java - FileObserver 中的 NullPointerException - Android

Question

我现在正在编写 FileObserver 代码。我有以下代码，如果运行它，我将收到 NullPointerException。

public class FileModificationService extends Service{
    private MyFileObserver fileOb;
    private static final int MAX_FO = 1;
    private List<MyFileObserver> fileOb_list = new ArrayList<MyFileObserver>();
    @Override
    public void onCreate() {
        if (!EnvironmentUtilsStatic.is_external_storage_available()) {
            Toast.makeText(FileModificationService.this, "SDCARD is not available!", Toast.LENGTH_SHORT).show();
            return;
        }
        File sdcard = new File("/storage/sdcard/DCIM/Camera/SAMPLE IMAGES/");
        if (sdcard == null) {
            return;
        } else {
            fileOb_list.clear();
            num_of_fos = 0;
            createFileObs(sdcard);
        }
    }

    //only create fileobserver for folders
    int num_of_fos = 0;
    private void createFileObs(File f) {
        if (num_of_fos > MAX_FO) {
            return;
        }
        if (!f.isDirectory()) {
            MyFileObserver aFileOb = new MyFileObserver(f.getAbsolutePath());
            fileOb_list.add(aFileOb);
        } else {
            fileOb = new MyFileObserver(f.getAbsolutePath());
            MyFileObserver aFileOb = new MyFileObserver(f.getAbsolutePath());
            fileOb_list.add(aFileOb);
            num_of_fos++;
            try {
                for (File currentFile : f.listFiles()) {
                    createFileObs(currentFile);
                }
            } catch (Exception e) {
                Log.e("Error", e.toString());
            }
        }
    }

    @Override
    public void onStart(Intent intent, int startid) {
        fileOb.startWatching();
fileOb = new MyFileObserver("/storage/sdcard/DCIM/Camera/SAMPLE IMAGES/");// added code
        for (int i = 0; i < fileOb_list.size(); ++i) { //This is line no. 60
            fileOb_list.get(i).startWatching();
        }
        Toast.makeText(this.getApplicationContext(), "start monitoring file modification", Toast.LENGTH_SHORT).show();
    }
    @Override
    public void onDestroy() {
        fileOb.stopWatching();
        for (int i = 0; i < fileOb_list.size(); ++i) {
            fileOb_list.get(i).stopWatching();
        }
        Toast.makeText(this.getApplicationContext(), "stop monitoring file modification", Toast.LENGTH_SHORT).show();
    }

    @Override
    public IBinder onBind(Intent arg0) {
        // TODO Auto-generated method stub
        return null;
    }
}

我收到以下 logcat -

09-17 13:05:55.915: E/AndroidRuntime(2596): FATAL EXCEPTION: main
09-17 13:05:55.915: E/AndroidRuntime(2596): java.lang.RuntimeException: Unable to start service roman10.tutorial.fileobserver.FileModificationService@b11293c0 with Intent { cmp=roman10.tutorial.fileobserver/.FileModificationService }: java.lang.NullPointerException
09-17 13:05:55.915: E/AndroidRuntime(2596):     at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2721)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at android.app.ActivityThread.access$1900(ActivityThread.java:141)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1353)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at android.os.Looper.loop(Looper.java:137)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at android.app.ActivityThread.main(ActivityThread.java:5103)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at java.lang.reflect.Method.invokeNative(Native Method)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at java.lang.reflect.Method.invoke(Method.java:525)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at dalvik.system.NativeStart.main(Native Method)
09-17 13:05:55.915: E/AndroidRuntime(2596): Caused by: java.lang.NullPointerException
09-17 13:05:55.915: E/AndroidRuntime(2596):     at roman10.tutorial.fileobserver.FileModificationService.onStart(FileModificationService.java:60)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at android.app.Service.onStartCommand(Service.java:450)
09-17 13:05:55.915: E/AndroidRuntime(2596):     at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2704)
09-17 13:05:55.915: E/AndroidRuntime(2596):     ... 10 more

这段代码之前实际上是有效的。我没有对代码进行任何更改。但是，当我尝试运行此代码时，将模拟器从 Google 更改为 Intel x86 后，问题就出现了。但是，我尝试将模拟器转换回 Google。而且，该代码不起作用。

我不明白，为什么这段代码不起作用。

请指导我，提供一些建议，如果可能的话，提供教程。提前致谢。

Answer 1

在onStart()的fileOb.startWatching()中使用fileOb之前需要先初始化它