AsyncTask源码分析

Sun式源码分析步骤：

1、这个东西怎么用？

（1）定义具体的AsyncTask

    /**     * 三个参数传入的参数 AsyncTask<Params, Progress, Result>：要处理的数据类型，进度更新的类型（对应onProgressUpdate（）），处理结果的类型     */    class MyTask extends AsyncTask<String, Integer, String> {        private TextView textView;        public MyTask(TextView textView) {            this.textView = textView;        }        //在execute(Params... params)被调用后立即执行，一般用来在执行后台任务前对UI做一些标记        @Override        protected void onPreExecute() {            dialog.setTitle("请稍等");            dialog.setMessage("网页正在加载中。。。");            dialog.setProgressStyle(ProgressDialog.STYLE_SPINNER);            dialog.setButton(DialogInterface.BUTTON_NEUTRAL, "取消", new DialogInterface.OnClickListener() {                @Override                public void onClick(DialogInterface dialog, int which) {                    dialog.cancel();                    //取消任务                    MyTask.this.cancel(true);                }            });            dialog.show();        }        //在onPreExecute()完成后立即执行，用于执行较为费时的操作，此方法将接收输入参数和返回计算结果。在执行过程中可以调用publishProgress(Progress... values)来更新进度信息。        @Override        protected String doInBackground(String... params) {            int i = 10;            while (true) {                publishProgress((int) i);                if (i == 0) break;                try {                    Thread.sleep(1000);                } catch (InterruptedException e) {                    e.printStackTrace();                }                i--;            }            return null;        }        //在调用publishProgress(Progress... values)时，此方法被执行，直接将进度信息更新到UI组件上        @Override        protected void onProgressUpdate(Integer... values) {            textView.setText(values + "");            super.onProgressUpdate(values);        }        // 任务执行的结果作为此方法的返回参数result其实是doInBackgSround(Object... params)的返回值        @Override        protected void onPostExecute(String s) {            super.onPostExecute(s);        }    }

（2）启动Task

new MyTask(tv).execute("");

2、找到分析的切入点：这句启动代码就是我们的切入点，开始源码分析

new了一个对象，那么就必定途径构造函数。子类MyTask构造函数没啥分析的，那么就轮到父类AsyncTask的构造函数了。

    public AsyncTask() {        mWorker = new WorkerRunnable<Params, Result>() {            public Result call() throws Exception {                mTaskInvoked.set(true);                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);                //noinspection unchecked                Result result = doInBackground(mParams);                Binder.flushPendingCommands();                return postResult(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);                }            }        };    }

mWorker、mFuture是啥？mWorker是一个实现Callable接口的抽象类。Callable+FutureTask+ThreadPoolExecutor就是多线程的第三种实现方式（另外两种是：Thread、Runnable）

  private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {        Params[] mParams;    }

mWorker=Callable，mFuture=FutureTask，那ThreadPoolExecutor在哪？Look below！

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();    private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;    private static final int KEEP_ALIVE_SECONDS = 30;    private static final ThreadFactory sThreadFactory = new ThreadFactory() {        private final AtomicInteger mCount = new AtomicInteger(1);        public Thread newThread(Runnable r) {            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());        }    };    private static final BlockingQueue<Runnable> sPoolWorkQueue =            new LinkedBlockingQueue<Runnable>(128);    public static final Executor THREAD_POOL_EXECUTOR;    static {        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,                sPoolWorkQueue, sThreadFactory);        threadPoolExecutor.allowCoreThreadTimeOut(true);        THREAD_POOL_EXECUTOR = threadPoolExecutor;    }    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;    private static class SerialExecutor implements Executor {        //ArrayDeque是数组队列，offer()代表插入元素，poll()代表取出并移除头元素（pop()只是取出）        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);            }        }    }

这么多的代码就是创建一个线程池。致此，new MyTask的过程就结束了，就是为了凑齐Callable+FutureTask+ThreadPoolExecutor三大将来完成多线程的准备。
接下来分析.execute(“”)这部分。

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

简单易懂， sDefaultExecutor就是之前创建的那个线程池调度器，传入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);        return this;    }

我们可以看到，出现在我们的眼前的便是第一个覆写的函数onPreExecute()，这个函数主要是用来进行一些预处理操作的，它运行在主线程。紧接着，mWork接受传入的参数。然后线程池开始执行mFuture。那么我们再次回到mFuture这段代码。

mWorker = new WorkerRunnable<Params, Result>() {            public Result call() throws Exception {                mTaskInvoked.set(true);                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);                //noinspection unchecked                Result result = doInBackground(mParams);                Binder.flushPendingCommands();                return postResult(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);                }            }        };

mFuture的执行传入的mWorker。而mWork中就正式开始执行doInBackground(mParams)，这个方法就是运行在非UI线程的，你可以覆写此方法，进行一些耗时操作，然后result获得到了该方法的运行结果，并通过postResult方法将结果传出去，接下来看postResult方法。

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

postResult方法比较好理解，将接收到的结果封装到AsyncTaskResult中，通过消息机制发送出去。那么发送到哪里去了呢？我们看到，是getHandler发送出去的，那么看一下该方法。

private static Handler getHandler() {        synchronized (AsyncTask.class) {            if (sHandler == null) {                sHandler = new InternalHandler();            }            return sHandler;        }    }

我们看到，原来是获取了一个InternalHandler单例，好了，去找InternalHandler吧。

private static class InternalHandler extends Handler {        public InternalHandler() {            super(Looper.getMainLooper());        }        @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;            }        }    }

这里，将会对MESSAGE_POST_RESULT进行处理，处理过程是result.mTask.finish(result.mData[0])。result.mTask就是内部包装类AsyncTaskResult中的AsyncTask实例。那么finish就是AsyncTask中的方法，我们来看看它。

   private void finish(Result result) {        if (isCancelled()) {            onCancelled(result);        } else {            onPostExecute(result);        }        mStatus = Status.FINISHED;    }

可以看到，如果Task已经被取消了，则执行Cancel相关处理，如果没有被取消，则通过onPostExecute方法将结果回调给我们。同样publishProgress方也是通过InternalHandler将进度回调给我们的。
至此，我们就简单的将AsyncTask“运行”了一遍，其中更多的细节，需要自己耐心的去品味了，毕竟我只是想分享一下阅读源码的思维。