Known Direct Subclasses
PriorityAsyncTask<Params, Progress, Result>
|
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
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 afteronPreExecute()
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 usepublishProgress(Progress...)
to publish one or more units of progress. These values are published on the UI thread, in theonProgressUpdate(Progress...)
step.onProgressUpdate(Progress...)
, invoked on the UI thread after a call topublishProgress(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(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:
- The AsyncTask class must be loaded on the UI thread. This is done
automatically as of
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 in such a way that the following operations are safe without explicit synchronizations.
- Set member fields in the constructor or
onPreExecute()
, and refer to them indoInBackground(Params...)
. - Set member fields in
doInBackground(Params...)
, and refer to them inonProgressUpdate(Progress...)
andonPostExecute(Result)
.
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 |
void |
onCancelled(Result result)
Runs on the UI thread after |
void |
onPostExecute(Result result)
Runs on the UI thread after |
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
Fields
public static final Executor SERIAL_EXECUTOR
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
An Executor
that can be used to execute tasks in parallel.
Public Constructors
public AsyncTask ()
Creates a new asynchronous task. This constructor must be invoked on the UI thread.
Public Methods
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(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
See Also
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 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 static void execute (Runnable runnable)
Convenience version of execute(Object)
for use with
a simple Runnable object. See execute(Object[])
for more
information on the order of execution.
Parameters
runnable |
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.
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 . |
See Also
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.
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 ()
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 ()
Returns the current status of this task.
Returns
- The current status.
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(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)
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 ()
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 onCancelled (Result result)
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 |
See Also
protected void onPostExecute (Result result)
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 onProgressUpdate (Progress... values)
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)
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 note be called if the task has been
canceled.
Parameters
values | The progress values to update the UI with. |