Android：无法在广播接收器中与 MainActivity 通信

Question

我正在研究广播接收器。我的代码如下：

Settings2类

public void set_notification_task()
    {
        Utilities.initParse(Settings2.this);
        get_notification_info();
    }

    public void get_notification_info()
    {
        ...
    }

    private void setAlarm_manager(Calendar c)
    {
        Intent intent = new Intent(Settings2.this, AlarmBroadcastReceiver.class);   
        PendingIntent pendingIntent = PendingIntent.getBroadcast(Settings2.this, ALARM_RUEST, intent, 0);
        AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
        alarmManager.setRepeating(AlarmManager.RTC_WAKEUP, c.getTimeInMillis(), AlarmManager.INTERVAL_DAY, pendingIntent);
    } 
}

`AlarmBroadcastReceiver

public class AlarmBroadcastReceiver extends BroadcastReceiver {
    final public static String ONE_TIME = "onetime";
    SharedPreferences settings;
    Settings2 setting_activity;

    public AlarmBroadcastReceiver() { }

    public AlarmBroadcastReceiver(Settings2 setting_Context){
        setting_activity =setting_Context;
    }

     @Override
     public void onReceive(Context context, Intent intent) 
     {
         PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
         PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "YOUR TAG");
         //Acquire the lock
         wl.acquire();

         //You can do the processing here.
         Bundle extras = intent.getExtras();
         StringBuilder msgStr = new StringBuilder();

         if(extras != null && extras.getBoolean(ONE_TIME, Boolean.FALSE)){
          //Make sure this intent has been sent by the one-time timer button.
          msgStr.append("One time Timer : ");
         }
         Format formatter = new SimpleDateFormat("hh:mm:ss a");
         msgStr.append(formatter.format(new Date()));

         Toast.makeText(context, "hello", Toast.LENGTH_LONG).show();
         wl.release();  //Release the lock

         setting_activity.set_notification_task(); //line 49
     }
}

Logcat：

11-07 00:28:02.514: E/AndroidRuntime(25045): FATAL EXCEPTION: main
11-07 00:28:02.514: E/AndroidRuntime(25045): Process: com.abc.wmn, PID: 25045
11-07 00:28:02.514: E/AndroidRuntime(25045): java.lang.RuntimeException: Unable to start receiver com.abc.utilities.AlarmBroadcastReceiver: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.abc.wmn.Settings2.set_notification_task()' on a null object reference
11-07 00:28:02.514: E/AndroidRuntime(25045):    at android.app.ActivityThread.handleReceiver(ActivityThread.java:2739)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at android.app.ActivityThread.access$1800(ActivityThread.java:153)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1428)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at android.os.Handler.dispatchMessage(Handler.java:102)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at android.os.Looper.loop(Looper.java:148)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at android.app.ActivityThread.main(ActivityThread.java:5438)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at java.lang.reflect.Method.invoke(Native Method)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629)
11-07 00:28:02.514: E/AndroidRuntime(25045): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.abc.wmn.Settings2.set_notification_task()' on a null object reference
11-07 00:28:02.514: E/AndroidRuntime(25045):    at com.abc.utilities.AlarmBroadcastReceiver.onReceive(AlarmBroadcastReceiver.java:49)
11-07 00:28:02.514: E/AndroidRuntime(25045):    at android.app.ActivityThread.handleReceiver(ActivityThread.java:2732)
11-07 00:28:02.514: E/AndroidRuntime(25045):    ... 8 more

问题：

得到了上面的logcat问题：java.lang.RuntimeException: Unable to start receiver com.abc.utilities.AlarmBroadcastReceiver: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.abc.wmn.Settings2.set_notification_task()' on a null object reference，怎么解决？

谢谢！

Answer 1

这里的问题是setting_activity是null，这里：

setting_activity.set_notification_task();

这是因为设置setting_activity 的AlarmBroadcastReceiver 构造函数永远不会被调用。由于PendingIntent 用于调用您的广播，它将是AlarmBroadcastReceiver 的默认构造函数，而不是您的自定义构造函数，它在发送广播时被调用。因此，setting_activity 变量永远不会被初始化，因此NullPointerException。

Answer 2

我已解决如下：

public class AlarmBroadcastReceiver extends BroadcastReceiver {
    final public static String ONE_TIME = "onetime";
    SharedPreferences settings;
    Settings2 setting_activity;

    public AlarmBroadcastReceiver() { }

    public AlarmBroadcastReceiver(Settings2 setting_Context){
        setting_activity =setting_Context;
    }

     @Override
     public void onReceive(Context context, Intent intent) 
     {
         PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
         PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "YOUR TAG");
         //Acquire the lock
         wl.acquire();

         //You can do the processing here.
         Bundle extras = intent.getExtras();
         StringBuilder msgStr = new StringBuilder();

         if(extras != null && extras.getBoolean(ONE_TIME, Boolean.FALSE)){
          //Make sure this intent has been sent by the one-time timer button.
          msgStr.append("One time Timer : ");
         }
         Format formatter = new SimpleDateFormat("hh:mm:ss a");
         msgStr.append(formatter.format(new Date()));

         Toast.makeText(context, "hello", Toast.LENGTH_LONG).show();
         wl.release();  //Release the lock

         setting_activity = new Settings2();
         setting_activity.set_notification_task();
     }
}

Answer 3

正如 Pathfinderelite 所说，您不能只从接收器访问您的活动。您可以做的是通过依赖注入使您的活动在接收器中可用，或者更简单地，通过在活动中创建指向自身实例的静态链接。 Receiver 然后可以获取实例，但它确实需要检查 null，因为不能保证 Activity 可用并且在调用 onReceive 时不为 null。 （我很抱歉没有格式化，键盘反引号坏了）。