在Android中的应用程序类与数据库问题

我无法访问我的Android应用程序中的SQLite数据库。我创建了一个包含SQLiteOpenHelper实例的应用程序类，但是每当我尝试在另一个活动中打开数据库时，它都会崩溃。在Android中的应用程序类与数据库问题

这里是我的应用程序类：

public class GlobalVariables extends Application { 

    private LocationsData locations; 

    public void onCreate(Context context) { 
      locations = new LocationsData(context); 
    } 

    public LocationsData getLocations() { 
      return locations; 
    } 
}

这里是我SQLiteOpenHelper类：

public class LocationsData extends SQLiteOpenHelper{ 
private static final String DATABASE_NAME = "locations.db"; 
private static final int DATABASE_VERSION = 1; 

public static final String TABLE_LOCATIONS = "locations"; 
public static final String COLUMN_ID = "_id"; 
public static final String COLUMN_NICKNAME = "nickname"; 

private static final String DATABASE_CREATE = "create table " 
     + TABLE_LOCATIONS + "(" + COLUMN_ID 
     + " integer primary key autoincrement, " + COLUMN_NICKNAME 
     + " text not null);"; 

public LocationsData(Context context) { 
    super(context, DATABASE_NAME, null, DATABASE_VERSION); 
} 

@Override 
public void onCreate(SQLiteDatabase db) { 
    db.execSQL(DATABASE_CREATE); 

} 

@Override 
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_LOCATIONS); 
    onCreate(db); 

}

}

，当我尝试访问该数据库的另一个动作是这样，我得到出错：

@Override 
    public void onResume() 
    { 
     locationsData=((GlobalVariables)getApplication()).getLocations(); 
     SQLiteDatabase db = locationsData.getReadableDatabase(); 
     super.onResume(); 
    }

任何帮助？谢谢！

Log Cat: 
05-10 02:00:50.146: D/AndroidRuntime(532): Shutting down VM 
05-10 02:00:50.146: W/dalvikvm(532): threadid=1: thread exiting with uncaught exception (group=0x40015560) 
05-10 02:00:50.179: E/AndroidRuntime(532): FATAL EXCEPTION: main 
05-10 02:00:50.179: E/AndroidRuntime(532): java.lang.RuntimeException: Unable to resume activity {com.John.seniorDesign/com.John.seniorDesign.LocationsTabActivity}: java.lang.NullPointerException 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2120) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:138) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:654) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.widget.TabHost.setCurrentTab(TabHost.java:326) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.widget.TabHost$2.onTabSelectionChanged(TabHost.java:132) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.widget.TabWidget$TabClickListener.onClick(TabWidget.java:456) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.view.View.performClick(View.java:2485) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.view.View$PerformClick.run(View.java:9080) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.os.Handler.handleCallback(Handler.java:587) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.os.Handler.dispatchMessage(Handler.java:92) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.os.Looper.loop(Looper.java:130) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.app.ActivityThread.main(ActivityThread.java:3683) 
05-10 02:00:50.179: E/AndroidRuntime(532): at java.lang.reflect.Method.invokeNative(Native Method) 
05-10 02:00:50.179: E/AndroidRuntime(532): at java.lang.reflect.Method.invoke(Method.java:507) 
05-10 02:00:50.179: E/AndroidRuntime(532): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) 
05-10 02:00:50.179: E/AndroidRuntime(532): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597) 
05-10 02:00:50.179: E/AndroidRuntime(532): at dalvik.system.NativeStart.main(Native Method) 
05-10 02:00:50.179: E/AndroidRuntime(532): Caused by: java.lang.NullPointerException 
05-10 02:00:50.179: E/AndroidRuntime(532): at com.John.seniorDesign.LocationsTabActivity.onResume(LocationsTabActivity.java:75) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1150) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.app.Activity.performResume(Activity.java:3832) 
05-10 02:00:50.179: E/AndroidRuntime(532): at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2110) 
05-10 02:00:50.179: E/AndroidRuntime(532): ... 17 more

来源

2012-05-10 Seuby

从logcat发布错误的堆栈跟踪。 – superfell

我认为它可能与GlobalVariables类中的onCreate（）调用中的上下文不正确有关。但我不确定... – Seuby

发布LocationsTabActivity.onResume的代码 – superfell

在GlobalVariables变化背景下getApplicationContext（）在新LocationsData和删除的onCreate语境 - 这个签名不存在。

编辑：

public class GlobalVariables extends Application { 

    private LocationsData locations; 

    @Override 
    public void onCreate() { 
      locations = new LocationsData(getApplicationContext()); 
    } 

    public LocationsData getLocations() { 
      return locations; 
    } 
}

来源

2012-05-10 05:54:44

@Override会抓住这个。 – superfell

当然，确实添加到我的答案。谢谢。 –

不知道是什么原因，但在这里是如何我通常实现我的分贝开/关功能：

在我的DB类：

public GroceryDB(Context ctx) { 
    this.mCtx = ctx; 
} 

public GroceryDB open() throws SQLException { 
    mDBHelper = new DBHelper(mCtx); 
    mDb = mDBHelper.getWritableDatabase(); 
    return this; 
} 

public void close() { 
    mDBHelper.close(); 
}

然后，我把它叫做任何其他类：

mDbHelper = new GroceryDB(getActivity()); 
    mDbHelper.open();

希望这有助于！

来源

2012-05-10 05:58:54 Barak

在您的应用程序生命周期中，您总是希望确保您正在处理单个数据库实例。看看这个answer到一个类似的问题。它给出了一些如何在应用程序中正确打开/关闭数据库的例子。

来源

2012-05-11 07:29:19

在Android中的应用程序类与数据库问题

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