DataSnapShot对象的值在getvalue（boolean.class）上返回null

Question

我不明白出了什么问题，编码这个的人让它完美地工作。

异常发生在以下行：if（datasnapshot.getvalue（boolean.class））{

当我将此记录在屏幕上时，datasnapshot对象有一个键但没有值

//firebase
DatabaseReference onlineRef,currentUserRef,counterRef;
FirebaseRecyclerAdapter<User,ListOnlineViewHolder> adapter;

//View
RecyclerView listOnline;
RecyclerView.LayoutManager layoutManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list_online);

    //setting the recyclerview
    listOnline = (RecyclerView)findViewById(R.id.listOnlineRecyclerview);
    listOnline.setHasFixedSize(true);
    layoutManager = new LinearLayoutManager(this);
    listOnline.setLayoutManager(layoutManager);

    //set toolbar and menu / join,logout
    Toolbar toolbar = (Toolbar)findViewById(R.id.toolbarID);
    toolbar.setTitle("Presence System");
    setSupportActionBar(toolbar);

    //firebase
    onlineRef = FirebaseDatabase.getInstance().getReference().child("info/connected");
    counterRef = FirebaseDatabase.getInstance().getReference("lastOnline"); //create new child name lastOnline
    currentUserRef = FirebaseDatabase.getInstance().getReference().child(FirebaseAuth.getInstance().getCurrentUser().getUid());

    setupSystem();
    //after setup we load all users and display in recyclerview
    //this is online list
    updateList();
}

private void updateList() {
    adapter = new FirebaseRecyclerAdapter<User, ListOnlineViewHolder>(
            User.class,R.layout.user_layout,ListOnlineViewHolder.class,counterRef
    ) {
        @Override
        protected void populateViewHolder(ListOnlineViewHolder viewHolder, User model, int position) {
            viewHolder.emailTextView.setText(model.getEmail());
        }

    };
    adapter.notifyDataSetChanged();
    listOnline.setAdapter(adapter);
}

private void setupSystem() {
    onlineRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
                if(dataSnapshot.getValue(Boolean.class)){
                    currentUserRef.onDisconnect().removeValue();
                    //set online user in list
                    counterRef.child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                            .setValue(FirebaseAuth.getInstance().getCurrentUser().getEmail(),"Online");
                    adapter.notifyDataSetChanged();
                }
            }



        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
    counterRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot postSnapshot:dataSnapshot.getChildren()){
                User user = postSnapshot.getValue(User.class);
                Log.d("LOG",""+user.getEmail()+"is "+user.getStatus());
            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater menuInflater = getMenuInflater();
    menuInflater.inflate(R.menu.main_menu,menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()){
        case R.id.action_join:
            counterRef.child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                    .setValue(FirebaseAuth.getInstance().getCurrentUser().getEmail(),"Online");
            break;
        case R.id.action_logout:
            currentUserRef.removeValue();

    }
    return super.onOptionsItemSelected(item);
}

public class User {
private String email,status;


public User(String email, String status) {
    this.email = email;
    this.status = status;
}

public User() {

}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getStatus() {
    return status;
}

public void setStatus(String status) {
    this.status = status;
}}

主体活动

public class MainActivity extends AppCompatActivity {

Button signInButton;
private final static int LOGIN_PERMISSION = 1000;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    signInButton = (Button) findViewById(R.id.signInButton);
    signInButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startActivityForResult(AuthUI.getInstance().createSignInIntentBuilder().setAllowNewEmailAccounts(true).build(),LOGIN_PERMISSION);

        }

    });
}

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {

    if(requestCode == LOGIN_PERMISSION){
        startNewActivity(resultCode,data);
    }
}

private void startNewActivity(int resultcode, Intent data) {

    if(resultcode == RESULT_OK){
        Intent intent = new Intent(MainActivity.this,ListOnline.class);
        startActivity(intent);
        finish();

    }
    else{
        Toast.makeText(this,"login failed!!",Toast.LENGTH_SHORT).show();
    }
}}

Answer 1

在setupsystem()方法中，将监听器附加到onlineref(info/connected节点），然后将返回的值封送到boolean值。

但是，如果数据库中的指定位置没有数据，datasnapshot#getvalue()将返回null。如果发生这种情况，datasnapshot.getValue(Boolean.class)调用将创建一个Boolean变量，该变量的值为null，因此在当前If语句中不能检查该变量是否为true（请参见检查null Boolean是否为true将导致异常）。

您可以通过在if语句中添加null-check来检查getvalue()是否首先返回null:

if(dataSnapshot.getValue() != null && dataSnapshot.getValue(Boolean.class)){
    // ...
}

或者使用datasnapshot#exists()检查该位置是否存在：

if(dataSnapshot.exists() && dataSnapshot.getValue(Boolean.class)){
    // ...
}

但是，如果您试图检测连接状态，您是否打算将侦听器附加到.info/connected节点？根据文档：

对于许多与状态相关的功能，了解应用程序何时在线或离线是很有用的。Firebase Realtime Database在/.info/connected提供了一个特殊位置，该位置在Firebase Realtime Database客户端的连接状态每次更改时都会更新。下面是一个例子：

DatabaseReference connectedRef = FirebaseDatabase.getInstance().getReference(".info/connected");
connectedRef.addValueEventListener(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot snapshot) {
    boolean connected = snapshot.getValue(Boolean.class);
    if (connected) {
      System.out.println("connected");
    } else {
      System.out.println("not connected");
    }
  }

  @Override
  public void onCancelled(DatabaseError error) {
    System.err.println("Listener was cancelled");
  }
});