Develop Reference

What is the meaning of use FLAG_ONE_SHOT in a pendingintent which will be used to a notification? Is it necessary?

Intent intent = new Intent(this, MainActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0 /* Request code */, intent,
PendingIntent.FLAG_ONE_SHOT);
NotificationCompat.Builder notificationBuilder =
new NotificationCompat.Builder(this, channelId)
.setContentIntent(pendingIntent);
Does it mean that I can't jump to the mainActivity multiple times by repeatedly clicking this notification? Is pendingIntent.send() called internally when I clicks the notification? Does this flag become meaningless in the notifications?

PendingIntent.FLAG_ONE_SHOT means that the PendingIntent will be deleted after it is used once. Normally, a PendingIntent can hang around if there are other references to it in the system.
This flag can be used to prevent multiple uses of the PendingIntent, although my personal experience is that this flag generally causes more problems than it solves.
You asked:
Is pendingIntent.send() called internally when I clicks the notification?
Yes, that's basically what happens when you click on the UI element associated with the PendingIntent in the Notification.

Android app does not open on tapping notifications appearing on top of device screen

When the user drags down all notifications and then tap on my app notification, then the app opens.
However, if a fresh notification appears on top on device screen and the user taps it, it does not open.
What could be the issue?
Intent intent = Activity1.createIntent(app.getApplicationContext());
intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_SINGLE_TOP);
Activity2.createIntent(app.getApplicationContext(), new ArrayList<>(participants));
PendingIntent pendingIntent =
PendingIntent.getActivity(app.getApplicationContext(), 1000, intent, PendingIntent.FLAG_CANCEL_CURRENT);
Uri defaultSoundUri= RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
Bitmap bm = BitmapFactory.decodeResource(app.getResources(), R.mipmap.appiconmain);
NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(app.getApplicationContext())
.setSmallIcon(R.drawable.new_item_arrival, Notification.PRIORITY_HIGH)
.setLargeIcon(bm)
.setContentTitle(displayNotificationData.getTitle())
.setContentText(displayNotificationData.getSubTitle())
.setAutoCancel(true)
.setSound(defaultSoundUri)
.setPriority(Notification.PRIORITY_HIGH)
.setContentIntent(pendingIntent);
NotificationManager notificationManager =
(NotificationManager) app.getSystemService(Context.NOTIFICATION_SERVICE);
notificationManager.notify(0, notificationBuilder.build());

You need to look at overriding Activity's onNewIntent(Intent intent) event handler, on this method you should be able to perform whatever tasks you need
Additionally, according to the official docs you can still get the original intent through via getIntent(), therefore still inside onNewIntent() you should consider using setIntent (Intent newIntent) so you can use this new intent anywhere in the activity
Note that getIntent() still returns the original Intent. You can use setIntent(Intent) to update it to this new Intent.
Take a look at this quote from a precise and concise answer:
For managing if the activity is already running you have two ways:
Add FLAG_ACTIVITY_SINGLE_TOP flag to the Intent when launching the
activity, and then in the activity class implement onNewIntent(Intent
intent) event handler, that way you can access the new intent that was
called for the activity (which is not the same as just calling
getIntent(), this will always return the first Intent that launched
your activity.
Same as number one, but instead of adding a flag to the
Intent you must add "singleTop" in your activity AndroidManifest.xml.
If you use intent extras, remeber to call PendingIntent.getActivity()
with the flag PendingIntent.FLAG_UPDATE_CURRENT, otherwise the same
extras will be reused for every notification.
I hope this helps

Use this :-
PendingIntent pendingIntent =
PendingIntent.getActivity(app.getApplicationContext(), 1000, intent, PendingIntent.FLAG_UPDATE_CURRENT);
and overrride onNewIntent method in your MainActivity.

Pending Intent in Notification not getting cleared

Please have a look at my Pending Intent code.
notificationIntent.putExtra("is_from_notification", true);
notificationIntent.putExtra("push_message_id", push_message_id);
notificationIntent.putExtra("open_target", open_target);
notificationIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
PendingIntent contentIntent = PendingIntent.getActivity(context,notifyID, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);
Notification noti = new NotificationCompat.Builder(context)
.setSmallIcon(icon_small)
.setTicker(message)
.setLargeIcon(largeIcon)
.setWhen(System.currentTimeMillis())
.setContentTitle(title)
.setContentText(message)
.setContentIntent(contentIntent)
.setAutoCancel(true).build();
NotificationManager notificationManager = (NotificationManager) context
.getSystemService(Context.NOTIFICATION_SERVICE);
notificationManager.notify(notifyID, noti);
My Problem is that, where and how should I clear this pending intent. It stays as far as app is in recent tasks.

Just call notificationManager.cancel(notifyId);
Or
call notificationManager.cancelAll();
Hope , it helps.

To clear a pendingIntent use:
Edit:
PendingIntent.getActivity(context,notifyID, contentIntent, PendingIntent.FLAG_UPDATE_CURRENT).cancel();
Place this line somewhere in your code, where u want to cancel the pendingintent.
Note: The intent must have the same name and ID as your pendingIntent, in your case: notifyID and contentIntent
Note: A PendingIntent itself is simply a reference to a token maintained by the system describing the original data used to retrieve it. This means that, even if its owning application's process is killed, the PendingIntent itself will remain usable from other processes that have been given it. If the creating application later re-retrieves the same kind of PendingIntent (same operation, same Intent action, data, categories, and components, and same flags), it will receive a PendingIntent representing the same token if that is still valid, and can thus call cancel() to remove it.
cancel() - Cancel a currently active PendingIntent. Only the original application owning a PendingIntent can cancel it.
Should help u out

How maintain GCM push message in Android, its replaced by new one

My Android application receives push notification with some text messages.If I tap a push it redirects me to desired activity with latest push message (intent message) but I want to show my desired activity with corresponding push messages.
For example If I receives 10 push notifications and I tap 3rd notification, my code redirects me to the specified activity with 10th push notification's message, but I want to show 3rd intent push notification's message.
I know PendingIntent.FLAG_UPDATE_CURRENT replace the intent message, how can I redirect with corresponding message instead last message?
I have tried the following:
Intent intent = new Intent(this, TestActivity2.class);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK
| Intent.FLAG_ACTIVITY_CLEAR_TASK);
intent.putExtra("uid", uid);
PendingIntent resultPendingIntent = PendingIntent.getActivity(this, 0,
intent, PendingIntent.FLAG_UPDATE_CURRENT);
NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(
getApplicationContext());
Notification notification = mBuilder
.setSmallIcon(R.drawable.ic_launcher)
.setTicker(textMsg)
.setWhen(0)
.setAutoCancel(true)
.setContentTitle(textMsg)
.setStyle(
new NotificationCompat.BigTextStyle().bigText(textMsg))
.setContentIntent(resultPendingIntent).setContentText(textMsg)
.build();
NotificationManager notificationManager = (NotificationManager) getApplicationContext()
.getSystemService(Context.NOTIFICATION_SERVICE);
notificationManager.notify((int) System.currentTimeMillis(),
notification);

Use FLAG_ONE_SHOT instead and change the second parameter in getActivity which is set to 0. This answer will make it clear.

You should Change the pendingIntend's content for Different Messages.
Here is the snippet from PendingIntent document.
"A PendingIntent itself is simply a reference to a token maintained by the system describing the original data used to retrieve it. This means that, even if its owning application's process is killed, the PendingIntent itself will remain usable from other processes that have been given it. If the creating application later re-retrieves the same kind of PendingIntent (same operation, same Intent action, data, categories, and components, and same flags), it will receive a PendingIntent representing the same token if that is still valid, and can thus call cancel() to remove it."
In simple words...try passing different id(Something like currentEpochTime) for each pendingIntent.

Android Notification Click - don't backstack activity

I have a service in background which gives a notification.
I want the next thing:
If I click the notification and the app is not opened open the XActivity.
If the app is opened and the XActivity is created, go there and don't recreate the activity (because if this happen, on back key i will see the same activity again).
My notification code
NotificationCompat.Builder mBuilder =
new NotificationCompat.Builder(context)
.setSmallIcon(R.drawable.ic_launcher)
.setContentTitle("New posts!")
.setContentText("New funny posts had just arrived! Click here to see them!");
mBuilder.setAutoCancel(true);
Intent resultIntent = new Intent(context, XActivity.class);
PendingIntent resultPendingIntent =
PendingIntent.getActivity(
context,
0,
resultIntent,
PendingIntent.FLAG_UPDATE_CURRENT
);
mBuilder.setContentIntent(resultPendingIntent);
int mNotificationId = 001;
// Gets an instance of the NotificationManager service
NotificationManager mNotifyMgr =
(NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
// Builds the notification and issues it.
mNotifyMgr.notify(mNotificationId, mBuilder.build());
I start the service from the XActivity. (just a activity name example)
Thank you.

i had the same problem, what i did was , i added this line android:launchMode="singleTop" to my activity in manifest, and it worked.

Problem for duplicate activity in your case is because of standard structure design of Activity in Android. For creating only single instance of activity (no duplication) you need to give launchMode property to your activity in your manifest file. You can use singleTop property for this behaviour. You can read more in provided link.
In Addition, for new data access you can use onNewIntent() for new intent coming to this activity and set intent using setIntent() and continue working with new data.
Hope this helps.
Thanks.