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Difference between Process,Activity,Threads and Tasks in Android

What is the difference between all the above?
I found various posts which were helpful but also quite confusing. According to my understanding in short this is what I came upto:
Threads are tasks that share same resources
Processes are tasks which have independent rersources. A process can
have multiple threads.
Tasks are the instructions being executed
Now this is where I get confused. How is an activity related to all
these three in android. Activity can have multiple tasks so it must be something like a process. But then what is the difference between activity and process. Moreover I read somewhere that tasks are stack of activities. It got me all confused. Ive also read that all activities run on UI thread which just make the distinction a little more confusing.

You should differentiate between Processes & Threads vs. Activities vs. Task. They aren't even really in the same category.
Let's start with the simplest one, Task's. Assuming you are not talking abouy any actual class, i.e. TimerTask, the basic concept of a Task is the following.
When a user starts your app for the first time, a new Tasks is created. You can see this by pressing the "OverviewButton", represented by a Square for the software buttons. (on Android 5.0 an higher)
A Task will not get disposed of, unless the User actually removes(swipes left/right) it from the Overview screen.
So a Task is really just a high-level abstraction for the user.
Like you alluded to, a Tasks has an Activity backstack, which is just a normal stack that is used to keep track of the "history" for the user. For example, your App is launched, your MainActivity will be at the bottom of the stack, the User enters some values and then goes on to a new Activity. Now this new Activity is above the previous one, and the user can press the "back button" at -hopefully- any time to get back to the previous activity.
Now for Processes &Thread's, a Processes under Android is very similar to a linux process, your app will usually only be working within one single process. A process gets assigned a certain part of the memory by the OS, if you're familiar with languages like C, attempting to acess memory that does not belong to your process wil cause a
"segmentation fault".
Like you said, a process may have any number of Threads, assuming the OS can manage the required Overhead.
A process will at least have one Thread, under android this is called Main-Thread or UI-Thread. Threads, very basically, allow you to do some work in parallel. You will most likely need to make use of them, for example when performing network operations.
Now for Activities, they have no direct relationship to multithreading. The currently "active Activity" is the one that is run on the UI-Thread. So all of its callbacks will be run on the UI-Thread, unless specifically documented not to.
An Activity is an abstraction used by the android framework, it exists at a fundamentally different level than a Processes & Thread's. You can call a method defined in a Activity, from any Thread you want.

A really nice question, from my little experience with android development, I'd like to contribute. Let's start from..
Processes
Ever opened task manager on windows to see open apps? Those are processes. On android, when an app is launched, a new process is opened and allocation of memory etc is given.
The activity classes,imports and threads all make a process in the Android system. sometimes you see an error message when an app crashes "unfortunately com.android.bakerapp has stopped."
This means an error causes the whole process of threads, imports, activities to close. So basically processes are parts of an app or an app in general that's running.
Activity
An activity is the heart and soul of all android apps, all Threads, preferences, views and layouts are opened by android activity class. It is the container object that holds views, passes information around and runs threads too. Activities communicate with each other through intents, objects in the class extends and methods.
Activity is the piece of code that creates and communicates UI and everything a user sees and uses. It is used to create threads. Which is discussed below.
Threads
This one is easy, a thread is basically a process to get something done, it lives and dies after its work. Imagine you have an activity with a view of picture on the screen and you want to automatically set your apps theme color to the most common color on the picture using a library.
The best method to do this without the user knowing and also confusing the main thread responsible for loading the picture into a view from a website is to open a thread using an Asynchronous task (something that runs in background) is an example of a thread.
So a thread is basically a life cycle of a task to be done, it can be continuous (Main activity views and list views) or short (Find a dominant color in a picture) or fun and multitasking (downloading a picture from a group chat while at thesame time chatting with your girlfriend on WhatsApp).
Threads are the most essential part of all activities and processes and can send,receive and process data.
Activities cannot work without threads because the setContentview and UI itself is just another thread, you can have multiple threads in one activity.
Happy coding!
https://developer.android.com/guide/components/processes-and-threads.html

I know this is old, but you can also say that a thread is the smallest unit of execution of code. Threads are scheduled to run on the CPU. A process can have one or more threads.

Starting singleTask activity every few seconds gives ANR in Android

In the app I have an activity which has launch mode as singleTask. There are number of use cases which pass through this activity and hence it's called number of times. On stress testing the app by running monkeyrunner script and calling this activity every few seconds causes ANR's.
I guess, the way it's designed where most of the use cases pass through this activity is not correct but I am not in a position to change this design.
Is there anyway ANR's can be suppressed? I mean, adding UI operations to event queue so that it doesn't block main UI thread and doesn't give ANR.

It is unclear from the question what your activity is (or should be) doing. Probably you need a service instead.
It is common to perform time-consuming operations in background threads and deliver the results to the UI thread.
You may use the classes Handler/Looper (it it easir to send Runnables rather than messages), or use an AsyncTask. The AsyncTask is nevertheless tricky, this is discussed here: Is AsyncTask really conceptually flawed or am I just missing something? . AFAIK Google tried to fix the typical bugs and made the new behavior incompatible with the old one (namely, I have seen some misbehavior on the newer Androids that may be explained by the assumption that since some version threads doing asynctask jobs get killed after the activity that started them goes out of the screen).
I can guess that singleTask is your way to fight the fact that an activity dies when the screen turns, and a new one comes. I suggest you use singletons (they survive screen rotation but do not survive a process restart, one more thing that sometimes happens in Android). (The user switches to an app like Camera, takes a big photo, returns back -- and the activity is restarted because Camera needed memory. Have seen this in practice, but that time I did not try to find out if the whole process was restarted.)
Anyway, please add logging telling you when your activity in entered and left, including onNewIntent() and other lifecycle functions (to be on the safe side, I recommend to print the thread names as well). Then you will probably see what is going on.

Multiple async tasks not executing properly

I am trying to start 6 async tasks at a time in onCreate() of the activity. But I noticed following:
a) If I stay on same activity all the async tasks' doInBackground() execute properly.
b) If I switch to some other activity, only 4 or 5 async tasks' doInBackground() executes. Last async task's doInBackground() never executes.
Can someone tell what I might be doing wrong. I am staring different asynctasks in a for loop. If I do this in onStart(), then all the async tasks are executed again if I switch to this activity. Please help.
Here is the sample code:
For(int i=0;i<7;i++){
webServiceTask= WebServiceTask.getInstance();
webServiceTask.execute("");
}

Maybe you should consider some of the following points:
Is the WebServiceTask retrieving any information that is worth persisting (i.e. not very volatile, with a high risk of the user requesting the same data over and over again): in that case you can offload your work to an (Intent)Service and communicate the result back to your app via a ResultReceiver, or a ContentProvider (just to name a few).
AsyncTasks are not guaranteed to run to completion if no Activity or Service is around to keep your app's process alive.
If it is ok that the WebServiceTasks run after eachother, then you can probably also change your code to use only one AsyncTask that sequentially performs those tasks. You can even consider implementing AsyncTask's progress reporting mechanism.
If the operations performed by the AsyncTask(s) have no meaning once the Activity is closed, be sure to .cancel() them in your onStop() or onPause() lifecycle methods.
Keep bullet 3 of Ed Rowland's answer in mind, which I also posted in my earlier comment to your question.
Happy coding!

You need a Service of some kind to keep your process alive after the user switches away. Once your activity loses focus, Android is free to shut down your process altogether. Or your Activity. Either will cause problems, particularly if you are using the context of an Activity that has been shut down.
The Right Thing to Do (tm) is to implement a Service, and pass the operations off to the service for execution.
There are any of a bunch of reasons why only four tasks are running concurrently. Off the top of my head:
HttpConnection pools connections to servers, and throttles the
maximum number of connetions to any given server to some reasonable
value. 4 sounds about right.
your target server is throttling the number of simultaneous connections.
Your thread pool isn't as large as you think it is. Starting an API 16 (I think) the default threadpool size is one thread! (!!) Rationale: apparently Android OS developers got fed up with Android app developers doing threading wrong. Is it possible your tasks are executing serially? That would more or less fit the symptoms you describe.
But that's kind of a separate issue.

Android AsyncTask for long running operations

Quoting the documentation for AsyncTask found here, it says:
AsyncTasks should ideally be used for short operations (a few seconds at the most.) If you need to keep threads running for long periods of time, it is highly recommended you use the various APIs provided by the java.util.concurrent pacakge such as Executor, ThreadPoolExecutor and FutureTask.
Now my question arises: why? The doInBackground() function runs off the UI thread so what harm is there by having a long running operation here?

It is a very good question, it takes time as an Android Programmer to fully understand the issue. Indeed AsyncTask have two main issues that are related :
They are poorly tied to the activity life cycle
They create memory leaks very easily.
Inside the RoboSpice Motivations app (available on Google Play) we answer that question in detail. It will give an in-depth view of AsyncTasks, Loaders, their features and drawbacks and also introduce you to an alternative solution for network requests : RoboSpice.
Network requests are a common requirement in Android and are by nature long running operations
.
Here is an excerpt from the app :
The AsyncTask and Activity life cycle
AsyncTasks don't follow Activity instances' life cycle. If you start an AsyncTask inside an Activity and you rotate the device, the Activity will be destroyed and a new instance will be created. But the AsyncTask will not die. It will go on living until it completes.
And when it completes, the AsyncTask won't update the UI of the new Activity. Indeed it updates the former instance of the activity that
is not displayed anymore. This can lead to an Exception of the type java.lang.IllegalArgumentException: View not attached to window manager if you
use, for instance, findViewById to retrieve a view inside the Activity.
Memory leak issue
It is very convenient to create AsyncTasks as inner classes of your Activities. As the AsyncTask will need to manipulate the views
of the Activity when the task is complete or in progress, using an inner class of the Activity seems convenient : inner classes can
access directly any field of the outer class.
Nevertheless, it means the inner class will hold an invisible reference on its outer class instance : the Activity.
On the long run, this produces a memory leak : if the AsyncTask lasts for long, it keeps the activity "alive"
whereas Android would like to get rid of it as it can no longer be displayed. The activity can't be garbage collected and that's a central
mechanism for Android to preserve resources on the device.
It is really a very very bad idea to use AsyncTasks for long running operations. Nevertheless, they are fine for short living ones such as updating a View after 1 or 2 seconds.
I encourage you to download the RoboSpice Motivations app, it really explains this in-depth and provides samples and demonstrations of the different ways to do some background operations.

why ?
Because AsyncTask, by default, uses a thread pool that you did not create. Never tie up resources from a pool that you did not create, as you do not know what that pool's requirements are. And never tie up resources from a pool that you did not create if the documentation for that pool tells you not to, as is the case here.
In particular, starting with Android 3.2, the thread pool used by AsyncTask by default (for apps with android:targetSdkVersion set to 13 or higher) has only one thread in it -- if you tie up this thread indefinitely, none of your other tasks will run.

Aysnc task are specialized threads that are still meant to be used with your apps GUI but whilst keeping resource-heavy tasks of the UI thread. So when stuff like updating lists, changing your views etc require you to do some fetch operations or update operations, you should use async tasks so that you can keep these operations off the UI thread but note that these operations are still connected to the UI somehow.
For longer-running tasks, which don't require UI updation, you can use services instead because they can live even without a UI.
So for short tasks, use async tasks because they can get killed by the OS after your spawning activity dies (usually will not die mid-operation but will complete its task). And for long and repetitive tasks, use services instead.
for more info, See threads:
AsyncTask for longer than a few seconds?
and
AsyncTask won't stop even when the activity has destroyed

The problem with AsyncTask is that if it is defined as non-static inner class of the activity, it will have a reference to activity. In the scenario where activity the container of async task finishes, but the background work in AsyncTask continues, the activity object will not be garbage collected as there is a reference to it, this causes the memory leak.
The solution to fix this is define async task as static inner class of activity and use weak reference to context.
But still, it is a good idea to use it for simple and quick background tasks. To develop app with clean code, it is better to use RxJava to run complex background tasks and updating UI with results from it.

android design considerations: AsyncTask vs Service (IntentService?)

I'm designing an android app which will need to do the following steps:
user pushes a button or otherwise indicates to "sync data".
sync process will use REST web services to move data to and from the server.
the data will be stored locally in a sqlite database.
the sync process should provide status updates/messages to the UI
the user should not be allowed to wander off to other parts of the application and do more work during the sync process.
The first time the sync process runs, it may take 10-20 minutes.
After the initial sync, less data will be transferred and stored and
I expect the process to take 1-2 minutes or less.
I've been doing a lot of reading about android's AsyncTask and various examples of using a Service ... But I don't fully understand the design considerations and trade-offs of choosing one design over the other. I currently have my demo project stubbed out using an AsyncTask. After watching (most of) Developing Android REST client applications: http://code.google.com/events/io/2010/sessions/developing-RESTful-android-apps.html# I'm left confused the design patterns described here feel overly
complex, perhaps because I just "don't get it" yet.
I come from a java, spring, web and desktop application background. Thinking and designing in terms of a handheld device is quite new to me. (What happens when the screen layout is changed? What happens when the phone rings while I'm running a sync?) Taking 2 steps back, if the initial sync IS going to be such a long running process, is there a better way for me to think about the problem->solution, the user experience, the user expectations of an application running on a phone?
Would love to hear from some more experienced android developers out there who have already wrestled with these questions.

In my opinion this is the most tricky/hard part of a mainstream/average Android development. For instance on BlackBerry this is IN TIMES easier.
Definitely you need to use a Service.
AsyncTask does not suit, because it is tightly "bound" to your Activity via a Context handle (otherwise you would not be able to update UI of the Activity from your AsyncTask). However an Activity can be killed by OS once the Activity went in background. An example reason of going to background can be an incoming call - user switches to Phone application so your Activity becomes invisible. In this case (depending on the current RAM state) OS may decide to kill one of the background (invisible to the user) activities.
Some devs workaround this by arranging a static stuff for having a long-running actions inside of. Some recommend to use Application instance. This is because static stuff and Application exist while the whole app process exists. However those are incorrect workarounds. Processes in Android are also may be killed when OS decides it is time to. Android OS have its own considerations about what it can kill and in what order. All processes are devided to 5 levels of "killability". Here is the doc where those levels are specified. It is interesting to read there:
Because a process running a service is
ranked higher than one with background
activities, an activity that initiates
a long-running operation might do well
to start a service for that operation,
rather than simply spawn a thread —
particularly if the operation will
likely outlast the activity. Examples
of this are playing music in the
background and uploading a picture
taken by the camera to a web site.
Using a service guarantees that the
operation will have at least "service
process" priority, regardless of what
happens to the activity.
Your Activity where users initiate a long-running action should show a ProgressDialog to make sure user does not do anything else while the action is running. The guide is here.
Also, you'd most likely want to use the NotificationManager for notifying the user about your long-running action completion (or failure) if your Activity is currently invisible. Here is the NotificationManager info to start from.

There are multiple considerations that you must weigh in order to best decide how to approach your situation. It sounds like you need a good comparison between the two approaches... So here is a list of similarities, and differences and additional considerations that must be taken into account when working on a handheld device.
A Service is a part of your Application that has no UI. It may be called by a UI(Activity) to be started, or may be started by any other component of your Application. When developing, you have the freedom to place it on a different thread, or even run it in a different Task or Process. This allows you to ultimately separate it from your UI. Additionally, you may start the Service to run independently (startService) or bind your activity to it (bindService) depending upon your needs. By using custom Handlers, you can set callbacks to update the UI with your progress. A Service does not necessarily end if a User changes Activities, but may be ended at ANY time by the OS.
A AsyncTask is always instantiated from the UI thread. It only allows specific callbacks, but simplifies the process of multi-threading for the purposes of relatively short transactions (as compared to dedicated separate threaded services) that are inherently tied to actions performed by an Activity. Whenever a User changes Activities, the AsyncTask is put on "pause" and may even die because there is no UI thread for your Activity any longer.
The thing that I would be most concerned about is if the app is going to take 10-20 minutes the first time, I would ASSUME that the User will either change tasks temporarily or set the phone down until it completes (which can cause all of the same complications if the phone sleeps). Given this consideration, a threaded service bound to your activity may be your best choice. To protect your UI, I would make a Progress Dialog for your Activity that receives your progress callbacks. This limits user input in YOUR app and allows your service to continue the way that it needs to. Then override the Activity onResume to check the status of your Service and if it is running. Then you can reset the Dialog immediately.
Given that this is my preferred method, I would also take into account that the OS may kill the App at any time anyway. So make sure to have some way to detect an incomplete or partial sync. Then you may resume automatically when your Activity or Service restarts.

With AsyncTask if the user goes to another Activity you can't transfer that object to the other Activity so it dies. There are tricks you can play when say the user rotates the screen or something like that, but that doesn't extend to general purpose destruction. AsyncTask can randomly die.
Google Sync is run as a Service in the background because syncing can take a while to complete. You might need to follow their path and create your own sync service that you can communicate with. Here is some thoughts how to accomplish that:
http://mylifewithandroid.blogspot.com/2008/01/about-binders.html
You can definitely communicate between Service and Activity, but it's tricky to do it right.

The choice is mainly dependent on the app design. Since both AsyncTask and IntentService stands their ground, what you may want from the app(user experience) is more important and then choose either or both. Some scenarios are mentioned below (mostly what I experienced while developing apps)
Assume apps that have feeds pages - where more than one api calls are made to make the page presentable ( /getFriends, /getDates, /getPictures etc.) you can warp all such api calls to a single AsyncTask with executor which is multithreaded and the sequence of execution doesn't matter. In contrast to IntentService which runs all calls in sequence in a single worker thread. For a high end device with multi-core the call from AsyncTask is more effective. And if you start the AsyncTask on UI thread then updating IU is a piece of cakes(read less boiler plate code). And even if an user leaves the page, with intelligent use of not holding on to the context the app doesn't crash.
Assuming you are trying to write an app which doesn't need the user to be on view/activity/fragment and the total execution time to show something is not mission critical (assume sync service or user notification/alarm) then IntentService is a better choice. (no hassle to start Asynctask on UI thread so that you don't need to write a Handler to force changes on UI etc. etc. and less boiler plate code)
From my experience - write small app for both and compare the pros and cons to get a better idea. (p.s I'd suggest take a look at the iosched app from google to get a better idea - they use both Asynctask and IntentService)

I tend to prefer the IntentService + BroadcastReceiver combo because they give you a really strong degree of control
You definitely have to make sure the UI is running if you are updating something on the screen. ASyncTask crashes were at once reported to be one of the top causes of Android crashes. This can be avoided by keeping some sort of "activityIsAlive" variable and skipping or delaying a UI update if the activity is dead.
The IntentService + BroadcastReceiver combo is a little more resistant to the crash because most tutorials tell you to shut off the BroadcastReceiver onPause or onStop. If you do not do this, again you'll have to turn off the UI update. There's a runOnUiThread command somewhere that will help you do UI updates.
The IntentService + BroadcastReceiver combo is also more extensible. You can create functions and extend BroadcastReceiver to make a more elegant REST processing solution. However, it does require more plumbing vs an ASyncTask
If you do delay the UI update, you may be able to rig it on OnWindowFocusChangedListener. When that function receives true, it means that the UI is alive.
tldr; Make sure the Activity and/or Fragment is alive before updating the UI if you are running something in the background
2015 Edit: check out Loaders as well. A little harder to grasp because there's a lot going on behind the scenes