Handler vs AsyncTask vs Thread [closed]

Answer 1

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

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.