AsyncTask问题：多个task是串行执行还是并行的

在封装Retrofit下载文件时：
 final SaveFileTask task = new SaveFileTask(REQUEST, SUCCESS);
//继承AsyncTask
                    task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, DOWNLOAD_DIR, EXTENSION, responseBody, NAME);
为什么这个我们不是调用excute（）方法?往下看

AsyncTask的execute（）函数

  @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
调用executeOnExecutor（）方法
    }

@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);//这一行是重点

        return this;
    }

• exec是什么？是调用exceute（）时传入的sDefaultExecutor
• sDefaultExecutor是什么？ 是它的一个实例。 new SerialExecutor();
• sDefaultExecutor是SerialExecutor的一个实例，而且它是个静态变量。也就是说，一个进程里面所有AsyncTask对象都共享同一个SerialExecutor对象。
  那么所有的秘密就在于SerialExecutor的execute函数了。

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);
            }
        }
    }

• SerialExecutor是一个一个执行任务的，而所有的对象共享一个 SerialExecutor对象（静态成员），所以多个AsyncTask是串行的。demo：

final AsyncTask task = new AsyncTask() {
            @Override
            protected Object doInBackground(Object[] params) {
                Log.e("tag", "task 1 " + "  start");
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                Log.e("tag", "task 1 " + "  end");
                return null;
            }
        };
        task.execute();
        // task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);


        final AsyncTask task2 = new AsyncTask() {
            @Override
            protected Object doInBackground(Object[] params) {
                Log.e("tag", "task 2 " + "  start");
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                Log.e("tag", "task 2 " + "  end");
                return null;
            }
        };
        task2.execute();
        // task2.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);

1.png

如何并行执行多个AsyncTask对象

使用THREAD_POOL_EXECUTOR
 public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);
线程池中核心线程数、最大线程数。。。。。怎么调度的再议，但是有一点可以肯定，它不是排队在一个线程里面执行的

 final AsyncTask task = new AsyncTask() {
            @Override
            protected Object doInBackground(Object[] params) {
                Log.e("tag", "task 1 " + "  start");
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                Log.e("tag", "task 1 " + "  end");
                return null;
            }
        };
        //   task.execute();
        task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);


        final AsyncTask task2 = new AsyncTask() {
            @Override
            protected Object doInBackground(Object[] params) {
                Log.e("tag", "task 2 " + "  start");
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                Log.e("tag", "task 2 " + "  end");
                return null;
            }
        };
        //   task2.execute();
        task2.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);

    }

2.png