[Android] Thread Constructs: Comparisons

In this series of posts we have seen the following thread constructs:
1. Basic threads and communication between them [see article]
2. Understanding the Main thread or the UI thread [see article]
3. IntentService [see article]
4. AsyncTask [see article]

NOTE:  These are Android specific constructs. Android also includes the java.util.concurrent package which can be leveraged for concurrent tasks. That is out of the scope of my discussion.]

So, when to use what ? What is the exact purpose of each ? What are the drawbacks of using one over the other ? Which one is easier to program ?

I came across all these questions when trying to understand each of these. In due course of time, I have come up with some guidelines which I discuss in this post.

NOTE: Any of the recommendations made in this post and the table below are not comprehensive and final. There may be better and alternate ways of doing things. I would love to know any different views and get an insight into a different thought process. The views expressed in this post are my personal views.

Service
Before proceeding further, let me touch on the Service class. Traditionally speaking, the notion of service reminds us of task running in the background while we can work and interact with the UI. This causes confusion for newbies. Because in Android, even if the Service runs in the background, it runs on the Main Thread of the application. So, if at the same time if you have an activity displayed, the running service will take the main thread and the activity will seem slow. It is important to note that a Service is just a way of telling Android that something needs to run without a user interface in the background while the user may not interacting with your application. So, if you expect the user to be interacting with the application while the service is running and you have a long task to perform in a service, you need to create a worker thread in the Service to carry out the task.
So, even if Service is not a threading construct, its a way of executing a task at hand. Hence I have included it in the comparison.

The table below tries to summarize various aspects of four task execution mechanisms : Service, Thread, IntentService, AsyncTask.

Most of the points in the table are self-explanatory. However, some points that need an explanation are numbered and explained  below the table. Also, this table is just to summarize about the concepts discussed in the previous posts. So, if anything is still unclear, I recommend to go through each individual posts in this series.

	Service	Thread	IntentService	AsyncTask
When to use ?	Task with no UI, but shouldn't be too long. Use threads within service for long tasks.	- Long task in general. - For tasks in parallel use Multiple threads (traditional mechanisms)	- Long task usually with no communication to main thread. (Update)- If communication is required, can use main thread handler or broadcast intents[3] - When callbacks are needed (Intent triggered tasks).	- Long task having to communicate with main thread. - For tasks in parallel use multiple instances OR Executor [1]
Trigger	Call to method onStartService()	Thread start() method	Intent	Call to method execute()
Triggered From (thread)	Any thread	Any Thread	Main Thread (Intent is received on main thread and then worker thread is spawed)	Main Thread
Runs On (thread)	Main Thread	Its own thread	Separate worker thread	Worker thread. However, Main thread methods may be invoked in between to publish progress.
Limitations / Drawbacks	May block main thread	- Manual thread management - Code may become difficult to read	- Cannot run tasks in parallel. - Multiple intents are queued on the same worker thread.	- one instance can only be executed once (hence cannot run in a loop) [2] - Must be created and executed from the Main thread

[1] API Level 11 (Android 3.0) introduces the executeOnExecutor() method, that runs multiple tasks on a thread pool managed by AsyncTask. Below API Level 11, we need to create multiple instances of AsyncTask and call execute() on them in order to start parallel execution of multiple tasks.

[2] Once you create an object of an AsyncTask and call execute, you cannot call execute on that object again. Hence, trying to run an AsyncTask inside a loop will require you to each time create a new object in the loop before calling the execute on it.
To be very precise, you cannot do something like :
        TestAsyncTask myATask = new TestAsyncTask();
        for (int i = 0; i < count; i++) {
            myATask.execute("one", "two", "three", "four");
        }

But you can do :

        for (int i = 0; i < count; i++) {
            TestAsyncTask myATask = new TestAsyncTask();
            myATask.execute("one", "two", "three", "four");
        }

[3] Thanks to comment posted by () : "there's nothing stopping an IntentService from communicating back to activities. A broadcast Intent is good for that, particularly with setPackage() to keep it constrained to your app (available on Android 2.1, IIRC). An ordered broadcast can be particularly useful, as you can rig it up such that if none of your activities are on-screen, that some manifest-registered BroadcastReceiver picks up the broadcast and raises a Notification."

Conclusion:
With a pretty understanding of the AsyncTask and the IntentService classes provided by Android, it would be a good idea to leverage these for most of the tasks as against trying to manage threads manually or implement your class as a Service unless there is really a need to do so.

Tricky Things:
There are some tricky scenarios that need some special handling. After all, we are dealing with a mobile environment and things are pretty unpredictable. For instance, the user opens your activity in which you start an AsyncTask, but before completion of the task, the user decides to exit the application (say by pressing the back button). Or your activity may be terminated (read kicked off) due to say an incoming call, when you are running a long task in the background. In such cases, where your application is not shutdown, but any foreground tasks have been closed or changed, all the background tasks need to know of this and try to exit gracefully.