java - 如何修复 Firebase 身份验证中的空指针异常？

Question

我需要这方面的帮助。每次我运行该应用程序时，每次在登录 Activity 中打开时它都会崩溃。请帮忙，我坚持这个。 我已经遵循了很多教程，我认为代码是相同的，但仍然出现空指针异常。我不知道如何修复这个空指针。

日志猫:

java.lang.NullPointerException: Attempt to invoke interface method 'void com.google.firebase.auth.FirebaseAuth$AuthStateListener.onAuthStateChanged(com.google.firebase.auth.FirebaseAuth)' on a null object reference
    at com.google.firebase.auth.zzi.run(Unknown Source:2)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at com.google.android.gms.internal.firebase_auth.zzj.dispatchMessage(Unknown Source:6)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:6718)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:495)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

我的登录 Activity 代码:

public class Login extends AppCompatActivity {

    private FirebaseAuth mAuth;
    FirebaseAuth.AuthStateListener mAuthListner;

    Button btnlogin;
    TextView btndaftar;
    MaterialEditText mEmail, pass;

    @Override
    protected void onStart() {
        super.onStart();
        FirebaseUser currentUser = mAuth.getCurrentUser();
        updateUI(currentUser);
    }

    private void updateUI(FirebaseUser currentUser) {
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        mAuth = FirebaseAuth.getInstance();

        mAuth.addAuthStateListener(mAuthListner);

        mAuthListner = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {
                    Intent intent = new Intent(Login.this, NavigationMenu.class);
                    startActivity(intent);
                    finish();
                }
            }
        };

        btnlogin=findViewById(R.id.btnLogin);
        mEmail=findViewById(R.id.edt_email);
        pass=findViewById(R.id.edt_pasword);
        btndaftar=findViewById(R.id.btnDaftar);

        btndaftar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent daftar = new Intent(Login.this, Daftar.class);
                startActivity(daftar);
            }
        });



        btnlogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String email = mEmail.getText().toString();
                String password = pass.getText().toString();

                mAuth.signInWithEmailAndPassword(email, password)
                        .addOnCompleteListener(Login.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
                                    Intent intent = new Intent(Login.this, NavigationMenu.class);
                                    startActivity(intent);
                                    finish();
                                    FirebaseUser currentUser = mAuth.getCurrentUser();
                                    updateUI(currentUser);
                                } else {
                                    // If sign in fails, display a message to the user.
                                    Toast.makeText(Login.this, "Login gagal !",
                                            Toast.LENGTH_SHORT).show();
                                    updateUI(null);
                                }
                            }
                        });
           }
        });
    }
}

应用程序打开后，它就崩溃了。知道为什么会发生这种情况吗？

Answer 1

当您将监听器添加到 onCreate() 中的 addAuthStateListener() 方法时，它还没有值，因此您的应用会崩溃

在添加到mAuth之前初始化mAuthListner

您需要像这样反转代码:

   // First create your listener instance
   mAuthListner = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {
                    Intent intent = new Intent(Login.this, NavigationMenu.class);
                    startActivity(intent);
                    finish();
                }
            }
        };

   // And so, add it to the addAuthStateListener() method
   mAuth.addAuthStateListener(mAuthListner);