Dao 類中 Query 的房間數據庫問題

Question

所以在我的房間查詢中，我得到錯誤無法解析符號“照片”和無法解析符號“image_id”，例如房間無法識別此表和字段，錯誤發生在執行 MyImages 數據庫之后

我的圖像類：

import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity(tableName = "photos")
public class MyImages {
    @PrimaryKey(autoGenerate = true)
    public int image_id;
    public String image_title;
    public String image_description;
    public byte[] image;

    public MyImages(String image_title, String image_description, byte[] image) {
        this.image_title = image_title;
        this.image_description = image_description;
        this.image = image;
    }

    public void setImage_id(int image_id) {
        this.image_id = image_id;
    }

    public int getImage_id() {
        return image_id;
    }

    public String getImage_title() {
        return image_title;
    }

    public String getImage_description() {
        return image_description;
    }

    public byte[] getImage() {
        return image;
    }
}

MyImagesViewModel：

import android.app.Application;

import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;

import java.util.List;

public class MyImagesViewModel extends AndroidViewModel {
    private MyImagesRepository repository;
    private LiveData<List<MyImages>> myImages;

    public MyImagesViewModel(@NonNull Application application) {
        super(application);
        repository = new MyImagesRepository(application);
        myImages = repository.getAllImages();
    }

    public void insert(MyImages myImage) {
        repository.insert(myImage);
    }

    public void update(MyImages myImage) {
        repository.update(myImage);
    }

    public void delete(MyImages myImage) {
        repository.delete(myImage);
    }

    public LiveData<List<MyImages>> getAllMyImages() {
        return myImages;
    }
}

我的圖像存儲庫：

package com.example.photoalbum;

import android.app.Application;

import androidx.lifecycle.LiveData;

import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class MyImagesRepository {
    private MyImagesDao myImagesDao;
    private LiveData<List<MyImages>> imagesList;
    ExecutorService executorService= Executors.newSingleThreadExecutor();
    public MyImagesRepository(Application application)
    {
        MyImagesDatabase database=MyImagesDatabase.getInstance(application);
        myImagesDao=database.myImagesDao();
        imagesList= myImagesDao.getAllImages();
    }
    public void insert(MyImages myImages){
        executorService.execute(new Runnable() {
            @Override
            public void run() {
                myImagesDao.insert(myImages);
            }
        });
    }
    public void delete(MyImages myImages){
        executorService.execute(new Runnable() {
            @Override
            public void run() {
                myImagesDao.delete(myImages);
            }
        });
    }
    public void update(MyImages myImages){
        executorService.execute(new Runnable() {
            @Override
            public void run() {
                myImagesDao.update(myImages);
            }
        });
    }
    public LiveData<List<MyImages>>getAllImages(){
        return imagesList;
    }
}

我的圖片道：

import androidx.lifecycle.LiveData;
import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Update;

import java.util.List;

@Dao
public interface MyImagesDao {
    @Insert
    void insert(MyImages myImages);

    @Delete
    void delete(MyImages myImages);

    @Update
    void update(MyImages myImages);

    @Query("SELECT * FROM photos order by image_id asc")
    LiveData<List<MyImages>> getAllImages();
}

我的圖像數據庫：

package com.example.photoalbum;

import android.content.Context;

import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
@Database(entities = MyImages.class,version = 1)
public abstract class MyImagesDatabase extends RoomDatabase {
    private static MyImagesDatabase instance;
    public abstract MyImagesDao myImagesDao();
    public static synchronized MyImagesDatabase getInstance(Context context){
        if(instance==null)
        {
            Room.databaseBuilder(context.getApplicationContext(), MyImagesDatabase.class,"photos_database")
                    .fallbackToDestructiveMigration()
                    .build();

        }
        return instance;
    }
}

我以前寫過類似的代碼，我很困惑為什么它現在不起作用

錯誤：

    Process: com.example.photoalbum, PID: 17024
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.photoalbum/com.example.photoalbum.MainActivity}: java.lang.RuntimeException: Cannot create an instance of class com.example.photoalbum.MyImagesViewModel
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3835)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4011)
        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$H.handleMessage(ActivityThread.java:2325)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:246)
        at android.app.ActivityThread.main(ActivityThread.java:8633)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
     Caused by: java.lang.RuntimeException: Cannot create an instance of class com.example.photoalbum.MyImagesViewModel
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.kt:236)
        at com.example.photoalbum.MainActivity.onCreate(MainActivity.java:42)
        at android.app.Activity.performCreate(Activity.java:8207)
        at android.app.Activity.performCreate(Activity.java:8191)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3808)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4011) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2325) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:246) 
        at android.app.ActivityThread.main(ActivityThread.java:8633) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130) 
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Constructor.newInstance0(Native Method)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343)
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.kt:228)
        at com.example.photoalbum.MainActivity.onCreate(MainActivity.java:42) 
        at android.app.Activity.performCreate(Activity.java:8207) 
        at android.app.Activity.performCreate(Activity.java:8191) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3808) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4011) 
        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$H.handleMessage(ActivityThread.java:2325) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:246) 
        at android.app.ActivityThread.main(ActivityThread.java:8633) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130) 
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.example.photoalbum.MyImagesDao com.example.photoalbum.MyImagesDatabase.myImagesDao()' on a null object reference
        at com.example.photoalbum.MyImagesRepository.<init>(MyImagesRepository.java:18)
        at com.example.photoalbum.MyImagesViewModel.<init>(MyImagesViewModel.java:17)
        at java.lang.reflect.Constructor.newInstance0(Native Method) 
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343) 
        at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.kt:228) 
        at com.example.photoalbum.MainActivity.onCreate(MainActivity.java:42) 
        at android.app.Activity.performCreate(Activity.java:8207) 
        at android.app.Activity.performCreate(Activity.java:8191) 
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309) 
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3808) 
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4011) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2325) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:246) 
        at android.app.ActivityThread.main(ActivityThread.java:8633) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130) ```

Answer 1

我相信這個問題並不是你所看到的真正問題：-

你可以忽略它。 但是，您可能會遇到其他問題：-

1. 在MyImagesDatabase中，您沒有設置instance ，因此它將始終為空。

2. 在MyImagesDao中使用default似乎總是返回 null，而不是 MyImages 列表。

使用（為簡潔和方便添加到 MyImagedDatabase 的 .allowMainThreadQueries ）和兩個額外的 @Query 注釋方法：-

/* ADDED for testing on Main Thread */
@Query("SELECT * FROM photos ORDER BY image_id ASC")
default List<MyImages> testGetAllImages() {
  return null;
}
@Query("SELECT * FROM photos ORDER BY image_id ASC")
List<MyImages> test2GetAllImages();

• 注意與未找到符號的相同問題

並使用以下活動代碼：-

public class MainActivity extends AppCompatActivity {

    MyImagesDatabase db;
    MyImagesDao dao;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        db = MyImagesDatabase.getInstance(this);
        dao = db.myImagesDao();

        dao.insert(new MyImages("Image1","The first image", new byte[]{0,1,2,3,4,5,6,7,8,9,0,1,2,3,4,5,6,7,8,9,}));
        dao.insert(new MyImages("Image2","The second image", new byte[]{0,9,8,7,6,5,4,3,2,1,0,9,8,7,6,5,4,3,2,1}));
        List<MyImages> milist = dao.testGetAllImages();
        if (milist != null) {
            for (MyImages mi : milist) {
                Log.d("DBINFO_TEST1", "TITLE=" + mi.image_title + " DESC=" + mi.image_description + "ImageData=" + byteArrayToString(mi.image));
            }
        } else {
            Log.d("DBINFO_TEST2","NULL returned from extract.");
        }

        milist = dao.test2GetAllImages();
        if (milist != null) {
            for (MyImages mi : milist) {
                Log.d("DBINFO_TEST2", "TITLE=" + mi.image_title + " DESC=" + mi.image_description + "ImageData=" + byteArrayToString(mi.image));
            }
        } else {
            Log.d("DBINFO_TEST1","NULL returned from extract.");
        }
    }

    String byteArrayToString(byte[] ba) {
        StringBuilder sb = new StringBuilder();
        for (byte b: ba) {
            sb.append(b);
        }
        return sb.toString();
    }
}

然后輸出到日志的結果是：-

D/DBINFO_TEST1: NULL returned from extract.
D/DBINFO_TEST2: TITLE=Image1 DESC=The first imageImageData=01234567890123456789
D/DBINFO_TEST2: TITLE=Image2 DESC=The second imageImageData=09876543210987654321

即default返回NULL，而沒有數據則按預期返回。

額外的

現在您已經包含了堆棧跟蹤，您將看到您有一個 NPE（空指針異常）

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'com.example.photoalbum.MyImagesDao com.example.photoalbum.MyImagesDatabase.myImagesDao()' on a null object reference

如前所述。 如果您使用， instance將始終為空：-

@Database(entities = MyImages.class,version = 1)
public abstract class MyImagesDatabase extends RoomDatabase {
    private static MyImagesDatabase instance;
    public abstract MyImagesDao myImagesDao();
    public static synchronized MyImagesDatabase getInstance(Context context){
        if(instance==null)
        {
            Room.databaseBuilder(context.getApplicationContext(), MyImagesDatabase.class,"photos_database")
                    .fallbackToDestructiveMigration()
                    .build();

        }
        return instance;
    }
}

相反，您想使用：-

@Database(entities = MyImages.class,version = 1)
public abstract class MyImagesDatabase extends RoomDatabase {
    private static MyImagesDatabase instance;
    public abstract MyImagesDao myImagesDao();
    public static synchronized MyImagesDatabase getInstance(Context context){
        if(instance==null)
        {
            /* FOLLOWING LINE CHANGED to actual set the variable instance to not be null */
            instance = Room.databaseBuilder(context.getApplicationContext(), MyImagesDatabase.class,"photos_database")
                    .fallbackToDestructiveMigration()
                    .build();

        }
        return instance;
    }
}

• 閱讀評論（您不需要包含在代碼中）