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ViewModel实例-活动UI-Android体系结构组件

分类：编程问答

如何解决ViewModel实例-活动UI-Android体系结构组件

 public class Wordviewmodel extends Androidviewmodel {

    private WordRepository mRepository;
    private LiveData<List<Word>> mAllWords;

    public Wordviewmodel(@NonNull 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);
    }
}

mWordviewmodel = new viewmodelProvider(this,new viewmodelProvider.AndroidviewmodelFactory(getApplication()))
                .get(Wordviewmodel.class);

public class WordRepository {

    private WordDao mWordDao;
    private LiveData<List<Word>> mAllWords;

    public WordRepository(Application application) {

        WordRoomDatabase db = WordRoomDatabase.getDatabase(application);
        mWordDao = db.wordDao();
        mAllWords = mWordDao.getAllWords();
    }

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

    public void insert(Word word) {
        new insertAsyncTask(mWordDao).execute(word);
    }

    private static class insertAsyncTask extends AsyncTask<Word,Void,Void> {

        private WordDao mAsyncTaskDao;

        insertAsyncTask(WordDao dao) {
            mAsyncTaskDao = dao;
        }

        @Override
        protected Void doInBackground(final Word... params) {
            mAsyncTaskDao.insert(params[0]);
            return null;
        }
    }
}

@Database(entities = {Word.class},version = 1,exportSchema = false)
public abstract class WordRoomDatabase extends RoomDatabase {

    public abstract WordDao wordDao();

    private static WordRoomDatabase INSTANCE;

    public static WordRoomDatabase getDatabase(Context context) {

        if (INSTANCE == null) {
            synchronized (WordRoomDatabase.class) {
                if (INSTANCE == null) {
                    Room.databaseBuilder(context.getApplicationContext(),WordRoomDatabase.class,"word_database")
                            .fallbackToDestructiveMigration()
                            .addCallback(sRoomDatabaseCallback)
                            .build();
                }
            }
        }

        return INSTANCE;
    }

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

    /**
     * Populate the database in the background.
     */
    private static class PopulateDbAsync extends AsyncTask<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;
        }
    }
}

我正在尝试在MainActivity的 onCreate 方法中实例化viewmodel类，但是它显示以下错误。

无法创建com.example.roomwords_sample.Wordviewmodel类的实例

我还尝试使用 viewmodelProviders.of（this）.get（viewmodel.class），但它仍显示相同的错误。我也正在寻找其他人在类似问题上的答案，但它没有用。我已经尝试了几乎所有东西。请帮我。

添加了堆栈跟踪

2020-09-20 01:51:37.103 13578-13578/com.example.roomwords_sample E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.roomwords_sample,PID: 13578
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.roomwords_sample/com.example.roomwords_sample.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.roomwords_sample.Wordviewmodel
        at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.roomwords_sample.Wordviewmodel
        at androidx.lifecycle.viewmodelProvider$AndroidviewmodelFactory.create(viewmodelProvider.java:275)
        at androidx.lifecycle.viewmodelProvider.get(viewmodelProvider.java:187)
        at androidx.lifecycle.viewmodelProvider.get(viewmodelProvider.java:150)
        at com.example.roomwords_sample.MainActivity.onCreate(MainActivity.java:32)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Constructor.newInstance0(Native Method)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343)
        at androidx.lifecycle.viewmodelProvider$AndroidviewmodelFactory.create(viewmodelProvider.java:267)
        at androidx.lifecycle.viewmodelProvider.get(viewmodelProvider.java:187) 
        at androidx.lifecycle.viewmodelProvider.get(viewmodelProvider.java:150) 
        at com.example.roomwords_sample.MainActivity.onCreate(MainActivity.java:32) 
        at android.app.Activity.performCreate(Activity.java:8000) 
        at android.app.Activity.performCreate(Activity.java:7984) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309) 
        at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3422) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.example.roomwords_sample.WordDao com.example.roomwords_sample.WordRoomDatabase.wordDao()' on a null object reference
        at com.example.roomwords_sample.WordRepository.<init>(WordRepository.java:18)
        at com.example.roomwords_sample.Wordviewmodel.<init>(Wordviewmodel.java:18)
        at java.lang.reflect.Constructor.newInstance0(Native Method) 
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343) 
        at androidx.lifecycle.viewmodelProvider$AndroidviewmodelFactory.create(viewmodelProvider.java:267) 
        at androidx.lifecycle.viewmodelProvider.get(viewmodelProvider.java:187) 
        at androidx.lifecycle.viewmodelProvider.get(viewmodelProvider.java:150) 
        at com.example.roomwords_sample.MainActivity.onCreate(MainActivity.java:32) 
        at android.app.Activity.performCreate(Activity.java:8000) 
        at android.app.Activity.performCreate(Activity.java:7984) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309) 
        at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:3422) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

解决方法

问题出在您的WordRoomDatabase.getDatabase(application)方法上

撰写时：

if (INSTANCE == null) {
    Room.databaseBuilder(context.getApplicationContext(),WordRoomDatabase.class,"word_database")
        .fallbackToDestructiveMigration()
        .addCallback(sRoomDatabaseCallback)
        .build();
}

您创建了数据库，但实际上并未将其分配给您的INSTANCE变量。这意味着您的INSTANCE始终为 空，而WordRoomDatabase.getDatabase(application)实际不会返回任何空值。

您可以通过设置NullPointerException变量来解决问题和由此产生的INSTANCE：

if (INSTANCE == null) {
    INSTANCE = Room.databaseBuilder(context.getApplicationContext(),"word_database")
        .fallbackToDestructiveMigration()
        .addCallback(sRoomDatabaseCallback)
        .build();
}

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