Activity 中关闭游标和数据库对象的位置没有任何异常

Question

在我的应用程序中，有很多数据库操作，所以我创建了很多游标来检索数据，并且异步任务中使用的游标也很少。现在我的问题是在哪里关闭游标和数据库。

当我在异步操作后关闭onCreate 中的游标时，我得到Null Pointer Exception，如果我在onPause 中关闭，我仍然得到Null pointer exception，所以我已经关闭了@987654326 中的游标和数据库@。

现在我很困惑这种方法是否正确。我想了解在活动中关闭游标和数据库而不影响数据库操作的最佳方法。

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.stockdetail);
        //Helptext.setVisibility(View.INVISIBLE);
        Declerations();

        d=Database.getInstance(getApplicationContext()).getWritableDatabase();



        new popdailydata().execute();
// Above is async task so I have closed after above statement, but if I close cursor getting Null pointer exception

protected void onDestroy() {
        // TODO Auto-generated method stub
    super.onDestroy();
    selectdate.close();
    Database.getInstance(getApplicationContext()).close();

    }

    @Override
    protected void onPause() {
        // TODO Auto-generated method stub
        super.onPause();
        //selectdate.close(); --- At this point also I am getting nullpointer exception
        Database.getInstance(getApplicationContext()).close();
    }

---

04-01 17:03:48.045: D/dalvikvm(1288): Late-enabling CheckJNI
04-01 17:03:50.021: D/libEGL(1288): loaded /system/lib/egl/libEGL_genymotion.so
04-01 17:03:50.113: D/(1288): HostConnection::get() New Host Connection established 0xb90bb5c8, tid 1288
04-01 17:03:50.177: D/libEGL(1288): loaded /system/lib/egl/libGLESv1_CM_genymotion.so
04-01 17:03:50.181: D/libEGL(1288): loaded /system/lib/egl/libGLESv2_genymotion.so
04-01 17:03:50.773: W/EGL_genymotion(1288): eglSurfaceAttrib not implemented
04-01 17:03:50.797: E/OpenGLRenderer(1288): Getting MAX_TEXTURE_SIZE from GradienCache
04-01 17:03:50.861: E/OpenGLRenderer(1288): Getting MAX_TEXTURE_SIZE from Caches::initConstraints()
04-01 17:03:50.865: D/OpenGLRenderer(1288): Enabling debug mode 0
04-01 17:03:54.645: D/dalvikvm(1288): GC_FOR_ALLOC freed 102K, 1% free 16948K/17072K, paused 89ms, total 96ms
04-01 17:03:54.709: D/AndroidRuntime(1288): Shutting down VM
04-01 17:03:54.725: W/dalvikvm(1288): threadid=1: thread exiting with uncaught exception (group=0xa4c45648)
04-01 17:03:54.745: E/AndroidRuntime(1288): FATAL EXCEPTION: main
04-01 17:03:54.745: E/AndroidRuntime(1288): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.portfoliomanager/com.example.StockDirector.stockmanager}: java.lang.NullPointerException
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.os.Looper.loop(Looper.java:137)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.app.ActivityThread.main(ActivityThread.java:5103)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at java.lang.reflect.Method.invokeNative(Native Method)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at java.lang.reflect.Method.invoke(Method.java:525)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at dalvik.system.NativeStart.main(Native Method)
04-01 17:03:54.745: E/AndroidRuntime(1288): Caused by: java.lang.NullPointerException
04-01 17:03:54.745: E/AndroidRuntime(1288):     at com.example.StockDirector.stockmanager.onCreate(stockmanager.java:81)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.app.Activity.performCreate(Activity.java:5133)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-01 17:03:54.745: E/AndroidRuntime(1288):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
04-01 17:03:54.745: E/AndroidRuntime(1288):     ... 11 more
04-01 17:03:57.889: I/Process(1288): Sending signal. PID: 1288 SIG: 9

Answer 1

对于 android 数据库操作的整体操作，这里是使用内容提供程序的重要信息来源： http://www.vogella.com/tutorials/AndroidSQLite/article.html

这是一个如何实现 LoaderManager 的示例。您可以在此处找到各种示例：http://developer.android.com/reference/android/app/LoaderManager.html

对于一个基本的实现思路，它应该是这样的：

public class MainActivity extends Activity implements LoaderManager.LoaderCallbacks<Cursor> {

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

@Override
public Loader<Cursor> onCreateLoader(int id, Bundle args) {
    return new CursorLoader(this,
            ObjectEntity.URI,   // Database uri
            new String[]{"id"}, // Columns to select
            "id=?",               // 'where' field
            new String[]{"3"},  // Values for 'where' '?' field
            null);
}

@Override
public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
    // Do anything you want here with the cursor
}

@Override
public void onLoaderReset(Loader<Cursor> loader) {

}

}

Answer 2

您应该尽快关闭数据库/光标连接，这意味着一旦您从中提取数据，您应该关闭所有内容，以免锁定数据库。

保持数据库连接是不好的做法，尤其是对于多线程应用程序