在内容提供商中定义并在ongreate()中初始化的全局变量在query()中为null

Question

我定义了一个内容提供商，并使我的数据库助手成为一个全局变量，我在onCreate方法中初始化。但是，在query((方法中，该全局变量的值为null。

这是我的内容提供商：

    /** Tag for the log messages */
    public static final String LOG_TAG = FeedingProvider.class.getSimpleName();
    // Declaring a FeedingDbHelper variable;
    private FeedingDbHelper mDbHelper;
    /** URI matcher code for the content URI for the pets table */
    private static final int FEEDINGS = 100;
    /** URI matcher code for the content URI for a single pet in the pets table */
    private static final int FEEDING_ID = 101;
    /**
     * UriMatcher object to match a content URI to a corresponding code.
     * The input passed into the constructor represents the code to return for the root URI.
     */
    private static final UriMatcher sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
    // Static initializer. This is run the first time anything is called from this class.
    static {
        // The calls to addURI() go here, for all of the content URI patterns that the provider
        // should recognize. All paths added to the UriMatcher have a corresponding code to return
        // when a match is found.
        sUriMatcher.addURI(FeedingContract.CONTENT_AUTHORITY, FeedingContract.PATH_FEEDINGS,
                FEEDINGS);
        sUriMatcher.addURI(FeedingContract.CONTENT_AUTHORITY, FeedingContract.PATH_FEEDINGS +
                "/#", FEEDING_ID);
    }
    /**
     * Initialize the provider and the database helper object.
     */
    @Override
    public boolean onCreate() {
        mDbHelper = new FeedingDbHelper(this.getContext());
        // Make sure the variable is a global variable, so it can be referenced from other
        // ContentProvider methods.
        return true;
    }
    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs,
                                    String sortOrder) {
        Log.d(LOG_TAG, "Test in query: " + (mDbHelper == null));
        SQLiteDatabase db = mDbHelper.getReadableDatabase();
        Cursor cursor;
        int match = sUriMatcher.match(uri);
        switch (match) {
            case FEEDINGS:
                cursor = db.query(FeedingContract.FeedingEntry.TABLE_NAME, projection, selection,
                        selectionArgs, null, null, sortOrder);
                break;
            case FEEDING_ID:
                selection = FeedingEntry._ID + "=?";
                selectionArgs = new String[] {
                        String.valueOf(ContentUris.parseId(uri))
                };
                cursor = db.query(FeedingContract.FeedingEntry.TABLE_NAME, projection, selection,
                        selectionArgs, null, null, sortOrder);
                break;
            default:
                throw new IllegalArgumentException("Can't find uri in PetProvider.query()");
        }
        Log.e("Gerke", "Cursor: " + DatabaseUtils.dumpCursorToString(cursor));
        return cursor;
    }

结果错误消息是：

"java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase com.example.android.smartfeed.data.FeedingDbHelper.getReadableDatabase()'上的null对象参考"

">

我的问题是：为什么全局变量mdbhelper null而不是我的dbhelper的实例？还尝试将上下文保存到全局变量，但这也不起作用。

Answer 1

我发现自己的错误：ContentProvider的定义很好，但是当使用它时，我尝试创建一个新实例，这是错误的。

所以对于其他所有有问题的人来说：与ContentProvider的通信是通过ContentResolver完成的。要获取ContentResolver的实例，请致电GetContentResolver((;