Added in API level 3
Deprecated in API level 30

AsyncTask

abstract class AsyncTask<Params : Any!, Progress : Any!, Result : Any!>
kotlin.Any
   ↳ android.os.AsyncTask

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 Executors 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.

Usage

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

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:

  1. Params, the type of the parameters sent to the task upon execution.
  2. Progress, the type of the progress units published during the background computation.
  3. 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:

  1. 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.
  2. doInBackground, 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 to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate step.
  3. onProgressUpdate, invoked on the UI thread after a call to publishProgress. 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.
  4. onPostExecute, 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:

Memory observability

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

Order of execution

When first introduced, AsyncTasks were executed serially on a single background thread. Starting with android.os.Build.VERSION_CODES#DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with android.os.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

Nested classes

Indicates the current status of the task.
Public constructors

Creates a new asynchronous task.
Public methods
Boolean
cancel(mayInterruptIfRunning: Boolean)

Attempts to cancel execution of this task.
AsyncTask<Params, Progress, Result>!
execute(vararg params: Params)

Executes the task with the specified parameters.
open static Unit
execute(runnable: Runnable!)

Convenience version of execute(java.lang.Object...) for use with a simple Runnable object.
AsyncTask<Params, Progress, Result>!
executeOnExecutor(exec: Executor!, vararg params: Params)

Executes the task with the specified parameters.
Result
get()

Waits if necessary for the computation to complete, and then retrieves its result.
Result
get(timeout: Long, unit: TimeUnit!)

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result.
AsyncTask.Status!

Returns the current status of this task.
Boolean

Returns true if this task was cancelled before it completed normally.
Protected methods
abstract Result
doInBackground(vararg params: Params)

Override this method to perform a computation on a background thread.
open Unit
onCancelled(result: Result)

Runs on the UI thread after cancel(boolean) is invoked and doInBackground(java.lang.Object[]) has finished.
open Unit

Applications should preferably override onCancelled(java.lang.Object).
open Unit
onPostExecute(result: Result)

Runs on the UI thread after doInBackground.
open Unit

Runs on the UI thread before doInBackground.
open Unit
onProgressUpdate(vararg values: Progress)

Runs on the UI thread after publishProgress is invoked.
Unit
publishProgress(vararg values: Progress)

This method can be invoked from doInBackground to publish updates on the UI thread while the background computation is still running.
Properties
static Executor!

An Executor that executes tasks one at a time in serial order.
static Executor!

An Executor that can be used to execute tasks in parallel.

Public constructors

AsyncTask

Added in API level 3
AsyncTask()

Creates a new asynchronous task. This constructor must be invoked on the UI thread.

Public methods

cancel

Added in API level 3
fun cancel(mayInterruptIfRunning: Boolean): Boolean

Deprecated: Deprecated in Java.

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

Calling this method will result in onCancelled(java.lang.Object) being invoked on the UI thread after doInBackground(java.lang.Object[]) returns. Calling this method guarantees that onPostExecute(Object) is never subsequently invoked, even if cancel returns false, but onPostExecute has not yet run. To finish the task as early as possible, check isCancelled() periodically from doInBackground(java.lang.Object[]).

This only requests cancellation. It never waits for a running background task to terminate, even if mayInterruptIfRunning is true.

Parameters
mayInterruptIfRunning Boolean: true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete.
Return
Boolean false if the task could not be cancelled, typically because it has already completed normally; true otherwise

execute

Added in API level 3
fun execute(vararg params: Params): AsyncTask<Params, Progress, Result>!

Deprecated: Deprecated in Java.

Executes the task with the specified parameters. The task returns itself (this) so that the caller can keep a reference to it.

Note: this function schedules the task on a queue for a single background thread or pool of threads depending on the platform version. When first introduced, AsyncTasks were executed serially on a single background thread. Starting with android.os.Build.VERSION_CODES#DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting android.os.Build.VERSION_CODES#HONEYCOMB, tasks are back to being executed on a single thread to avoid common application errors caused by parallel execution. If you truly want parallel execution, you can use the executeOnExecutor version of this method with THREAD_POOL_EXECUTOR; however, see commentary there for warnings on its use.

This method must be invoked on the UI thread.
This method must be called from the main thread of your app.

Parameters
params Params: The parameters of the task.
Return
AsyncTask<Params, Progress, Result>! This instance of AsyncTask.
Exceptions
java.lang.IllegalStateException If getStatus() returns either AsyncTask.Status#RUNNING or AsyncTask.Status#FINISHED.

execute

Added in API level 11
Deprecated in API level 30
open static fun execute(runnable: Runnable!): Unit

Deprecated: Deprecated in Java.

Convenience version of execute(java.lang.Object...) for use with a simple Runnable object. See execute(java.lang.Object[]) for more information on the order of execution.
This method must be called from the main thread of your app.

executeOnExecutor

Added in API level 11
Deprecated in API level 30
fun executeOnExecutor(
    exec: Executor!, 
    vararg params: Params
): AsyncTask<Params, Progress, Result>!

Deprecated: Deprecated in Java.

Executes the task with the specified parameters. The task returns itself (this) so that the caller can keep a reference to it.

This method is typically used with THREAD_POOL_EXECUTOR to allow multiple tasks to run in parallel on a pool of threads managed by AsyncTask, however you can also use your own Executor for custom behavior.

Warning: Allowing multiple tasks to run in parallel from a thread pool is generally not what one wants, because the order of their operation is not defined. For example, if these tasks are used to modify any state in common (such as writing a file due to a button click), there are no guarantees on the order of the modifications. Without careful work it is possible in rare cases for the newer version of the data to be over-written by an older one, leading to obscure data loss and stability issues. Such changes are best executed in serial; to guarantee such work is serialized regardless of platform version you can use this function with SERIAL_EXECUTOR.

This method must be invoked on the UI thread.
This method must be called from the main thread of your app.

Parameters
exec Executor!: The executor to use. THREAD_POOL_EXECUTOR is available as a convenient process-wide thread pool for tasks that are loosely coupled.
params Params: The parameters of the task.
Return
AsyncTask<Params, Progress, Result>! This instance of AsyncTask.
Exceptions
java.lang.IllegalStateException If getStatus() returns either AsyncTask.Status#RUNNING or AsyncTask.Status#FINISHED.

get

Added in API level 3
fun get(): Result

Deprecated: Deprecated in Java.

Waits if necessary for the computation to complete, and then retrieves its result.

Return
Result The computed result.
Exceptions
java.util.concurrent.CancellationException If the computation was cancelled.
java.util.concurrent.ExecutionException If the computation threw an exception.
java.lang.InterruptedException If the current thread was interrupted while waiting.

get

Added in API level 3
fun get(
    timeout: Long, 
    unit: TimeUnit!
): Result

Deprecated: Deprecated in Java.

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result.

Parameters
timeout Long: Time to wait before cancelling the operation.
unit TimeUnit!: The time unit for the timeout.
Return
Result The computed result.
Exceptions
java.util.concurrent.CancellationException If the computation was cancelled.
java.util.concurrent.ExecutionException If the computation threw an exception.
java.lang.InterruptedException If the current thread was interrupted while waiting.
java.util.concurrent.TimeoutException If the wait timed out.

getStatus

Added in API level 3
fun getStatus(): AsyncTask.Status!

Deprecated: Deprecated in Java.

Returns the current status of this task.

Return
AsyncTask.Status! The current status.

isCancelled

Added in API level 3
fun isCancelled(): Boolean

Deprecated: Deprecated in Java.

Returns true if this task was cancelled before it completed normally. If you are calling cancel(boolean) on the task, the value returned by this method should be checked periodically from doInBackground(java.lang.Object[]) to end the task as soon as possible.

Return
Boolean true if task was cancelled before it completed

See Also

Protected methods

doInBackground

Added in API level 3
protected abstract fun doInBackground(vararg params: Params): Result

Deprecated: Deprecated in Java.

Override this method to perform a computation on a background thread. The specified parameters are the parameters passed to #execute by the caller of this task. This will normally run on a background thread. But to better support testing frameworks, it is recommended that this also tolerates direct execution on the foreground thread, as part of the #execute call. This method can call publishProgress to publish updates on the UI thread.
This method may take several seconds to complete, so it should only be called from a worker thread.

Parameters
params Params: The parameters of the task.
Return
Result A result, defined by the subclass of this task.

onCancelled

Added in API level 11
Deprecated in API level 30
protected open fun onCancelled(result: Result): Unit

Deprecated: Deprecated in Java.

Runs on the UI thread after cancel(boolean) is invoked and doInBackground(java.lang.Object[]) has finished.

The default implementation simply invokes onCancelled() and ignores the result. If you write your own implementation, do not call super.onCancelled(result).


This method must be called from the main thread of your app.
Parameters
result Result: The result, if any, computed in doInBackground(java.lang.Object[]), can be null

onCancelled

Added in API level 3
protected open fun onCancelled(): Unit

Deprecated: Deprecated in Java.

Applications should preferably override onCancelled(java.lang.Object). This method is invoked by the default implementation of onCancelled(java.lang.Object). The default version does nothing.

Runs on the UI thread after cancel(boolean) is invoked and doInBackground(java.lang.Object[]) has finished.


This method must be called from the main thread of your app.

onPostExecute

Added in API level 3
protected open fun onPostExecute(result: Result): Unit

Deprecated: Deprecated in Java.

Runs on the UI thread after doInBackground. The specified result is the value returned by doInBackground. To better support testing frameworks, it is recommended that this be written to tolerate direct execution as part of the execute() call. The default version does nothing.

This method won't be invoked if the task was cancelled.


This method must be called from the main thread of your app.
Parameters
result Result: The result of the operation computed by doInBackground.

onPreExecute

Added in API level 3
protected open fun onPreExecute(): Unit

Deprecated: Deprecated in Java.

Runs on the UI thread before doInBackground. Invoked directly by #execute or executeOnExecutor. The default version does nothing.
This method must be called from the main thread of your app.

onProgressUpdate

Added in API level 3
protected open fun onProgressUpdate(vararg values: Progress): Unit

Deprecated: Deprecated in Java.

Runs on the UI thread after publishProgress is invoked. The specified values are the values passed to publishProgress. The default version does nothing.
This method must be called from the main thread of your app.

Parameters
values Progress: The values indicating progress.

publishProgress

Added in API level 3
protected fun publishProgress(vararg values: Progress): Unit

Deprecated: Deprecated in Java.

This method can be invoked from doInBackground to publish updates on the UI thread while the background computation is still running. Each call to this method will trigger the execution of onProgressUpdate on the UI thread. onProgressUpdate will not be called if the task has been canceled.
This method may take several seconds to complete, so it should only be called from a worker thread.

Parameters
values Progress: The progress values to update the UI with.

Properties

SERIAL_EXECUTOR

Added in API level 11
Deprecated in API level 30
static val SERIAL_EXECUTOR: Executor!

Deprecated: Globally serializing tasks results in excessive queuing for unrelated operations.

An Executor that executes tasks one at a time in serial order. This serialization is global to a particular process.

THREAD_POOL_EXECUTOR

Added in API level 11
Deprecated in API level 30
static val THREAD_POOL_EXECUTOR: Executor!

Deprecated: Using a single thread pool for a general purpose results in suboptimal behavior for different tasks. Small, CPU-bound tasks benefit from a bounded pool and queueing, and long-running blocking tasks, such as network operations, benefit from many threads. Use or create an Executor configured for your use case.

An Executor that can be used to execute tasks in parallel.