public abstract class AsyncTask<Params,Progress,Result>
extends java.lang.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 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.
For more information about using tasks and threads, read the Processes and Threads developer guide.
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);
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> { ... }
When an asynchronous task is executed, the task goes through 4 steps:
onPreExecute(), invoked on the UI thread immediately after 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 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.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.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.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.)
There are a few threading rules that must be followed for this class to work properly:
Build.VERSION_CODES.JELLY_BEAN.execute(Params...) must be invoked on the UI thread.onPreExecute(), onPostExecute(Result),
doInBackground(Params...), onProgressUpdate(Progress...) manually.AsyncTask guarantees that all callback calls are synchronized in such a way that the following operations are safe without explicit synchronizations.
onPreExecute(), and refer to them
in doInBackground(Params...).
doInBackground(Params...), and refer to them in
onProgressUpdate(Progress...) and onPostExecute(Result).
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, Object[]) with
THREAD_POOL_EXECUTOR.
| Modifier and Type | Class and Description |
|---|---|
static class |
AsyncTask.Status
Indicates the current status of the task.
|
| Modifier and Type | Field and Description |
|---|---|
static java.util.concurrent.Executor |
DUAL_THREAD_EXECUTOR |
static java.util.concurrent.Executor |
SERIAL_EXECUTOR
An
Executor that executes tasks one at a time in serial
order. |
static java.util.concurrent.Executor |
THREAD_POOL_EXECUTOR
An
Executor that can be used to execute tasks in parallel. |
| Constructor and Description |
|---|
AsyncTask()
Creates a new asynchronous task.
|
| Modifier and Type | Method and Description |
|---|---|
boolean |
cancel(boolean mayInterruptIfRunning)
Attempts to cancel execution of this task.
|
protected abstract Result |
doInBackground(Params... params)
Override this method to perform a computation on a background thread.
|
AsyncTask<Params,Progress,Result> |
execute(Params... params)
Executes the task with the specified parameters.
|
static void |
execute(java.lang.Runnable runnable)
Convenience version of
execute(Object...) for use with
a simple Runnable object. |
AsyncTask<Params,Progress,Result> |
executeOnExecutor(java.util.concurrent.Executor exec,
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(long timeout,
java.util.concurrent.TimeUnit unit)
Waits if necessary for at most the given time for the computation
to complete, and then retrieves its result.
|
AsyncTask.Status |
getStatus()
Returns the current status of this task.
|
static void |
init() |
boolean |
isCancelled()
Returns true if this task was cancelled before it completed
normally.
|
protected void |
onCancelled()
Applications should preferably override
onCancelled(Object). |
protected void |
onCancelled(Result result)
Runs on the UI thread after
cancel(boolean) is invoked and
doInBackground(Object[]) has finished. |
protected void |
onPostExecute(Result result)
Runs on the UI thread after
doInBackground(Params...). |
protected void |
onPreExecute()
Runs on the UI thread before
doInBackground(Params...). |
protected void |
onProgressUpdate(Progress... values)
Runs on the UI thread after
publishProgress(Progress...) is invoked. |
protected 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. |
static void |
setDefaultExecutor(java.util.concurrent.Executor exec) |
public static final java.util.concurrent.Executor THREAD_POOL_EXECUTOR
Executor that can be used to execute tasks in parallel.public static final java.util.concurrent.Executor SERIAL_EXECUTOR
Executor that executes tasks one at a time in serial
order. This serialization is global to a particular process.public static final java.util.concurrent.Executor DUAL_THREAD_EXECUTOR
public AsyncTask()
public static void init()
public static void setDefaultExecutor(java.util.concurrent.Executor exec)
public final AsyncTask.Status getStatus()
protected abstract Result doInBackground(Params... params)
execute(Params...)
by the caller of this task.
This method can call publishProgress(Progress...) to publish updates
on the UI thread.params - The parameters of the task.onPreExecute(),
onPostExecute(Result),
publishProgress(Progress...)protected void onPreExecute()
doInBackground(Params...).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.
result - The result of the operation computed by doInBackground(Params...).onPreExecute(),
doInBackground(Params...),
onCancelled(Object)protected void onProgressUpdate(Progress... values)
publishProgress(Progress...) is invoked.
The specified values are the values passed to publishProgress(Progress...).values - The values indicating progress.publishProgress(Progress...),
doInBackground(Params...)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).
result - The result, if any, computed in
doInBackground(Object[]), can be nullcancel(boolean),
isCancelled()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.
onCancelled(Object),
cancel(boolean),
isCancelled()public final boolean isCancelled()
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.cancel(boolean)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.
mayInterruptIfRunning - true if the thread executing this
task should be interrupted; otherwise, in-progress tasks are allowed
to complete.isCancelled(),
onCancelled(Object)public final Result get() throws java.lang.InterruptedException, java.util.concurrent.ExecutionException
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.public final Result get(long timeout, java.util.concurrent.TimeUnit unit) throws java.lang.InterruptedException, java.util.concurrent.ExecutionException, java.util.concurrent.TimeoutException
timeout - Time to wait before cancelling the operation.unit - The time unit for the timeout.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.public final AsyncTask<Params,Progress,Result> execute(Params... params)
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(java.util.concurrent.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.
params - The parameters of the task.java.lang.IllegalStateException - If getStatus() returns either
AsyncTask.Status.RUNNING or AsyncTask.Status.FINISHED.executeOnExecutor(java.util.concurrent.Executor, Object[]),
execute(Runnable)public final AsyncTask<Params,Progress,Result> executeOnExecutor(java.util.concurrent.Executor exec, Params... params)
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.
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.java.lang.IllegalStateException - If getStatus() returns either
AsyncTask.Status.RUNNING or AsyncTask.Status.FINISHED.execute(Object[])public static void execute(java.lang.Runnable runnable)
execute(Object...) for use with
a simple Runnable object. See execute(Object[]) for more
information on the order of execution.protected final void publishProgress(Progress... values)
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.values - The progress values to update the UI with.onProgressUpdate(Progress...),
doInBackground(Params...)