Java DBHelper.GetWirtableDatabase引发空异常

我正在尝试为用户编写自己的内容提供商代码。当我尝试验证用户是否存在时，当DBHelper尝试创建数据库时，它抛出一个null异常

以下是内容提供商：

public class MyUserProvider extends ContentProvider {
    private UserDBHelper db;

    private static final int USERS = 10;
    private static final int USER_ID = 20;

    private static final String AUTHORITY =
            "net.ifo420.ritc.agenda.userprovider";

    private static final String BASE_PATH = "users";
    public static final Uri CONTENT_URI = Uri.parse("content://" + AUTHORITY
                                        + "/" + BASE_PATH);

    public static final String CONTENT_TYPE =
            ContentResolver.CURSOR_DIR_BASE_TYPE + "/users";

    public static final String CONTENT_ITEM_TYPE =
            ContentResolver.CURSOR_ITEM_BASE_TYPE + "/user";

    private static final UriMatcher uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);

    static {
        uriMatcher.addURI(AUTHORITY, BASE_PATH, USERS);
        uriMatcher.addURI(AUTHORITY, BASE_PATH + "/#", USER_ID);
    }

    public MyUserProvider() {
    }

    @Override
    public boolean onCreate() {
        db = new UserDBHelper(getContext());
        return false;
    }

    @Override
    public Cursor query(Uri uri, String[] projection, String selection,
                        String[] selectionArgs, String sortOrder) {
        SQLiteQueryBuilder queryBuilder = new SQLiteQueryBuilder();

        checkColumns(projection);

        queryBuilder.setTables(UserTable.TABLE);

        int uriType = uriMatcher.match(uri);
        switch (uriType) {
            case USERS:
                break;
            case USER_ID:
                queryBuilder.appendWhere(UserTable.COLUMN_ID + " = " + uri.getLastPathSegment());
                break;
            default:
                throw new IllegalArgumentException("Uknown uri " + uri);
        }
        SQLiteDatabase db1 = db.getReadableDatabase();
        Cursor cursor = queryBuilder.query(db1, projection, selection, selectionArgs,
                null, null, sortOrder);

        cursor.setNotificationUri(getContext().getContentResolver(), uri);

        return cursor;
    }

    @Override
    public String getType(Uri uri) {
        return null;
    }

    @Override
    public Uri insert(Uri uri, ContentValues values) {
        int uriType = uriMatcher.match(uri);
        SQLiteDatabase database = db.getReadableDatabase();
        long id = 0;
        switch (uriType) {
            case USERS:
                id = database.insert(UserTable.TABLE, null, values);
                break;
            default:
                throw new IllegalArgumentException("Unknown URI: " + uri);
        }
        getContext().getContentResolver().notifyChange(uri, null);
        return Uri.parse(BASE_PATH + "/" + id);
    }

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
        int uriType = uriMatcher.match(uri);
        SQLiteDatabase sqLiteDatabase = db.getReadableDatabase();
        int rowsDeleted = 0;
        switch (uriType) {
            case USERS:
                rowsDeleted = sqLiteDatabase.delete(UserTable.TABLE, selection,
                        selectionArgs);
                break;
            case USER_ID:
                String id = uri.getLastPathSegment();
                if(TextUtils.isEmpty(selection)) {
                    rowsDeleted = sqLiteDatabase.delete(UserTable.TABLE,
                            UserTable.COLUMN_ID + " = " + id, null);
                } else {
                    rowsDeleted = sqLiteDatabase.delete(UserTable.TABLE,
                            UserTable.COLUMN_ID + " = " + id + " and " +
                    selection, selectionArgs);
                }
                break;
            default:
                throw new IllegalArgumentException("Unknown URI: " + uri);
        }
        getContext().getContentResolver().notifyChange(uri, null);
        return rowsDeleted;
    }

    @Override
    public int update(Uri uri, ContentValues values, String selection,
                      String[] selectionArgs) {
        int uriType = uriMatcher.match(uri);
        SQLiteDatabase sqLiteDatabase = db.getReadableDatabase();
        int rowsUpdated = 0;
        switch (uriType) {
            case USERS:
                rowsUpdated = sqLiteDatabase.update(UserTable.TABLE,
                        values,
                        selection,
                        selectionArgs);
                break;
            case USER_ID:
                String id = uri.getLastPathSegment();
                if (TextUtils.isEmpty(selection)) {
                    rowsUpdated = sqLiteDatabase.update(UserTable.TABLE,
                            values,
                            UserTable.COLUMN_ID + "=" + id,
                            null);
                } else {
                    rowsUpdated = sqLiteDatabase.update(UserTable.TABLE,
                            values,
                            UserTable.COLUMN_ID + "=" + id
                                    + " and "
                                    + selection,
                            selectionArgs);
                }
                break;
            default:
                throw new IllegalArgumentException("Unknown URI: " + uri);
        }
        getContext().getContentResolver().notifyChange(uri, null);
        return rowsUpdated;
    }

    private void checkColumns(String[] projection) {
        String[] available = {UserTable.COLUMN_USERNAME, UserTable.COLUMN_PASSWORD,
                            UserTable.COLUMN_ID};

        if (projection != null) {

            HashSet<String> requestedColumns =
                    new HashSet<String>(Arrays.asList(projection));
            HashSet<String> availableColumns =
                    new HashSet<String>(Arrays.asList(available));

            if (!availableColumns.containsAll(requestedColumns)) {
                throw new IllegalArgumentException("Unknown columns in projection");
            }
        }
    }

以下是logcat给我的信息：

java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase net.info420.ritc.agendacegep.UserDBHelper.getReadableDatabase()' on a null object reference
  at net.info420.ritc.agendacegep.MyUserProvider.query(MyUserProvider.java:70)
  at net.info420.ritc.agendacegep.LoginActivity.utilisateurExiste(LoginActivity.java:69)
  at net.info420.ritc.agendacegep.LoginActivity.onClick(LoginActivity.java:57)
  at android.view.View.performClick(View.java:5610)
  at android.view.View$PerformClick.run(View.java:22260)
  at android.os.Handler.handleCallback(Handler.java:751)
  at android.os.Handler.dispatchMessage(Handler.java:95)
  at android.os.Looper.loop(Looper.java:154)
  at android.app.ActivityThread.main(ActivityThread.java:6077)
  at java.lang.reflect.Method.invoke(Native Method)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

---

您不能直接实例化和调用ContentProvider—您必须通过内容解析器。否则它不会经过提供者的生命周期，也不会调用onCreate。

好吧，我有自己的答案。是的，我不能实例化我的ContentProvider，但不仅仅如此。我更改了代码，意识到我只是没有真正理解查询中的selection和SelectionAgs参数

所以它是从：

private boolean userExists() {
    String username = nomUtilisateur.getText().toString();
    String password = motPasse.getText().toString();

    Cursor users = userProvider.query(MyUserProvider.CONTENT_URI,
            null,
            " WHERE username = " + nomUtilisateur.getText().toString() +
            " AND password = " + motPasse.getText().toString(), null, null);

    if (users.getCount() != 0) {
        return true;
    } else {
        return false;
    }
}

致：

---

因此，游标为空是因为我没有进行正确的查询，这就是为什么没有创建数据库的原因：因为我在代码的末尾不需要它。

在

onCreate

中尝试

返回true

，它仍然不起作用。现在，当我尝试获取游标返回的结果数时，它会抛出异常。它说它是空的，当我试图查看数据库时，它没有被创建。好的，我已经用标记运行了一些测试，我注意到onCreate方法是在我的应用程序开始时调用的，而不是在我想使用getWritableDatabase（）时。而且query（）方法即使在我调用它的时候也不会被调用，所以游标为null是正常的，但是数据库仍然没有被创建。我得到了答案，部分问题就是你提到的。谢谢

private boolean userExists() {
    String username = nomUtilisateur.getText().toString();
    String password = motPasse.getText().toString();

    Cursor users = userProvider.query(MyUserProvider.CONTENT_URI,
            null,
            " WHERE username = " + nomUtilisateur.getText().toString() +
            " AND password = " + motPasse.getText().toString(), null, null);

    if (users.getCount() != 0) {
        return true;
    } else {
        return false;
    }
}

private boolean userExists() {
    String username = nomUtilisateur.getText().toString();
    String password = motPasse.getText().toString();
    String[] selection = {username, password};

    Cursor users = getApplicationContext().getContentResolver().query
            (Agenda.UserEntry.CONTENT_URI,
            null,
            "username = ? AND password = ?", selection, null);

    if (users.getCount() != 0) {
        return true;
    } else {
        return false;
    }
}