何處關閉在Activity中關閉遊標和數據庫對象沒有任何異常

Question

在我的應用程序中有很多數據庫操作，所以我創建了許多遊標來檢索數據，並且在Async Task中也使用了很少的遊標。現在我的問題是關閉遊標和數據庫的地方。

當我關閉遊標onCreate的異步操作我得到Null Pointer Exception後，也如果我關閉在還是我得到Null pointer exception，所以我在onDestroy關閉遊標和數據庫。

現在我很困惑這種方法是否正確..我想了解什麼是關閉活動中游標和數據庫的最佳方式，而不會對數據庫操作產生任何影響。

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

你應該收於數據庫/光標連接儘快，這意味着一旦你拉從它的數據，你應該關閉的一切，讓你不鎖定數據庫。

持有到數據庫連接是不好的做法，特別是對多線程應用程序

Answer 2

有關Android的數據庫操作的整體操作，這裏是信息的重要來源，使用內容提供商： http://www.vogella.com/tutorials/AndroidSQLite/article.html

這裏是一個如何實現LoaderManager的例子。你可以在這裏找到各種實例：http://developer.android.com/reference/android/app/LoaderManager.html

對於基本實現的想法，這是應該的樣子：

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) { 

} 

}