无法使用 AndroidViewModel 超类和 Room 获取数据库的实例答案

【问题标题】：Can't get an INSTANCE of a DB using the AndroidViewModel superclass and Room无法使用 AndroidViewModel 超类和 Room 获取数据库的实例
【发布时间】：2019-09-19 22:08:34
【问题描述】：

在我的 MVVM 应用程序中，我正在扩展 AndroidViewModel 并通过 Repository 类，我试图获取一个 DB 实例（在扩展 RoomDatabase 超类的单独抽象类中）。我通过ViewModelProviders 方法从MainActivity 类中调用ViewModel。我收到以下错误：

这是异常的整个堆栈跟踪：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.roomlistingapllicate, PID: 31766
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.roomlistingapllicate/com.example.roomlistingapllicate.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.roomlistingapllicate.NOtesViewModel
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2572)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2654)
        at android.app.ActivityThread.access$900(ActivityThread.java:175)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1488)
        at android.os.Handler.dispatchMessage(Handler.java:111)
        at android.os.Looper.loop(Looper.java:207)
        at android.app.ActivityThread.main(ActivityThread.java:5728)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.roomlistingapllicate.NOtesViewModel
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:238)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:164)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:130)
        at com.example.roomlistingapllicate.MainActivity.onCreate(MainActivity.java:19)
        at android.app.Activity.performCreate(Activity.java:6309)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1113)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2519)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2654) 
        at android.app.ActivityThread.access$900(ActivityThread.java:175) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1488) 
        at android.os.Handler.dispatchMessage(Handler.java:111) 
        at android.os.Looper.loop(Looper.java:207) 
        at android.app.ActivityThread.main(ActivityThread.java:5728) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679) 
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Constructor.newInstance(Native Method)
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:230)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:164) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:130) 
        at com.example.roomlistingapllicate.MainActivity.onCreate(MainActivity.java:19) 
        at android.app.Activity.performCreate(Activity.java:6309) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1113) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2519) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2654) 
        at android.app.ActivityThread.access$900(ActivityThread.java:175) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1488) 
        at android.os.Handler.dispatchMessage(Handler.java:111) 
        at android.os.Looper.loop(Looper.java:207) 
        at android.app.ActivityThread.main(ActivityThread.java:5728) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679) 
     Caused by: java.lang.RuntimeException: cannot find implementation for com.example.roomlistingapllicate.NotesDatabase. NotesDatabase_Impl does not exist
        at androidx.room.Room.getGeneratedImplementation(Room.java:94)
        at androidx.room.RoomDatabase$Builder.build(RoomDatabase.java:851)
        at com.example.roomlistingapllicate.NotesDatabase.getInstance(NotesDatabase.java:23)
        at com.example.roomlistingapllicate.NoteRepository.<init>(NoteRepository.java:15)
        at com.example.roomlistingapllicate.NOtesViewModel.<init>(NOtesViewModel.java:15)
        at java.lang.reflect.Constructor.newInstance(Native Method) 
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:230) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:164) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:130) 
        at com.example.roomlistingapllicate.MainActivity.onCreate(MainActivity.java:19) 
        at android.app.Activity.performCreate(Activity.java:6309) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1113) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2519) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2654) 
        at android.app.ActivityThread.access$900(ActivityThread.java:175) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1488) 
        at android.os.Handler.dispatchMessage(Handler.java:111) 
        at android.os.Looper.loop(Looper.java:207) 
        at android.app.ActivityThread.main(ActivityThread.java:5728) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:679)

我正在开发 AndroidX 架构。以下是我认为问题所在的部分代码：

import androidx.appcompat.app.AppCompatActivity;

import androidx.lifecycle.ViewModelProviders;

import android.os.Bundle;


public class MainActivity extends AppCompatActivity {

    private NOtesViewModel nOtesViewModel;

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

        nOtesViewModel = ViewModelProviders.of(this).get(NOtesViewModel.class);

    }
}


import android.app.Application;
import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;
import java.util.List;

public class NOtesViewModel extends AndroidViewModel {
    private NoteRepository noteRepository;
    private LiveData<List<Note>> allNotes;

    public NOtesViewModel(@NonNull Application application) {
        super(application);
        noteRepository = new NoteRepository(application);
        allNotes = noteRepository.getAllNotes();
    }


import android.app.Application;
import android.os.AsyncTask;

import androidx.lifecycle.LiveData;

import java.util.List;

public class NoteRepository {
    private NoteDAO noteDAO;
    private LiveData<List<Note>> allNotes;

    public NoteRepository(Application application) {
        NotesDatabase notesDatabase = NotesDatabase.getInstance(application.getApplicationContext());
        noteDAO = notesDatabase.noteDAO();
        allNotes = noteDAO.getAllNotes();
    }


import android.content.Context;
import android.os.AsyncTask;

import androidx.annotation.NonNull;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
import androidx.sqlite.db.SupportSQLiteDatabase;

@Database(entities = {Note.class}, version = 1)
public abstract class NotesDatabase extends RoomDatabase {

    private static NotesDatabase instance;
    public abstract NoteDAO noteDAO();

    public static synchronized NotesDatabase getInstance(Context context) {
        if (instance == null) {
            instance = Room.databaseBuilder(context.getApplicationContext(), NotesDatabase.class,
                    "note_database").fallbackToDestructiveMigration()
                    .addCallback(roomCallback)
                    .build();
        }
        return instance;
    }

【问题讨论】：

Android ViewModel additional arguments的可能重复
如果有重复我看不到。顺便说一句，我尝试过像下面的 application.getApplicationContext() 那样缩小论点，但它没有用（我知道它不会但仍然......）。但是是的，当我将参数从存储库文件 (NoteRepository.class) 传递给 RoomDatabase 扩展超类抽象类文件时发生错误：NoteDatabase noteDatabase = NoteDatabase.getInstanceOfTheDB(application)，因此 getInstanceOfTheDB 方法请求（上下文上下文）作为参数。
你能告诉我们异常的整个堆栈跟踪吗？
@JensV 我在添加异常的堆栈跟踪时编辑了我的答案

标签： android android-room android-viewmodel android-mvvm

【解决方案1】：

堆栈跟踪显示您收到此错误，因为 Room 找不到您的 Notes 数据库抽象类的实现。

请确保您已在应用的 build.gradle 文件中添加房间注释处理器依赖项。

annotationProcessor "androidx.room:room-compiler:$room_version"

编辑：如果您使用 Kotlin，请使用 kapt 而不是 annotationProcessor，并确保在应用的 build.gradle 文件顶部添加 apply plugin: 'kotlin-kapt'。

【讨论】：

非常感谢。我使用的是 androidTestImplementatin 而不是 annotationProcessor。问题解决了。