AsyncTask的线程池使用

这几天写了一个demo，利用asynctask来异步加载手机相册的照片，小米2S，4.1.1，但是发现相片是一张接一张加载出来的，而不是好几张同时加载的，说明我起的那些异步的线程在线程池里面排起队了，一个队，一个一个服务。

于是在网上搜了一下，才发现asynctask的线程池机制是跟系统版本有关系的，参考http://blog.csdn.net/hitlion2008/article/details/7983449

大致就是2.3以前的版本asynctask的线程池默认是Executors.newFixedThreadPool(5)，即是5个服务的线程池，而3.0之后是由一个队列来维护的单队伍的线程池。

/**
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

    /**
     * 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();

一个并行，一个串行。

private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

3.0之后默认的是串行的。

    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);
            }
        }
    }

串行线程池实现，就是维护了一个队列，等队头执行完了，再执行下一个，而干苦力的依然是并行的线程池

THREAD_POOL_EXECUTOR

所幸，asynctask提供了一个用户自己来指定线程池的方法

public 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;
    }

所以使用的时候可以使用THREAD_POOL_EXECUTOR，也可以使用自己定义的线程池，如

private ExecutorService mExecutor = Executors.newFixedThreadPool(5);
    ……
private void loaddata(){
    ……
    task.executeOnExecutor(mExecutor, imageUrl);
}