当 firebase 规则设置为 true 时，为什么我的应用程序会崩溃？

Question

我制作了一个应用程序，它从 firebase 实时数据库中检索数据并将其显示在 RecyclerView 中。

主活动.java：

package com.example.myapplication;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.widget.Button;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;
import android.widget.TextView;

import com.google.firebase.FirebaseApp;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    RecyclerView recyclerview;
    ArrayList<Dishes> list;
    DatabaseReference reference;
    Food_RecyclerViewAdapter adapter;
    Button button;

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

        recyclerview = findViewById(R.id.recycler);
        reference = FirebaseDatabase.getInstance().getReference("Dishes");
        recyclerview.setHasFixedSize(true);
        recyclerview.setLayoutManager(new LinearLayoutManager(this));
        button = findViewById(R.id.button);

        list = new ArrayList<>();
        adapter = new Food_RecyclerViewAdapter(this, list);
        recyclerview.setAdapter(adapter);


        reference.addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull  DataSnapshot snapshot) {

                        for (DataSnapshot ds : snapshot.getChildren()){
                            //Add firebase data to Recyclerview adapter
                            Dishes dish = ds.getValue(Dishes.class);
                            list.add(dish);

                        }
                        adapter.notifyDataSetChanged();

                    }

                    @Override
                    //when the connection is cancelled call the Display() method
                    public void onCancelled(@NonNull  DatabaseError error) {
                        Display();
                    }
                });
        }

    private void Display() {
//Display the toast message "Error"
        Toast.makeText(this, "Error", Toast.LENGTH_SHORT).show();
    }
}

RecyclerView 的 Model class：

package com.example.myapplication;

class Dishes {
    String name;
    int price;
    float rating;


    public Dishes(String name, float rating, int price) {
        this.name = name;
        this.price = price;
        this.rating = rating;
    }

    public String getName() {
        return name;
    }

    public int getPrice() {
        return price;
    }

    public float getRating() {
        return rating;
    }


}

RecyclerView 适配器：

package com.example.myapplication;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.RatingBar;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import java.util.ArrayList;

class Food_RecyclerViewAdapter extends RecyclerView.Adapter<Food_RecyclerViewAdapter.MyViewHolder> {
    Context context;
    ArrayList<Dishes> models;


    public Food_RecyclerViewAdapter(Context context, ArrayList<Dishes> models){
        this.context = context;
        this.models = models;
    }

    @NonNull
    @Override
    public Food_RecyclerViewAdapter.MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        LayoutInflater inflater = LayoutInflater.from(context);
        View view = inflater.inflate(R.layout.item, parent, false);

        return new Food_RecyclerViewAdapter.MyViewHolder(view);
    }

    @Override
    public void onBindViewHolder(@NonNull Food_RecyclerViewAdapter.MyViewHolder holder, int position) {
        Dishes dish = models.get(position);

        holder.name.setText(dish.getName());
        holder.price.setText("Price" + " $ " + dish.getPrice());
        holder.rating.setRating(dish.getRating());

    }

    @Override
    public int getItemCount() {
        return models.size();
    }

    public static class MyViewHolder extends RecyclerView.ViewHolder{
        TextView name, price;
        Button button;
        RatingBar rating;

        public MyViewHolder(@NonNull View ItemView) {
            super(ItemView);

            name = ItemView.findViewById(R.id.food_name);
            price = ItemView.findViewById(R.id.pri);
            button = ItemView.findViewById(R.id.btn);
            rating = ItemView.findViewById(R.id.ratingBar);
        }
    }
}

依赖项：

dependencies {

    implementation 'androidx.appcompat:appcompat:1.3.0'
    implementation 'com.google.android.material:material:1.4.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.4'
    implementation 'com.google.firebase:firebase-database:20.0.3'
    testImplementation 'junit:junit:4.+'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}

但是，当我在我的设备“Samsung”上运行该应用程序时，数据不会加载，并且在使用这些规则时会调用 Oncancelled 方法：

{
  "rules": {
    ".read": false,
    ".write": false
  }
}

当 read 和 write 设置为 true 时，应用程序就会崩溃：

{
  "rules": {
    ".read": true,
    ".write": true
  }
}

我猜我做错了一件事或者错过了什么，那是什么？ 我是 firebase 的新手，所以非常感谢您的解释和帮助，谢谢。

编辑：logcat 上的堆栈跟踪读取：02-14 16:40:29.800 32134-32142/? W/SQLiteConnectionPool：数据库 '+data+data+com_android_vending+databases+resource_data_db' 的 SQLiteConnection object 被泄露。 请修复您的应用程序以正确结束正在进行的事务并在不再需要时关闭数据库。

edit2：当应用程序崩溃时，我得到了这个错误：02-14 18:42:33.075 14095-14095/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main com.google.firebase.database.DatabaseException: 8874725584801889 .example.myapplication.Dishes 没有定义无参构造函数。 如果您使用 ProGuard，请确保这些构造函数未被剥离。 at com.google.firebase.database.core.utilities.encoding.CustomClassMapper$BeanMapper.deserialize(CustomClassMapper.java:570) at com.google.firebase.database.core.utilities.encoding.CustomClassMapper$BeanMapper.deserialize(CustomClassMapper. java:563) at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertBean(CustomClassMapper.java:433) at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.deserializeToClass(CustomClassMapper. java:232) at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertToCustomClass(CustomClassMapper.java:80) at com.google.firebase.database.DataSnapshot.getValue(DataSnapshot.java:203) at com .example.myapplication.MainActivity$1.onDataChange(MainActivity.java:54) 在 com.google.firebase.database.core。 ValueEventRegistration.fireEvent(ValueEventRegistration.java:75) at com.google.firebase.database.core.view.DataEvent.fire(DataEvent.java:63) at com.google.firebase.database.core.view.EventRaiser$1.run (EventRaiser.java:55) at android.os.Handler.handleCallback(Handler.java:615) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java: 137) at android.app.ActivityThread.main(ActivityThread.java:4947) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android .internal.os.ZygoteInit$MethodAndArgsCaller.run（ZygoteInit.java:1038）在 com.android.internal.os.ZygoteInit.main（ZygoteInit.java:805）（ZygoteInit.java:805）。

编辑 3：我在做 snachsms answer 后解决了问题，在 dishes 中添加了另一个空构造函数：

public Dishes(){}

    public Dishes(String name, float rating, int price) {
        this.name = name;
        this.price = price;
        this.rating = rating;
    }

非常感谢男人和所有做出贡献的人。

Answer 1

Class com.example.myapplication.Dishes 未定义无参数构造函数。

所以Dishes没有无参构造函数，这是正确的，因为Dishes中只有一个构造函数 class

public Dishes(String name, float rating, int price) {

所以你还需要一个根本没有 arguments 的，所以

public Dishes() {}

public Dishes(String name, float rating, int price) {
   ... rest of code

为什么会这样？ 也许 Firebase 出于某些内部目的使用空构造函数创建Dishes object，但那是猜测......