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android - 尝试获取现有数据库，Room Android

问题描述

下午好。我在应用程序中创建了一个现有数据库。我想通过房间向她讲话，但我收到了错误消息IllegalArgumentException: Cannot provide null context for the database.。我的代码如下所示：

    @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

我想知道我做错了什么，也许能听到一些提示，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();

日志猫：

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)

标签： androiddatabaseandroid-roomandroid-assets

解决方案

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

此行正在重新调整上下文 null。上下文尚未附加到片段。如果您在片段的构造函数中调用 getContext()，就会发生这种情况。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);
        }

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