AsyncTask

• Allows to perform background operations and publish results on the UI thread without having to manipulate threads and/or handlers.
• Should ideally be used for short operations. (use java.util.concurrent package such as Executor, ThreadPoolExecutor and FutureTask for long running thread).
• Define 3 generic types, called Params, Progress and Result, and 4 steps, called onPreExecute, doInBackground, onProgressUpdate and onPostExecute.
• Async Task uses ThreadPoolExecutor to run multiple tasks at same time. total thread size = CPU Count * 2 + 1.
• AsyncTask must be subclassed to be used. 
• The task instance must be created on the UI thread.
• AsyncTask callback methods are thread safe.

Generic Types.
AsyncTask<Params, Progress, Result>

params....
execute(params....)  (ex: execute(url1, url2, url3))

• params are passed to doInBackground method which is being executed in worker thread

Progress....
publishProgress(Progress...) 

• being called inside doInBackground, Progress... value passed to method onProgressUpdate running in UI Thread.

result.....

• return result.... return value from doInBackground passed to onPostExecute

Cancelling a Task.
isCancelled() 

• returns true is task been cancelled using API cancel(boolean) 

Execution.

• Before DONUT - no thread pool in AsyncTask
• After DONUT - Thread Pool introduced and by default task are executed in multiple thread.
• After HONECOMP - by default tasks are being executed in single thread to avoid error. use executeOnExecutor(Executor exec, Params... params) API to enable true parallel execution.
• AsyncTask has two static Executor instance SERIAL_EXECUTOR & THREAD_POOL_EXECUTOR