AsyncTask

public abstract class AsyncTask extends Object

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 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(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.

Nested Class Summary

enum AsyncTask.Status Indicates the current status of the task. 

Field Summary

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 Constructor Summary

AsyncTask()
Creates a new asynchronous task.

Public Method Summary

final boolean
cancel(boolean mayInterruptIfRunning)

Attempts to cancel execution of this task.

final AsyncTask<Params, Progress, Result>
execute(Params... params)
Executes the task with the specified parameters.
static void
execute(Runnable runnable)
Convenience version of execute(Object) for use with a simple Runnable object.
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 Method Summary

abstract Result
doInBackground(Params... params)
Override this method to perform a computation on a background thread.
void
onCancelled()

Applications should preferably override onCancelled(Object).

void
onCancelled(Result result)

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

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.

Inherited Method Summary