源码分析--AsyncTask

ゝ一纸荒年。 2022-08-06 15:07 130阅读 0赞

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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使适当的和易用的用户界面线程。这个类允许执行后台操作，在UI线程上发布的结果而无需操纵线程和/或处理程序。

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 forshort 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`.

AsyncTask被设计成Thread和Handler的一个辅助类，不构成一个通用的线程框架。asynctasks应用于短操作（最多几秒钟）如果你需要保持线程长时间运行的，高度推荐你使用各种的java.util.concurrent包提供如执行 `Executor`, `ThreadPoolExecutor`和`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`.

一个异步任务的耗时计算运行是在后台线程，其执行结果在UI线程。一个异步任务由3个泛型类型定义，称为 `Params`, `Progress` 和 Result，和4个步骤，称为它的

onPreExecute

doInBackground

`onProgressUpdate`

onPostExecute

### 文档给我们提供的例子 ###

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);

好了从文档我们可以对AsyncTask类有了一个大概的了解，我们看一下他的结构

![SouthEast][]

具体看一下上面提到的4个方法

//Runs on the UI thread before doInBackground. 运行在UI线程，在doInBackground之前执行
    onPreExecute
    
    //Override this method to perform a computation on a background thread.  重写该方法来实现在一个后台线程中耗时操作
    //The specified parameters are the parameters passed to execute by the caller of this task.  该方法的参数是又task执行execute方法时传递进来的参数
    //This method can call publishProgress to publish updates on the UI thread. 该方法会调用publishProgress去在子线程更新UI
    doInBackground
    
    //Runs on the UI thread after doInBackground. The specified result is the value returned by doInBackground. 运行在UI线程，在doInBackground之后执行，result是doInBackgroud方法返回的
    //This method won't be invoked if the task was cancelled. 如果task被取消了将不会执行该方法
    onPostExecute
    
    //Runs on the UI thread after publishProgress is invoked.  运行在UI线程，在publishProgress之后执行
    //The specified values are the values passed to publishProgress.//values 是由publicProgress传递过来的
    onProgressUpdate(Void... values) 
    
    //Runs on the UI thread after cancel(boolean) is invoked and doInBackground(Object []) has finished. 运行在UI线程，在doInBackground执行完毕后被调用
     onCancelled

我写的一个小例子，从网络下载一张图片，下载过程中更新进度条，下载完成后隐藏进度条，显示图片

public class MainActivity extends Activity {
 
 private Context context;
 private ImageView iv;
 private Handler handler;
 
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_main);
 context = this;
 iv = (ImageView) findViewById(R.id.iv);
 new GetImageTask().execute("http://p18.qhimg.com/bdr/__85/d/_open360/1217xiaoqingxin/3.jpg");
 }
 
 private ProgressDialog dialog;
 
 class GetImageTask extends AsyncTask<String, Integer, File> {
 @Override
 protected void onPreExecute() {
 
 dialog = new ProgressDialog(context);
 dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
 dialog.show();
 super.onPreExecute();
 }
 
 @Override
 protected File doInBackground(String... params) {
 
 try {
 File file = new File(context.getCacheDir() + "/img.jpg");
 
 HttpClient client = new DefaultHttpClient();
 HttpResponse response = client.execute(new HttpGet(params[0]));
 HttpEntity entity = response.getEntity();
 long lenghtOfFile = entity.getContentLength();
 InputStream is = entity.getContent();
 FileOutputStream fos = new FileOutputStream(file);
 // 计算文件长度
 long total = 0;
 byte[] buffer = new byte[2014];
 int len = 0;
 while ((len = is.read(buffer)) != -1) {
 fos.write(buffer, 0, len);
 total += len;
 publishProgress((int) (total * 100.0f / lenghtOfFile));
 }
 fos.flush();
 fos.close();
 is.close();
 
 return file;
 
 } catch (IOException e) {
 e.printStackTrace();
 }
 return null;
 }
 
 @Override
 protected void onProgressUpdate(Integer... values) {
 super.onProgressUpdate(values);
 dialog.setProgress(values[0]);
 }
 
 @Override
 protected void onCancelled(File result) {
 super.onCancelled(result);
 }
 
 @Override
 protected void onCancelled() {
 super.onCancelled();
 }
 
 @Override
 protected void onPostExecute(File result) {
 super.onPostExecute(result);
 if (dialog != null && dialog.isShowing()) {
 dialog.dismiss();
 }
 iv.setImageBitmap(BitmapFactory.decodeFile(result.getAbsolutePath()));
 }
 
 }
 
 }

分析AsyncTask，我们从构造方法入手

/**
 * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
 */
 public AsyncTask() {
 mWorker = new WorkerRunnable<Params, Result>() {//这里创建了一个WorkerRunnable，他是一个实现了Callable接口的抽象类
 public Result call() throws Exception {
 mTaskInvoked.set(true);
 //设置为后台进程
 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
 //noinspection unchecked
 return postResult(doInBackground(mParams)); //这里执行了doInBackground方法
 }
 };
 //创建一个FutureTask（实现了Runnable接口），将上面的Callable对象传递进来，会在FutureTask的run方法中调用Callable的call()方法，所以doInBackground是后台线程执行的
 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);
 }
 }
 };
 }

创建一个FutureTask（实现了Runnable接口），将上面的Callable对象传递进来，会在FutureTask的run方法中调用Callable的call()方法，所以doInBackground是后台线程执行的，我们继续看postResult

private Result postResult(Result result) {
 @SuppressWarnings("unchecked")
 Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
 new AsyncTaskResult<Result>(this, result));
 message.sendToTarget();
 return result;
 }

结合sHandler，发送了一个Message

private static final InternalHandler sHandler = new InternalHandler();

private static class InternalHandler extends Handler {
            @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
            @Override
            public void handleMessage(Message msg) {
                AsyncTaskResult result = (AsyncTaskResult) msg.obj;
                switch (msg.what) { //下面是在UI线程的了
                    case MESSAGE_POST_RESULT: //调用finish
                        // There is only one result
                        result.mTask.finish(result.mData[0]);
                        break;
                    case MESSAGE_POST_PROGRESS: //回调onProgressUpdate
                        result.mTask.onProgressUpdate(result.mData);
                        break;
                }
            }

finish()方法

private void finish(Result result) {
            if (isCancelled()) {
                onCancelled(result);  //回调 onCancelled(result)
            } else {
                onPostExecute(result); //回调  onPostExecute(result);上面Handler是UI线程了
            }
            mStatus = Status.FINISHED; //把状态置为FINISHED
        }

再看看publishProgress，同样时Handler

protected final void publishProgress(Progress... values) {
 if (!isCancelled()) {
 sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
 new AsyncTaskResult<Progress>(this, values)).sendToTarget();
 }
 }

好吧，这几个方法的执行时间大概算是了解了

下面看AsyncTask的execute()方法

/**
 * 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 {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
 * to a pool of threads allowing multiple tasks to operate in parallel. Starting
 * {@link android.os.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 {@link #executeOnExecutor} version of this method
 * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings
 * on its use.
 * This method must be invoked on the UI thread.
 * @param params The parameters of the task.
 * @return This instance of AsyncTask.
 * @throws IllegalStateException If {@link #getStatus()} returns either
 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
 * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
 * @see #execute(Runnable)
 *注意：这个函数让任务是以单线程队列方式或线程池队列方式运行，依赖于平台版本而有所不同。AasyncTask首次引入时，这个函数会让任务以后台单线程串行方式执行 *从android.os.Build.VERSION_CODES.DONUT（android 1.6）开始，它让允许任务在线程池中多任务并行执行。但在 android.os.Build.VERSION_CODES.HONEYCOMB *（android 3.0）之后，它又回去了，变成了单线程执行的模式，原因是多线程并行执行容易引发问题。 如果你真想并行执行任务，你可以使用另外一个版本： *使用THREAD_POOL_EXECUTOR参数的executeOnExecutor方法，但要注意使用警告提示
 */
 public final AsyncTask<Params, Progress, Result> execute(Params... params) {
 return executeOnExecutor(sDefaultExecutor, params);
 }

executeOnExecutor

/**
 * 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 {@link #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 {@link 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 {@link #SERIAL_EXECUTOR}.
 *
 * This method must be invoked on the UI thread.
 *
 * @param exec The executor to use. {@link #THREAD_POOL_EXECUTOR} is available as a
 * convenient process-wide thread pool for tasks that are loosely coupled.
 * @param params The parameters of the task.
 *
 * @return This instance of AsyncTask.
 *
 * @throws IllegalStateException If {@link #getStatus()} returns either
 * {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
 *
 * @see #execute(Object[])
 */
 public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
 Params... params) {
 if (mStatus != Status.PENDING) { //首先判断AsyncTask的状态，首次默认的是PENDING
 switch (mStatus) {
 case RUNNING: //如果当前时RUNNING，也就是正在执行会抛出下面的异常
 throw new IllegalStateException("Cannot execute task:"
 + " the task is already running.");
 case FINISHED://如果状态为FINISHED，也就是说已经执行完了，会抛下面的异常
 throw new IllegalStateException("Cannot execute task:"
 + " the task has already been executed "
 + "(a task can be executed only once)");
 }
 }
 
 mStatus = Status.RUNNING; //状态置为RUNNING
 
 onPreExecute(); //调用该方法还是属于UI线程
 
 mWorker.mParams = params;//传递参数
 exec.execute(mFuture);
 
 return this;
 }

**上面的方法可以看出，**

**1. 一个正在执行的AsyncTask任务，再次调用execute方法会抛 Cannot execute task: the task is already running异常，**

**2.一个执行过的AsyncTask任务，再次调用execute方法会抛 Cannot execute task: the task has already been executed (a task can be executed only once异常，**

我们继续往下看， 最终调用了 exec.execute(mFuture);

从execute()方法中看出参数exec即一个名为 sDefaultExecutor的变量，

private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

即 exec是一个SerialExecutor类，它里面的execute方法

private static class SerialExecutor implements Executor {
 final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); //创建了一个队列
 Runnable mActive;
 //会执行下面的方法
 public synchronized void execute(final Runnable r) {
 mTasks.offer(new Runnable() { //向队列中添加一个Runnable
 public void run() {
 try {
 r.run(); 
 } finally {
 scheduleNext();
 }
 }
 });
 if (mActive == null) {
 scheduleNext();
 }
 }
 	//从队列中取出一个Runnable对象赋值为mActive,再调用THREAD_POOL_EXECUTOR.execute(mActive);
 protected synchronized void scheduleNext() {
 if ((mActive = mTasks.poll()) != null) {
 THREAD_POOL_EXECUTOR.execute(mActive);
 }
 }
 }

线程池，

/**
         * An {@link 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);

线程池的初始化

private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors(); //CPU_COUNT进程数
        private static final int CORE_POOL_SIZE = CPU_COUNT + 1; //核心线程数=CPU_COUNT数+1
        private static final int MAXIMUM_POOL_SIZCPU_COUNT= CPU_COUNT * 2 + 1; 
        private static final int KEEP_ALIVE = 1;

AsyncTask的版本不同，实现原理不同

详细区别 [http://blog.csdn.net/pipisky2006/article/details/8535454][http_blog.csdn.net_pipisky2006_article_details_8535454]

[SouthEast]: /images/20220805/4d5e445bf9624a21b3c4422158daa7bd.png
[http_blog.csdn.net_pipisky2006_article_details_8535454]: http://blog.csdn.net/pipisky2006/article/details/8535454