來源：https://developer.android.com/reference/android/os/AsyncTask

AsyncTask

public abstract class AsyncTask 
extends Object 

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

 

---

 

AsyncTask enables proper and easy use of the UI thread. This class allows you 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:

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 ofonPostExecute(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 fromdoInBackground(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 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 in doInBackground(Params...).
• Set member fields in doInBackground(Params...), and refer to them in onProgressUpdate(Progress...) and onPostExecute(Result).

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, Object[]) with THREAD_POOL_EXECUTOR.

 

Summary

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

Fields
public static finalExecutor	SERIAL_EXECUTOR An Executor that executes tasks one at a time in serial order.
public static finalExecutor	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.
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 methods
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 methods
From class java.lang.Object

Fields

SERIAL_EXECUTOR

added in API level 11

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.

 

THREAD_POOL_EXECUTOR

added in API level 11

public static final Executor THREAD_POOL_EXECUTOR

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

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 fromdoInBackground(Object[]) to finish the task as early as possible.

 

 

Parameters
mayInterruptIfRunning	boolean: true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete.

Returns
boolean	false if the task could not be cancelled, typically because it has already completed normally; trueotherwise

See also:

• isCancelled()
• onCancelled(Object)

execute

added in API level 3

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.

Parameters
params	Params: The parameters of the task.

Returns
AsyncTask<Params, Progress, Result>	This instance of AsyncTask.

Throws
IllegalStateException	If getStatus() returns either AsyncTask.Status.RUNNING or AsyncTask.Status.FINISHED.

See also:

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

execute

added in API level 11

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.

This method must be called from the main thread of your app.

Parameters
runnable	Runnable

See also:

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

executeOnExecutor

added in API level 11

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.

Parameters
exec	Executor: The executor to use. THREAD_POOL_EXECUTOR is available as a convenient process-wide thread pool for tasks that are loosely coupled.
params	Params: The parameters of the task.

Returns
AsyncTask<Params, Progress, Result>	This instance of AsyncTask.

Throws
IllegalStateException	If getStatus() returns either AsyncTask.Status.RUNNING or AsyncTask.Status.FINISHED.

See also:

• execute(Object[])

get

added in API level 3

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	long: Time to wait before cancelling the operation.
unit	TimeUnit: The time unit for the timeout.

Returns
Result	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.

get

added in API level 3

public final Result get ()

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

 

Returns
Result	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.

getStatus

added in API level 3

public final AsyncTask.Status getStatus ()

Returns the current status of this task.

 

Returns
AsyncTask.Status	The current status.

isCancelled

added in API level 3

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
boolean	true if task was cancelled before it completed

See also:

• cancel(boolean)

Protected methods

doInBackground

added in API level 3

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.

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

Parameters
params	Params: The parameters of the task.

Returns
Result	A result, defined by the subclass of this task.

See also:

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

onCancelled

added in API level 3

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.

 

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

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).

 

This method must be called from the main thread of your app.

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

See also:

• cancel(boolean)
• isCancelled()

onPostExecute

added in API level 3

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.

 

This method must be called from the main thread of your app.

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

See also:

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

onPreExecute

added in API level 3

protected void onPreExecute ()

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

This method must be called from the main thread of your app.

See also:

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

onProgressUpdate

added in API level 3

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...).

This method must be called from the main thread of your app.

Parameters
values	Progress: The values indicating progress.

See also:

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

publishProgress

added in API level 3

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 ofonProgressUpdate(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.

Parameters
values	Progress: The progress values to update the UI with.

See also:

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