//AsyncTask从本质上讲,是对ThreadPool和handler的封装.

在学习线程池相关的知识时,看到书中提到AsyncTask的实现中使用到了ThreadPool,于是把源码翻了出来,

源码并不长,顺便分析了一下.小白挑战,大牛勿喷(好像大牛也不会看到...)

成员变量的解析

我打算先把成员变量分析一下(AsyncTask源码不是很复杂,可以这么做,算是特殊情况吧),算是为后面分析AsyncTask的工作流程做铺垫.

线程池 THREAD_POOL_EXECUTOR

    //cpu核心数

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();

    //核心线程数

    private static final int CORE_POOL_SIZE = CPU_COUNT + 1;

    //最大线程数

    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;

    //非核心线程数闲置时的超时时长

    private static final int KEEP_ALIVE = 1;

    //线程工厂,为线程池提供创建新线程的功能只有一个方法newThread(),具体实现就是创建一个thread并返回

    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());

        }

    };

    //线程池中的任务队列,通过线程池的execute方法提交的Runnable对象会存储在这个参数中,此处LinkedBlockingQueue是一个单向链表实现的阻塞队列

    private static final BlockingQueue<Runnable> sPoolWorkQueue =

            new LinkedBlockingQueue<Runnable>(128);

    //使用以上参数创建的线程池

    public static final Executor THREAD_POOL_EXECUTOR

            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,

                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

sDefaultExecutor (特别强调:串行)

    private static class SerialExecutor implements Executor {

        //这里使用了一个双端队列

        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);

            }

        }

    }

这里实现了一个串行的exectuor,这里的实现很巧妙,仔细观察可以发现,每次调用execute()方式都必然会调用scheduleNext(),

使得mTasks中的所有Runnable对象依次执行.

还有一点需要注意的是:

这里的execute()方法的作用只是用于给任务排队,真正执行任务的是THREAD_POOL_EXECUTOR.

然后看一下成员变量的声明:

    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

SERIAL_EXECUTOR 唯一的作用是把SerialExecutor对象传递给sDefaultExecutor,我觉得这种写法很奇怪,于是尝试了一下发现

final和volatile不能同时修饰一个变量,其实从它们各自的作用也可以看出同时使用这两个修饰符是矛盾的.

为什么这里要这样使用呢?

我觉得原因在于下面这个隐藏方法:

    /** @hide */

    public static void setDefaultExecutor(Executor exec) {

        sDefaultExecutor = exec;

    }

也就是说AsyncTask中有一个sDefaultExecutor的默认实现,但是还提供了一个隐藏方法,可以对其进行修改.

sHandler

首先先看一个内部类的实现,这个内部类是用来包装执行结果的,其中封装了一个AsyncTask实例和某种类型的数据集.

    @SuppressWarnings({"RawUseOfParameterizedType"})

    private static class AsyncTaskResult<Data> {

        final AsyncTask mTask;

        final Data[] mData;

        AsyncTaskResult(AsyncTask task, Data... data) {

            mTask = task;

            mData = data;

        }

    }

接下来看InternalHandler的具体实现

    private static final int MESSAGE_POST_RESULT = 0x1;//返回结果

    private static final int MESSAGE_POST_PROGRESS = 0x2;//返回进度

    private static InternalHandler sHandler;

    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]);//具体实现在下面

                    break;

                case MESSAGE_POST_PROGRESS:

                    result.mTask.onProgressUpdate(result.mData);//这个就是我们自定义AsyncTask时需要实现的onProgressUpdate()

                    break;

            }

        }

    }

    private void finish(Result result) {

        if (isCancelled()) {

            onCancelled(result);//这个同样需要用户来实现

        } else {

            onPostExecute(result);//这个就是我们自定义AsyncTask时需要实现的onPostExecute()

        }

        mStatus = Status.FINISHED;//任务执行完成的状态

    }

    //这里的实现是单例模式

    private static Handler getHandler() {

        synchronized (AsyncTask.class) {

            if (sHandler == null) {

                sHandler = new InternalHandler();

            }

            return sHandler;

        }

    }

onProgressUpdate,onPostExecute,onCancelled这三个回调方法会在这里集中处理,结合AsyncTask的使用方法,我们可以得出以下结论:

在doInBackground()执行过程中或者结束后,会由该handler发送消息到主线程,调用相关回调方法,可以认为,handler是AsyncTask整个工作流程的倒数第二站.

mWorker&FutureTask

    private final WorkerRunnable<Params, Result> mWorker;

    private final FutureTask<Result> mFuture;

    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {

        Params[] mParams;

    }

Callable

WorkRunnable实现了Callble接口,Callable与Runnable的功能大致相似,不同的是Callable是一个泛型接口,它有一个泛型参数V,该接口中只有一个call()方法,该方法返回类型为V的值.

Callable可以简单理解为有返回值的Runnable.

可以对比一下

public interface Callable<V> {

    /**

     * Computes a result, or throws an exception if unable to do so.

     *

     * @return computed result

     * @throws Exception if unable to compute a result

     */

    V call() throws Exception;

}

public interface Runnable {

    /**

     * Starts executing the active part of the class' code. This method is

     * called when a thread is started that has been created with a class which

     * implements {@code Runnable}.

     */

    public void run();

}

FutureTask

FutureTask稍微复杂一些,FutureTask实现了 RunnableFuture接口,而RunnableFuture继承Runnable, Future

Future提供了对Runnable或者Callable任务的执行结果进行取消,查询是否完成,获取结果的操作,FutureTask是它的实现类

public interface Future<V> {

    boolean cancel(boolean mayInterruptIfRunning);

    boolean isCancelled();

    boolean isDone();

    V get() throws InterruptedException, ExecutionException;

    V get(long timeout, TimeUnit unit)

        throws InterruptedException, ExecutionException, TimeoutException;

}

下面是FutureTask的一个构造函数,在AsyncTask的构造函数中会使用其创建一个FutureTask实例

    public FutureTask(Callable<V> callable) {

        if (callable == null)

            throw new NullPointerException();

        this.callable = callable;

        this.state = NEW;       // ensure visibility of callable

    }

下面看一下构造函数对这两个成员变量的初始化(构造函数唯一做的事情)

    /**

     * Creates a new asynchronous task. This constructor must be invoked on the UI thread.

     */

    public AsyncTask() {

        mWorker = new WorkerRunnable<Params, Result>() {

            public Result call() throws Exception {

                mTaskInvoked.set(true);//设置mTask已经被调用过

                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);

                //noinspection unchecked

                Result result = doInBackground(mParams);//调用doInBackground()!!!!!!!!

                Binder.flushPendingCommands();

                //调用postResult将结果投递给UI线程

                return postResult(result);

            }

        };

        //将mWorker作为参数创建FutureTask实例

        //在mFuture实例中,将会调用mWorker做后台任务,完成后调用done方法

        mFuture = new FutureTask<Result>(mWorker) {

            @Override

            protected void done() {

                try {

                    postResultIfNotInvoked(get());//确保postResult的执行

                } 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);

                }

            }

        };

    }

    //确保postResult的执行

    private void postResultIfNotInvoked(Result result) {

        final boolean wasTaskInvoked = mTaskInvoked.get();

        if (!wasTaskInvoked) {

            postResult(result);

        }

    }

    //这里最终的结果是调用onCancelled()或者onPostExecute()

    private Result postResult(Result result) {

        @SuppressWarnings("unchecked")

        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,

                new AsyncTaskResult<Result>(this, result));

        message.sendToTarget();

        return result;

    }

doInBackground和handler的使用都在这里!!!!!

最后的三个成员变量(标志位)

    //枚举 执行状态

    private volatile Status mStatus = Status.PENDING;

    //是否取消

    private final AtomicBoolean mCancelled = new AtomicBoolean();

    //mTask是否被调用过

    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();

    public enum Status {

        /**

         * Indicates that the task has not been executed yet.

         */

        PENDING,

        /**

         * Indicates that the task is running.

         */

        RUNNING,

        /**

         * Indicates that {@link AsyncTask#onPostExecute} has finished.

         */

        FINISHED,

    }

至此,AsyncTask中的所有成员变量分析完毕,各个关键回调方法的调用地点也都清楚了,下面我们从execute()方法入手,分析一下AsyncTask的工作流程.

    @MainThread

    public final AsyncTask<Params, Progress, Result> execute(Params... params) {

        return executeOnExecutor(sDefaultExecutor, params);//这里的参数现在看起来已经很清晰了,sDefaultExecutor是用于将任务加入队列的executor,params是用户提供的参数.

    }

继续往下看executeOnExecutor

    @MainThread

    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,

            Params... params) {

        //首先,判断状态,只有PENDING才能继续执行

        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)");

            }

        }

        //修改状态为RUNNING

        mStatus = Status.RUNNING;

        onPreExecute();//onPreExectue首先执行

        //设置mWorker的参数

        mWorker.mParams = params;

        //这里的exec就是sDefaultExecutor,

        exec.execute(mFuture);

        return this;

    }

通过上文的分析,我们知道这里的exec.execute(mFuture)最终调用到了THREAD_POOL_EXECUTOR中的execute()方法.

线程池内部的具体细节暂不分析,execute()方法执行的最终结果是将mFuture中的run()方法启动.

FutureTask中的run():

 public void run() {

        if (state != NEW ||

            !U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))

            return;

        try {

            Callable<V> c = callable;

            if (c != null && state == NEW) {

                V result;

                boolean ran;

                try {

                    result = c.call();//这里调用了mWorker的call()方法,而mWorker的call()调用了doInBackground()方法.至此,线程真正启动了.

                    ran = true;

                } catch (Throwable ex) {

                    result = null;

                    ran = false;

                    setException(ex);

                }

                if (ran)

                    set(result);//成功获取结果后,调用set()方法

            }

        } 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);

        }

    }

    protected void set(V v) {

        if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {

            outcome = v;

            U.putOrderedInt(this, STATE, NORMAL); // final state

            finishCompletion();//set()方法又调用finishCompletion()方法

        }

    }

 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();//finishCompletion()方法最终调用了done()方法,而上文中已经提到,调用done()方法最终的结果是通过handler调用onCancelled()或者opPostExecute();

        callable = null;        // to reduce footprint

    }

至此,AsyncTask的整个工作流程分析完毕,一次奇妙的源码之旅get,接下来可以去拜读各位大牛对于AsyncTask更加深入的分析了...