/* 说到AsyncTask这个类,好多人其实不太了解。最近看了下代码,把心得分享给大家。 AsyncTask的execute的执行流程为 先调用ThreadPoolExecutor.execute(mFuture); 然后ThreadPoolExecutor.execute(mFuture) 会调用ThreadPoolExecutor.addWorker(mFuture); 最后ThreadPoolExecutor.addWorker(mFuture)会调用mFuture的run()方法,run方法中就是该线程要执行操作的地方 到此我们来关注一下mFuture,AsyncTask中的mFuture是一个FutureTask,FutureTask实现了Future<V>, Runnable两个接口, Future 表示异步计算的结果。它提供了检查计算是否完成的方法,以等待计算的完成,并获取计算的结果,计算完成后只能使用 get 方法来获取结果。 mFuture以mWorker作为参数 现看mFuture的构造方法: public void run() { sync.innerRun(); } sync是什么呢?Sync类是一个内部类,我们看看它的初始化 public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); sync = new Sync(callable); } 在看看sync.innerRun()方法: void innerRun() { if (!compareAndSetState(READY, RUNNING)) return; runner = Thread.currentThread(); if (getState() == RUNNING) { // recheck after setting thread V result; try { result = callable.call(); } catch (Throwable ex) { setException(ex); return; } set(result); } else { releaseShared(0); // cancel } } 从代码可以看到,其实最终是调用了callable.call()这个方法。 从AsyncTask中我们可以知道,我们传入的Callable是我们的WorkerRunnable 所以,我们会调用WorkerRunnable的call()方法,在call方法里面 返回postResult(doInBackground(mParams)); 通知UI线程更新,这就是调用过程 Notes: 1: 因为AsyncTask里面的内部handler和Executor都是静态变量,所以,他们控制着所有的子类。 2: 我们可以通过AsyncTask.execute()方法来调用系统默认的线程池来处理当前的任务, 系统默认的线程池用的是SerialExecutor.这个线程池控制所有任务按顺序执行。也就是一次只执行一条. 当前执行完了,才执行下一条.2.3平台以前是所有的任务并发执行,这会导致一种情况,就是其中一条任务执行出问题了,会引起其他任务 出现错误. 3: AsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR)你也可以采用这个系统提供的线程池来处理你的任务 默认这个线程池是并发处理任务的,也就是不按顺序来.核心为5条,最大128条 4: 你也可以使用自定义的线程池,这样就可以即使的执行你的任务需求,而不是用系统的。因为用系统默认的线程池可以需要等待,它默认 是按顺序执行(THREAD_POOL_EXECUTOR)或者最多执行5个(SerialExecutor). 自己使用自定义线程池方式如下: new AsyncTask.executeOnExecutor((ExecutorService)Executors.newCachedThreadPool()). 5: 不要随意使用AsyncTask,除非你必须要与UI线程交互.默认情况下使用Thread即可,要注意需要将线程优先级调低. 从google官方文档你也可以看到,AsyncTasks should ideally be used for short operations (a few seconds at the most.) AsyncTask适合处理短时间的操作,长时间的操作,比如下载一个很大的视频,这就需要你使用自己的线程来下载,不管是断点下载还是其它的. 当然,如果你需要开启自定义的很多线程来处理你的任务,切记你此时可以考虑自定义线程池 */ public abstract class AsyncTask<Params, Progress, Result> { private static final String LOG_TAG = "AsyncTask"; // 核心线程数是要 private static final int CORE_POOL_SIZE = 5; // 最大线程数支持128 private static final int MAXIMUM_POOL_SIZE = 128; // 这个参数的的意思是当前线程池里面的thread如果超过了规定的核心线程5,如果有线程的空闲时间超过了这个数值, // 数值的单位自己指定,就回收该线程的资源,达到动态调整线程池资源的目的. private static final int KEEP_ALIVE = 1; // ThreadFactory是用来在线程池中构建新线程的方法.可以看到每次构建一个方法,名字都不同.为"AsyncTask # 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()); } }; // 线程池所使用的缓冲队列.FIFO,它用于存放如果当前线程池中核心线程已满,此时来的任务都被放到缓冲队列中等待被处理. // 初始化容量为10 private static final BlockingQueue<Runnable> sPoolWorkQueue = new LinkedBlockingQueue<Runnable>(10); /** * An {@link Executor} that can be used to execute tasks in parallel. */ // 线程池的初始化,指定了核心线程5,最大线程128,超时1s,缓冲队列等, 你在使用asyncTask的时候,可以传入这个参数, // 就可以让多条线程并发的执行了.比如:executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR) public static final Executor THREAD_POOL_EXECUTOR = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory); /** * An {@link Executor} that executes tasks one at a time in serial * order. This serialization is global to a particular process. */ // 从这个线程池内部看,已经不是并行执行任务,而是一次只执行一个. public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); // 消息数值 private static final int MESSAGE_POST_RESULT = 0x1; private static final int MESSAGE_POST_PROGRESS = 0x2; // 这个InternalHandler就是用来是UI线程打交道的。可以看到它是个静态的变量。也就是说谁第一次调用它,下一次另一个 // 线程来调用,也不会实例话这个常量.关于这个handler,默认asynctask都是从主线程中调用的,所以,这个Handler默认 // 获得了主线程的Looper,所以就能和主线程来交互. Notes:假如你在一个子线程中构建了自己的Looper并使用Asynctask, // 应该会出问题,因为此时这个Handler就属于子线程了,就不能去操控UI的操作.这应该算是AsyncTask的Bug.网上有人说 // 在4.0上运行没问题,2.3会有问题,原因是因为4.0中的ActivityThread.main方法里面最先用主线程的Looper来初始化了这个 // AsyncTask。理论上Asynctask应该判断当前的Looper如果不是MainThread的Looper的话,抛出异常,遗憾的是, // google没有考虑到这里,只是在文档中要求必须在主线程中调用,其实,很不好! private static final InternalHandler sHandler = new InternalHandler(); private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR; // 自定义的静态内部类 private final WorkerRunnable<Params, Result> mWorker; // 其实就是也一个Runnable,实现了这个接口 private final FutureTask<Result> mFuture; // 默认为pending状态。 private volatile Status mStatus = Status.PENDING; // 原子操作,专门用来处理并发访问,就可以不用synchronized private final AtomicBoolean mCancelled = new AtomicBoolean(); private final AtomicBoolean mTaskInvoked = new AtomicBoolean(); private static class SerialExecutor implements Executor { // ArrayDeque是一个双向队列,我们来理解下这个线程池是如何做到一次只 // 执行一条任务的.比如此时有多处先后都调用了AsyncTask.execute()方法, // 对第一条最先到的任务来说,首先自己被假如到了队列中,因为第一次mActive == null成立, // 所以执行THREAD_POOL_EXECUTOR.execute(mActive).且mActive 此时不等于Null. // 所以第二条任务来的时候,只是被加入到了队列中,并不会执行.除非第一条任务执行完了,在它的finnally方法中 // 调用scheduleNext()去再次从对列中取出下一条任务来执行.这样就实现了所有任务按顺序执行的功能. final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>(); Runnable mActive; public synchronized void execute(final Runnable r) { // 把线程offer到队列中 mTasks.offer(new Runnable() { public void run() { try { r.run(); } finally { // 一条执行完了,执行下一条任务 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 {@link AsyncTask#onPostExecute} has finished. */ FINISHED, } /** @hide Used to force static handler to be created. */ public static void init() { sHandler.getLooper(); } /** @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() { //初始化mWorker并复写call方法,后面会介绍什么时候调用 mWorker = new WorkerRunnable<Params, Result>() { // 这个方法就是当你嗲用excutor.excute()方法后执行的方法。至于是如何执行的,我们后面会分析 public Result call() throws Exception { mTaskInvoked.set(true); // 将线程优先级设置为后台线程,默认和主线程优先级一样,如果不这样做,也会降低程序性能.因为会优先 // 抢占cpu资源.所以,如果你在程序中不使用asyncTask而是自己new 一条线程出来,记得把线程的优先级设置为 // 后台线程 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //这个地方调用了我们自己实现的doInBackground return postResult(doInBackground(mParams)); } }; // 用mWorker创建一个可取消的异步计算任务 mFuture = new FutureTask<Result>(mWorker) { @Override // 当任务不管是正常终止、异常或取消而完成的,都回调此方法, 即isDone()为true时,isDone不管成功还是失败都 // 返回true protected void done() { try { // 如果当前的task没有被invoke,就被finish掉 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); } } // 当doInBackground结束了,调用PostResult发布结果 private Result postResult(Result result) { @SuppressWarnings("unchecked") Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult<Result>(this, result)); message.sendToTarget(); return result; } /** * Returns the current status of this task. * * @return The current status. */ // 获得当前的状态 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 {@link #execute} * by the caller of this task. * * This method can call {@link #publishProgress} to publish updates * on the UI thread. * * @param params The parameters of the task. * * @return A result, defined by the subclass of this task. * * @see #onPreExecute() * @see #onPostExecute * @see #publishProgress */ // 用户自己实现 protected abstract Result doInBackground(Params... params); /** * Runs on the UI thread before {@link #doInBackground}. * * @see #onPostExecute * @see #doInBackground */ // 用户自己实现 protected void onPreExecute() { } /** * <p>Runs on the UI thread after {@link #doInBackground}. The * specified result is the value returned by {@link #doInBackground}.</p> * * <p>This method won't be invoked if the task was cancelled.</p> * * @param result The result of the operation computed by {@link #doInBackground}. * * @see #onPreExecute * @see #doInBackground * @see #onCancelled(Object) */ @SuppressWarnings({"UnusedDeclaration"}) // 用户自己实现 protected void onPostExecute(Result result) { } /** * Runs on the UI thread after {@link #publishProgress} is invoked. * The specified values are the values passed to {@link #publishProgress}. * * @param values The values indicating progress. * * @see #publishProgress * @see #doInBackground */ @SuppressWarnings({"UnusedDeclaration"}) // 用户自己实现 protected void onProgressUpdate(Progress... values) { } /** * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and * {@link #doInBackground(Object[])} has finished.</p> * * <p>The default implementation simply invokes {@link #onCancelled()} and * ignores the result. If you write your own implementation, do not call * <code>super.onCancelled(result)</code>.</p> * * @param result The result, if any, computed in * {@link #doInBackground(Object[])}, can be null * * @see #cancel(boolean) * @see #isCancelled() */ @SuppressWarnings({"UnusedParameters"}) protected void onCancelled(Result result) { onCancelled(); } /** * <p>Applications should preferably override {@link #onCancelled(Object)}. * This method is invoked by the default implementation of * {@link #onCancelled(Object)}.</p> * * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and * {@link #doInBackground(Object[])} has finished.</p> * * @see #onCancelled(Object) * @see #cancel(boolean) * @see #isCancelled() */ protected void onCancelled() { } /** * Returns <tt>true</tt> if this task was cancelled before it completed * normally. If you are calling {@link #cancel(boolean)} on the task, * the value returned by this method should be checked periodically from * {@link #doInBackground(Object[])} to end the task as soon as possible. * * @return <tt>true</tt> if task was cancelled before it completed * * @see #cancel(boolean) */ public final boolean isCancelled() { return mCancelled.get(); } /** * <p>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 <tt>cancel</tt> is called, * this task should never run. If the task has already started, * then the <tt>mayInterruptIfRunning</tt> parameter determines * whether the thread executing this task should be interrupted in * an attempt to stop the task.</p> * * <p>Calling this method will result in {@link #onCancelled(Object)} being * invoked on the UI thread after {@link #doInBackground(Object[])} * returns. Calling this method guarantees that {@link #onPostExecute(Object)} * is never invoked. After invoking this method, you should check the * value returned by {@link #isCancelled()} periodically from * {@link #doInBackground(Object[])} to finish the task as early as * possible.</p> * * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this * task should be interrupted; otherwise, in-progress tasks are allowed * to complete. * * @return <tt>false</tt> if the task could not be cancelled, * typically because it has already completed normally; * <tt>true</tt> otherwise * * @see #isCancelled() * @see #onCancelled(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. * * @return The computed result. * * @throws CancellationException If the computation was cancelled. * @throws ExecutionException If the computation threw an exception. * @throws 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. * * @param timeout Time to wait before cancelling the operation. * @param unit The time unit for the timeout. * * @return The computed result. * * @throws CancellationException If the computation was cancelled. * @throws ExecutionException If the computation threw an exception. * @throws InterruptedException If the current thread was interrupted * while waiting. * @throws 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. * * <p>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. * * <p>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) */ // 这个方法就是用户调用的excute方法,默认采用asynctask自带的线程池串行执行任务 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. * * <p>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. * * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from * a thread pool is generally <em>not</em> 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}. * * <p>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) { // 可以看出同一个任务只能执行一次 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; // 调用用户--UI线程---自己实现的方法 onPreExecute(); mWorker.mParams = params; // 这个方法就会调用前面的mWorker的call方法 exec.execute(mFuture); return this; } /** * Convenience version of {@link #execute(Object...)} for use with * a simple Runnable object. See {@link #execute(Object[])} for more * information on the order of execution. * * @see #execute(Object[]) * @see #executeOnExecutor(java.util.concurrent.Executor, Object[]) */ public static void execute(Runnable runnable) { sDefaultExecutor.execute(runnable); } /** * This method can be invoked from {@link #doInBackground} to * publish updates on the UI thread while the background computation is * still running. Each call to this method will trigger the execution of * {@link #onProgressUpdate} on the UI thread. * * {@link #onProgressUpdate} will note be called if the task has been * canceled. * * @param values The progress values to update the UI with. * * @see #onProgressUpdate * @see #doInBackground */ protected final void publishProgress(Progress... values) { if (!isCancelled()) { sHandler.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; } // 与UI交互 private static class InternalHandler extends Handler { @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"}) // 存储异步执行结果的类 private static class AsyncTaskResult<Data> { final AsyncTask mTask; final Data[] mData; AsyncTaskResult(AsyncTask task, Data... data) { mTask = task; mData = data; } } }