尝试获取现有的数据库，Room Android

Question

下午好。我在应用中使用一个现有的数据库。我想通过 Room 来操作它，但是我遇到了错误 IllegalArgumentException: Cannot provide null context for the database.。 我的代码如下：

    @Database(entities = {Series.class}, version = 1, exportSchema = false)
    public abstract class AppDatabase extends RoomDatabase {
    private static AppDatabase INSTANCE;
    private static final String DB_NAME = "base.db";

    public abstract CymbalDao cymbalDao();

    public static AppDatabase getDatabase(final Context context) {
        if (INSTANCE == null) {
            synchronized (AppDatabase.class) {
                if (INSTANCE == null) {
                    INSTANCE = Room.databaseBuilder(context,
                            AppDatabase.class, DB_NAME)
                            .allowMainThreadQueries()
                            .build();
                }
            }
        }
        return INSTANCE;
    }

}

数据库位置：

..\src\main\assets\base.db

我希望知道我做错了什么，并听取一些建议，谢谢！

由片段调用：

public class CymbalsFragment extends Fragment {

    List<CymbalSeries> cymbalSeries = new ArrayList<>();
    CymbalsAdapter cymbalsAdapter;

    final AppDatabase db = AppDatabase.getDatabase(getContext());
    final CymbalDao cymbalDao = db.cymbalDao();

日志输出:

08-15 15:46:23.247 31769-31769/ru.alexsuvorov.paistewiki W/System.err: 
java.lang.IllegalArgumentException: Cannot provide null context for the 
database.
08-15 15:46:23.248 31769-31769/ru.alexsuvorov.paistewiki W/System.err:     at     android.arch.persistence.room.RoomDatabase$Builder.build(RoomDatabase.java:600)
    at ru.alexsuvorov.paistewiki.db.AppDatabase.getDatabase(AppDatabase.java:25)
    at ru.alexsuvorov.paistewiki.fragments.CymbalsFragment.<init>(CymbalsFragment.java:29)
    at java.lang.Class.newInstance(Native Method)
    at ru.alexsuvorov.paistewiki.StartDrawer.onNavigationItemSelected(StartDrawer.java:70)
    at android.support.design.widget.NavigationView$1.onMenuItemSelected(NavigationView.java:154)
    at android.support.v7.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:822)
    at android.support.v7.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:171)
    at android.support.v7.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:973)
    at 
 android.support.design.internal.NavigationMenuPresenter$1.onClick(NavigationMenuPresenter.java:342)

- AlexS

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这是一个活动吗？你能发更多的代码吗？ - iamkdblue

@kdblue，我正在编辑这个问题。 - AlexS

Answer 1

final AppDatabase db = AppDatabase.getDatabase(getContext());

这一行代码返回了null。因为Context尚未附加到片段上。如果您在片段的构造函数中调用getContext()，就会发生这种情况。请在onCreateView或onViewCreated()中调用getContext()。

示例代码：

 @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            // Inflate the layout for this fragment

            Context context = getContext();
            final AppDatabase db = AppDatabase.getDatabase(context);
            return inflater.inflate(R.layout.fragment_profile, container, false);
        }

- user3626630

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是的，运行得非常完美！你能告诉我如何使用现有的数据库吗？我做得对吗？ - AlexS