Java 如果应用程序已打开其他活动，则通知在后台不起作用

抱歉，我是Android studio的新手，但我需要了解如何使此通知保存以及如何使其每天循环

我添加了我的4个java活动，它们处理所有的通知操作，我还添加了将通知发送到xml的文本，但在我关闭应用程序或打开其他活动后，它消失了

package com.example.moksleiviopinigai;

import android.annotation.TargetApi;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.content.Context;
import android.content.ContextWrapper;
import android.os.Build;

import androidx.core.app.NotificationCompat;

public class NotificationHelper extends ContextWrapper {
    public static final String channelID = "channelID";
    public static final String channelName = "Channel Name";
    private NotificationManager mManager;
    public NotificationHelper(Context base) {
        super(base);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            createChannel();
        }
    }
    @TargetApi(Build.VERSION_CODES.O)
    private void createChannel() {
        NotificationChannel channel = new NotificationChannel(channelID, channelName, NotificationManager.IMPORTANCE_HIGH);
        getManager().createNotificationChannel(channel);
    }
    public NotificationManager getManager() {
        if (mManager == null) {
            mManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        }
        return mManager;
    }
    public NotificationCompat.Builder getChannelNotification() {
        return new NotificationCompat.Builder(getApplicationContext(), channelID)
                .setContentTitle("Jau laikas")
                .setContentText("Užpildyti savo dienos ataskaitą!!!")
                .setSmallIcon(R.mipmap.ic_launcher_round);
    }
}

还有我的其他活动

package com.example.moksleiviopinigai;

import android.app.AlarmManager;
import android.app.PendingIntent;
import android.app.TimePickerDialog;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.TimePicker;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.DialogFragment;

import java.text.DateFormat;
import java.util.Calendar;

public class Notification extends AppCompatActivity implements TimePickerDialog.OnTimeSetListener {

        private TextView mTextView;
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_notification);
            mTextView = findViewById(R.id.textView);
            Button buttonTimePicker = findViewById(R.id.button_timepicker);
            buttonTimePicker.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    DialogFragment timePicker = new TimePickerFragment();
                    timePicker.show(getSupportFragmentManager(), "time picker");
                }
            });
            Button buttonCancelAlarm = findViewById(R.id.button_cancel);
            buttonCancelAlarm.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    cancelAlarm();
                }
            });
        }
        @Override
        public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
            Calendar c = Calendar.getInstance();
            c.set(Calendar.HOUR_OF_DAY, hourOfDay);
            c.set(Calendar.MINUTE, minute);
            c.set(Calendar.SECOND, 0);
            updateTimeText(c);
            startAlarm(c);
        }
        private void updateTimeText(Calendar c) {
            String timeText = "Alarm set for: ";
            timeText += DateFormat.getTimeInstance(DateFormat.SHORT).format(c.getTime());
            mTextView.setText(timeText);
        }
        private void startAlarm(Calendar c) {
            AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
            Intent intent = new Intent(this, AlertReceiver.class);
            PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 1, intent, 0);
            if (c.before(Calendar.getInstance())) {
                c.add(Calendar.DATE, 1);
            }
            alarmManager.setExact(AlarmManager.RTC_WAKEUP, c.getTimeInMillis(), pendingIntent);
        }
        private void cancelAlarm() {
            AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
            Intent intent = new Intent(this, AlertReceiver.class);
            PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 1, intent, 0);
            alarmManager.cancel(pendingIntent);
            mTextView.setText("Alarm canceled");
        }
    }

及

及

package com.example.moksleiviopinigai;

import android.app.Dialog;
import android.app.TimePickerDialog;
import android.os.Bundle;
import android.text.format.DateFormat;

import androidx.annotation.NonNull;
import androidx.fragment.app.DialogFragment;

import java.util.Calendar;

public class TimePickerFragment extends DialogFragment {
    @NonNull
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        Calendar c = Calendar.getInstance();
        int hour = c.get(Calendar.HOUR_OF_DAY);
        int minute = c.get(Calendar.MINUTE);
        return new TimePickerDialog(getActivity(), (TimePickerDialog.OnTimeSetListener) getActivity(), hour, minute, DateFormat.is24HourFormat(getActivity()));
    }
}

package com.example.moksleiviopinigai;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;

import androidx.core.app.NotificationCompat;

public class AlertReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        NotificationHelper notificationHelper = new NotificationHelper(context);
        NotificationCompat.Builder nb = notificationHelper.getChannelNotification();
        notificationHelper.getManager().notify(1, nb.build());
    }
}