Question

当我从Fragment中获取asyntake并访问sqlite时，我遇到了以下错误。

引起：java.lang.NullPointerException 在android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked（SQLiteOpenHelper.java:224）

这是我的编码。我不知道为什么我收到此错误，即使我已经声明“DatabaseUtil dbUtil = new DatabaseUtil（ctx）;”

  public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        .
        ..

    }



    public static class ContentFragment extends Fragment {


        public ContentFragment() {

        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.list_layout, container, false);
            MainActivity myActivity = new MainActivity();
            MainActivity.GetDataTask asyncTask = myActivity.new GetDataTask(); 

            return rootView;
        }
    }    

    private class GetDataTask extends AsyncTask<Object, Void, Integer> {

        protected void onPreExecute() {

        }       

        @Override
        protected Integer doInBackground(Object... args) {

            DatabaseUtil dbUtil = new DatabaseUtil(ctx);
                dbUtil.open();
                        dbUtil.insertRecord(....);
                dbUtil.close();

            return 1;
        }

        @Override
        protected void onPostExecute(Integer result) {
            super.onPostExecute(result);
        }
    }

Answer 1

你在做什么是错的

    MainActivity myActivity = new MainActivity();

您正在创建活动类的实例。活动由startActivtiy(new Intent(params))开始。活动有ui和生命周期。因此，不需要创建实例od活动类。

建议在你的片段类中有asynctask

另外

   DatabaseUtil dbUtil = new DatabaseUtil(getActivtiy());

编辑问题

  public class MainActivity extends Activity {

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    .
    ..

  }

然后在ContentFragment.java

中

    public class ContentFragment extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.list_layout, container, false);
         new GetDataTask(getActivity()).execute(params);
        return rootView;
    }
}    

 class GetDataTask extends AsyncTask<Object, Void, Integer> {

    Context mContext;  
   public GetDataTask(Activity activity) {
    // TODO Auto-generated constructor stub
    mContext = activity;
}

    @Override 
    protected void onPreExecute() {
        super.onPreExecute();
    }       

    @Override
    protected Integer doInBackground(Object... args) {

        DatabaseUtil dbUtil = new DatabaseUtil(mContext);
            dbUtil.open();
                    dbUtil.insertRecord(....);
            dbUtil.close();

        return 1;
    }

    @Override
    protected void onPostExecute(Integer result) {
        super.onPostExecute(result);
    }
 }

Answer 2

您可以为 GetDataTask ....

制作构造函数

Context ctx;

public GetDataTask(Context c){

 this.ctx = c;

}

现在您可以使用此 ctx 变量

DatabaseUtil dbUtil = new DatabaseUtil(ctx);

从Fragment获取错误访问sqlite

2 个答案: