編輯:關於Android編程
AsyncTask的基本用法這裡就不在贅述了,是個安卓開發者就會。
1.android 3.0以前的 AsyncTask
private static final int CORE_POOL_SIZE = 5; private static final int MAXIMUM_POOL_SIZE = 128; private static final it KEEP_ALIVE = 10; …… private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);
在這裡我們又看到了ThreadPoolExecutor,它的原理我已經在上一篇介紹過了http://blog.csdn.net/itachi85/article/details/44874511。
在這裡同一時刻能夠運行的線程數為5個,線程池總大小為128,當線程數大於核心時,終止前多余的空閒線程等待新任務的最長時間為10秒。在3.0之前的AsyncTask可以同時有5個任務在執行,而3.0之後的AsyncTask同時只能有1個任務在執行。
2.讓我們來看看android 4.3版本的 AsyncTask
AsyncTask構造函數:
/** * Creates a new asynchronous task. This constructor must be invoked on the UI thread. */ public AsyncTask() { mWorker = new WorkerRunnable() { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked return postResult(doInBackground(mParams)); } }; mFuture = new FutureTask (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); } } }; }
這段代碼初始化了兩個變量,mWorker和mFuture,並在初始化mFuture的時候將mWorker作為參數傳入。mWorker是一個Callable對象,mFuture是一個FutureTask對象,這兩個變量會暫時保存在內存中,稍後才會用到它們。
我們要運用AsyncTask時,大多時候會調用execute()方法,來看看execute()的源碼:
public final AsyncTaskexecute(Params... params) { return executeOnExecutor(sDefaultExecutor, params);}
返回了executeOnExecutor並傳進去sDefaultExecutor(默認的線程池)。先看看executeOnExecutor的源碼:
public final AsyncTaskexecuteOnExecutor(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; }
傳入的線程池exec調用了execute方法並將上文提到的mFuture傳了進去。
這個傳進來的線程池sDefaultExecutor就是默認的線程池SerialExecutor也就是調用了SerialExecutor的execute()方法:
public static final Executor SERIAL_EXECUTOR = new SerialExecutor(); private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
SerialExecutor的源碼:
private static class SerialExecutor implements Executor { final ArrayDequemTasks = new ArrayDeque (); 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); } } }
調用SerialExecutor的execute方法這裡可以看到傳進來一個Runnable,這個Runnable就是上文提到的mFuture(FutureTask),第九行執行了FutureTask的run方法:
public void run() { if (state != NEW || !UNSAFE.compareAndSwapObject(this, runnerOffset, null, Thread.currentThread())) return; try { Callablec = callable; if (c != null && state == NEW) { V result; boolean ran; try { result = c.call(); ran = true; } catch (Throwable ex) { result = null; ran = false; setException(ex); } if (ran) set(result); } } finally { // runner must be non-null until state is settled to // prevent concurrent calls to run() runner = null; // state must be re-read after nulling runner to prevent // leaked interrupts int s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } }
在run方法中執行了c.call,這裡的c就是我們上文提到的mWorker(WorkerRunnable)。執行WorkerRunnable的call方法:
mWorker = new WorkerRunnable() { public Result call() throws Exception { mTaskInvoked.set(true); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //noinspection unchecked return postResult(doInBackground(mParams)); }
最後一行postResult()方法源碼:
private Result postResult(Result result) { @SuppressWarnings(unchecked) Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT, new AsyncTaskResult(this, result)); message.sendToTarget(); return result; }
我們發現就是發送了一個消息,上面的代碼發送的消息由這裡接受:
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; } } }
消息是MESSAGE_POST_RESULT所以會執行 result.mTask.finish(result.mData[0]) ,finish源碼:
private void finish(Result result) { if (isCancelled()) { onCancelled(result); } else { onPostExecute(result); } mStatus = Status.FINISHED; }
當被取消時會執行 onCancelled(result);否則就會調用 onPostExecute(result);這樣我們就可以在onPostExecute方發中得到我們需要的結果result來進行下一步的處理了。
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