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Download file on AsyncTask or Service?

What would be the best approach for download on Android. You would be using AsyncTask or Service. I see several example being made of two ways, but what would be the best approach?

That's not the right question to ask, and the answer may be both. An AsyncTask is a separate thread of execution. A Service is just a piece of your app that runs in the background and can live past the end of an Activity.
You need to download any files on a non-UI thread, so either a Thread or AsyncTask is necessary whether you use a Service or not. So you'll always use one of those two. The question of whether or not to also use a service is a question of whether you need the file downloaded even if the user goes to another Activity (there's a few other reasons why you may want to use a Service, but this is the main one).

i suggest ,it depends upon the size of file.for small content file ,asynch is great but if is gonna be a longer task then you can go for service because service also comes with restart feature (start_sticky) in case user kill your app or your app got killed by the android os(LMK : low memory killer) in case of memory crises

What is the difference between thread and service in android , when downloading an image

What is the difference between thread and service in android,when downloading an image

There are lots of difference between Normal thread and a Service
Service: Due to android component it runs on UI thread but without any view shown to user. It runs in background with the same kind of property that an activity have, like you cannot run network operations (downloading image, calling web service) in service for that you have to use Thread which will run on worker thread other than UI thread.
Thread: Its an independent path of execution which can consist network operation, complex coding, huge amount of data transfer and accept. Thread is not related to android but in android it is used to perform different task. You can download an image on Thread but to show it on any UI part you have to update downloaded image on UI thread using runOnUIThread method
Please let me know if this explanation clears your doubt. If not let me know which part you did not understand and what exactly is your question.

With rare exceptions, you should never explicitly create a Thread. Threads are expensive and prone to programmer error. Use AsyncTask because it handles the complexity of thread safety and provides the optimization of thread pooling. Or better yet, if network activity is your reason for doing work outside the main thread, use one of the many network libraries that manages all of these concerns for you. Which approach is fastest is not something that can be answered generally, and should never even be a concern until you've tried the simple and clear solution and demonstrated that its performance is inadequate.
Regardless of how you make your network activity asynchronous, any network activity that is not started and completed (or cancelled) within the lifetime of a single Activity instance needs to be hosted in something else. If it only needs to survive across configuration changes, host it in a retained Fragment. If it needs to survive between different activities, host it in a Service. When choosing between these options, remember that your Activity may be destroyed any time it goes into the background or backstack.

Async task or threads

I have the following task:
I have to do a Wi-Fi scan and then send some RSSs to the a server to obtain a location from the server. After that I should display the location on a map.
I have one activity in my application. What is the best way to implement it? I haven't implemented a thread nor AsyncTask in android before. So I don't know a lot about it. I have read about threads and AsyncTask on android developer website. But I still can't understand everything.
I would appreciate it if anyone can tell me when its the best to use threads, and how AsynTask is different from threads and when to use AsyncTask instead of threads.
EDIT: the task that I have to implement should run only when the user click on actioBar item
.

If you can't understand the read documentation, I'd suggest starting with an AsyncTask, as it already implements a Thread when doInBackground() is fired and it already implements some mechanisms that you would probably have to implement by hand when using Threads (concurrency, Thread start/stop processes, sending information to UI thread, ...).
However, you don't specify what exactly want to do. If you plan to do a long-running process, instead of Threads or AsyncTasks, it's recommended using Service that would start a Thread as implementation, as contrary to popular beliefs, AsyncTasks are meant for short-lasting tasks.
In any case, if you're starting with this, I'd recommend using AsyncTask first, and when understood what it's doing and how, decide by yourself what's the best way to implement what you're planning to do: if running a Service with a Thread inside, a Thread itself or an AsyncTask.

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 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