAuthListener = new FirebaseAuth.AuthStateListener() {
           @Override
           public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
               FirebaseUser user = firebaseAuth.getCurrentUser();
               if (user != null) {
                   // User is signed in
                   Log.d(TAG, "onAuthStateChanged:signed_in:" + user.getUid());
               } else {
                   // User is signed out
                   Log.d(TAG, "onAuthStateChanged:signed_out");
               }
               // ...
           }
       };
       // ...
   }
   
   @Override
   public void onStart() {
       super.onStart();
       mAuth.addAuthStateListener(mAuthListener);
   }
   
   @Override
   public void onStop() {
       super.onStop();
       if (mAuthListener != null) {
           mAuth.removeAuthStateListener(mAuthListener);
       }
   }
   AnonymousAuthActivity.java
   

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

   mAuth.signInAnonymously()
           .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
               @Override
               public void onComplete(@NonNull Task<AuthResult> task) {
                   Log.d(TAG, "signInAnonymously:onComplete:" + task.isSuccessful());
   
                   // If sign in fails, display a message to the user. If sign in succeeds
                   // the auth state listener will be notified and logic to handle the
                   // signed in user can be handled in the listener.
                   if (!task.isSuccessful()) {
                       Log.w(TAG, "signInAnonymously", task.getException());
                       Toast.makeText(AnonymousAuthActivity.this, "Authentication failed.",
                               Toast.LENGTH_SHORT).show();
                   }
   
                   // ...
               }
           });
   AnonymousAuthActivity.java
   

   If sign-in succeeds, the AuthStateListener runs the onAuthStateChanged callback, in which 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

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

   Facebook Login

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

   Email-password sign-in

   AuthCredential credential = EmailAuthProvider.getEmailAuthCredential(email, password);
   

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

   mAuth.getCurrentUser().linkWithCredential(credential)
           .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
               @Override
               public void onComplete(@NonNull Task<AuthResult> task) {
                   Log.d(TAG, "linkWithCredential:onComplete:" + task.isSuccessful());
   
                   // If sign in fails, display a message to the user. If sign in succeeds
                   // the auth state listener will be notified and logic to handle the
                   // signed in user can be handled in the listener.
                   if (!task.isSuccessful()) {
                       Toast.makeText(AnonymousAuthActivity.this, "Authentication failed.",
                               Toast.LENGTH_SHORT).show();
                   }
   
                   // ...
               }
           });
   AnonymousAuthActivity.java
   

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.