在成功调用.build()之后，未调用RoomDatabase.Callback()的onCreate()

Question

我已经使用Room Persisdent库创建了MovieDatabase。根据RoomDatabase.Callback中onCreate((方法的文档，onCreate(在第一次创建数据库并且创建了所有表之后调用。

在我调用buildPersistentDB((之后，我收到了MoviesDatabase类的日志，这个类用@Database注释，但RoomDatabase中onCreate((的日志。回调尽管我打过电话，但从未打过电话：

this.mMovieDatabase = this.mMovieDBPersistentBuilder.fallbackToDestructiveMigration().build();

buildPersistentDB((方法清楚地显示了我创建数据库的方式。我还发布了MovieDatabase类，如下所示。

我想知道为什么我没有收到来自Room中onCreate((方法的日志。尽管数据库已成功创建，但回调

code_1：

public void buildPersistentDB() {
Log.v(TAG_LOG, "->buildPersistentDB");
this.mMovieDBPersistentBuilder = Room
.databaseBuilder(getApplicationContext(), MovieDatabase.class, ActMain.DATA_BASE_NAME);
this.mMovieDBPersistentBuilder.addCallback(new RoomDatabase.Callback() {
@Override
public void onCreate(@NonNull SupportSQLiteDatabase db) {
super.onCreate(db);
Log.v(TAG_LOG + "->onCreate", " buildPersistentDB->DB is created, all tables has been created");
Log.d(TAG_LOG + "->onCreate", " buildPersistentDB->DB db.getPath(): " + db.getPath());
Log.d(TAG_LOG + "->onCreate", " buildPersistentDB->DB db.toString(): " + db.toString());
Log.d(TAG_LOG + "->onCreate", " buildPersistentDB->DB db.isOpen(): " + db.isOpen());
Log.d(TAG_LOG + "->onCreate", " buildPersistentDB->DB db.isReadOnly(): " + db.isReadOnly());
}
@Override
public void onOpen(@NonNull SupportSQLiteDatabase db) {
super.onOpen(db);
Log.v(TAG_LOG + "->onOpen", " buildPersistentDB->onCreate");
Log.d(TAG_LOG + "->onOpen", " buildPersistentDB->DB has been opened");
}
});
this.mMovieDatabase = this.mMovieDBPersistentBuilder.fallbackToDestructiveMigration().build();
Log.d(TAG_LOG + "->buildPersistentDB", "->this.mMovieDatabase.isOpen(): " + this.mMovieDatabase.isOpen());
}//eof-buildPersistentDB

code_2：

//class - The abstract class which is annotated with Database and extends RoomDatabase.
@Database(entities = {Movie.class}, version = 1, exportSchema = false)
public abstract class MovieDatabase extends RoomDatabase {
private final static String TAG_LOG = MovieDatabase.class.getSimpleName();
public abstract IDatabaseAccessObject dao();
public MovieDatabase() {
super();
Log.w(TAG_LOG, "->MovieDatabase constructor is called.");
}
@Override
public void init(@NonNull DatabaseConfiguration configuration) {
super.init(configuration);
Log.w(TAG_LOG, "->init is called.");
}
@NonNull
@Override
public SupportSQLiteOpenHelper getOpenHelper() {
Log.w(TAG_LOG, "->init is called.");
return super.getOpenHelper();
}
@NonNull
@Override
protected SupportSQLiteOpenHelper createOpenHelper(DatabaseConfiguration config) {
Log.w(TAG_LOG, "->createOpenHelper is called.");
Log.d(TAG_LOG, "->createOpenHelper->config.name:" + config.name);
Log.d(TAG_LOG, "->createOpenHelper->config.callbacks:" + config.callbacks);
Log.d(TAG_LOG, "->createOpenHelper->config.requireMigration:" + config.requireMigration);
return null;
}
@NonNull
@Override
protected InvalidationTracker createInvalidationTracker() {
Log.w(TAG_LOG, "->createInvalidationTracker is called.");
return null;
}
@Override
public void clearAllTables() {
Log.w(TAG_LOG, "->clearAllTables is called.");
}
}

logcat：

2018-12-13 14:37:54.665 8949-8949 V/ActMain: ->initPersistentDBHandlerThread <-
2018-12-13 14:37:54.666 8949-8949 V/ActMain->RoomPersistentDBHandlerThread: RoomPersistentDBHandlerThread constructor is called.
2018-12-13 14:37:54.666 8949-8949 V/ActMain->RoomPersistentDBHandlerThread: initHandler is called
2018-12-13 14:37:54.667 8949-8967 V/ActMain->RoomPersistentDBHandlerThread: onLooperPrepared is called. [getLooper: Looper (ROOM_PERSISTENT_DB_HANDLER_THREAD, tid 356) {cbd0ffd}] ++++++++++++++++++++++++ 
2018-12-13 14:37:54.667 8949-8967 V/ActMain->RoomPersistentDBHandlerThread: ++++++++++++++++++++++++  [getLooper: Looper (ROOM_PERSISTENT_DB_HANDLER_THREAD, tid 356) {cbd0ffd}] ++++++++++++++++++++++++ 
2018-12-13 14:37:54.670 8949-8949 V/ActMain->RoomPersistentDBHandlerThread: enqueueMessage is called for what = 1
2018-12-13 14:37:54.670 8949-8967 V/ActMain->RoomPersistentDBHandlerThread: handleMessage is called for msg.what = 1
2018-12-13 14:37:54.670 8949-8967 V/ActMain: ->buildPersistentDB <-
2018-12-13 14:37:54.673 8949-8967 W/MovieDatabase: ->MovieDatabase constructor is called. <-
2018-12-13 14:37:54.673 8949-8967 W/MovieDatabase: ->init is called. <-
2018-12-13 15:02:54.802 9384-9403 D/ActMain->buildPersistentDB: ->this.mMovieDatabase.isOpen(): false

Answer 1

没有调用OnCreate的原因是，在第一次创建数据库时只调用一次，然后只要数据库存在，就再也不会调用。

如果您删除应用程序的数据或卸载应用程序，然后重新运行它，您将看到随后调用onCreate。

例如

private void BuildDatabase() {
RoomDatabase.Builder roombuilder = Room.databaseBuilder(this, Database.class,"mydb");
roombuilder.addCallback(new RoomDatabase.Callback() {
@Override
public void onCreate(@NonNull SupportSQLiteDatabase db) {
super.onCreate(db);
Log.d("ONCREATE","Database has been created.");
}
@Override
public void onOpen(@NonNull SupportSQLiteDatabase db) {
super.onOpen(db);
Log.d("ONOPEN","Database has been opened.");
}
});
mRoomDB = (Database) roombuilder.build();
}

删除应用程序的数据后，结果是：-

2018-12-20 06:36:23.045 2271-2287/so53839431.so53839431roomrelationship D/ONCREATE: Database has been created.
2018-12-20 06:36:23.055 2271-2287/so53839431.so53839431roomrelationship D/ONOPEN: Database has been opened.

Answer 2

Oncreate方法在创建数据库后被调用。Room数据库是SQLiteOpenHelper的抽象。调用getReadableDatabase((或getWriteableDatabase(((时创建的数据库。所以，在执行一些具体操作之前，比如调用一个命中数据库的@Dao方法，数据库将不会创建。

要解决此问题，您应该应用以下选项之一。

• 执行一些操作，包括@delete、@insert或@update

  或

• 在卸载应用之前，删除手机设置中的应用数据

  或

• 手动删除您的数据库文件(Android Studio上的设备文件资源管理器&rar；data&rar；数据&rar；com.company.yourapp&rar；数据库(。

  或

• 在您的项目中调用以下代码：

  public void deleteDatabaseFile(String databaseName) { 
  File databases = 
  new File(getApplicationInfo().dataDir + "/databases");
  File db = new File(databases, databaseName);
  if (db.delete())
  Timber.d("Database deleted successfully");
  else
  Timber.d("Failed to delete database");
  }