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Android中具有数据库问题的应用程序类

sqliteclasssqliteopenhelperandroid
3

我在我的安卓应用中遇到了访问SQLite数据库的问题。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，它就会崩溃。

这是我的应用程序类：

public class GlobalVariables extends Application {

    private LocationsData locations;

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

    public LocationsData getLocations() {
           return locations;
    }
}

这是我的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);

}

当我尝试在另一个活动中访问数据库时，出现了错误：

代码如下：

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

需要帮忙吗？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
将错误的堆栈跟踪从logcat中发布。 - superfell
我认为这可能与GlobalVariables类中onCreate()调用的上下文不正确有关。但我不确定... - Seuby
1
请发布LocationsTabActivity.onResume代码。 - superfell
这是我上次写的最后一部分，不过我会编辑一下，让它更清晰明了。 - Seuby
第75行出现了空指针异常：Caused by: java.lang.NullPointerException 05-10 02:00:50.179: E/AndroidRuntime(532): at com.John.seniorDesign.LocationsTabActivity.onResume(LocationsTabActivity.java:75) - theelfismike
3个回答
4
在新的LocationsData中，将GlobalVariables中的context更改为getApplicationContext()，并从onCreate中删除Context - 此签名不存在。
public class GlobalVariables extends Application {

    private LocationsData locations;

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

    public LocationsData getLocations() {
           return locations;
    }
}
@Override 会捕捉到这个错误。 - superfell
当然，我已经将它添加到我的回答中。谢谢。 - Harald Wilhelm
0
在应用程序的生命周期中，您始终希望确保处理单个数据库实例。请查看答案以获取类似问题的几个示例。它提供了如何在应用程序中正确打开/关闭数据库的方法。
0

不确定原因是什么，但这是我通常实现我的数据库打开/关闭功能的方法：

在我的数据库类中：

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();
}

然后我可以在任何其他类中这样调用它：

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

希望这可以帮到你！

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