java.lang.NullPointerException：不能在 child() 中为参数“pathString”传递 null

Question

Structure of database

首先，在用户登录或完全注册时进行测试时，没有异常或错误，但当用户退出并重新打开应用程序时，应用程序崩溃了。 logcat 基本上是在指出错误：

 UsersRef.child(currentUserID).addValueEventListener(new ValueEventListener()

但如果我将 currentUserID 更改为 Users .. 应用程序会停止崩溃并平稳运行，但不会加载位于导航抽屉。

ma​​inactivity.java

public class MainActivity extends AppCompatActivity {
    private NavigationView navigationView;
    private DrawerLayout drawerLayout;
    private RecyclerView postlist;
    private Toolbar mToolbar;
    private ActionBarDrawerToggle actionBarDrawerToggle;
    private FirebaseAuth mAuth;
    private FirebaseUser FirebaseUser;
    private DatabaseReference  UsersRef;
    private CircleImageView NavProfileImage;
    private ImageButton AddNewPostButton;
    private TextView NavProfileUserName;
    String currentUserID;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mAuth = FirebaseAuth.getInstance();
        final FirebaseUser mFirebaseUser = mAuth.getCurrentUser();
        if (mFirebaseUser != null) {
            currentUserID = mFirebaseUser.getUid();
        }




        mToolbar =(Toolbar) findViewById(R.id.main_page_toolbar);
        setSupportActionBar(mToolbar);
        getSupportActionBar().setTitle("Home");

        drawerLayout = (DrawerLayout) findViewById(R.id.drawable_layout);
       actionBarDrawerToggle =  new ActionBarDrawerToggle(MainActivity.this,drawerLayout,R.string.drawer_open, R.string.drawer_close);
        navigationView = (NavigationView)findViewById(R.id.navigation_view);
       AddNewPostButton = (ImageButton)findViewById(R.id.add_new_post_button);

        drawerLayout.addDrawerListener(actionBarDrawerToggle);
       actionBarDrawerToggle.syncState();
       getSupportActionBar().setDisplayHomeAsUpEnabled(true);


        View navView = navigationView.inflateHeaderView(R.layout.nav_header);
        NavProfileImage = (CircleImageView)navView.findViewById(R.id.nav_profile_image);
        NavProfileUserName = (TextView) navView.findViewById(R.id.nav_user_full_name);

        UsersRef = FirebaseDatabase.getInstance().getReference().child("Users");
        UsersRef.child(currentUserID).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot)
            {
                if(dataSnapshot.exists())
                {   if (dataSnapshot.hasChild("fullname")){
                    String fullname = dataSnapshot.child("fullname").getValue().toString();
                    NavProfileUserName.setText(fullname);
                }if (dataSnapshot.hasChild("profileimage")) {
                    String image = dataSnapshot.child("profileimage").getValue().toString();
                    Picasso.with(MainActivity.this).load(image).placeholder(R.drawable.profile).into(NavProfileImage);

                }else {
                    Toast.makeText(MainActivity.this, "Profile name do not exists...", Toast.LENGTH_SHORT).show();
                }




                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item)
            { UserMenuSelector(item);

                return false;
            }
        });
        AddNewPostButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                SendUserToPostActivity();

            }
        });
    }

    private void SendUserToPostActivity() {
        Intent addNewPostIntent = new Intent (MainActivity.this,PostActivity.class);
        startActivity(addNewPostIntent);
    }

    @Override
    protected void onStart() {
        super.onStart();
        FirebaseUser currentUser = mAuth.getCurrentUser();
        if (currentUser == null)
        {
            sendUserToLoginActivity();
        }else{
            CheckUserExistance();
        }
    }

    private void CheckUserExistance()
    {
        final String current_user_id = mAuth.getCurrentUser().getUid();
          UsersRef.addValueEventListener(new ValueEventListener() {
              @Override
              public void onDataChange(DataSnapshot dataSnapshot) {
                  if (!dataSnapshot.hasChild(current_user_id)){
                      sendUserToSetupActivity();
                  }
              }

              @Override
              public void onCancelled(DatabaseError databaseError) {

              }
          });
    }

    private void sendUserToSetupActivity() {
        Intent setupIntent = new Intent(MainActivity.this, SetupActivity.class);
        setupIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        startActivity(setupIntent);
        finish();

    }

    private void sendUserToLoginActivity()
    {
        Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
         loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
         startActivity(loginIntent);
         finish();

    }

Answer 1

在注销mFirebaseUser 后获取 null 值，因此此 currentUserID 值也为 null 以避免 nullpointerexception 在使用 currentUserID 之前执行 null 检查，如下代码所示

public class MainActivity extends AppCompatActivity {
private NavigationView navigationView;
private DrawerLayout drawerLayout;
private RecyclerView postlist;
private Toolbar mToolbar;
private ActionBarDrawerToggle actionBarDrawerToggle;
private FirebaseAuth mAuth;
private FirebaseUser FirebaseUser;
private DatabaseReference  UsersRef;
private CircleImageView NavProfileImage;
private ImageButton AddNewPostButton;
private TextView NavProfileUserName;
String currentUserID;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mAuth = FirebaseAuth.getInstance();
    final FirebaseUser mFirebaseUser = mAuth.getCurrentUser();
    if (mFirebaseUser != null) {
        currentUserID = mFirebaseUser.getUid();
    }




    mToolbar =(Toolbar) findViewById(R.id.main_page_toolbar);
    setSupportActionBar(mToolbar);
    getSupportActionBar().setTitle("Home");

    drawerLayout = (DrawerLayout) findViewById(R.id.drawable_layout);
   actionBarDrawerToggle =  new ActionBarDrawerToggle(MainActivity.this,drawerLayout,R.string.drawer_open, R.string.drawer_close);
    navigationView = (NavigationView)findViewById(R.id.navigation_view);
   AddNewPostButton = (ImageButton)findViewById(R.id.add_new_post_button);

    drawerLayout.addDrawerListener(actionBarDrawerToggle);
   actionBarDrawerToggle.syncState();
   getSupportActionBar().setDisplayHomeAsUpEnabled(true);


    View navView = navigationView.inflateHeaderView(R.layout.nav_header);
    NavProfileImage = (CircleImageView)navView.findViewById(R.id.nav_profile_image);
    NavProfileUserName = (TextView) navView.findViewById(R.id.nav_user_full_name);

    UsersRef = FirebaseDatabase.getInstance().getReference().child("Users");
      // replace below code
     if (mFirebaseUser != null) {

    UsersRef.child(currentUserID).addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot)
        {
            if(dataSnapshot.exists())
            {   if (dataSnapshot.hasChild("fullname")){
                String fullname = dataSnapshot.child("fullname").getValue().toString();
                NavProfileUserName.setText(fullname);
            }if (dataSnapshot.hasChild("profileimage")) {
                String image = dataSnapshot.child("profileimage").getValue().toString();
                Picasso.with(MainActivity.this).load(image).placeholder(R.drawable.profile).into(NavProfileImage);

            }else {
                Toast.makeText(MainActivity.this, "Profile name do not exists...", Toast.LENGTH_SHORT).show();
            }




            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

    }else{
     sendUserToLoginActivity();
    }

    navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item)
        { UserMenuSelector(item);

            return false;
        }
    });
    AddNewPostButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            SendUserToPostActivity();

        }
    });
}

private void SendUserToPostActivity() {
    Intent addNewPostIntent = new Intent (MainActivity.this,PostActivity.class);
    startActivity(addNewPostIntent);
}

@Override
protected void onStart() {
    super.onStart();
    FirebaseUser currentUser = mAuth.getCurrentUser();
    if (currentUser == null)
    {
        sendUserToLoginActivity();
    }else{
        CheckUserExistance();
    }
}

private void CheckUserExistance()
{
    final String current_user_id = mAuth.getCurrentUser().getUid();
      UsersRef.addValueEventListener(new ValueEventListener() {
          @Override
          public void onDataChange(DataSnapshot dataSnapshot) {
              if (!dataSnapshot.hasChild(current_user_id)){
                  sendUserToSetupActivity();
              }
          }

          @Override
          public void onCancelled(DatabaseError databaseError) {

          }
      });
}

private void sendUserToSetupActivity() {
    Intent setupIntent = new Intent(MainActivity.this, SetupActivity.class);
    setupIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
    startActivity(setupIntent);
    finish();

}

private void sendUserToLoginActivity()
{
    Intent loginIntent = new Intent(MainActivity.this, LoginActivity.class);
     loginIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
     startActivity(loginIntent);
     finish();

}

或者你也可以直接在这里查看

 if (mFirebaseUser != null) {
        currentUserID = mFirebaseUser.getUid();
    }else{
        sendUserToLoginActivity();
    }

Answer 2

我认为您在currentUserId 中获得null，然后将null 传递给firebase。可能是您没有获得user id，请确保用户已登录，然后检查您是否从firebase 获得user id。如果您从 firebase 获得user id，那么试试这个。

UsersRef.child("Users").child(currentUserID).addValueEventListener(new 
       ValueEventListener(){ }

并确保您的用户模型类中的所有密钥都是公开的，我定期在 firebase 上工作了大约一个月，我遇到了很多这样的问题。

Answer 3

如果您检查我相信您没有任何登录用户，这就是您在currentUserID 中获得null 值的原因。

您正在检查用户是否在OnStart() 中登录，您还必须在 onCreate() 中检查。因为在活动生命周期中onCreate() 在onStart() 之前被调用。

这是您的 onCreate 函数的外观：

@Override
protected void onCreate(Bundle savedInstanceState) {
    FirebaseUser currentUser = mAuth.getCurrentUser();
    if (currentUser == null)
    {
        sendUserToLoginActivity();
    }else{
        CheckUserExistance();
    }
    //Your code ...
}

Answer 4

您收到java.lang.NullPointerException: Can't pass null for argument 'pathString' in child() 是因为当您注销用户时，您并没有将他发送到登录活动，这就是此错误的原因。解决这个问题的最简单方法是使用监听器。假设您有两个活动，LoginActivity 和 MainActivity。可以在LoginActivity 中创建的监听器应该是这样的：

FirebaseAuth.AuthStateListener authStateListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
        if (firebaseUser != null) {
            Intent intent = new Intent(LoginActivity.this, MainActivity.class);
            startActivity(intent);
            finish();
        }
    }
};

这基本上意味着如果用户已登录，则跳过LoginActivity 并转到MainActivity。

实例化FirebaseAuth 对象：

FirebaseAuth firebaseAuth = FirebaseAuth.getInstance();

然后开始监听 onStart() 方法的变化，如下所示：

@Override
protected void onStart() {
    super.onStart();
    firebaseAuth.addAuthStateListener(authStateListener);
}

在MainActivity，你应该做同样的事情：

FirebaseAuth.AuthStateListener authStateListener = new FirebaseAuth.AuthStateListener() {
    @Override
    public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
        FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
        if (firebaseUser == null) {
            Intent intent = new Intent(MainActivity.this, LoginActivity.class);
            startActivity(intent);
        }
    }
};

这基本上意味着如果用户未登录，请跳过MainActivity 并转到LoginActivity。在这个活动中你应该做和LoginActivity一样的事情，你应该开始监听onStart()的变化。

这样，你就再也不会遇到这样的错误了。