從原始碼解析-掌握AsyncTask工作原理 為什麼序列執行和記憶體洩漏
什麼是AsyncTask
它本質上是一個封裝了執行緒池和Handler的非同步框架；它內部使用一個執行緒池，序列執行每一個執行緒，執行緒生命週期不用開發者管理，用來執行非同步任務，通過Handler來進行回撥更新UI
這一套業務也可以使用Thread來做，但是使用Thread有些麻煩，每次使用都要自己new一個執行緒，要自己管理其生命週期，而且android還不允許在非主執行緒的執行緒更新UI，這樣的話thread執行得到的結果如果需要更新UI可能還需要發一個handler通知主執行緒更新介面，程式碼寫起來比較凌亂不如asynctask一目瞭然好維護。
AsyncTask的原始碼
public abstract class AsyncTask<Params, Progress, Result> {
private static final String LOG_TAG = "AsyncTask";
//獲取CPU數量
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
//確定執行緒池核心執行緒數量
private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
//確定執行緒池執行緒最大數量
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE = 1;
//執行緒工廠，下面例項化執行緒池要使用
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
//提供原子操作的Integer類，確保getAndIncrement()執行緒安全
private final AtomicInteger mCount = new AtomicInteger(1);
//重寫該方法，確定新增加的執行緒的執行緒名，就是增加一個標識，表明這個執行緒是AsyncTask的
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
/**
* BlockingQueue 是Concurrent包中的一種阻塞佇列
* 當BlockingQueue為空， 從佇列取資料時會讓執行緒等待狀態，直到能取出非空的資料，執行緒會被喚醒。
* 當BlockingQueue是滿的，存資料到佇列時執行緒也會進入等待狀態，直到有空間，執行緒才會被喚醒。
* 在這裡是存放待執行的Runnable，容量為128
*/
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
/**
* An {@link Executor} that can be used to execute tasks in parallel.
* 根據上面的引數例項化執行緒池
*/
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.
* AsyncTask內部實現的執行緒池，序列執行執行緒
*/
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
/**
* Handler訊息的what
*/
private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
//用原子符修飾sDefaultExecutor，並且將SERIAL_EXECUTOR賦值給sDefaultExecutor
//AsyncTask內部使用它進行執行任務
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
//主執行緒的Handler
private static InternalHandler sHandler;
/**
* AstncTask內部類，實現了Callable介面
* 內部例項了一個儲存引數的mParams陣列
* Callable介面內部有一個call方法，方法裡是開發者具體邏輯實現，與Runnable介面類似，
* 只不過Runnable裡的run方法沒有返回值，而call方法有返回值
*/
private final WorkerRunnable<Params, Result> mWorker;
/**
* FutureTask類實現了Runnable介面和Future介面，且內部維護了一個Callable物件，
* FutureTask的建構函式中需要傳入一個Callable物件以對其進行例項化
* Executor的execute方法接收一個Runnable物件，由於FutureTask實現了Runnable介面，
* 所以可以把一個FutureTask物件傳遞給Executor的execute方法去執行。
* 當任務執行完畢的時候會執行FutureTask的done方法，我們可以在這個方法中寫一些邏輯處理。
* 在任務執行的過程中，我們也可以隨時呼叫FutureTask的cancel方法取消執行任務，任務取消後也會執行FutureTask的done方法。
* 我們也可以通過FutureTask的get方法阻塞式地等待任務的返回值（即Callable的call方法的返回值），
* 如果任務執行完了就立即返回執行的結果，否則就阻塞式等待call方法的完成。
*/
private final FutureTask<Result> mFuture;
//標識當前正在執行的執行緒的狀態
private volatile Status mStatus = Status.PENDING;
/**
* 提供原子操作的Boolean型別，保證執行緒安全
* mCancelled表示任務是否被取消
* mTaskInvoked是否開始執行了
*/
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
/**
* 自定義一個執行緒池，序列執行任務
* 序列執行邏輯就是：
* 首先通過execute(Params... params)方法執行到這個類的execute；
* 然後將mFuture封裝成一個Runnable（裡面的run方法的邏輯就是先執行mFuture的run方法，
* 執行結束後就通過scheduleNext()方法取出下一個任務執行，迴圈往復）新增到佇列裡；
* 但是一開始mActive肯定為null，所以就通過scheduleNext()方法取出上一步新增的任務去執行；
* 以後就迴圈執行run方法裡的邏輯
*/
private static class SerialExecutor implements Executor {
/**
* ArrayDeque是一個雙向佇列，能夠同時在兩端進行插入刪除操作，
* 不過它的內部沒有做同步操作，雖然效率上要高於linkedList, vector等，但是存在併發問題
* 需要開發者自己做維護，這裡SerialExecutor就是序列執行，所以沒有併發問題
* 在這裡存放Runnable，佇列容量沒有限制
*/
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
//當前正在執行的任務
Runnable mActive;
//實現Executor的execute方法，用於序列執行任務
public synchronized void execute(final Runnable r) {
//通過佇列的offer方法將封裝後的r新增到佇列的最後
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
//r執行結束後執行下一個任務
scheduleNext();
}
}
});
//當前沒有執行任務，就執行這個方法去取任務執行
if (mActive == null) {
scheduleNext();
}
}
//返回並刪除佇列的頭部第一個元素,然後把執行緒交給執行緒池執行
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.
* 表示任務還沒有開始執行
*/
PENDING,
/**
* Indicates that the task is running.
* 表示任務正在執行
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
* 表示任務執行結束
*/
FINISHED,
}
//獲取Handler
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() {
//例項化mWorker物件，
mWorker = new WorkerRunnable<Params, Result>() {
//實現call方法，執行任務
public Result call() throws Exception {
//將mTaskInvoked設定為true，表明任務開始執行
mTaskInvoked.set(true);
//將該執行緒設定為後臺執行緒
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//這個方法就是我們具體邏輯的實現，然後獲取返回結果
Result result = doInBackground(mParams);
Binder.flushPendingCommands();
//將返回結果傳遞給postResult
return postResult(result);
}
};
//當mFuture被sDefaultExecutor執行的時候，FutureTask內部run方法中的callable物件會呼叫call方法，
// 就會走到上面的mWorker物件的call方法，方法走完，FutureTask內部run方法中會呼叫set(result)方法或者setException(ex)方法
// 這兩個方法裡都會回撥done()方法。
mFuture = new FutureTask<Result>(mWorker) {
//任務執行結束或者被取消都會呼叫done方法
@Override
protected void done() {
try {
//任務正常執行完成
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
//任務出現異常
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
//任務執行出現異常
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
//任務取消
postResultIfNotInvoked(null);
}
}
};
}
private void postResultIfNotInvoked(Result result) {
final boolean wasTaskInvoked = mTaskInvoked.get();
if (!wasTaskInvoked) {
//call方法沒有被呼叫執行這個
postResult(result);
}
}
/**
* AsyncTaskResult是一個內部類，裡面包含執行任務的AsyncTask物件和返回的資料
* 將結果封裝到AsyncTaskResultHandler中傳送出去
* @param result
* @return
*/
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.
*
* @return 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 {@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
*/
@WorkerThread
protected abstract Result doInBackground(Params... params);
/**
* Runs on the UI thread before {@link #doInBackground}.
*
* @see #onPostExecute
* @see #doInBackground
*/
@MainThread
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"})
@MainThread
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"})
@MainThread
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"})
@MainThread
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()
*/
@MainThread
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)
*/
@MainThread
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[])
*/
@MainThread
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
//如果這個AsyncTask已經執行了還執行這個方法就丟擲異常
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
//如果這個AsyncTask已經執行結束了還執行這個方法就丟擲異常
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
//將該任務標識為正在執行
mStatus = Status.RUNNING;
//回撥該方法，開發者可在這個方法裡做一些準備工作
onPreExecute();
//將開發者傳入的引數賦值給mWorker物件的內部變數
mWorker.mParams = params;
//讓sDefaultExecutor執行mFuture，其實就是SerialExecutor這個類去執行execute(final Runnable r)方法，
//因為mFuture實現了Runnable介面，所以執行緒池可以執行這個物件
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[])
*/
@MainThread
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 not be called if the task has been
* canceled.
*
* @param values The progress values to update the UI with.
*
* @see #onProgressUpdate
* @see #doInBackground
*/
@WorkerThread
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()) {
//如果任務被取消了，回撥該方法
onCancelled(result);
} else {
//如果任務正常走完，就回撥該方法
onPostExecute(result);
}
//將該任務狀態設定為結束
mStatus = Status.FINISHED;
}
private static class InternalHandler extends Handler {
public InternalHandler() {
//獲取主執行緒的Looper，將InternalHandler與主執行緒繫結
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
//釋出最終結果
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
//開發者通過呼叫publishProgress，會轉到這裡將過程回撥給開發者
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}
/**
* 將AsyncTask和返回資料封裝成一個物件，用於Handler通訊時使用
* @param <Data>
*/
@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
//具體執行任務的AsyncTask
final AsyncTask mTask;
//儲存資料，一個可變長度的陣列
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
}
AsyncTask為什麼是序列執行任務，其中的原理在上面的註釋中已經解釋了
而有的文章說它是並行執行任務是為什麼呢？你們可以看到上面有一個方法
/** @hide */
public static void setDefaultExecutor(Executor exec) {
sDefaultExecutor = exec;
}
這個方法我們肯定是沒有呼叫過的，那是誰呼叫的呢？其實是應用剛啟動的時候在ActivityThread的handleBindApplication方法呼叫的（原文出處），裡面有這樣一句
// 如果sdk版本在Android3.1.x(API12)或更早版本，需要設定Asynctask中使用的執行緒池，這樣任務是並行執行，在高版本是序列執行
if (data.appInfo.targetSdkVersion <= android.os.Build.VERSION_CODES.HONEYCOMB_MR1) {
AsyncTask.setDefaultExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
}
顯然這個THREAD_POOL_EXECUTOR是這樣的
/**
* An {@link Executor} that can be used to execute tasks in parallel.
* 根據上面的引數例項化執行緒池
*/
public static final Executor THREAD_POOL_EXECUTOR
= new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
所以AsyncTask在API12及以前是序列執行任務的
總結機制原理
- 獲取cpu數量，核心執行緒數量，執行緒池最大執行緒數量，例項化了一個執行緒工廠(標識新執行緒的名稱)，一個存放執行緒池的阻塞佇列，最後構建一個執行緒池，這些東西都是靜態的，不會每次new Asynctask都會例項化
- AsyncTask內部自己實現了一個執行緒池SerialExecutor類，從這個名字也可以看出來，這是一個序列執行器，平時看到很多分析部落格還在說AsyncTask可以同時執行多個執行緒，其實這是不對的，只有老早的版本才會這樣。至於序列的實現原理，上面註釋已經很清楚了。之後把這個類的引用賦值給sDefaultExecutor
- 接下來就是AsyncTask構造方法了，做了兩件事，第一件就是例項化了一個WorkerRunnable物件，它是內部類，實現了Callable介面，並且定義了一個存放開發者傳入的引數的陣列，重寫了call方法，這裡就是耗時操作的執行地，並且將執行結果返回，最終會回撥到onPostExecute(Result result)；第二件事就是例項化一個FutureTask物件，重寫done方法，當任務執行結束或者出現異常會回撥這個方法。
- 最後就是兩個最重要的方法了，開發者new了一個AsyncTask物件後呼叫它的execute(Params… params)方法，這裡走到了executeOnExecutor(Executor exec,Params… params)方法，第一個引數是sDefaultExecutor，也就是SerialExecutor物件，第二個引數是開發者傳入的引數。非同步任務執行邏輯將FutureTask丟到SerialExecutor的類的execute方法中，這個方法將FutureTask又封裝成了一個Runnable，這個Runnable的run方法邏輯就是執行FutureTask的run方法（這裡面會回撥WorkerRunnable的call方法，這裡面會回撥doInBackground；執行完畢後呼叫FutureTask的done方法），執行結束後呼叫scheduleNext()方法迴圈往復。
AsyncTask注意事項
- 記憶體洩漏
其實這個問題跟Handler記憶體洩漏原因是一樣的，都是非靜態內部類持有外部類activity的引用
具體原因和解決辦法可以去這篇部落格檢視
Android之Handler原始碼分析/Looper，Message，Messagequeue三者關係/Handler引起的記憶體洩漏 - 生命週期
AsyncTask的生命週期並不是跟隨Activity的生命週期，當Activity銷燬的時候，需要處理AsyncTask，以防內部的子執行緒造成記憶體洩漏，所以需要及時呼叫AsyncTask.cancel(boolean mayInterruptIfRunning)取消非同步任務 - 結果丟失
當應用發生記憶體重啟的時候，之前的AsyncTask會持有之前的activity的引用，這時候再呼叫Activity就會異常，這時候需要進行判斷Activity是否位null - 序列 並行
在Android1.6之前的版本是序列的，在1.6-2.3改成了並行，在2.3以後為了系統穩定又改成了序列，但是也提供了一個方法execute(Runnable runnable)達到並行的需求
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