Handler vs AsyncTask vs Thread [closed]

Answer 1

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.

Answer 2

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:

1. To schedule messages and runnables to be executed as some point in the future;

2. 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:

1. 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.

2. 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

Answer 3

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

Answer 4

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.

Answer 5

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:

1. Fetching some data from web services and display over the layout.
2. Database query.
3. 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:

1. It allows you to do message queuing.
2. 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.

Answer 6

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<FlickrItem> items = new ArrayList<FlickrItem>();
    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<Void, Void, Void> {



        @Override
        protected Void doInBackground(Void... params) {
            FlickrGetter getter = new FlickrGetter();

            ArrayList<FlickrItem> 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.