Question

我的应用正在尝试获取USB设备列表。我的班级extends Service但是当我尝试使用AsyncTask时，会出现NullPointerException。

@Override 
protected String doInBackground(Void... params) {
     Context context = MainActivity.getMainActivity();

     UsbManager manager = (UsbManager) getSystemService(Context.USB_SERVICE);

     HashMap<String, UsbDevice> deviceList = manager.getDeviceList();
     Iterator<UsbDevice> deviceIterator = deviceList.values().iterator();
     while (deviceIterator.hasNext()) {
        UsbDevice device = deviceIterator.next();

        String ACTION_USB_PERMISSION = "my packagename.USB_PERMISSION";
        PendingIntent mPermissionIntent = PendingIntent.getBroadcast(context, 0, new Intent(ACTION_USB_PERMISSION), 0); //NULLPOINTEXCEPTION;
        manager.requestPermission(device, mPermissionIntent);
        .
        .
       }
  }

logcat的

Process: blackeyeonandroid.iosystem.co.kr.toastama, PID: 15069
   java.lang.RuntimeException: An error occurred while executing doInBackground()
       at android.os.AsyncTask$3.done(AsyncTask.java:309)
       at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
       at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
       at java.util.concurrent.FutureTask.run(FutureTask.java:242)
       at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
       at java.lang.Thread.run(Thread.java:818)
    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
       at android.app.PendingIntent.getBroadcastAsUser(PendingIntent.java:526)
       at android.app.PendingIntent.getBroadcast(PendingIntent.java:515)
       at blackeyeonandroid.iosystem.co.kr.toastama.MyService$DeviceDetectAsync.doInBackground(MyService.java:124)
       at blackeyeonandroid.iosystem.co.kr.toastama.MyService$DeviceDetectAsync.doInBackground(MyService.java:105)
       at android.os.AsyncTask$2.call(AsyncTask.java:295)
       at java.util.concurrent.FutureTask.run(FutureTask.java:237)
       at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234) 
       at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
       at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
       at java.lang.Thread.run(Thread.java:818)

我认为上下文可能是问题，但我不知道如何获得服务上下文。

Answer 1

您可以在服务中使用getApplicationContext()来获取应用程序上下文。

在您的情况下，您将尝试保存作为UI部分的主要活动上下文，但在后台服务中不起作用。

Answer 2

使用getApplicationContext();代替您的MainActivity用于上下文。

不要为你的活动制作吸气剂，因为一旦它们被销毁，就无法到达。它总是会产生一个nullpointer。对于上下文，您可以从应用程序中获取它，该应用程序贯穿应用程序的整个生命周期。希望这会有所帮助。

Answer 3

如果您的课程延长$selectedType = Type::where('slug', '=', $slug)->where('type', '=', 'studio')->with(['apartments' => function ($query) use ($request) { $query->where('status', '!=', 'sold'); if ($request->floor == 'low') { $query->where('floor', '<', '5'); } elseif ($request->floor == 'mid') { $query->where('floor', '>', '1')->where('floor', '<', '6'); } elseif ($request->floor == 'high') { $query->where('floor', '>', '4'); } if ($request->sun == 'morning_sun') { $query->where('sun', '=', 'morning'); } elseif ($request->sun == 'evening_sun') { $query->where('sun', '=', 'evening'); } if ($request->price == '1') { $query->where('price', '<', '900')->orWhere([['price_min', '!=', null], ['price_max', '!=', null], ['price_min', '<', '900']]); } elseif ($request->price == '2') { $query->where('price', '<', '999')->orWhere([['price_min', '!=', null], ['price_max', '!=', null], ['price_min', '<', '999']]); } }])->first();，则表示使用服务上下文时请不要使用Service上下文，并在从serivce传递上下文初始化Activity时

通过传递上下文来启动任务

AsyncTask

new DoSomething(this).execute();

课程中的

就是这样的

AsyncTask

如何在Android服务类中使用PendingIntent上下文？

3 个答案: