Handler vs AsyncTask vs Thread

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.

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.

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.