高并发-AsyncTask

1.AsyncTask是google 设计出来，便于开发者使用的线程池，属于模板设计的模式;

2.AsyncTask内部其实是使用了 java提供的线程池类库的实现，如下代码：

private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
// We want at least 2 threads and at most 4 threads in the core pool,
// preferring to have 1 less than the CPU count to avoid saturating
// the CPU with background work
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;

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());
    }
};

private static final BlockingQueue<Runnable> sPoolWorkQueue =
        new LinkedBlockingQueue<Runnable>(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;
}

3.其中高度封装了线程池，开发者主要关注一下几个步骤即可：

3.1实例化AsyncTask，以及onPreExecute、doInBackground、onPostExecute，最后call execute 即可;

3.2一旦call execute 函数，AsyncTask框架会将客户端的任务交给SerialExecutor处理：

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);
        }
    }
}

3.3最终，丢给ThreadPoolExecutor线程池，进行处理;

3.4在将任务丢到线程池的 任务队列（LinkedBlockQueue）后，等待任务被线程池的线程处理;最终会回调 在AyncTask构造函数中初始化的 Callable.call() 和 futureTask.done()相关的接口，如下：

/**
 * Creates a new asynchronous task. This constructor must be invoked on the UI thread.
 */
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);
            }
        }
    };
}