Android(安卓)AsyncTask原理分析

本章我们来理解一下AsyncTask工作原理

AsyncTask.javaprivate 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());}};private static final BlockingQueue sPoolWorkQueue =    new LinkedBlockingQueue(128);/*** An {@link Executor} that can be used to execute tasks in parallel.*/public static final Executor THREAD_POOL_EXECUTOR;static {ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(        CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,        sPoolWorkQueue, sThreadFactory);threadPoolExecutor.allowCoreThreadTimeOut(true);THREAD_POOL_EXECUTOR = threadPoolExecutor;}/*** An {@link Executor} that executes tasks one at a time in serial* order.  This serialization is global to a particular process.*/public static final Executor SERIAL_EXECUTOR = new SerialExecutor();private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;private final WorkerRunnable mWorker;private final FutureTask mFuture;private volatile Status mStatus = Status.PENDING;private static class SerialExecutor implements Executor {final ArrayDeque mTasks = 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);    }}

1、AsyncTask里面使用了static类型的处理器：THREAD_POOL_EXECUTOR（ThreadPoolExecutor），一个process共享这同一个
2、使用了SerialExecutor把要处理的task进行压入队列，所以使用AsyncTask进行异步任务时，一次只能运行一个task

 @MainThreadpublic final AsyncTask execute(Params... params) {    return executeOnExecutor(sDefaultExecutor, params);} @MainThreadpublic static void execute(Runnable runnable) {    sDefaultExecutor.execute(runnable);}@MainThreadpublic final AsyncTask 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;}

所以，由上面的分析可以知道，在使用AsyncTask的时候，请避免处理一些及时的task，因为同时处理一个以上的task时，AsyncTask是窜行的，非并行的。

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