深入理解为什么AsyncTask只能被执行一次

看该篇文章前，我推荐了另外一篇博客，看完再来看该篇博客。

 

Android源码分析—带你认识不一样的AsyncTask

接下来，就来分析为什么AsyncTask只能被执行一次：

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

我们都知道，当外界调用AsyncTask.execute的时候，他就会执行doInBackground中的代码。而该函数又调用了该函数

    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);

        return this;
    }

该函数中，首先对状态进行了判断，如果状态不是pending的话就会抛出异常，要么是正在执行，要么是已经被执行过的异常。

然后我将该段代码注释掉

//        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)");
//            }
//        }

然后让该AsyncTask执行两遍（调用execute两次）。发现并没有报错，但是发现doInBackground中的代码并没有被执行两遍，而是执行了一遍（第一次调用Execute被执行，第二次没有执行）。

再回看该函数，里面执行doInBackground中最重要的代码就是

exec.execute(mFuture);

那我猜想应该是mFuture出现了问题。看如下该图

Callable和Runnable是类似的，只是Runnable是调用run函数，Callable调用call函数，而且call函数会返回执行结果，FutureTask就是去获取执行结果的一个类。具体可以看这篇文章： Runnable、Callable、Executor、Future、FutureTask关系解读

FutureTask主要是在FutureTask的构造函数中传入callable。当通过ThreadPoolExecutor.execute(FutureTask)后，会调用FutureTask.run，在run中会调用Callable.call函数，执行完后会将callable清空，所以FutureTask只能被执行一次，第二次执行时会发现callable已经为空。看下代码

    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

在set(Rsult)和setException(ex)中会调用finishCompletion()。

    private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, 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
    }

看到了吗，在最后一句会将callable置成null。讲了这么多最主要的就是FutureTask只能被执行一次。

而且在AsyckTask中FutureTask被声明称final的，即只能被初始化一次。后来我做了如下改变，将FutureTask和Callable去掉final，在execute函数中去初始化它，如下代码所示：

    public final MyAsyncTask<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 = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);

                android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_BACKGROUND);
                //noinspection unchecked
                return postResult(doInBackground(mParams));
            }
        };
        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 occured while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                }
            }
        };

        mWorker.mParams = params;
        exec.execute(mFuture);

        return this;
    }

现在无论外面调用execute多少次，都可以执行多遍了。