06.源码阅读（AsyncTask深度分析-api26）

AsyncTask作为android原生的处理异步任务的方案，问题很多，比如不允许多个任务同时运行，关于这一点，网上有很多说法，早期的说法是AsyncTask最初被引入的时候只支持一个任务同时运行，后期增加到了5个，我今天特意验证了一下看看：

private void start(View view){
        for (int i = 0; i < 100; i++) {
            new Task().execute();
        }
    }

    class Task extends AsyncTask<Void,Void,Void>{

        @Override
        protected Void doInBackground(Void... voids) {
            try {
                int times = 100;
                for (int i = 0; i < times; i++) {
                    System.out.println("thread"+Thread.currentThread()+i+"");
                    Thread.sleep(10);
                }
            } catch (Exception e) {

            }
            return null;
        }
    }

在for循环中开启了一百个异步任务，每个任务打印从0到99，每次打印sleep10毫秒，发现结果出乎意料：

03-29 16:36:09.323 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]0
03-29 16:36:09.334 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]1
03-29 16:36:09.344 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]2
03-29 16:36:09.354 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]3
03-29 16:36:09.365 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]4
03-29 16:36:09.375 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]5
03-29 16:36:09.385 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]6
03-29 16:36:09.396 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]7
03-29 16:36:09.406 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]8
03-29 16:36:09.416 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]9
03-29 16:36:09.426 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]10
03-29 16:36:09.437 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]11
03-29 16:36:09.447 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]12
03-29 16:36:09.457 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]13
03-29 16:36:09.468 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]14
03-29 16:36:09.478 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]15
03-29 16:36:09.488 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]16
03-29 16:36:09.499 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]17
03-29 16:36:09.509 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]18
03-29 16:36:09.520 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]19
03-29 16:36:09.531 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]20
03-29 16:36:09.541 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]21
03-29 16:36:09.552 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]22
03-29 16:36:09.562 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]23
03-29 16:36:09.573 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]24
03-29 16:36:09.583 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]25
03-29 16:36:09.594 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]26
03-29 16:36:09.606 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]27
03-29 16:36:09.617 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]28
03-29 16:36:09.627 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]29
03-29 16:36:09.638 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]30
03-29 16:36:09.649 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]31
03-29 16:36:09.659 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]32
03-29 16:36:09.670 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]33
03-29 16:36:09.681 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]34
03-29 16:36:09.692 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]35
03-29 16:36:09.703 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]36
03-29 16:36:09.713 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]37
03-29 16:36:09.724 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]38
03-29 16:36:09.734 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]39
03-29 16:36:09.745 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]40
03-29 16:36:09.755 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]41
03-29 16:36:09.766 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]42
03-29 16:36:09.776 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]43
03-29 16:36:09.787 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]44
03-29 16:36:09.797 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]45
03-29 16:36:09.808 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]46
03-29 16:36:09.818 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]47
03-29 16:36:09.828 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]48
03-29 16:36:09.839 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]49
03-29 16:36:09.849 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]50
03-29 16:36:09.860 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]51
03-29 16:36:09.870 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]52
03-29 16:36:09.880 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]53
03-29 16:36:09.891 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]54
03-29 16:36:09.902 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]55
03-29 16:36:09.913 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]56
03-29 16:36:09.923 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]57
03-29 16:36:09.934 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]58
03-29 16:36:09.944 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]59
03-29 16:36:09.955 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]60
03-29 16:36:09.965 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]61
03-29 16:36:09.976 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]62
03-29 16:36:09.986 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]63
03-29 16:36:09.997 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]64
03-29 16:36:10.007 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]65
03-29 16:36:10.018 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]66
03-29 16:36:10.028 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]67
03-29 16:36:10.038 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]68
03-29 16:36:10.049 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]69
03-29 16:36:10.059 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]70
03-29 16:36:10.070 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]71
03-29 16:36:10.080 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]72
03-29 16:36:10.091 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]73
03-29 16:36:10.101 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]74
03-29 16:36:10.112 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]75
03-29 16:36:10.122 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]76
03-29 16:36:10.133 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]77
03-29 16:36:10.143 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]78
03-29 16:36:10.153 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]79
03-29 16:36:10.164 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]80
03-29 16:36:10.174 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]81
03-29 16:36:10.184 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]82
03-29 16:36:10.195 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]83
03-29 16:36:10.206 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]84
03-29 16:36:10.216 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]85
03-29 16:36:10.227 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]86
03-29 16:36:10.237 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]87
03-29 16:36:10.248 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]88
03-29 16:36:10.258 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]89
03-29 16:36:10.269 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]90
03-29 16:36:10.279 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]91
03-29 16:36:10.290 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]92
03-29 16:36:10.301 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]93
03-29 16:36:10.311 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]94
03-29 16:36:10.322 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]95
03-29 16:36:10.332 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]96
03-29 16:36:10.343 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]97
03-29 16:36:10.353 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]98
03-29 16:36:10.364 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]99
03-29 16:36:10.375 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]0
03-29 16:36:10.386 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]1
03-29 16:36:10.396 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]2
03-29 16:36:10.407 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]3
03-29 16:36:10.417 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]4
03-29 16:36:10.428 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]5
03-29 16:36:10.438 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]6
03-29 16:36:10.448 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]7
03-29 16:36:10.459 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]8
03-29 16:36:10.469 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]9
03-29 16:36:10.480 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]10
03-29 16:36:10.491 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]11
03-29 16:36:10.502 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]12
03-29 16:36:10.512 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]13
03-29 16:36:10.523 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]14
03-29 16:36:10.534 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]15
03-29 16:36:10.544 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]16
03-29 16:36:10.555 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]17
03-29 16:36:10.565 10746-10813/com.example.rzm.appmarket I/System.out: threadThread[AsyncTask #4,5,main]18
......

在一个AsyncTask执行完成之前，另一个任务不会开始，也就是目前仍然，AsyncTask只能同时执行一个后台线程

接下来看源码

我们把AsyncTask的几个方法作为源码阅读的入口

execute 开始执行
cancel 取消执行

execute

//@MainThread 只能在主线程中开启
@MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }

我们知道，AsyncTask是通过线程池，Handler，Runnable集合来处理的,sDefaultExecutor继承自Executor，用来处理异步任务

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
......
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

......
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);
            }
        }
    }

executeOnExecutor

@MainThread
    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
            Params... params) {
        //判断当前任务的状态，如果不是未开始执行，而是正在执行或者已经执行
        //完成就抛出异常，所以一个任务只能执行一次，这就是一个任务调用两次execute方法会报错的原因
        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)");
            }
        }
        //更新当前运行状态
        mStatus = Status.RUNNING;
        //回调onPreExecute，重写此方法可进行一些初始化操作
        onPreExecute();
        //将参数存入worker的数组中
        mWorker.mParams = params;
        //这个exec就是刚刚传入的那个线程池类sDefaultExecutor，
        //将mFuture传入，执行sDefaultExecutor的execute方法
        exec.execute(mFuture);

        return this;
    }

这里看到了一个mFuture，这个mFuture是什么呢，它是在AsyncTask的构造方法中初始化的

public AsyncTask(@Nullable Looper callbackLooper) {
        mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);

        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);
                }
            }
        };
    }

构造方法中初始化了一个Handler，用于接收子线程的消息，mWorker和mFuture是结合在一起使用的，mWorker是一种继承自Callable实现的线程，这种线程不同于Runnable和Thread,第一，可以有返回值，第二，可以抛出异常信息

private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
        //之前传入的参数就是存在这里的
        Params[] mParams;
    }

FutureTask对Callable进行了一层包装，以保证这个线程可以被线程池执行，exec.execute(mFuture)线程池execute方法可以接受一个FutureTask对象

我们回到线程池Executor的execute方法中

private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
            //execute方法执行，将这个Runnable任务加入mTasks
            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);
            }
        }
    }

我们可以看到ArrayDeque是一个集合

public class ArrayDeque<E> extends AbstractCollection<E>

mTasks.offer是将这个任务加入了集合的尾部，相当于一个任务队列，

public boolean offer(E e) {
        return offerLast(e);
    }

/**
     * Inserts the specified element at the end of this deque.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Deque#offerLast})
     * @throws NullPointerException if the specified element is null
     */
    public boolean offerLast(E e) {
        addLast(e);
        return true;
    }

看到这里我们已经接近了为什么AsyncTask只能同时执行一个任务这个问题的答案了，看这里

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
......

private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {

            //这里的操作只是将Runnable加入了mTasks
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            //任务加入集合之后，执行这个方法，使用线程池
            //来执行集合中的runnable任务，THREAD_POOL_EXECUTOR.execute(mActive);
            
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

SerialExecutor是一个静态类，那么一旦初始化之后就会一直存在了，所有的AysyncTask共享这一个Executor，在第一个任务加入线程队列之后，取出第一个task开始执行第一个任务，并且给mActive附了值，此时mActive已经不再满足mActive==null的判断条件，也就是，在这个任务执行完成之前，如果有新的任务过来被加入任务队列，那么由于mActive==null的条件不成立，scheduleNext方法没法执行，这个任务就不会马上被执行，而是等待上一个任务完成之后再次调用scheduleNext开启下一个任务

当第一个Runnable中的异步任务执行完成的时候，会再执行scheduleNext()方法，从线程队列中取出下一个任务开始执行，所以说AsyncTask的任务执行顺序就是加入线程队列的前后顺序，到这里，最初的问题解决了，AsyncTask的确是一次只能同时执行一个任务

大概有人看到这里会觉得疑惑，对于这句话”当第一个Runnable中的异步任务执行完成的时候，会再执行scheduleNext()方法“中的Runnable中的异步任务指的是什么觉得不解，如果刚才的流程还记得的话，你会知道这个Runnable中的异步任务其实就是exec.execute(mFuture)传过来的FutureTask中包装的Callable,FutureTask也是最终实现了Runnable接口，所以r.run()中的r就是这个FutureTask。那么既然是异步任务，为什么要说当他执行完成的时候才会执行scheduleNext方法呢，按常理讲，应该是同时进行的，这里就涉及到了Callable线程的一种特性

Callable线程运行过程中，会阻塞当前线程，直到执行完成获得执行结果

这也是为什么将Runnable往线程队列中加的时候，要在这个Callable线程的外边再加一个Runnable，为什么不直接把这个Callable加入队列中，因为Callable有这个特性，导致他执行的时候会阻塞当前线程，那么如果直接加入集合，执行的时候直接执行这个Callable，那么就会导致主线程被阻塞，因为当前还是主线程，所以必须在Runnable中执行这个Callable

所以当他运行的时候，当前线程被阻塞，那么

finally {
       scheduleNext();
}

不会被立即执行，而是等待他执行完成,一个任务执行完成之后，调用scheduleNext执行下一个任务

这时候我们再回到WorkerRunnable中,这里就是每一个任务执行的地方

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
                    //回调doInBackground方法，执行我们的异步任务
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

当任务执行完成后，执行finally中的postResult方法,发送消息到Handler主线程

private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

private static class InternalHandler extends Handler {
        public InternalHandler(Looper looper) {
            super(looper);
        }

        @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
        @Override
        public void handleMessage(Message msg) {
            AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
            switch (msg.what) {
                case MESSAGE_POST_RESULT:
                    // There is only one result
                    result.mTask.finish(result.mData[0]);
                    break;
                case MESSAGE_POST_PROGRESS:
                    result.mTask.onProgressUpdate(result.mData);
                    break;
            }
        }
    }

调用AsyncTask的finish方法,并传递任务执行结果

private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            //回调onPostExecute，一个任务完成
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

下面我们再从cancel方法这个切入点去看源码

public final boolean cancel(boolean mayInterruptIfRunning) {
        mCancelled.set(true);
        return mFuture.cancel(mayInterruptIfRunning);
    }

调用的是FutureTask的cancel方法

public boolean cancel(boolean mayInterruptIfRunning) {
        if (!(state == NEW &&
              U.compareAndSwapInt(this, STATE, NEW,
                  mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
            return false;
        try {    // in case call to interrupt throws exception
            if (mayInterruptIfRunning) {
                try {
                    Thread t = runner;
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    U.putOrderedInt(this, STATE, INTERRUPTED);
                }
            }
        } finally {
            finishCompletion();
        }
        return true;
    }

finishCompletion

private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (U.compareAndSwapObject(this, WAITERS, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }
        
        done();

        callable = null;        // to reduce footprint
    }

最终还是走到done方法中