警报管理器无法在后台运行

Question

我在程序中使用了警报管理器，但是第一次运行它，它只在我的程序打开时起作用，而在关闭时它在后台不起作用，但是第二次在后台运行。背景。

MainActivity：

public class MainActivity extends AppCompatActivity {
    AlarmManager alarmManager;
    Intent intent;
    PendingIntent pendingIntent;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        intent = new Intent(MainActivity.this, AlertReceiver.class);
        pendingIntent = PendingIntent.getBroadcast(MainActivity.this, 0, intent, 0);
        alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
        alarmManager.set(AlarmManager.RTC_WAKEUP, System.currentTimeMillis()+60000, pendingIntent);

    }
}

AlertReceiver：

public class AlertReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {

        Log.d("Log", "onReceiver!!");
   
    }
}

AndroidManifest：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
package="com.example.yadame">

<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />
<uses-permission android:name="com.android.alarm.permission.SET_ALARM" />
<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".MainActivity" android:theme="@style/AppTheme.NoActionBar">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <receiver android:name=".AlertReceiver"/>

</application>