sdk3.0前，使用内部的线程池，多线程并发执行。线程池大小等于5，最大达128

sdk3.0后，使用默认的serial线程池，执行完一个线程，再顺序执行下一个线程。sdk4.3时 线程池大小等于5，最大达128

sdk4.4后线程池大小等于 cpu count + 1，最大值为cpu count * 2 + 1

sdk3.0后有两种线程池的实现，默认为 Serial 线程池

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static void setDefaultExecutor(Executor exec) {//设置默认线程池
        sDefaultExecutor = exec;
}

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);
            }
        }
    }

asynctask.setDefaultExecutor(AsyncTask.THREAD_POOL_EXECUTOR); //设置使用 并发线程池

asynctask.executeOnExecutor(executor); //可以自定义 线程池     使用这两种方法