Handler vs AsyncTask vs Thread

Question

I got slightly confused about the differences between Handlers  AsyncTask and Threads in Android  I ve read quite a few blogs and questions here in StackOverflow    Handler are background threads that provide you to communicate with the UI  Updating a progress bar  for instance  should be done via Handler  Using Handlers you have the advantage of MessagingQueues  so if you want to schedule messages or update multiple UI elements or have repeating tasks   AsyncTask are similar  in fact  they make use of Handler  but doesn t run in the UI thread  so it s good for fetching data  for instance fetching web services  Later you can interact with the UI   Thread however can t interact with the UI  provide more  basic  threading and you miss all the abstractions of AsyncTask   However  I would like to have a socket connection run in service  Should this be run in a handler or a thread  or even an AsyncTask  UI interaction is not necessary at all  Does it make a difference in terms of performance which I use   Meanwhile  the documentation has been majorly improved

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If we look at the source code  we will see AsyncTask and Handler is purely written in Java   There are some exceptions  though  But that is not an important point   So there is no magic in AsyncTask or Handler  These classes make our life easier as a developer   For example  If Program A calls method A    method A   could run in a different thread with Program A  We can easily verify by following code   Thread t   Thread currentThread        int id   t getId      Why should we use a new thread for some tasks  You can google for it  Many many reasons e g  lifting heavily  long-running works   So  what are the differences between Thread  AsyncTask  and Handler   AsyncTask and Handler are written in Java  internally they use a Thread   so everything we can do with Handler or AsyncTask  we can achieve using a Thread too   What can Handler and AsyncTask really help   The most obvious reason is communication between the caller thread and the worker thread   Caller Thread  A thread which calls the Worker Thread to perform some tasks  A caller thread doesn t necessarily have to be the UI thread   Of course  we can communicate between two threads in other ways  but there are many disadvantages  and dangers  because of thread safety   That is why we should use Handler and AsyncTask  These classes do most of the work for us  we only need to know which methods to override   The difference between Handler and AsyncTask is  Use AsyncTask when Caller thread is a UI Thread  This is what android document says      AsyncTask enables proper and easy use of the UI thread  This class   allows to perform background operations and publish results on the UI   thread without having to manipulate threads and or handlers   I want to emphasize two points   1  Easy use of the UI thread  so  use when caller thread is UI Thread    2  No need to manipulate handlers   means  You can use Handler instead of AsyncTask  but AsyncTask is an easier option    There are many things in this post I haven t said yet  for example  what is UI Thread  or why it s easier  You must know some methods behind each class and use it  you will completely understand the reason why      when you read the Android document  you will see        Handler allows you to send and process Message and Runnable objects   associated with a thread s MessageQueue   This description might seem strange at first  We only need to understand that each thread has each message queue  like a to-do list   and the thread will take each message and do it until the message queue is empty  just like we finish our work and go to bed   So  when Handler communicates  it just gives a message to caller thread and it will wait to process    Complicated  Just remember that Handler can communicate with the caller thread safely

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Let me try and answer the question here with an example    - MyImageSearch  Kindly refer the image here of the main activity screen - containing an edit text   search button   grid view     Description of MyImageSearch - Once user enters the details on the edit text field and clicks on the search button  we will search images on the internet via the web services provided by flickr  you only need to register there to get a key secret token  - for searching we send an HTTP Request and GET JSON Data back in response containing the url s of individual images which we will then use to load the grid view   My Implementation - In the main activity I will define a inner class which extends the AsyncTask to send the HTTP Request in doInBackGround Method and fetch the JSON Response and update my local ArrayList of FlickrItems which I am going to use to update my GridView via the FlickrAdapter  extends the BaseAdapter  and call the adapter notifyDataSetChanged   in the onPostExecute   of AsyncTask to reload the grid view  Note that here the HTTP Request is a blocking call because of which I have done it via the AsyncTask  And  I can cache the items in adapter to increase the performance or store them on SDCard  The grid that I will be inflating in the FlickrAdapter contains in my implementation a progressbar and image view  Below you can find the code for mainActivity which I used   Answer to the Question Now -  So once we have the JSON data for fetching individual Images we can implement the logic of getting the images in background via Handlers or Threads or AsyncTask  We should note here that since my images once downloaded must be displayed on the UI main thread we cannot simply use threads as it is since they don t have access to the context  In the FlickrAdapter  the choices I could think of    Choice 1  Create a LooperThread  extends thread  - and keep on downloading images sequentially in one thread by keeping this thread open  looper loop    Choice 2  Make use of a Thread Pool and post the runnable via myHandler which contains reference to my ImageView  but since the views in Grid View are recycled  again the problem might arise where image at index 4 is displayed at index 9  download may take more time  Choice 3  I used this   Make use of a Thread Pool and send a message to myHandler  which contains data related to ImageView s index and ImageView itself  so while doing handleMessage   we will update the ImageView only if currentIndex matches the index of the Image we tried to download  Choice 4  Make use of AsyncTask to download the images in background  but here I will not have access to the number of threads I want in  the thread pool and it varies with different android version  but in Choice 3 I can make of conscious decision of the size of thread pool depending on device configuration being used    Here the source code   public class MainActivity extends ActionBarActivity        GridView imageGridView      ArrayList lt FlickrItem gt  items   new ArrayList lt FlickrItem gt         FlickrAdapter adapter        Override     protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout activity main            imageGridView    GridView  findViewById R id gridView1           adapter   new FlickrAdapter this  items           imageGridView setAdapter adapter                 To avoid a memory leak on configuration change making it a inner class     class FlickrDownloader extends AsyncTask lt Void  Void  Void gt                Override         protected Void doInBackground Void    params                FlickrGetter getter   new FlickrGetter                 ArrayList lt FlickrItem gt  newItems   getter fetchItems                    clear the existing array             items clear                    add the new items to the array             items addAll newItems                   is this correct   - Wrong rebuilding the list view and should not be done in background               adapter notifyDataSetChanged                 return null                      Override         protected void onPostExecute Void result                super onPostExecute result                adapter notifyDataSetChanged                          public void search View view               get the flickr data         FlickrDownloader downloader   new FlickrDownloader            downloader execute                Override     public boolean onCreateOptionsMenu Menu menu               Inflate the menu  this adds items to the action bar if it is present          getMenuInflater   inflate R menu main  menu           return true              Override     public boolean onOptionsItemSelected MenuItem item               Handle action bar item clicks here  The action bar will            automatically handle clicks on the Home Up button  so long            as you specify a parent activity in AndroidManifest xml          int id   item getItemId            if  id    R id action settings                return true                    return super onOptionsItemSelected item             I hope my answer though long will help in understanding some of the finer details

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An AsyncTask is used to do some background computation and publish the result to the UI thread  with optional progress updates   Since you re not concerned with UI  then a Handler or Thread seems more appropriate   You can spawn a background Thread and pass messages back to your main thread by using the  Handler s post method

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Thread   You can use the new Thread for long-running background tasks without impacting UI Thread  From java Thread  you can t update UI Thread    Since normal Thread is not much useful for Android architecture  helper classes for threading have been introduced    You can find answers to your queries in Threading performance documentation page   Handler   A Handler allows you to send and process Message and Runnable objects associated with a thread s MessageQueue  Each Handler instance is associated with a single thread and that thread s message queue   There are two main uses for a Handler    To schedule messages and runnables to be executed as some point in the future   To enqueue an action to be performed on a different thread than your own    AsyncTask   AsyncTask enables proper and easy use of the UI thread  This class allows you to perform background operations and publish results on the UI thread without having to manipulate threads and or handlers   Drawbacks    By default  an app pushes all of the AsyncTask objects it creates into a single thread  Therefore  they execute in serial fashion  and   as with the main thread   an especially long work packet can block the queue  Due to this reason   use AsyncTask to handle work items shorter than 5ms in duration  AsyncTask objects are also the most common offenders for implicit-reference issues  AsyncTask objects present risks related to explicit references  as well    HandlerThread   You may need a more traditional approach to executing a block of work on a long-running thread  unlike AsyncTask  which should be used for 5ms workload   and some ability to manage that workflow manually   A handler thread is effectively a long-running thread that grabs work from a queue and operates on it   ThreadPoolExecutor   This class manages the creation of a group of threads  sets their priorities  and manages how work is distributed among those threads  As workload increases or decreases  the class spins up or destroys more threads to adjust to the workload   If the workload is more and single HandlerThread is not suffice  you can go for ThreadPoolExecutor       However I would like to have a socket connection run in service  Should this be run in a handler or a thread  or even an AsyncTask  UI interaction is not necessary at all  Does it make a difference in terms of performance which I use    Since UI interaction is not required  you may not go for AsyncTask  Normal threads are not much useful and hence HandlerThread is the best option  Since you have to maintain socket connection  Handler on the main thread is not useful at all  Create a HandlerThread and get a Handler from the looper of HandlerThread     HandlerThread handlerThread   new HandlerThread  SocketOperation     handlerThread start     Handler requestHandler   new Handler handlerThread getLooper      requestHandler post myRunnable      where myRunnable is your Runnable object     If you want to communicate back to UI thread  you can use one more Handler to process response    final Handler responseHandler   new Handler Looper getMainLooper               Override         public void handleMessage Message msg                  txtView setText  String  msg obj               Toast makeText MainActivity this                       Foreground task is completed    String msg obj                      Toast LENGTH LONG                       show                       in your Runnable  you can add  responseHandler sendMessage msg     More details about implementation can be found here   Android  Toast in a thread

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After looking in-depth  it s straight forward   AsyncTask    It s a simple way to use a thread without knowing anything about the java thread model  AsyncTask gives various callbacks respective to the worker thread and main thread   Use for small waiting operations like the following    Fetching some data from web services and display over the layout  Database query  When you realize that running operation will never  ever be nested    Handler   When we install an application in android  then it creates a thread for that application called MAIN UI Thread   All activities run inside that thread   By the android single thread model rule  we can not access UI elements  bitmap  textview  etc    directly for another thread defined inside that activity   A Handler allows you to communicate back with the UI thread from other background threads  This is useful in android as android doesn   t allow other threads to communicate directly with UI thread  A handler can send and process Message and Runnable objects associated with a thread   s MessageQueue  Each Handler instance is associated with a single thread and that thread   s message queue  When a new Handler is created  it is bound to the thread message queue of the thread that is creating it   It s the best fit for    It allows you to do message queuing  Message scheduling    Thread   Now it s time to talk about the thread   Thread is the parent of both AsyncTask and Handler  They both internally use thread  which means you can also create your own thread model like AsyncTask and Handler  but that requires a good knowledge of Java s Multi-Threading Implementation

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As the Tutorial on Android background processing with Handlers  AsyncTask and Loaders on the Vogella site puts it   The Handler class can be used to register to a thread and provides a simple channel to send data to this thread   The AsyncTask class encapsulates the creation of a background process and the synchronization with the main thread  It also supports reporting progress of the running tasks   And a Thread is basically the core element of multithreading which a developer can use with the following disadvantage      If you use Java threads you have to handle the following requirements   in your own code          Synchronization with the main thread if you post back results to the user interface   No default for canceling the thread   No default thread pooling   No default for handling configuration changes in Android      And regarding the AsyncTask  as the Android Developer s Reference puts it      AsyncTask enables proper and easy use of the UI thread  This class   allows to perform background operations and publish results on the UI   thread without having to manipulate threads and or handlers       AsyncTask is designed to be a helper class around Thread and Handler   and does not constitute a generic threading framework  AsyncTasks   should ideally be used for short operations  a few seconds at the   most   If you need to keep threads running for long periods of time    it is highly recommended you use the various APIs provided by the   java util concurrent package such as Executor  ThreadPoolExecutor and   FutureTask    Update May 2015  I found an excellent series of lectures covering this topic      This is the Google Search  Douglas Schmidt lecture android concurrency and synchronisation      This is the video of the first lecture on YouTube      All this is part of the CS 282  2013   Systems Programming for Android from the Vanderbilt University  Here s the YouTube Playlist      Douglas Schmidt seems to be an excellent lecturer   Important  If you are at a point where you are considering to use AsyncTask to solve your threading issues  you should first check out ReactiveX RxAndroid for a possibly more appropriate programming pattern  A very good resource for getting an overview is Learning RxJava 2 for Android by example

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Thread   When you start an app  a process is created to execute the code  To efficiently use computing resource  threads can be started within the process so that multiple tasks can be executed at the time  So threads allow you to build efficient apps by utilizing cpu efficiently without idle time   In Android  all components execute on a single called main thread  Android system queue tasks and execute them one by one on the main thread  When long running tasks are executed  app become unresponsive   To prevent this  you can create worker threads and run background or long running tasks   Handler  Since android uses single thread model  UI components are created non-thread safe meaning only the thread it created should access them that means UI component should be updated on main thread only  As UI component run on the main thread  tasks which run on worker threads can not modify UI components  This is where Handler comes into picture  Handler with the help of Looper can connect to new thread or existing thread and run code it contains on the connected thread   Handler makes it possible for inter thread communication  Using Handler  background thread can send results to it and the handler which is connected to main thread can update the UI components on the main thread   AsyncTask  AsyncTask provided by android uses both thread and handler to make running simple tasks in the background and updating results from background thread to main thread easy   Please see android thread  handler  asynctask and thread pools for examples

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Thread  Android supports standard Java Threads  You can use standard Threads and the tools from the package    java util concurrent    to put actions into the background  The only limitation is that you cannot directly update the UI from the a background process   If you need to update the UI from a background task you need to use some Android specific classes  You can use the class    android os Handler    for this or the class    AsyncTask     Handler  The class    Handler    can update the UI  A handle provides methods for receiving messages and for runnables  To use a handler you have to subclass it and override handleMessage   to process messages  To process Runable  you can use the method post    You only need one instance of a handler in your activity   You thread can post messages via the method sendMessage Message msg  or sendEmptyMessage    AsyncTask  If you have an Activity which needs to download content or perform operations that can be done in the background AsyncTask allows you to maintain a responsive user interface and publish progress for those operations to the user   For more information you can have a look at these links   http   mobisys in blog 2012 01 android-threads-handlers-and-asynctask-tutorial   http   www slideshare net HoangNgoBuu android-thread-handler-and-asynctask

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AsyncTask is designed to perform not more than few seconds operation to be done in background  not recommended for megabytes of file downloading from server or compute cpu intensive task such as file IO operations    If you need to execute a long running operation   you have been  strongly advised to use java native threads  Java gives you various thread related classes to do what you need  Use Handler to  update the UI Thread

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public class RequestHandler        public String sendPostRequest String requestURL                                    HashMap lt String  String gt  postDataParams             URL url           StringBuilder sb   new StringBuilder            try               url   new URL requestURL                HttpURLConnection conn    HttpURLConnection  url openConnection                conn setReadTimeout 15000               conn setConnectTimeout 15000               conn setRequestMethod  POST                conn setDoInput true               conn setDoOutput true                 OutputStream os   conn getOutputStream                BufferedWriter writer   new BufferedWriter                      new OutputStreamWriter os   UTF-8                 writer write getPostDataString postDataParams                 writer flush                writer close                os close                int responseCode   conn getResponseCode                 if  responseCode    HttpsURLConnection HTTP OK                    BufferedReader br   new BufferedReader new InputStreamReader conn getInputStream                      sb   new StringBuilder                    String response                  while   response   br readLine       null                       sb append response                                              catch  Exception e                e printStackTrace                      return sb toString               private String getPostDataString HashMap lt String  String gt  params  throws UnsupportedEncodingException           StringBuilder result   new StringBuilder            boolean first   true          for  Map Entry lt String  String gt  entry   params entrySet                  if  first                  first   false              else                 result append   amp                  result append URLEncoder encode entry getKey     UTF-8                 result append                   result append URLEncoder encode entry getValue     UTF-8                        return result toString

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Handler - is communication medium between threads  In android it is mostly used to communicate with main thread by creating and sending messages through handler  AsyncTask - is used to perform long running applications in a background thread  With nAsyncTask you get can do the operation in a background thread and get the result in the main thread of the application    Thread - is a light weight process  to achieve concurrency and maximum cpu utilization  In android you can use thread to perform activities which does not touch UI of the app

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It depends which one to chose is based on the requirement   Handler is mostly  used to switch from  other thread to main thread  Handler is attached to a looper on which it post its runnable task in queue  So If you are already in other thread and switch to main thread then you need handle instead of async task or other thread   If Handler created in other than main thread which is not a looper is will not give error as  handle is created the  thread  that thread  need to be made a lopper   AsyncTask is used to execute code for few seconds which run on background thread and gives its result to main thread      AsyncTask Limitations 1  Async Task is not  attached to life cycle of activity and it keeps run even if its activity destroyed whereas loader doesn t have this limitation  2  All Async Tasks share the same background thread for execution which also impact  the app performance  Thread is used in app for background  work also but it doesn t have any call back on main thread  If requirement suits some threads instead of one thread and which need to give task many times  then thread pool executor is better option Eg Requirement of   Image loading from multiple url like glide

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In my opinion threads aren t the most efficient way of doing socket connections but they do provide the most functionality in terms of running threads  I say that because from experience  running threads for a long time causes devices to be very hot and resource intensive  Even a simple while true  will heat a phone in minutes  If you say that UI interaction is not important  perhaps an AsyncTask is good because they are designed for long-term processes  This is just my opinion on it   UPDATE  Please disregard my above answer  I answered this question back in 2011 when I was far less experienced in Android than I am now  My answer above is misleading and is considered wrong  I m leaving it there because many people commented on it below correcting me  and I ve learned my lesson   There are far better other answers on this thread  but I will at least give me more proper answer  There is nothing wrong with using a regular Java Thread  however  you should really be careful about how you implement it because doing it wrong can be very processor intensive  most notable symptom can be your device heating up   AsyncTasks are pretty ideal for most tasks that you want to run in the background  common examples are disk I O  network calls  and database calls   However  AsyncTasks shouldn t be used for particularly long processes that may need to continue after the user has closed your app or put their device to standby  I would say for most cases  anything that doesn t belong in the UI thread  can be taken care of in an AsyncTask

[android] Handler vs AsyncTask vs Thread

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