android - SQlite 数据库 : Unable to find context

显然，上下文和下一行存在一些问题，因为我收到了 NullPointerException 。我认为上下文有问题。此外，当我尝试从两个 Activity 中打开和关闭数据库时，它会抛出 CannotOpenORCloseDatabase 错误。请帮忙。

DBHelper dbHelper = new DBHelper(this.getApplicationContext());

知道为什么吗？或者，如果有人可以提出解决方法，那就太好了。

public static final String EXT_CATEGORY = "category";

public static final String EXT_URL = "url";

private String tableName = DBHelper.tableName;

private SQLiteDatabase MyDb;

private int category;
Cursor c = null;

public static final String EXT_POS = "position";
private String url;
private int position;
WebView mWebView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    Log.v(LOG_TAG, "onCreate() called");
    super.onCreate(savedInstanceState);

    openAndQueryDatabase();

}

private void openAndQueryDatabase() {

    position = 1;
    category = 0;
    Log.v(LOG_TAG, "onCreate() called 3");
    try {

        DBHelper dbHelper = DBHelper.getInstance(getBaseContext());
        MyDb = dbHelper.getWritableDatabase();


        c = MyDb.rawQuery("SELECT * FROM " + tableName
                + " where Category =" + category + "AND Position ="
                + position, null);
        Log.v(LOG_TAG, "onCreate() called 4");
        // url = c.getString(c.getColumnIndex("Image"));
        url = "www.google.com";
        Log.v(url, " URL in Recipe ");
    } catch (SQLiteException se) {
        Log.e(getClass().getSimpleName(),
                "Could not create or Open the database");
    }

最佳答案

您应该将 DBHelper 声明为静态实例(单例)，以确保在任何给定时间只存在一个 DBHelper。

 public class DBHelper extends SQLiteOpenHelper {

    private static DBHelper mInstance = null;

    public static DBHelper getInstance(Context activityContext) {

    // Get the application context from the activityContext to prevent leak
    if (mInstance == null) {
      mInstance = new DBHelper (activityContext.getApplicationContext());
    }
    return mInstance;
  }

  /**
   * Private, use the static method "DBHelper.getInstance(Context)" instead.
   */
  private DBHelper (Context applicationContext) {
    super(applicationContext, DATABASE_NAME, null, DATABASE_VERSION);
  }
}

要打开 SQLiteDatabase 使用:

SQLiteDatabase mDataBase = DBHelper.getInstance(context).getWritableDatabase();

并关闭它

mDataBase.close();