NullPointerException/IllegalStateException OutputStream.write（）

Question

我似乎有我的方法來備份當前數據庫的問題。方法如下：

當使用nexus 4時，運行4.2.2該方法工作正常，但是當使用模擬器來模擬版本API 15時，會發生錯誤。發生錯誤的行是os.write（）謝謝。

public void backupDatabase() throws IOException { 

    File file = context.getDatabasePath(DATABASE_NAME); 
    FileInputStream fis = null; 
    try { 
     fis = new FileInputStream(file); 
    } catch (FileNotFoundException e) { 
     e.printStackTrace(); 
    } 
    File appDir = new File(Environment.getExternalStorageDirectory() + "/AppName/"); 
    appDir.mkdir(); 
    File outputDB = new File(appDir, "BackupFile.file"); 
    //String outputDB = Environment.getExternalStorageDirectory() +"/AppName/BackupFile.file"; 
    OutputStream os = null; 
    try { 
     os = new FileOutputStream(outputDB); 
    } catch (FileNotFoundException e) { 
     e.printStackTrace(); 
    } 

    // Transfer bytes from the inputfile to the outputfile 
    byte[] buffer = new byte[1024]; 
    int length; 
    while ((length = fis.read(buffer))>0) { 
      os.write(buffer, 0, length); //<---Error.... (Only on emulator) 
    } 
    // Close the streams 
    os.flush(); 
    os.close(); 
    fis.close(); 

    Toast.makeText(context, "Data successfully backed up!", Toast.LENGTH_SHORT).show(); 
} 


01-18 18:57:38.077: E/ACRA(1961): PACKAGE_NAME fatal error : Could not execute method of the activity 
01-18 18:57:38.077: E/ACRA(1961): java.lang.IllegalStateException: Could not execute method of the activity 
01-18 18:57:38.077: E/ACRA(1961): at android.view.View$1.onClick(View.java:3044) 
01-18 18:57:38.077: E/ACRA(1961): at android.view.View.performClick(View.java:3511) 
01-18 18:57:38.077: E/ACRA(1961): at android.view.View$PerformClick.run(View.java:14105) 
01-18 18:57:38.077: E/ACRA(1961): at android.os.Handler.handleCallback(Handler.java:605) 
01-18 18:57:38.077: E/ACRA(1961): at android.os.Handler.dispatchMessage(Handler.java:92) 
01-18 18:57:38.077: E/ACRA(1961): at android.os.Looper.loop(Looper.java:137) 
01-18 18:57:38.077: E/ACRA(1961): at android.app.ActivityThread.main(ActivityThread.java:4424) 
01-18 18:57:38.077: E/ACRA(1961): at java.lang.reflect.Method.invokeNative(Native Method) 
01-18 18:57:38.077: E/ACRA(1961): at java.lang.reflect.Method.invoke(Method.java:511) 
01-18 18:57:38.077: E/ACRA(1961): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 
01-18 18:57:38.077: E/ACRA(1961): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 
01-18 18:57:38.077: E/ACRA(1961): at dalvik.system.NativeStart.main(Native Method) 
01-18 18:57:38.077: E/ACRA(1961): Caused by: java.lang.reflect.InvocationTargetException 
01-18 18:57:38.077: E/ACRA(1961): at java.lang.reflect.Method.invokeNative(Native Method) 
01-18 18:57:38.077: E/ACRA(1961): at java.lang.reflect.Method.invoke(Method.java:511) 
01-18 18:57:38.077: E/ACRA(1961): at android.view.View$1.onClick(View.java:3039) 
01-18 18:57:38.077: E/ACRA(1961): ... 11 more 
01-18 18:57:38.077: E/ACRA(1961): Caused by: java.lang.NullPointerException 
01-18 18:57:38.077: E/ACRA(1961): at  PACKAGE_NAME.backupDatabase(DbHelper.java:85) 

Answer 1

在我看來，這os是目前空。如果是這樣，那麼創建FileOutputStream投擲FileNotFoundException。如果有的話，你應該發佈任何日誌。

另外一定要設置permissionREAD_EXTERNAL_STORAGE和WRITE_EXTERNAL_STORAGE。

我的建議是檢查當前是否os不是null，然後才寫入，因爲文件打開可能會失敗。

此外，我不會依賴模擬器在實際設備上，因爲它只是模擬系統。如果您有任何可以測試的真實設備，請使用它們。