Service on android
A Service is an application component representing either an application’s desire to perform a longer-running operation while not interacting with the user or to supply functionality for other applications to use. Each service class must have a corresponding declaration in its package’s AndroidManifest.xml. Services can be started with Context.startService() and Context.bindService().
Note that services, like other application objects, run in the main thread of their hosting process. This means that, if your service is going to do any CPU intensive (such as MP3 playback) or blocking (such as networking) operations, it should spawn its own thread in which to do that work. More information on this can be found in Processes and Threads. The IntentService class is available as a standard implementation of Service that has its own thread where it schedules its work to be done.
There are two reasons that a service can be run by the system. If someone calls Context.startService() then the system will retrieve the service (creating it and calling its onCreate() method if needed) and then call its onStartCommand(Intent, int, int) method with the arguments supplied by the client. The service will at this point continue running until Context.stopService() or stopSelf() is called. Note that multiple calls to Context.startService() do not nest (though they do result in multiple corresponding calls to onStartCommand()), so no matter how many times it is started a service will be stopped once Context.stopService() or stopSelf() is called; however, services can use their stopSelf(int) method to ensure the service is not stopped until started intents have been processed.
Example in GitHub