SQLite数据库：找不到上下文 [英] SQlite Database: Unable to find context

问题描述

显然有一些问题的背景下，并在下面的行，因为我得到是一个 NullPointerException异常。我觉得有什么不妥的上下文。此外，当我米试图打开并从两个活动它扔 CannotOpenORCloseDatabase 错误，请帮忙。

关闭数据库

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

任何想法，为什么？或者，它会是巨大的，如果有人能提出一个解决方法。

 公共静态最后弦乐EXT_CATEGORY =类别;公共静态最后弦乐EXT_URL =URL;私有String tableName值= DBHelper.tableName;私人SQLiteDatabase MYDB;私人诠释类;
光标C = NULL;公共静态最后弦乐EXT_POS =位置;
私人字符串URL;
私人诠释的位置;
的WebView mWebView;@覆盖
保护无效的onCreate（捆绑savedInstanceState）{
    Log.v（LOG_TAG的onCreate（）被称为）;
    super.onCreate（savedInstanceState）;    openAndQueryDatabase（）;}私人无效openAndQueryDatabase（）{    位置= 1;
    类= 0;
    Log.v（LOG_TAG的onCreate（）呼吁3）;
    尝试{        DBHelper dbHelper = DBHelper.getInstance（getBaseContext（））;
        MYDB = dbHelper.getWritableDatabase（）;
        C = MyDb.rawQuery（SELECT * FROM+ tableName值
                +，其中分类=+类别+和位置=
                +位置，NULL）;
        Log.v（LOG_TAG的onCreate（）称为4）;
        // URL = c.getString（c.getColumnIndex（图像））;
        URL =www.google.com;
        Log.v（URL，URL在配方）;
    }赶上（SQLiteException SE）{
        Log.e（的getClass（）。getSimpleName（），
                无法创建或打开数据库）;
    }
 

解决方案

您应声明的DBHelper作为静态实例（单身），以确保只有一个DBHelper在任何给定的时间存在。

 公共类DBHelper扩展SQLiteOpenHelper {    私有静态DBHelper mInstance = NULL;    公共静态DBHelper的getInstance（上下文activityContext）{    //从activityContext至prevent泄漏的应用程序上下文
    如果（mInstance == NULL）{
      mInstance =新DBHelper（activityContext.getApplicationContext（））;
    }
    返回mInstance;
  }  / **
   *私人，使用静态方法DBHelper.getInstance（上下文）代替。
   * /
  私人DBHelper（上下文的applicationContext）{
    超（ApplicationContext一DATABASE_NAME，空，DATABASE_VERSION）;
  }
}
 

要打开SQLiteDatabase使用：

  SQLiteDatabase mDataBase = DBHelper.getInstance（上下文）.getWritableDatabase（）;
 

和关闭它。

  mDataBase.close（）;
 

Apparently there is some problem with the context and in the following line as I'm getting a NullPointerException. I think something is wrong with the context. Also when I"m trying to open and close database from two activities it's throwing CannotOpenORCloseDatabase Error. Please help.

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

Any idea why? Or it'd be great if someone could suggest a workaround.

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");
    } 

解决方案

You should declare your DBHelper as a static instance (singleton) to ensure that only one DBHelper exist at any given time.

 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);
  }
}

To open the SQLiteDatabase use:

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

and to close it

mDataBase.close();

这篇关于SQLite数据库：找不到上下文的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！