對比一下AsyncTask在子執行緒和在主執行緒初始化的區別

public class MainActivity extends AppCompatActivity {

    private static final String TAG = MainActivity.class.getCanonicalName();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        android.util.Log.e(TAG, "#onCreate, "
 
 + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());

        new Thread(new Runnable() {
            @Override
            public void run() {
                android.util.Log.e(TAG, "建立AsyncTask的子執行緒, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
                new
 
 AsyncTask<Void, Void, String>() {
                    @Override
                    protected void onPreExecute() {
                        android.util.Log.e(TAG, "子執行緒內初始化AsyncTask #onPreExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
                        super
 
.onPreExecute();
                    }

                    @Override
                    protected String doInBackground(Void... params) {
                        android.util.Log.e(TAG, "子執行緒內初始化AsyncTask #doInBackground, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
                        return null;
                    }

                    @Override
                    protected void onPostExecute(String result) {
                        android.util.Log.e(TAG, "子執行緒內初始化AsyncTask #onPostExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
                        super.onPostExecute(result);
                    }
                }.execute((Void) null);
            }
        }).start();

// Caused by: java.util.concurrent.RejectedExecutionException: 
// Task [email protected] rejected from  
// [email protected]
// [Running, pool size = 17, active threads = 17, queued tasks = 128, completed tasks = 0]

//        for(int i = 0;i<200;i++) {
//            new AsyncTask<Void, Void, Void>() {
//                @Override
//                protected Void doInBackground(Void... params) {
//                    // do something
//                    try {
//                        Thread.sleep(3000);
//                    } catch (InterruptedException e) {
//                        e.printStackTrace();
//                    }
//                    return null;
//                }
//            }.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
//        }

        new AsyncTask<Void, Void, String>() {
            @Override
            protected void onPreExecute() {
                android.util.Log.e(TAG, "主執行緒內初始化AsyncTask #onPreExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
                super.onPreExecute();
            }

            @Override
            protected String doInBackground(Void... params) {
                android.util.Log.e(TAG, "主執行緒內初始化AsyncTask #doInBackground, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
                return null;
            }

            @Override
            protected void onPostExecute(String result) {
                android.util.Log.e(TAG, "主執行緒內初始化AsyncTask #onPostExecute, " + Thread.currentThread().getName() + "--->" + Thread.currentThread().getId());
                super.onPostExecute(result);
            }
        }.execute((Void) null);
    }
}

在子執行緒初始化的列印日誌：

在主執行緒初始化的列印日誌：

總結：

• AsyncTask裡的Handler為InternalHandler，這裡直接使用的主執行緒的Looper( super(Looper.getMainLooper());)，如果去看API 22以下的程式碼，會發現它沒有這個建構函式，而是使用預設的；預設情況下，Handler會使用當前執行緒的Looper，如果你的AsyncTask是在子執行緒建立的，那麼很不幸，你的onPreExecute和onPostExecute並非在UI執行緒執行，而是被Handler post到建立它的那個執行緒執行；如果你在這兩個地方更新了UI，那麼直接導致崩潰，API 25的程式碼其實稍微更近一步（通過上面案例日誌分析），即使你在子執行緒中建立AsyncTask，onPostExecute也會在UI執行緒執行，但是onPreExecute還是在建立AsyncTask的執行緒中執行
• 所以 歸結為一點：為了避免不必要的麻煩，AsyncTask必須在主執行緒初始化，原始碼註釋也是這麼提示我們的
• 在Android 1.5剛引入的時候，AsyncTask的execute是序列執行的；到了Android 1.6直到Android 2.3.2，又被修改為並行執行了。Android 3.0以上，AsyncTask預設序列執行的
• 預設序列的設計是為了避免相同資源的同步訪問問題
• 如果希望AsyncTask可以並行處理的話，可以使用executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR)
• 如果任務過多，超過了工作佇列以及執行緒數目的限制導致這個執行緒池發生阻塞，預設的處理方式會直接丟擲一個異常導致程序掛掉 如上述案例模擬（當然可以重寫AsyncTask，在executeOnExecutor中對exec.execute(mFuture)做保護，捕獲RejectedExecutionException異常，這裡可以同時做一個統計，如果出現問題，說明AsyncTask不適合這種場景，需要考慮重構，既然知道有此問題，最好主動避免涉及）

AsyncTask原始碼

這裡是以API 25為例進行分析的：

public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    // 至少需要2個執行緒，最多需要4個執行緒在核心池中
    // 可以比CPU數量少一個，避免讓所有的CPU都在處理後臺工作
    // 這個計算方式不同版本表現不一樣，早期是CPU+1
    private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
    private static final int KEEP_ALIVE_SECONDS = 30;

    // 執行緒建立工廠
    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());
        }
    };

    // 工作佇列長度為128
    private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(128);
    // 可以用於並行執行任務的 Executor
    public static final Executor THREAD_POOL_EXECUTOR;
    // THREAD_POOL_EXECUTOR初始化
    static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                sPoolWorkQueue, sThreadFactory);
        // 核心執行緒空閒允許超時處理 待遇同非核心執行緒
        threadPoolExecutor.allowCoreThreadTimeOut(true);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }

    // 序列執行任務的Executor（按順序每次同時只能執行一個任務）
    // 指定程序全域性有效
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;
    //預設序列執行 而非並行執行
    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
    private static InternalHandler sHandler;

    private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture;

    private volatile Status mStatus = Status.PENDING;

    private final AtomicBoolean mCancelled = new AtomicBoolean();
    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();

    // SERIAL_EXECUTOR也使用THREAD_POOL_EXECUTOR實現的
    // 只不過通過佇列mTasks排程實現序列的控制
    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);
            }
        }
    }

    public enum Status {
        PENDING,
        RUNNING,
        FINISHED,
    }

    private static Handler getHandler() {
        synchronized (AsyncTask.class) {
            if (sHandler == null) {
                sHandler = new InternalHandler();
            }
            return sHandler;
        }
    }

    /** @hide */
    public static void setDefaultExecutor(Executor exec) {
        sDefaultExecutor = exec;
    }

    // 建立一個非同步任務，建構函式必須在UI執行緒呼叫
    public AsyncTask() {
        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //noinspection unchecked
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

        mFuture = new FutureTask<Result>(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 occurred while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                }
            }
        };
    }

    private void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }

    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

    public final Status getStatus() {
        return mStatus;
    }

    @WorkerThread
    protected abstract Result doInBackground(Params... params);

    @MainThread
    protected void onPreExecute() {
    }

    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onPostExecute(Result result) {
    }

    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onProgressUpdate(Progress... values) {
    }

    @SuppressWarnings({"UnusedParameters"})
    @MainThread
    protected void onCancelled(Result result) {
        onCancelled();
    }    

    @MainThread
    protected void onCancelled() {
    }

    public final boolean isCancelled() {
        return mCancelled.get();
    }

    public final boolean cancel(boolean mayInterruptIfRunning) {
        mCancelled.set(true);
        return mFuture.cancel(mayInterruptIfRunning);
    }

    public final Result get() throws InterruptedException, ExecutionException {
        return mFuture.get();
    }

    public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
            ExecutionException, TimeoutException {
        return mFuture.get(timeout, unit);
    }

     // 基於指定引數執行任務 預設序列執行
    @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }

    //基於指定引數執行任務，可以自定義Executor，也可以直接使用THREAD_POOL_EXECUTOR
    @MainThread
    public 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);

        return this;
    }

    //簡易版 可以直接執行一個Runnable物件
    @MainThread
    public static void execute(Runnable runnable) {
        sDefaultExecutor.execute(runnable);
    }

    @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()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

    private static class InternalHandler extends Handler {
        public InternalHandler() {
            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
                    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"})
    private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;

        AsyncTaskResult(AsyncTask task, Data... data) {
            mTask = task;
            mData = data;
        }
    }
}