After the written outline for a program is approved, the network will order a ______.

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 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.

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(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.
• Do not call onPreExecute(), onPostExecute(Result), doInBackground(Params...), onProgressUpdate(Progress...) 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...).
• The memory effects of doInBackground(Params...) are visible to onPostExecute(Result).
• 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.)
• 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

Fields

SERIAL_EXECUTOR

Added in API level 11
Deprecated in API level 30

public static final Executor SERIAL_EXECUTOR

This field was deprecated in API level 30.
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

public static final Executor THREAD_POOL_EXECUTOR

This field was deprecated in API level 30.
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.

Public constructors

AsyncTask

Added in API level 3

public AsyncTask ()

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

Public methods

cancel

Added in API level 3
Deprecated in API level 30

public final boolean cancel (boolean mayInterruptIfRunning)

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(Result) 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.

See also:

• isCancelled()
• onCancelled(Object)

execute

Added in API level 3
Deprecated in API level 30

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

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 Build.VERSION_CODES.DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting 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(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.
This method must be called from the main thread of your app.

See also:

• executeOnExecutor(java.util.concurrent.Executor, Object[])
• execute(Runnable)

execute

Added in API level 11
Deprecated in API level 30

public static void execute (Runnable runnable)

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.

See also:

• execute(Object[])
• executeOnExecutor(java.util.concurrent.Executor, Object[])

executeOnExecutor

Added in API level 11
Deprecated in API level 30

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

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.

See also:

• execute(Object[])

get

Added in API level 3
Deprecated in API level 30

public 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.

get

Added in API level 3
Deprecated in API level 30

public final Result get ()

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

getStatus

Added in API level 3
Deprecated in API level 30

public final AsyncTask.Status getStatus ()

Returns the current status of this task.

isCancelled

Added in API level 3
Deprecated in API level 30

public final boolean isCancelled ()

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.

See also:

• cancel(boolean)

Protected methods

doInBackground

Added in API level 3
Deprecated in API level 30

protected abstract Result doInBackground (Params... params)

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 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(Params...) call. This method can call publishProgress(Progress...) 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.

See also:

• onPreExecute()
• onPostExecute(Result)
• publishProgress(Progress...)

onCancelled

Added in API level 3
Deprecated in API level 30

protected void onCancelled ()

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.

See also:

• onCancelled(Object)
• cancel(boolean)
• isCancelled()

onCancelled

Added in API level 11
Deprecated in API level 30

protected void onCancelled (Result result)

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.

See also:

• cancel(boolean)
• isCancelled()

onPostExecute

Added in API level 3
Deprecated in API level 30

protected void onPostExecute (Result result)

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

See also:

• onPreExecute()
• doInBackground(Params...)
• onCancelled(Object)

onPreExecute

Added in API level 3
Deprecated in API level 30

protected void onPreExecute ()

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

See also:

• onPostExecute(Result)
• doInBackground(Params...)

onProgressUpdate

Added in API level 3
Deprecated in API level 30

protected void onProgressUpdate (Progress... values)

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

See also:

• publishProgress(Progress...)
• doInBackground(Params...)

publishProgress

Added in API level 3
Deprecated in API level 30

protected 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. 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.
This method may take several seconds to complete, so it should only be called from a worker thread.

See also:

• onProgressUpdate(Progress...)
• doInBackground(Params...)