Develop Reference

When to use and when not to use a Service in Android

I have been developing for Android for little less then 2 years, and I am still puzzled by this seemingly simple question.
When should one implement a service?
From my experience there are some rare cases but I am questioning this because on every phone there are quite a lot of them running and I doubt it's just a poor application design.
This is essentially core of my question but following are some of my experiences and thoughts about the subject which can explain my question in more detail.
In all apps that I have developed only one really required a service. It was a background sound recorder and I was using it as Foreground service with notification since I wanted buttons to be able to control it (like music players do for example).
Except this I never really saw a requirement for the constantly running service because:
A) Intent listeners (Manifest registered BroadcastReceivers) are quite a useful feature and using them as you know is usually enough for many use-cases (for example showing notifications).
B) If scheduled execution is a must one can subscribe to alarm events.
C) I know that service in Android is quite different then for example in Windows since in Android services are just a "package" to organize your code in and have a the system manage the lifetime of the object. Services use the Main Thread but it's customary to spawn new threads in them.
D) In the development documentation services are suggested for network communication and background calculations but I don't get why you should not just use AsyncTasks for that. I am a big fan of these and use them extensively for lot of things from downloading data from the internet to doing FFT calculations under time critical conditions.
E) I get the usefulness of Foreground services but why are people using background services so much (excluding the system apps).
Those are my thoughts about the SERVICE and I hope someone with more experience will be able to explain these PROS and CONS (along with others that I probably missed).

When should one implement a service?
When you have work -- delivering value to the user -- that:
Needs some time to complete, perhaps longer than you have time for in the component wishing the work to be done, or
Is delivering that value under user control (e.g., music player, controlled by play/pause buttons in a UI), or
In rare cases, needs to be running continuously, as it delivers value continuously
there are quite a lot of them running and I doubt it's just a poor application design
Some are likely to be poor implementations, either due to technical misunderstandings, or other concerns (e.g., making marketing happy) trumping making users happy.
It was a background sound recorder and I was using it as Foreground service with notification since I wanted buttons to be able to control it (like music players do for example)
That is a reasonable use for a service, IMHO.
Intent listeners are quite a useful feature and using them as you know is usually enough for many use-cases (for example showing notifications)
I assume that by "Intent listeners" you mean manifest-registered BroadcastReceivers. In that case, if the work to be done by the BroadcastReceiver will take more than a millisecond, that work should be delegated to an IntentService for completion. onReceive() is called on the main application thread, and it is not safe for a manifest-registered BroadcastReceiver to fork a bare thread, as the process could go away shortly after onReceive() returns. However, in these cases, the service is usually short-lived (e.g., do some network I/O and disk I/O, then go away).
In the development documentation services are suggested for network communication and background calculations but I don't get why you should not just use AsyncTasks for that
An AsyncTask is a fine solution for background work that is:
Requested by the UI (activity or fragment), and
Will take less than a second or so, and
Is non-critical
For example, if you are downloading avatars to show in a ListView, AsyncTask is probably a fine choice, whether you use them directly or use some image-fetching library that uses them internally.
Conversely, if the user buys an MP3 through your app, and you need to download that MP3 file, an AsyncTask is not a good solution. That could easily take over a second. While the download is going on, the user could switch away from the app (e.g., press HOME). At that point, your process is eligible to be terminated... perhaps before your download is complete. Using an IntentService to manage the download is a signal to the OS that you are really doing work here, adding value to the user, and so the process will be left alone for a little while.
Note that if the background work might take 15+ seconds, WakefulBroadcastReceiver or my WakefulIntentService is probably a good idea, so the device does not fall asleep while you are trying to wrap up this bit of work.

I can name some of the Service uses from my experience:
to implement
location listener,
sound module, generating various voices
in app content updates,
API, provide services to other apps
in app billing
Communication with webservices (if requests frequency is high)
actually (excluding 5.) they all are working for the whole app duration, they are using some of the other android services, also they manage their state. I suppose one of the important thing here is state management during application life cycle changes.
I prefer to look at AsyncTasks in a same way as Executors (ExecutorService), they should be executed sequentially and for small tasks.

In the android website, you can find a table when to use Service, Thread, or WorkManager (the new API for scheduling jobs, currently in alpha as of this comment posted). https://developer.android.com/guide/background/#table-choose
The website also state that you need to use started service only as last resort. The Android platform may not support started services in the future. Refer to this link https://developer.android.com/topic/performance/scheduling#services
You should avoid using started services that run perpetually or perform periodic work, since they continue to use device resources even when they aren't performing useful tasks. Instead, you should use other solutions that this page describes, and that provide native lifecycle management. Use started services only as a last resort. The Android platform may not support started services in the future.

If you consider UI and bound services, u would think that both can exist and not be doing anything for certian periods. In such scenarios, your UI can be recreated a lot of times however service does not. And this is where service is important. Lets say you are processing images and then rotate device you want processing to continue while UI is being recreated. You recording a voice and then rotate device. These are one of the places where I find service very important. (Having lot of heavy data processing, interaction with web, that could be few seconds)

what is the best/preferred approach to implement multi-threading in android app

I'm a beginner in android development and I'm trying to implement an android udp client, which connects to a java server and sends/receives some packets from it.In this process it collects some data (like round-trip delay etc), which is used to measure the QoS of that particular network. I have tried implementing the connection and sending/receiving data using Java Threads, but the application crashes, and hangs if i try to use more than 2 threads. So I'm looking for alternatives. While going through this site as well as some other links I found that in android multiple threads can be implemented using AsyncTask, Handler etc. Also I found that the Service class also helps to run a background service in an app. Please suggest which approach among these would be the best to achieve my purpose.
Thanks in advance.

You can use AasyncTask to do this and as you mentioned service may be useful too, where u can let your application do whatever it wants in background , if user needs to use application by its interface then AsyncTask must be used to avoid Crashing

There is not one right answer that can be applied as a broad stroke to how to do Android multi-threading. There are a few different ways to approach it based on what your specific needs are.
Any long running, blocking call, in Android will result in the application crashing.
The most common solution is to use an AsyncTask though. For example, when I want to make a call out to a web API endpoint for some XML data within an Activity I would in this case use an AsyncTask and kick off the calls from within doInBackground.
This is not an appropriate solution though if the wait time is longer, or possibly an unknown wait time. Or in a situation where there will always be waiting such as a message queuing service. In this type of situation it may be best to write a separate app based on extending the Service class. Then you can send/receive notifications to/from the service from your primary application in a similar manner to how you would communicate with a web service.

How to manage Android- to external device communication via web API?

I'm building a program which interfaces with a device which runs its own internal web server. I communicate with the device via a web API.
Basically what happens is that a GUI is presented to the user, where the user can make certain modifications to the device. These changes are communicated to the device, and results are returned through XML. The device needs to converse with the program in the background more or less continually (say every 15s or so) to update certain values to the user.
My structure that I'm envisioning is something like this:
UI - Main - Networking - XML Parser.
I'm looking for advice on how to manage these. I understand the UI thread should be separate to provide a smooth experience to users. I also understand that the networking should be at least an asynchronous task. I'm not so sure about how to handle their interaction, and make sure things are happening smoothly and effectively.
My idea is that Main will handle passing data around, telling the networker to send specific messages or changes, passing the returned XML to the parser, and then passing the parsed values to UI for handling.
I'm curious though for advice beyond that.

Have a look at creating a service that is created with your Activity. Without knowing the details of your plan, a Service looks like the optimal solution to perform all the heavy work.
UPDATE:
You could have the calls to web API run in a Service and, when needed, update the UI through an interface. You would have to instruct the Service to run on its own thread, so thread safety is an issue, but less trouble in the long run than using an AsyncTask.

Have a thought about using Google C2DM.
In your case,
Pros -> Less battery use, coordinated network traffic, Don't have to run a continues service and doesn't have the potential of being killed when the device runs out of resources.
Cons -> You have to post the results manually back to your internal server, and server should know which request the device is replying to. Communication is disconnected and may not be real-time. Requires a google account on the device and Google market.

Have I found a secret solution for passing thread data to a new activity? If it's bad what should I do instead? [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
I need a good way to get data from a thread to another activity.
I have one gaol: how to develop an Android app that plots bluetooth data forever in real-time.
I inherited a background thread which updates the screen with new data it receives over a bluetooth connection. When I launch my Plot activity I can see a bluetooth background thread continue to write bluetooth data to Logcat forever and ever. So I know for a fact the bluetooth background thread is still running when I launch my Plot activity.
I have succeeded as follows: since this bluetooth background thread seems to run fovever, I decided to use its update() method to call my static Plot.plotData() method to plot the bluetooth data. And this works. It will will run endlessly with out a problem - receiving bluetooth data and plotting it via periodic calls from the bluetooth background method update() to my static Plot.plotData() method.
The latest feedback I have received "It sounds like you're looking for an in-memory way to share data, and that's simply not the way the Android activity model works." In-memory???But this is rediculous. An Android phone has a limited amount of RAM for running Activities (one at a time), threads, handlers, Services, AnycTasks, etc. And an SD card for persisting data.
One critical person basically said: "In order to share data from a bluetooth background thread to my Plot activity (Plot.plotData()) that I must use the SD card." ??? This just sounds nuts, because I have it working using my static method Plot.plotData().
Frankly I don't see anything wrong with my solution primarily because those who criticize it do not follow up with a definitive alternative.
If you find my solution deficient please speak up and provide a definitive solution." Unless I use a new thread, handler, Service, AsyncTask, etc, everyone assumes my solution is not a good one. Why? Supposing the criticism is valid, PRECIOUSLY What should I do instead of what I am currently doing?

One more time: if you have background task running you should create Service. That's how Android expects applications to behave. Otherwise your thread may be terminated at any time (assuming none of the Activities are active).
From documentation:
A service is a component that runs in the background to perform long-running operations or to perform work for remote processes
This precisely describes your case.
Also carefully read Processes and Threads, specifically Process Lifecycle section.
Basically your application falls into Background Process or Empty Process group most of the time. That's why you need Service started in your application.

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