getExternalFilesDir（null）拋出NullPointerException

Question

我正在構建一個Android應用程序，並且我選擇在外部存儲中使用扁平的蒼蠅用於我的持久性方法。我有一個類擴展了應用程序與一些靜態實例方法。經過幾次調試後，我注意到沒有一個實例變量是null，而NullPointerException來自Context.getExternalFilesDir(null)。

我想以任何答案作爲前言，它不返回null。另外，我在清單中擁有正確的權限，並檢查SD卡是否安裝到設備上。

這是持有致電getExternalFilesDir(null)並引發錯誤的類。

public class WhileImOut extends Application { 

    public static TaskManager taskManager; 
    private static Context appContext; 

    public WhileImOut() { 
     super(); 
     appContext = this; 
     initialize(); 
    } 

    public static void initialize() { 
     if (taskManager == null && Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) { 
      try{ 
      File f = appContext.getExternalFilesDir(null); // Exception being thrown here 
      taskManager = new TaskManager(f.getAbsolutePath()); 
      }catch(NullPointerException e){ 
       Log.d("Bad","NPE"); 
      }catch(Exception e){ 

      } 
     } 
    } 
} 

這是我的AndroidManifest.xml：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> 

任何幫助將不勝感激。我一直在爲此奮鬥了幾個小時，我會採取任何建議。謝謝

編輯：logcat的信息

10-10 18:55:56.424: E/AndroidRuntime(964): FATAL EXCEPTION: main 
10-10 18:55:56.424: E/AndroidRuntime(964): java.lang.RuntimeException: Unable to resume activity {zsmith.capstone.whileimout/zsmith.capstone.whileimout.TaskListActivity}: java.lang.NullPointerException 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2575) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2603) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2089) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.access$600(ActivityThread.java:130) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.os.Handler.dispatchMessage(Handler.java:99) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.os.Looper.loop(Looper.java:137) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.main(ActivityThread.java:4745) 
10-10 18:55:56.424: E/AndroidRuntime(964): at java.lang.reflect.Method.invokeNative(Native Method) 
10-10 18:55:56.424: E/AndroidRuntime(964): at java.lang.reflect.Method.invoke(Method.java:511) 
10-10 18:55:56.424: E/AndroidRuntime(964): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786) 
10-10 18:55:56.424: E/AndroidRuntime(964): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
10-10 18:55:56.424: E/AndroidRuntime(964): at dalvik.system.NativeStart.main(Native Method) 
10-10 18:55:56.424: E/AndroidRuntime(964): Caused by: java.lang.NullPointerException 
10-10 18:55:56.424: E/AndroidRuntime(964): at zsmith.capstone.whileimout.TaskListActivity.onResume(TaskListActivity.java:58) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1184) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.Activity.performResume(Activity.java:5082) 
10-10 18:55:56.424: E/AndroidRuntime(964): at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2565) 

Answer 1

你是做在構造函數初始化的;這是行不通的。你應該在onCreate()中進行初始化。當調用構造函數時，上下文尚未正確初始化。

public class WhileImOut extends Application { 

    public static TaskManager taskManager; 
    private static Context appContext; 

    public void onCreate() { 
     super(); 
     appContext = this; 
     initialize(); 
    } 

    public static void initialize() { 
     if (taskManager == null && Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) { 
      try{ 
      File f = appContext.getExternalFilesDir(null); // Exception being thrown here 
      taskManager = new TaskManager(f.getAbsolutePath()); 
      }catch(NullPointerException e){ 
       Log.d("Bad","NPE"); 
      }catch(Exception e){ 

      } 
     } 
    } 
} 

（順便說一下，沒有任何理由擁有一個獨立的initialize()方法？如果是這樣，沒有任何理由爲它是public？）