問題由來：

? ? ?之前看到一篇博文，說AsyncTask不適合運行多任務(wù)， 多個任務(wù)不會異步執(zhí)行， 當(dāng)時只是印象里記住了一下也不確定， 今天把代碼看了看， 把原因?qū)懗鰜怼?/span>

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問題的代碼演示：

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1 public class AsyncTaskDemo extends AsyncTask<String, Integer, String>{ 2 private final static String TAG = "AsyncTaskTest"; 3 4 @Override 5 protected String doInBackground(String... params) { 6 Log.v(TAG, params[0] + "=================doInBackground===================PID = " + Thread.currentThread().getId()); 7 8 return params[0]; 9 } 10 11 12 }

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1 // 一個按鈕的onclick函數(shù) 2 public void test(View view) { 3 Log.v("AsyncTaskTest", "CPU_COUNT = " + Runtime.getRuntime().availableProcessors()); 4 5 new AsyncTaskDemo().execute("Hello1"); 6 new AsyncTaskDemo().execute("Hello2"); 7 new AsyncTaskDemo().execute("Hello3"); 8 new AsyncTaskDemo().execute("Hello4"); 9 } //logcat里打印出來的結(jié)果12-11 16:07:34.865 1979-1979/com.sabo.helloworld V/AsyncTaskTest﹕ CPU_COUNT = 1 12-11 16:07:34.871 1979-2102/com.sabo.helloworld V/AsyncTaskTest﹕ Hello1=================doInBackground===================PID = 161 12-11 16:07:34.872 1979-2103/com.sabo.helloworld V/AsyncTaskTest﹕ Hello2=================doInBackground===================PID = 162 12-11 16:07:34.872 1979-2103/com.sabo.helloworld V/AsyncTaskTest﹕ Hello3=================doInBackground===================PID = 162 12-11 16:07:34.872 1979-2103/com.sabo.helloworld V/AsyncTaskTest﹕ Hello4=================doInBackground===================PID = 162

實驗大致輔證了"AsyncTask不適合運行多任務(wù)"這個推斷， 另一個有趣的結(jié)果是非異步執(zhí)行的時候居然可能不是在一個線程里運行的【看我上面用紅色標(biāo)記的部分】。

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分析：

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1 @MainThread 2 public final AsyncTask<Params, Progress, Result> execute(Params... params) { 3 return executeOnExecutor(sDefaultExecutor, params); 4 }

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@MainThreadpublic final AsyncTask<Params, Progress, Result> executeOnExecutor(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);// exec --> sDefaultExecutorreturn this;}

如果熟悉Java的并發(fā)編程的話就知道sDefaultExecutor用于將要完成的任務(wù)交給內(nèi)部已經(jīng)實現(xiàn)的線程池去執(zhí)行（有興趣的話可以去看看Doug Lea寫的那本《Java并發(fā)編程》）

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;private static class SerialExecutor implements Executor {final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();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);}}}

這里的代碼就能解釋為什么多個AsyncTask一起執(zhí)行時序列化執(zhí)行而被異步的了，?

1 new AsyncTaskDemo().execute("Hello1"); //mActive == null 2 new AsyncTaskDemo().execute("Hello2"); //mActive != null 3 new AsyncTaskDemo().execute("Hello3"); //mActive != null 4 new AsyncTaskDemo().execute("Hello4"); //mActive != null

上面紅色標(biāo)記的代碼可以看出，只有第一個任務(wù)直接調(diào)用scheduleNext()--->THREAD_POOL_EXECUTOR.execute(mActive), 線程池直接將任務(wù)交由線程去執(zhí)行，而后面幾個任務(wù)先被放入ArrayDeque<Runnable> mTasks， 沒有交給任何線程去執(zhí)行，

每個任務(wù)執(zhí)行完之后又都要運行上面綠顏色標(biāo)記的scheduleNext()， 從而依次序序列化執(zhí)行任務(wù)。

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上面提到的有趣的事情，既然AsyncTask是序列化執(zhí)行任務(wù)的， 那么線程池里只要一個線程就能滿足要求了啊， 為什么會有兩個線程。

1 private static final int CORE_POOL_SIZE = CPU_COUNT + 1; 2 private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1; 3 private static final int KEEP_ALIVE = 1; 4 5 public static final Executor THREAD_POOL_EXECUTOR 6 = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE, 7 TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

上面我把CPU_COUNT的值打印出來是1， 所以這里線程池的線程數(shù)就可能是[2, 3]了， 然而對于AsyncTask來說1就夠了， 多余1的線程其實是沒有什么幫助的。

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P.S.

:-)第一次寫博客， 加上功力又有限， 歡迎大家指正canbin.zhang#qq.com

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轉(zhuǎn)載于:https://www.cnblogs.com/sabo/p/5040685.html

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