在封裝Retrofit下載文件時：
 final SaveFileTask task = new SaveFileTask(REQUEST, SUCCESS);
//繼承AsyncTask
                    task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, DOWNLOAD_DIR, EXTENSION, responseBody, NAME);
為什么這個我們不是調(diào)用excute（）方法?往下看

AsyncTask的execute（）函數(shù)

  @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
調(diào)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);//這一行是重點

        return this;
    }

• exec是什么？是調(diào)用exceute（）時傳入的sDefaultExecutor
• sDefaultExecutor是什么？ 是它的一個實例。 new SerialExecutor();
• sDefaultExecutor是SerialExecutor的一個實例，而且它是個靜態(tài)變量。也就是說，一個進程里面所有AsyncTask對象都共享同一個SerialExecutor對象。
  那么所有的秘密就在于SerialExecutor的execute函數(shù)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是一個一個執(zhí)行任務(wù)的，而所有的對象共享一個 SerialExecutor對象（靜態(tài)成員），所以多個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

如何并行執(zhí)行多個AsyncTask對象

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);
線程池中核心線程數(shù)、最大線程數(shù)。。。。。怎么調(diào)度的再議，但是有一點可以肯定，它不是排隊在一個線程里面執(zhí)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