I want to know what is the difference between a process and a service in an android app?
I tried to study about this topic a lot, but did not clear my basics yet...please help?
A process and a service are two different things:
What is a Service?
Most confusion about the Service class actually revolves around what it is not:
A Service is not a separate process. The Service object itself does not imply it is running in its own process; unless otherwise specified, it runs in the same process as the application it is part of.
A Service is not a thread. It is not a means itself to do work off of the main thread (to avoid Application Not Responding errors).
Thus a Service itself is actually very simple, providing two main features:
A facility for the application to tell the system about something it wants to be doing in the background (even when the user is not directly interacting with the application). This corresponds to calls to Context.startService(), which ask the system to schedule work for the service, to be run until the service or someone else explicitly stop it.
A facility for an application to expose some of its functionality to other applications. This corresponds to calls to Context.bindService(), which allows a long-standing connection to be made to the service in order to interact with it.
source: http://developer.android.com/reference/android/app/Service.html
What is a Process
When an application component starts and the application does not have any other components running, the Android system starts a new Linux process for the application with a single thread of execution. By default, all components of the same application run in the same process and thread (called the "main" thread). If an application component starts and there already exists a process for that application (because another component from the application exists), then the component is started within that process and uses the same thread of execution. However, you can arrange for different components in your application to run in separate processes, and you can create additional threads for any process.
source: http://developer.android.com/guide/components/processes-and-threads.html#Processes
Related
I was going through Bound Service in Android Developer website. I thought I understood the service enough but I just found another way of connecting service through Using a Messenger class especially for local service. There I got confused. Maybe I got the concept wrong.
Here is my understanding of Android Service. You create a service when
You want to do separate jobs in the background.
You want to make it a separate process.
You want to make it run in a lifecycle that's independent of the component that started it.
Confusion is the first item in the list, the definition of the background. Isn't the background a thread or process? I never thought that it can run on the main thread.
Here is the caution of service in the dev pages about.
Caution: A service runs in the main thread of its hosting process—the service does not create its own thread and does not run in a separate process (unless you specify otherwise). This means that, if your service is going to do any CPU intensive work or blocking operations (such as MP3 playback or networking), you should create a new thread within the service to do that work. By using a separate thread, you will reduce the risk of Application Not Responding (ANR) errors and the application's main thread can remain dedicated to user interaction with your activities.
Questions
Why does one choose to use service if the service function will anyway run on the main thread?
Do we have to write a service only to block ANR even if the time-consuming job is done in the main thread? Assume the service is only for my application.
Are there any practical cases or reasons to use a service as private and running in the same thread?
Application main thread is not always the UI thread. For example, when Activity is stopped, the onStop() is invoked, hence the UI thread is taken away from that Activity and moved to another Activity within the same or a different application. However it doesn't mean the application is no longer active, it can continue working in the background until it is closed either by OS or by user. Then who keeps it running in the background? It is the main thread and not the UI thread.
What are services
In Android, a Service is an application component that can perform
long-running operations in the background on the UI thread. By
background, it means that it doesn’t have a user interface. A Service
runs on the main thread of the calling Component’s process by default
(and hence can degrade responsiveness and cause ANRs), hence you
should create a new Thread to perform long running operations. A
Service can also be made to run in a completely different process.
Unlike Activity components, Services do not have any graphical
interfaces. Also Broadcast Receivers are for receiving broadcast
messages (broadcast, multicast, unicast) and perform short tasks
whereas Services are meant to do lengthy processing like streaming
music, network transactions, file I/O, interact with databases, etc.
When a Service is started by an application component like an Activity
it runs in the background and keeps running even if the user switches
to another application or the starting component is itself destroyed
Why use service
Services are given higher priority than other Background processes and
hence it’s less likely that Android will terminate it. Although it can
be configured to restart once there is ample resources available
again. You should go through the different processes and their
priority/important level in the documentation on processes and
threads. Assigning them the same priority as foreground activities is
definitely possible in which case it’ll need to have a visible
notification active (generally used for Services playing music).
Use IntentService if you don't want to fiddle with managing threads on your own. Otherwise, use AsyncTasks.
Please read this excellent article to understand more in detail and also read this answer.
Service basically runs in UI thread or main thread.But,if we are going to perform long running operations in service,we need to create a background thread and perform that task.
But why we have to use service?
Now let's think of Music Application.We need songs to be played continuously even if we leave music app.If we use activities,we can't achieve above requirement.So,service helps in these kind of scenarios.Even if the app is not in foreground, service keeps on running and we are able to listen to songs.This is why we use service even though it runs on main thread.
In short, Services runs on the background of the UI thread.
You can perform tasks like client-server authentication or write to a database where the tasks are done in the background with no graphical interface.
But if you're doing a really long processing tasks that could freeze the interface, you use a service on a separate thread.
eg of a Service on a separate thread is IntentService
In my android application I am using a foreground service which is spawning a thread as a worker. Sometimes I observed the application is killed but the thread is still trying to refer some of the values which are no more available.
Can android decide to kill my application leaving the service still running?
I am stopping the thread in onDestroy() method of my service and hence want to know when Android kills the application, would it call Service:onDestroy().
All the applications in Android runs as a separate process similar to a linux process. So if your service is running in a separate process then the Android might decide to shut down a process at some point, when memory is low and required by other processes that are more immediately serving the user. Application components running in the process that's killed are consequently destroyed. A process is started again for those components when there's again work for them to do.
When deciding which processes to kill, the Android system weighs their relative importance to the user. For example, it more readily shuts down a process hosting activities that are no longer visible on screen, compared to a process hosting visible activities. The decision whether to terminate a process, therefore, depends on the state of the components running in that process. The rules used to decide which processes to terminate is discussed below.
Also you can control that if your service can run in the same process as of your application or in a separate process by using the android:process attribute in the manifest.
For more information You can read about processes and threads on below link:
http://developer.android.com/guide/components/processes-and-threads.html
I am confused with android services and java thread.
Please help me to understand in which scenario i should use them.
As per my understanding
Service run in background ,so do thread.
Service is to be used for activities like network operation or playing mp3 in background,so do threads.
So whats actual difference between them and when to use each of them.
Let me give an analogy.
Activities and Service are like projects.
Activities are like external projects. This is what the clients(users) see.
Services are like internal projects. There might be several internal projects for 1 external project or none at all.
You can "pause" external project but the internal project that supports it can still continue.
Main Thread is like the boss in a company
The boss should never be held up by too much work since he shouldn't be late to meetings (UI freezing) or the client(user) will be unhappy.
Threads are like employees in a company.
The more you have, the more things you can do at the same time provided you have enough equipment(CPU speed) for all of them.
Multiple employees can work on the same project at the same time but the boss should really work only on the Activities.
Always: A service of your application is usable not only by other components of your application, but by other applications, too.
A service is for use in not-GUI parts of program.
Mostly: A service is something more independent, than a thread. Service is something more long-living than a thread. Service is something more complex than a thread.
BTW, threads do not run in background only. What runs in foreground, is a thread, too.
I believe the main difference is about Android system attitude. Service is a part of android infrastructure, so android recognizes service as a working part of application and considers killing service as a last option. Moreover, if your service is killed (e.g. because of memory lack) you can say system to restart it automatically, whenever resources available again. Moreover, you can tune up service priority in order to do it as important as foreground activity. As for threads, android does not recognize a thread as important part which must be kept. So usual threads has much more chances to be killed eventually.
For instance If you have an activity which start a working thread and then go background, as android do not recognize thread as a working part, it may think that application do nothing, because no activity or service running and kill the whole app, including the working thread.
Thus when you start a Service, you are telling system something like: "Hi. I'm doing some business here, don't kill me until I finish, please." and Android pay attention to your request.
Services are more analogous to a headless Activity.
The the important piece to understand is that a Service is about managing application lifetime and the ability to keep work running when your Application is not in the foreground (no UI visible). It is also about providing the ability to expose functionality to other apps.
http://developer.android.com/reference/android/app/Service.html#WhatIsAService
Typically when starting a Service you will also start a worker Thread. There are settings in the manifest that can cause a Service to be started in a new Process but generally you do not need to do this, it makes communication with your service more difficult.
Use a just Thread in your Activity when you need to offload work from the UI thread while the application is in the foreground, but this work can stop when you are no longer in the foreground. (It is possible that your app will continue to run when not it foreground but there is no guarantee depending on a number of factors) Generally speaking Android is free to kill your Activity if it is not in the foreground, and if your App process has no Activities or Services it can be killed.
Use a Service with a Thread to do work that will take place while your app is in the background and you want better guarantee about the lifetime.
Use a Service to expose non-UI functionality to other applications.
Android Service don't run in a separate process (by default) and even don't run in a separate thread! It runs in the main thread(UI thread) of the application, therefore if you would like to do something time consuming task in the Service start a separate thread yourself, or use IntentService.
As per Android Developer Guide (http://developer.android.com/guide/components/services.html#Basics) :
A service is simply a component that can run in the background even when the user is not interacting with your application. Thus, you should create a service only if that is what you need.
If you need to perform work outside your main thread, but only while the user is interacting with your application, then you should probably instead create a new thread and not a service. For example, if you want to play some music, but only while your activity is running, you might create a thread in onCreate(), start running it in onStart(), then stop it in onStop(). Also consider using AsyncTask or HandlerThread, instead of the traditional Thread class. See the Processes and Threading document for more information about threads.
Remember that if you do use a service, it still runs in your application's main thread by default, so you should still create a new thread within the service if it performs intensive or blocking operations.
Why we need service is to avoid resource crunch.
For example you opening an application after opening an another application so at the time your app added to the background task.
While opening multiple application, ur app may killed by android system. So if ur app has service it won't be killed by the system because service is higher priority , even it may killed the app has service so that we using constant return type in onStartCommand(). Method. That's START_STICKY,START_NOT_STICKY and DELIVER_INTENT.
What are the practical differences between putting a service in a separate process or keeping it in the app's main process? What would each scenario be used for?
When a service is running in the main process it will be stopped in case your application crashes for whatever reason. Put a service into it's own process is reasonable for some services which can be used from different applications or services which should run independently from your main app.
The only reasons I see for putting a service in another process is
The application is resource heavy and likely going to be killed quickly by the OS. Putting the service in a separate process will distribute the resources and if you application dies your service won't.
JUST in case your application has errors and dies your service will keep going.
However if you create a good application and use good programming you should not run into either of these issues. By having your service in a separate process it causes trouble with things like SharedPreferences and concurrent DB access... I would recommend not doing it.
Not to mention... another process means another DVM. This will take more resources than running in one DVM and slow things down.
Also putting service in another process makes your changes of static variables invisible for main process. You can get situation, when you assign a variable with some value, and it is not changed!! I spent whole day for this issue!
Following is a quote from Android Developer's web site.
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.
Jake points out that you can, thru manifest, control the Name of the process it is running. But following findings from Documentatioin:
Most confusion about the Service class actually revolves around what it is not:
A Service is not a separate process. The Service object itself does not imply it is running in its own process; unless otherwise specified, it runs in the same process as the application it is part of.
This is interesting, what is said Here is:
The name of the process where the service is to run. Normally, all components of an application run in the default process created for the application. It has the same name as the application package. The element's process attribute can set a different default for all components. But component can override the default with its own process attribute, allowing you to spread your application across multiple processes.
But anyway, if you need Service to be exposed to other applications, for example, you need to provide content (like phonebook) to other applications, setting service to run in different process is the reason.
Using the process attribute of a service is rejected by the manifest parser so it is rather misleading!
What are the advantages/disadvantages in placing a lengthy network access code in a thread in an activity or a thread in a service? How would it affect the application? I am writing a streaming audio player and from what I've read so far putting the code in a service will still end up blocking the application so a new thread is needed, does anyone know if it makes more sense to put this piece of code in a service.
Yes, a blocking operation in a Service will still block the application. Despite first appearances, Services are not simply for running tasks in the background. They are for running tasks with a lifecycle that is independent of the Activity lifecycle (IE, they may continue after the Activity is closed).
A Service that starts when an Activity starts and ends when the Activity ends is useless.
In your case, where you are streaming audio, you may want to stream audio even after the user closes the Activity, in which case, you should use a Service, but you'll still need a thread (or an AsyncTask) for blocking tasks.
From my experience (1+ years developing Android), there is no difference between running a new thread in a service or in an activity.
Try not to keep a reference to the Activity in the new thread - use the application context.
Also, the service's life-cycle didn't help at all because some methods are not guaranteed to be invoked :(
The only difference may be that the service can be destroyed without destroying the app completely - thus potentially destroying the new threads.
Why potentially? because on the practical side, this doesn't happen. The app ALWAYS gets killed without killing the service before that, meaning: the local service mechanism is useless!!!
Remote service is a different discussion - I was referring only to "where should I run a new thread?".
Good luck!!!
The difference is how the system manages your application process lifecycle. Running threads don't affect the application process lifecycle, but a service does.
To determine which processes should be killed when low on memory, Android places each process into an importance hierarchy based on the components running in them and the state of those components. If your app doesn't have a visible activity or a foreground service but has a background service, it's categorized as a service process and will be kept alive while there less priority cached processes exist. But if the app has neither a visible activity/fragment, foreground service nor a background service, it's categorized as a cached process and can be killed at any time to free system resources, whether it has a running thread or not.
But do not rush to create a background service, there are more modern approaches to deal with background tasks nowadays. Consider alternative solutions described below and in the background processing guide and keep in mind all restrictions associated with background services.
If a thread executes a task which result is required only by an activity, the thread lifecycle should be bound to the activity. In such a case no services are required. It's so called immediate tasks, ViewModel + Kotlin Coroutines + ViewModelScope is a great way to deal with it, see Guide to background processing for more details about different kinds of background tasks.
If the task should be completed whether the user closed you app or not and it's not required to execute it immediately, consider using WorkManager which is a great way to deal with such deferred tasks. See Android Work Manager vs Services? for more details.
Otherwise, if you have an immediate task which lifecycle isn't bound to an activity/fragment, may be a foreground service would be the best choice, especially in case of an audio player. There are some limitations considering background services since Android 8, the system stops an app's background services in several minutes after the app is closed, so it's not a place for a long running task.