Difference between Service, Threads & AsyncTask

Difference between Service, Threads & AsyncTask

Hi All.


After long time, again I have to something for write and share.

You can find many documents, post, blogs and discussion on this topic but I think if we have all the stuff at one place it is better to understand.

So I will try to my best to put the all things which covers the all thing for this topic.

So far, we have a way for applications to implicitly do work in the background, as long as the process doesn't get killed by Android as part of its regular memory management. This is fine for things like loading web pages in the background, but what about features with harder requirements? Background music playback, data synchronization, location tracking, alarm clocks, etc.

Service:

• Service is a Android component without UI.

• A Service allows an application to implement longer-running background operations. The fundamental purpose is for an application to "continue running even while in the background, until it done."

• While services do provide a rich client-server model, its use is optional. Upon starting an application's services, Android simply instantiates the component in the application's process to provide its context. How it is used after that is up to the application: it can put all of the needed code inside of the service itself without interacting with other parts of the application, make calls on other singleton objects shared with other parts of the app, directly retrieve the Service instance from elsewhere if needed, or run it in another process and do a full-blown RPC protocol if that is desired.

• Because an unbounded number of services can ask to be running for an unknown amount of time. There may not be enough RAM to have all of the requesting services run, so as a result no strong guarantees are made about being able to keep them running.

• If there is too little RAM, processes hosting services will be immediately killed like background processes are. However, if appropriate, Android will remember that these services wish to remain running, and restart their process at a later time when more RAM is available. For example, if the user goes to a web page that requires large amounts of RAM, Android may kill background service processes like sync until the browser's memory needs go down.

• Services can further negotiate this behaviour by requesting they be considered "foreground." This places the service in a "please don't kill" state, but requires that it include a notification to the user about it actively running. This is useful for services such as background music playback or car navigation, which the user is actively aware of; when you're playing music and using the browser, you can always see the music-playing glyph in the status bar. Android won't try to kill these services, but as a trade-off, ensures the user knows about them and is able to explicitly stop them when desired.

• Service never runs in a dedicated /thread/. By default it runs in the main application thread, and may optionally run in a separate /process.

• If you need to have something to keep on running while the user goes from one of your activities to the other, then you may also need a service.

• Thread must be "embedded" in the service, not the opposite. The service acts as an entry point, in which you can create. several threads if you like.

Note: 

• 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.

• Services are a way to tell Android that certain code in your application is important to the user, and Android should make an effort to keep it running even when it doesn't have active activities.

• Your service will want to spawn its own thread or asynctask to do your heavy work or else you will get a forceclose.

	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 - When callbacks are needed (Intent triggered tasks).	- Small task having to communicate with main thread. - For tasks in parallel use multiple instances OR Executor
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)  - Must be created and executed from the Main thread