AsyncyTask源碼

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 package android.os;

 import java.util.ArrayDeque;

 import java.util.concurrent.BlockingQueue;

 import java.util.concurrent.Callable;

 import java.util.concurrent.CancellationException;

 import java.util.concurrent.Executor;

 import java.util.concurrent.ExecutionException;

 import java.util.concurrent.FutureTask;

 import java.util.concurrent.LinkedBlockingQueue;

 import java.util.concurrent.ThreadFactory;

 import java.util.concurrent.ThreadPoolExecutor;

 import java.util.concurrent.TimeUnit;

 import java.util.concurrent.TimeoutException;

 import java.util.concurrent.atomic.AtomicBoolean;

 import java.util.concurrent.atomic.AtomicInteger;

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 java.lang.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 java.util.concurrent.Executor,java.util.concurrent.ThreadPoolExecutor and java.util.concurrent.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 andResult, 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(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));
             // 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 typejava.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 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(java.lang.Object[]), 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(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 topublishProgress(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.

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(java.lang.Object), instead of onPostExecute(java.lang.Object) will be invoked after doInBackground(java.lang.Object[]) returns. To ensure that a task is cancelled as quickly as possible, you should always check the return value ofisCancelled() periodically from doInBackground(java.lang.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 ofBuild.VERSION_CODES.JELLY_BEAN.
• The task instance must be created on the UI thread.
• execute(java.lang.Object[]) must be invoked on the UI thread.
• Do not call onPreExecute(), onPostExecute(java.lang.Object),doInBackground(java.lang.Object[]), onProgressUpdate(java.lang.Object[]) 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(java.lang.Object[]).
• Set member fields in doInBackground(java.lang.Object[]), and refer to them inonProgressUpdate(java.lang.Object[]) and onPostExecute(java.lang.Object).

Order of execution

When first introduced, AsyncTasks were executed serially on a single background thread. Starting withBuild.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 invokeexecuteOnExecutor(java.util.concurrent.Executor,java.lang.Object[]) with THREAD_POOL_EXECUTOR.

public abstract class AsyncTask<Params, Progress, Result> {

    private static final String LOG_TAG = "AsyncTask";

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();

    private static final int CORE_POOL_SIZE = CPU_COUNT + 1;

    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;

    private static final int KEEP_ALIVE = 1;

    private static final ThreadFactory sThreadFactory = new ThreadFactory() {

        private final AtomicInteger mCount = new AtomicInteger(1);

        public Thread newThread(Runnable r) {

            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());

        }

    };

    private static final BlockingQueue<Runnable> sPoolWorkQueue =

            new LinkedBlockingQueue<Runnable>(128);

    

An java.util.concurrent.Executor that can be used to execute tasks in parallel.

    public static final Executor THREAD_POOL_EXECUTOR

            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,

                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

    

An java.util.concurrent.Executor that executes tasks one at a time in serial order. This serialization is global to a particular process.

    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

    private static final int MESSAGE_POST_RESULT = 0x1;

    private static final int MESSAGE_POST_PROGRESS = 0x2;

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

    private static InternalHandler sHandler;

    private final WorkerRunnable<Params, Result> mWorker;

    private final FutureTask<Result> mFuture;

    private volatile Status mStatus = Status.PENDING;

    private final AtomicBoolean mCancelled = new AtomicBoolean();

    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();

    private static class SerialExecutor implements Executor {

        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();

        Runnable mActive;

        public synchronized void execute(final Runnable r) {

            mTasks.offer(new Runnable() {

                public void run() {

                    try {

                        r.run();

                    } finally {

                        scheduleNext();

                    }

                }

            });

            if (mActive == null) {

                scheduleNext();

            }

        }

        protected synchronized void scheduleNext() {

            if ((mActive = mTasks.poll()) != null) {

                THREAD_POOL_EXECUTOR.execute(mActive);

            }

        }

    }

    

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,

    }

    private static Handler getHandler() {

        synchronized (AsyncTask.class) {

            if (sHandler == null) {

                sHandler = new InternalHandler();

            }

            return sHandler;

        }

    }

    

Hide:

    public static void setDefaultExecutor(Executor exec) {

        sDefaultExecutor = exec;

    }

    

Creates a new asynchronous task. This constructor must be invoked on the UI thread.

    public AsyncTask() {

        mWorker = new WorkerRunnable<Params, Result>() {

            public Result call() throws Exception {

                mTaskInvoked.set(true);

                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);

                //noinspection unchecked

                return postResult(doInBackground(mParams));

            }

        };

        mFuture = new FutureTask<Result>(mWorker) {

            @Override

            protected void done() {

                try {

                    postResultIfNotInvoked(get());

                } catch (InterruptedException e) {

                    android.util.Log.w(LOG_TAG, e);

                } catch (ExecutionException e) {

                    throw new RuntimeException("An error occured while executing doInBackground()",

                            e.getCause());

                } catch (CancellationException e) {

                    postResultIfNotInvoked(null);

                }

            }

        };

    }

    private void postResultIfNotInvoked(Result result) {

        final boolean wasTaskInvoked = mTaskInvoked.get();

        if (!wasTaskInvoked) {

            postResult(result);

        }

    }

    private Result postResult(Result result) {

        @SuppressWarnings("unchecked")

        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,

                new AsyncTaskResult<Result>(this, result));

        message.sendToTarget();

        return result;

    }

    

Returns the current status of this task.

Returns:

The current status.

    public final Status getStatus() {

        return mStatus;

    }

    

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 callpublishProgress(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);

    

Runs on the UI thread before doInBackground(java.lang.Object[]).

See also:

onPostExecute(java.lang.Object)

doInBackground(java.lang.Object[])

    protected void onPreExecute() {

    }

    

Runs on the UI thread after doInBackground. The specified result is the value returned by doInBackground.

This method won't be invoked if the task was cancelled.

Parameters:

result The result of the operation computed by doInBackground.

See also:

onPreExecute

doInBackground

onCancelled(Object)

    @SuppressWarnings({"UnusedDeclaration"})

    protected void onPostExecute(Result result) {

    }

    

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

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 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:

cancel(boolean)

isCancelled()

    @SuppressWarnings({"UnusedParameters"})

    protected void onCancelled(Result result) {

        onCancelled();

    }    
    
    

Applications should preferably override onCancelled(java.lang.Object). This method is invoked by the default implementation of onCancelled(java.lang.Object).

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

See also:

onCancelled(java.lang.Object)

cancel(boolean)

isCancelled()

    protected void onCancelled() {

    }

    

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 fromdoInBackground(java.lang.Object[]) to end the task as soon as possible.

Returns:

true if task was cancelled before it completed

See also:

cancel(boolean)

    public final boolean isCancelled() {

        return mCancelled.get();

    }

    

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 whencancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunningparameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

Calling this method will result in onCancelled(java.lang.Object) being invoked on the UI thread afterdoInBackground(java.lang.Object[]) returns. Calling this method guarantees thatonPostExecute(java.lang.Object) is never invoked. After invoking this method, you should check the value returned by isCancelled() periodically from doInBackground(java.lang.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; trueotherwise

See also:

isCancelled()

onCancelled(java.lang.Object)

    public final boolean cancel(boolean mayInterruptIfRunning) {

        mCancelled.set(true);

        return mFuture.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 InterruptedException, ExecutionException {

        return mFuture.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 timeout, TimeUnit unit) throws InterruptedException,

            ExecutionException, TimeoutException {

        return mFuture.get(timeout, unit);

    }

    

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(java.util.concurrent.Executor,java.lang.Object[])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:

java.lang.IllegalStateException If getStatus() returns either AsyncTask.Status.RUNNING orAsyncTask.Status.FINISHED.

See also:

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

execute(java.lang.Runnable)

    public final AsyncTask<Params, Progress, Result> execute(Params... params) {

        return executeOnExecutor(sDefaultExecutor, 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 java.util.concurrent.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:

java.lang.IllegalStateException If getStatus() returns either AsyncTask.Status.RUNNING orAsyncTask.Status.FINISHED.

See also:

execute(java.lang.Object[])

    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,

            Params... params) {

        if (mStatus != Status.PENDING) {

            switch (mStatus) {

                case RUNNING:

                    throw new IllegalStateException("Cannot execute task:"

                            + " the task is already running.");

                case FINISHED:

                    throw new IllegalStateException("Cannot execute task:"

                            + " the task has already been executed "

                            + "(a task can be executed only once)");

            }

        }

        mStatus = Status.RUNNING;

        onPreExecute();

        mWorker.mParams = params;

        exec.execute(mFuture);

        return this;

    }

    

Convenience version of execute(java.lang.Object[]) for use with a simple Runnable object. Seeexecute(java.lang.Object[]) for more information on the order of execution.

See also:

execute(java.lang.Object[])

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

    public static void execute(Runnable runnable) {

        sDefaultExecutor.execute(runnable);

    }

    

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 ofonProgressUpdate(java.lang.Object[]) on the UI thread. onProgressUpdate(java.lang.Object[]) will not be called if the task has been canceled.

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) {

        if (!isCancelled()) {

            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,

                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();

        }

    }

    private void finish(Result result) {

        if (isCancelled()) {

            onCancelled(result);

        } else {

            onPostExecute(result);

        }

        mStatus = Status.FINISHED;

    }

    private static class InternalHandler extends Handler {

        public InternalHandler() {

            super(Looper.getMainLooper());

        }

        @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})

        @Override

        public void handleMessage(Message msg) {

            AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;

            switch (msg.what) {

                case MESSAGE_POST_RESULT:

                    // There is only one result

                    result.mTask.finish(result.mData[0]);

                    break;

                case MESSAGE_POST_PROGRESS:

                    result.mTask.onProgressUpdate(result.mData);

                    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) {

            mTask = task;

            mData = data;

        }

    }

}