Android APIs
public abstract class

AsyncTask

extends Object
java.lang.Object
   ↳ android.os.AsyncTask<Params, Progress, Result>

Class Overview

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 pacakge 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:

  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(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.
  3. 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.
  4. 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(Object), instead of onPostExecute(Object) will be invoked after doInBackground(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(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 in such a way that the following operations are safe without explicit synchronizations.

Order of execution

When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with 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, Object[]) with THREAD_POOL_EXECUTOR.

Summary

Nested Classes
enum AsyncTask.Status Indicates the current status of the task. 
Fields
public static final Executor SERIAL_EXECUTOR An Executor that executes tasks one at a time in serial order.
public static final Executor THREAD_POOL_EXECUTOR An Executor that can be used to execute tasks in parallel.
Public Constructors
AsyncTask()
Creates a new asynchronous task.
Public Methods
final boolean cancel(boolean mayInterruptIfRunning)

Attempts to cancel execution of this task.

static void execute(Runnable runnable)
Convenience version of execute(Object) for use with a simple Runnable object.
final AsyncTask<Params, Progress, Result> execute(Params... params)
Executes the task with the specified parameters.
final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec, Params... params)
Executes the task with the specified parameters.
final Result get(long timeout, TimeUnit unit)
Waits if necessary for at most the given time for the computation to complete, and then retrieves its result.
final Result get()
Waits if necessary for the computation to complete, and then retrieves its result.
final AsyncTask.Status getStatus()
Returns the current status of this task.
final boolean isCancelled()
Returns true if this task was cancelled before it completed normally.
Protected Methods
abstract Result doInBackground(Params... params)
Override this method to perform a computation on a background thread.
void onCancelled(Result result)

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

void onCancelled()

Applications should preferably override onCancelled(Object).

void onPostExecute(Result result)

Runs on the UI thread after doInBackground(Params...).

void onPreExecute()
Runs on the UI thread before doInBackground(Params...).
void onProgressUpdate(Progress... values)
Runs on the UI thread after publishProgress(Progress...) is invoked.
final void publishProgress(Progress... values)
This method can be invoked from doInBackground(Params...) to publish updates on the UI thread while the background computation is still running.
[Expand]
Inherited Methods
From class java.lang.Object

Fields

public static final Executor SERIAL_EXECUTOR

Added in API level 11

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

public static final Executor THREAD_POOL_EXECUTOR

Added in API level 11

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

Public Constructors

public AsyncTask ()

Added in API level 3

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

Public Methods

public final boolean cancel (boolean mayInterruptIfRunning)

Added in API level 3

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(Object) being invoked on the UI thread after doInBackground(Object[]) returns. Calling this method guarantees that onPostExecute(Object) is never invoked. After invoking this method, you should check the value returned by isCancelled() periodically from doInBackground(Object[]) to finish the task as early as possible.

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

public static void execute (Runnable runnable)

Added in API level 11

Convenience version of execute(Object) for use with a simple Runnable object. See execute(Object[]) for more information on the order of execution.

public final AsyncTask<Params, Progress, Result> execute (Params... params)

Added in API level 3

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 DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting 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(Executor, Params...) 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.

Parameters
params The parameters of the task.
Returns
  • This instance of AsyncTask.
Throws
IllegalStateException If getStatus() returns either RUNNING or FINISHED.

public final AsyncTask<Params, Progress, Result> executeOnExecutor (Executor exec, Params... params)

Added in API level 11

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.

Parameters
exec The executor to use. THREAD_POOL_EXECUTOR is available as a convenient process-wide thread pool for tasks that are loosely coupled.
params The parameters of the task.
Returns
  • This instance of AsyncTask.
Throws
IllegalStateException If getStatus() returns either RUNNING or FINISHED.

public final Result get (long timeout, TimeUnit unit)

Added in API level 3

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

Parameters
timeout Time to wait before cancelling the operation.
unit The time unit for the timeout.
Returns
  • The computed result.
Throws
CancellationException If the computation was cancelled.
ExecutionException If the computation threw an exception.
InterruptedException If the current thread was interrupted while waiting.
TimeoutException If the wait timed out.

public final Result get ()

Added in API level 3

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

Returns
  • The computed result.
Throws
CancellationException If the computation was cancelled.
ExecutionException If the computation threw an exception.
InterruptedException If the current thread was interrupted while waiting.

public final AsyncTask.Status getStatus ()

Added in API level 3

Returns the current status of this task.

Returns
  • The current status.

public final boolean isCancelled ()

Added in API level 3

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(Object[]) to end the task as soon as possible.

Returns
  • true if task was cancelled before it completed
See Also

Protected Methods

protected abstract Result doInBackground (Params... params)

Added in API level 3

Override this method to perform a computation on a background thread. The specified parameters are the parameters passed to execute(Params...) by the caller of this task. This method can call publishProgress(Progress...) to publish updates on the UI thread.

Parameters
params The parameters of the task.
Returns
  • A result, defined by the subclass of this task.

protected void onCancelled (Result result)

Added in API level 11

Runs on the UI thread after cancel(boolean) is invoked and doInBackground(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).

Parameters
result The result, if any, computed in doInBackground(Object[]), can be null

protected void onCancelled ()

Added in API level 3

Applications should preferably override onCancelled(Object). This method is invoked by the default implementation of onCancelled(Object).

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

protected void onPostExecute (Result result)

Added in API level 3

Runs on the UI thread after doInBackground(Params...). The specified result is the value returned by doInBackground(Params...).

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

Parameters
result The result of the operation computed by doInBackground(Params...).

protected void onPreExecute ()

Added in API level 3

protected void onProgressUpdate (Progress... values)

Added in API level 3

Runs on the UI thread after publishProgress(Progress...) is invoked. The specified values are the values passed to publishProgress(Progress...).

Parameters
values The values indicating progress.

protected final void publishProgress (Progress... values)

Added in API level 3

This method can be invoked from doInBackground(Params...) 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(Progress...) on the UI thread. onProgressUpdate(Progress...) will not be called if the task has been canceled.

Parameters
values The progress values to update the UI with.