Start line:  
End line:  

Snippet Preview

Snippet HTML Code

Stack Overflow Questions
  /*
   * Copyright (C) 2008 The Android Open Source Project
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 
 package android.os;
 

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.

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 begin, doInBackground, processProgress and end.

Usage

AsyncTask must be subclassed to be used. The subclass will override at least one method (doInBackground(java.lang.Object[])), and most often will override a second one (onPostExecute(java.lang.Object).)

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));
         }
         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 java.lang.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 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.
  2. doInBackground(java.lang.Object[]), 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(java.lang.Object[]) to publish one or more units of progress. These values are published on the UI thread, in the onProgressUpdate(java.lang.Object[]) step.
  3. onProgressUpdate(java.lang.Object[]), invoked on the UI thread after a call to publishProgress(java.lang.Object[]). 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(java.lang.Object), invoked on the UI thread after the background computation finishes. The result of the background computation is passed to this step as a parameter.

Threading rules

There are a few threading rules that must be followed for this class to work properly:

public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";
    private static final int CORE_POOL_SIZE = 5;
    private static final int MAXIMUM_POOL_SIZE = 128;
    private static final int KEEP_ALIVE = 10;
    private static final BlockingQueue<RunnablesWorkQueue =
            new LinkedBlockingQueue<Runnable>(10);
    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);
        public Thread newThread(Runnable r) {
            return new Thread(r"AsyncTask #" + .getAndIncrement());
        }
    };
    private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(,
    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;
    private static final int MESSAGE_POST_CANCEL = 0x3;
    private static final InternalHandler sHandler = new InternalHandler();
    private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture;
    private volatile Status mStatus = .;

    
Indicates the current status of the task. Each status will be set only once during the lifetime of a task.
    public enum Status {
        
Indicates that the task has not been executed yet.
        PENDING,
        
Indicates that the task is running.
        RUNNING,
        
Indicates that AsyncTask.onPostExecute(java.lang.Object) has finished.
        FINISHED,
    }

    
Creates a new asynchronous task. This constructor must be invoked on the UI thread.
    public AsyncTask() {
         = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                Process.setThreadPriority(.);
                return doInBackground();
            }
        };
         = new FutureTask<Result>() {
            @Override
            protected void done() {
                Message message;
                Result result = null;
                try {
                    result = get();
                } catch (InterruptedException e) {
                    android.util.Log.w(e);
                } catch (ExecutionException e) {
                    throw new RuntimeException("An error occured while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    message = .obtainMessage(,
                            new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));
                    message.sendToTarget();
                    return;
                } catch (Throwable t) {
                    throw new RuntimeException("An error occured while executing "
                            + "doInBackground()"t);
                }
                message = .obtainMessage(,
                        new AsyncTaskResult<Result>(AsyncTask.thisresult));
                message.sendToTarget();
            }
        };
    }

    
Returns the current status of this task.

Returns:
The current status.
    public final Status getStatus() {
        return ;
    }

    
Override this method to perform a computation on a background thread. The specified parameters are the parameters passed to execute(java.lang.Object[]) by the caller of this task. This method can call publishProgress(java.lang.Object[]) to publish updates on the UI thread.

Parameters:
params The parameters of the task.
Returns:
A result, defined by the subclass of this task.
See also:
onPreExecute()
onPostExecute(java.lang.Object)
publishProgress(java.lang.Object[])
    protected abstract Result doInBackground(Params... params);

    
    protected void onPreExecute() {
    }

    
Runs on the UI thread after doInBackground. The specified result is the value returned by doInBackground or null if the task was cancelled or an exception occured.

Parameters:
result The result of the operation computed by doInBackground.
See also:
onPreExecute
doInBackground
    @SuppressWarnings({"UnusedDeclaration"})
    protected void onPostExecute(Result result) {
    }

    
Runs on the UI thread after publishProgress is invoked. The specified values are the values passed to publishProgress.

Parameters:
values The values indicating progress.
See also:
publishProgress
doInBackground
    @SuppressWarnings({"UnusedDeclaration"})
    protected void onProgressUpdate(Progress... values) {
    }

    
Runs on the UI thread after cancel(boolean) is invoked.

    protected void onCancelled() {
    }

    
Returns true if this task was cancelled before it completed normally.

Returns:
true if task was cancelled before it completed
See also:
cancel(boolean)
    public final boolean isCancelled() {
        return .isCancelled();
    }

    
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.

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:
isCancelled()
onCancelled()
    public final boolean cancel(boolean mayInterruptIfRunning) {
        return .cancel(mayInterruptIfRunning);
    }

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

Returns:
The computed result.
Throws:
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() throws InterruptedExceptionExecutionException {
        return .get();
    }

    
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:
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 Result get(long timeoutTimeUnit unitthrows InterruptedException,
            ExecutionExceptionTimeoutException {
        return .get(timeoutunit);
    }

    
Executes the task with the specified parameters. The task returns itself (this) so that the caller can keep a reference to it. This method must be invoked on the UI thread.

Parameters:
params The parameters of the task.
Returns:
This instance of AsyncTask.
Throws:
java.lang.IllegalStateException If getStatus() returns either AsyncTask.Status.RUNNING or AsyncTask.Status.FINISHED.
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        if ( != .) {
            switch () {
                case :
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task is already running.");
                case :
                    throw new IllegalStateException("Cannot execute task:"
                            + " the task has already been executed "
                            + "(a task can be executed only once)");
            }
        }
         = .;
        onPreExecute();
        . = params;
        .execute();
        return this;
    }

    
This method can be invoked from doInBackground(java.lang.Object[]) 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(java.lang.Object[]) on the UI thread.

Parameters:
values The progress values to update the UI with.
See also:
onProgressUpdate(java.lang.Object[])
doInBackground(java.lang.Object[])
    protected final void publishProgress(Progress... values) {
                new AsyncTaskResult<Progress>(thisvalues)).sendToTarget();
    }
    private void finish(Result result) {
        if (isCancelled()) result = null;
        onPostExecute(result);
         = .;
    }
    private static class InternalHandler extends Handler {
        @SuppressWarnings({"unchecked""RawUseOfParameterizedType"})
        @Override
        public void handleMessage(Message msg) {
            AsyncTaskResult result = (AsyncTaskResultmsg.obj;
            switch (msg.what) {
                case :
                    // There is only one result
                    result.mTask.finish(result.mData[0]);
                    break;
                case :
                    result.mTask.onProgressUpdate(result.mData);
                    break;
                case :
                    result.mTask.onCancelled();
                    break;
            }
        }
    }
    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
        Params[] mParams;
    }
    @SuppressWarnings({"RawUseOfParameterizedType"})
    private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;
        AsyncTaskResult(AsyncTask task, Data... data) {
             = task;
             = data;
        }
    }
New to GrepCode? Check out our FAQ X