AsyncTask

Kotlin | Java

public abstract class AsyncTask

extends Object

This class was deprecated in API level 30.

Use the standard java.util.concurrent or Kotlin concurrency utilities instead.

AsyncTask was intended to enable proper and easy use of the UI thread. However, the most common use case was for integrating into UI, and that would cause Context leaks, missed callbacks, or crashes on configuration changes. It also has inconsistent behavior on different versions of the platform, swallows exceptions from doInBackground , and does not provide much utility over using Executor s directly.

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 .

An asynchronous task is defined by a computation that runs on a background thread and whose result is published on the UI thread. An asynchronous task is defined by 3 generic types, called Params , Progress and Result , and 4 steps, called onPreExecute , doInBackground , onProgressUpdate and onPostExecute .

Developer Guides For more information about using tasks and threads, read the Processes and Threads developer guide.

Usage

AsyncTask must be subclassed to be used. The subclass will override at least one method ( doInBackground(Params...) ), and most often will override a second one ( onPostExecute(Result) .)

Here is an example of subclassing:

private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> { protected Long doInBackground(URL... urls) { int count = urls.length; long totalSize = 0; for (int i = 0; i < count; i++) { totalSize += Downloader.downloadFile(urls[i]); publishProgress((int) ((i / (float) count) * 100)); // Escape early if cancel() is called if (isCancelled()) break; } return totalSize; } protected void onProgressUpdate(Integer... progress) { setProgressPercent(progress[0]); } protected void onPostExecute(Long result) { showDialog("Downloaded " + result + " bytes"); } }

Once created, a task is executed very simply:

new DownloadFilesTask().execute(url1, url2, url3);

AsyncTask's generic types

The three types used by an asynchronous task are the following:

Params , the type of the parameters sent to the task upon execution. Progress , the type of the progress units published during the background computation. Result , the type of the result of the background computation.

Not all types are always used by an asynchronous task. To mark a type as unused, simply use the type Void :

private class MyTask extends AsyncTask<Void, Void, Void> { ... }

The 4 steps

When an asynchronous task is executed, the task goes through 4 steps:

onPreExecute() , invoked on the UI thread before the task is executed. This step is normally used to setup the task, for instance by showing a progress bar in the user interface. doInBackground(Params...) , invoked on the background thread immediately after onPreExecute() finishes executing. This step is used to perform background computation that can take a long time. The parameters of the asynchronous task are passed to this step. The result of the computation must be returned by this step and will be passed back to the last step. This step can also use publishProgress(Progress...) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(Progress...) step. onProgressUpdate(Progress...) , invoked on the UI thread after a call to publishProgress(Progress...) . The timing of the execution is undefined. This method is used to display any form of progress in the user interface while the background computation is still executing. For instance, it can be used to animate a progress bar or show logs in a text field. onPostExecute(Result) , invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Cancelling a task

A task can be cancelled at any time by invoking cancel(boolean) . Invoking this method will cause subsequent calls to isCancelled() to return true. After invoking this method, onCancelled(java.lang.Object) , instead of onPostExecute(java.lang.Object) will be invoked after doInBackground(java.lang.Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value of isCancelled() periodically from doInBackground(java.lang.Object[]) , if possible (inside a loop for instance.)

Threading rules

There are a few threading rules that must be followed for this class to work properly:

The AsyncTask class must be loaded on the UI thread. This is done automatically as of Build.VERSION_CODES.JELLY_BEAN .

. The task instance must be created on the UI thread.

execute(Params...) must be invoked on the UI thread.

must be invoked on the UI thread. Do not call onPreExecute() , onPostExecute(Result) , doInBackground(Params...) , onProgressUpdate(Progress...) manually.

, , , manually. The task can be executed only once (an exception will be thrown if a second execution is attempted.)

Memory observability

AsyncTask guarantees that all callback calls are synchronized to ensure the following without explicit synchronizations.

The memory effects of onPreExecute() , and anything else executed before the call to execute(Params...) , including the construction of the AsyncTask object, are visible to doInBackground(Params...) .

, and anything else executed before the call to , including the construction of the AsyncTask object, are visible to . The memory effects of doInBackground(Params...) are visible to onPostExecute(Result) .

are visible to . Any memory effects of doInBackground(Params...) preceding a call to publishProgress(Progress...) are visible to the corresponding onProgressUpdate(Progress...) call. (But doInBackground(Params...) continues to run, and care needs to be taken that later updates in doInBackground(Params...) do not interfere with an in-progress onProgressUpdate(Progress...) call.)

preceding a call to are visible to the corresponding call. (But continues to run, and care needs to be taken that later updates in do not interfere with an in-progress call.) Any memory effects preceding a call to cancel(boolean) are visible after a call to isCancelled() that returns true as a result, or during and after a resulting call to onCancelled() .

Order of execution

When first introduced, AsyncTasks were executed serially on a single background thread. Starting with Build.VERSION_CODES.DONUT , this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with Build.VERSION_CODES.HONEYCOMB , tasks are executed on a single thread to avoid common application errors caused by parallel execution.

If you truly want parallel execution, you can invoke executeOnExecutor(java.util.concurrent.Executor, java.lang.Object[]) with THREAD_POOL_EXECUTOR .

Summary