java - 找不到 com.example.roomwordssample.WordRoomDatabase 的实现。 WordRoomDatabase_Impl 不存在

Question

我正在关注 CodeLab Google 的本教程 https://codelabs.developers.google.com/codelabs/android-training-livedata-viewmodel/index.html?index=..%2F..android -训练

但是，在遵循所有详细信息之后，最后我收到了此错误:

2019-10-04 07:05:11.310 10476-10476/com.example.roomwordssample E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.roomwordssample, PID: 10476
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.roomwordssample/com.example.roomwordssample.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.roomwordssample.WordViewModel
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2946)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:201)
        at android.app.ActivityThread.main(ActivityThread.java:6810)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.roomwordssample.WordViewModel
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:208)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:135)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:103)
        at com.example.roomwordssample.MainActivity.onCreate(MainActivity.java:36)
        at android.app.Activity.performCreate(Activity.java:7224)
        at android.app.Activity.performCreate(Activity.java:7213)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:201) 
        at android.app.ActivityThread.main(ActivityThread.java:6810) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) 
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Constructor.newInstance0(Native Method)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343)
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:200)
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:135) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:103) 
        at com.example.roomwordssample.MainActivity.onCreate(MainActivity.java:36) 
        at android.app.Activity.performCreate(Activity.java:7224) 
        at android.app.Activity.performCreate(Activity.java:7213) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:201) 
        at android.app.ActivityThread.main(ActivityThread.java:6810) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) 
     Caused by: java.lang.RuntimeException: cannot find implementation for com.example.roomwordssample.WordRoomDatabase. WordRoomDatabase_Impl does not exist
        at androidx.room.Room.getGeneratedImplementation(Room.java:94)
        at androidx.room.RoomDatabase$Builder.build(RoomDatabase.java:667)
        at com.example.roomwordssample.WordRoomDatabase.getDatabase(WordRoomDatabase.java:26)
        at com.example.roomwordssample.WordRepository.<init>(WordRepository.java:14)
        at com.example.roomwordssample.WordViewModel.<init>(WordViewModel.java:16)
        at java.lang.reflect.Constructor.newInstance0(Native Method) 
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343) 
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:200) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:135) 
        at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:103) 
        at com.example.roomwordssample.MainActivity.onCreate(MainActivity.java:36) 
        at android.app.Activity.performCreate(Activity.java:7224) 
        at android.app.Activity.performCreate(Activity.java:7213) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1272) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2926) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3081) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1831) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:201) 
        at android.app.ActivityThread.main(ActivityThread.java:6810) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) 

我尝试从以下链接寻找解决方案:

https://stackoverflow.com/questions/49224539/roomdatabase-impl-does-not-exist
https://stackoverflow.com/questions/47274677/room-cannot-find-implementation?noredirect=1&lq=1
https://stackoverflow.com/questions/55679940/database-impl-doesnt-exist-android-app-with-room
https://stackoverflow.com/questions/50453822/room-cannot-find-implementation-db-db-impl-does-not-exist

但是没有一个起作用，因为我使用的是 Java 而不是 Kotlin。上面的许多解决方案建议将annotationProcessor更改为kapt，然后添加apply plugin: 'kotlin-kapt'，但它只会给出另一个错误

build.grade(模块:应用程序)

apply plugin: 'com.android.application'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.2"
    defaultConfig {
        applicationId "com.example.roomwordssample"
        minSdkVersion 16
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.0.2'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation 'com.google.android.material:material:1.0.0'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.1.1'
    implementation 'androidx.room:room-runtime:2.0.0'
    annotationProcessor 'androidx.room:room-compiler:2.0.0'
    androidTestImplementation 'androidx.room:room-testing:2.0.0'
    implementation 'androidx.lifecycle:lifecycle-extensions:2.0.0'
    annotationProcessor 'androidx.lifecycle:lifecycle-compiler:2.0.0'
}

WordRoomDatabase.java

package com.example.roomwordssample;

import android.content.Context;
import android.os.AsyncTask;
import androidx.annotation.NonNull;
import androidx.room.Room;
import androidx.room.RoomDatabase;
import androidx.sqlite.db.SupportSQLiteDatabase;

public abstract class WordRoomDatabase extends RoomDatabase {

    public abstract WordDao wordDao();
    private static WordRoomDatabase INSTANCE;

    static WordRoomDatabase getDatabase(final Context context) {
        if (INSTANCE == null) {
            synchronized (WordRoomDatabase.class) {
                if (INSTANCE == null) {
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                        WordRoomDatabase.class, "word_database")
                        // Wipes and rebuilds instead of migrating
                        // if no Migration object.
                        // Migration is not part of this practical.
                        .fallbackToDestructiveMigration()
                        .addCallback(sRoomDatabaseCallback)
                        .build();
                }
            }
        }
        return INSTANCE;
    }

    private static RoomDatabase.Callback sRoomDatabaseCallback = new RoomDatabase.Callback(){
        @Override
        public void onOpen (@NonNull SupportSQLiteDatabase db){
            super.onOpen(db);
            new PopulateDbAsync(INSTANCE).execute();
        }
    };

    private static class PopulateDbAsync extends AsyncTask<Void, Void, Void> {
        private final WordDao mDao;
        String[] words = {"dolphin", "crocodile", "cobra"};
        PopulateDbAsync(WordRoomDatabase db) {
            mDao = db.wordDao();
        }
        @Override
        protected Void doInBackground(final Void... params) {
            // Start the app with a clean database every time.
            // Not needed if you only populate the database
            // when it is first created
            mDao.deleteAll();
            for (int i = 0; i <= words.length - 1; i++) {
                Word word = new Word(words[i]);
                mDao.insert(word);
            }
            return null;
        }
    }

}

WordViewModel.java

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

public class WordViewModel extends AndroidViewModel {

    private WordRepository mRepository;

    private LiveData<List<Word>> mAllWords;

    public WordViewModel (Application application) {
        super(application);
        mRepository = new WordRepository(application);
        mAllWords = mRepository.getAllWords();
    }

    LiveData<List<Word>> getAllWords() { return mAllWords; }

    public void insert(Word word) { mRepository.insert(word); }

}

MainActivity.java

package com.example.roomwordssample;

import android.os.Bundle;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.Snackbar;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;

import java.util.List;

public class MainActivity extends AppCompatActivity {

    private WordViewModel mWordViewModel;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        RecyclerView recyclerView = findViewById(R.id.recyclerview);
        final WordListAdapter adapter = new WordListAdapter(this);
        recyclerView.setAdapter(adapter);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));

        mWordViewModel = ViewModelProviders.of(this).get(WordViewModel.class);
        mWordViewModel.getAllWords().observe(this, new Observer<List<Word>>() {
            @Override
            public void onChanged(@Nullable final List<Word> words) {
                // Update the cached copy of the words in the adapter.
                adapter.setWords(words);
            }
        });

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Answer 1

我错过了抽象类WordRoomDatabase之前的@Database注释。添加 @Database(entities = {Word.class}, version = 1, exportSchema = false) 后，一切正常。