Asynctask解析以及注意事项
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;
}
}
}