源码分析之AsyncTask

AsyncTask在Android中是很常用的异步线程，那么AsyncTask和Thread有什么区别呢？这里将从源码角度深入理解AsyncTask的设计和工作原理，这里的AsyncTask基于SDK-25

分析知识准备

首先我们来看一个生产者与消费者模型的例子

public class ThreadTest {

    //产品
    static class ProductObject{
        public volatile static String value; //volatile线程操作变量可见
    }

    //生产者线程
    static class Producer extends Thread{
        Object lock;
        public Producer(Object lock) {
            this.lock = lock;
        }
        @Override
        public void run() {
            while(true){
                synchronized (lock) {
                    if(ProductObject.value != null){
                        try {
                            lock.wait(); //产品还没有被消费，等待
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    ProductObject.value = "NO:"+System.currentTimeMillis();
                    System.out.println("生产产品："+ProductObject.value);
                    lock.notify(); //生产完成，通知消费者消费
                }
            }
        }
    }

    //消费者线程
    static class Consumer extends Thread{
        Object lock;
        public Consumer(Object lock) {
            this.lock = lock;
        }
        @Override
        public void run() {
            while(true){
                synchronized (lock) {
                    if(ProductObject.value == null){
                        try {
                            lock.wait(); //等待，阻塞
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println("消费产品："+ProductObject.value);
                    ProductObject.value = null;
                    lock.notify(); //消费完成，通知生产者，继续生产
                }
            }
        }
    }

    public static void main(String[] args) {
        Object lock = new Object();
        new Producer(lock).start();
        new Consumer(lock).start();
    }
}

上面的例子关键点在于两个，其一是 volatile，使得线程间可见，第二个点在于互斥锁，这样就可以使得有商品的时候就要通知消费者消费，同时 wait，那么消费者收到消息开始消费，消费完毕通知生产者继续生产，从而不断生产，这样比轮询方式更加节省资源

在了解完上面的例子以后，我们就可以着手分析AsyncTask的源代码了

首先，我们在AsyncTask首先看其构造方法

private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
···
public AsyncTask() {
    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);
            }
        }
    };
}

这里首先给WorkerRunnable和Future进行了初始化，那么为何要初始化这两个变量呢？

这里就要说到常用的两个方法了，doInBackground()，这个方法是在子线程里面完成的，另一个方法就是onPostExecute()，而这个方法是存在于主线程的，那么也就是说子线程执行完将执行的结果传递到了主线程中，实现了线程间的通信，那么最关键的问题来了，这个通信是怎么实现的呢？
通常在子线程中执行的任务，是没有返回结果的，例如Runnable的源代码如下，就没有返回结果

public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

那么，要怎么才能得到返回值呢，这里首先想到的就是Callable接口，那么再看看Callable的源代码

@FunctionalInterface
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;
}

可以看到，这是一个泛型方法，是有返回值的，但是其本身确是不能直接执行的，需要借助其他类，接下来再看一看源代码中涉及到的Future接口

public interface Future<V> {

    /**
     * Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, has already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when {@code cancel} is called,
     * this task should never run.  If the task has already started,
     * then the {@code mayInterruptIfRunning} parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     *
     * <p>After this method returns, subsequent calls to {@link #isDone} will
     * always return {@code true}.  Subsequent calls to {@link #isCancelled}
     * will always return {@code true} if this method returned {@code true}.
     *
     * @param mayInterruptIfRunning {@code true} if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return {@code false} if the task could not be cancelled,
     * typically because it has already completed normally;
     * {@code true} otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

    /**
     * Returns {@code true} if this task was cancelled before it completed
     * normally.
     *
     * @return {@code true} if this task was cancelled before it completed
     */
    boolean isCancelled();

    /**
     * Returns {@code true} if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * {@code true}.
     *
     * @return {@code true} if this task completed
     */
    boolean isDone();

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    V get() throws InterruptedException, ExecutionException;

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

在Future类中，有好几个方法，而这些方法都是有返回值的，那么Runnable与Future和FutureTask有什么关系呢，产看源码便可得知，FutureTask实际上是实现了RunnableFuture接口

public class FutureTask<V> implements RunnableFuture<V>{ ··· }

而RunnableFuture又继承了Runnable和Future

public interface RunnableFuture<V> extends Runnable, Future<V> { void run(); }

那也就是说，FutureTask既可以在子线程中执行，也可以获得执行结果，下面使用一个例子来说明FutureTask

public class FutureTest {

    public static void main(String[] args) {
        Task work = new Task();
        FutureTask<Integer> future = new FutureTask<Integer>(work){
            @Override
            protected void done() { //异步任务执行完成，回调
                try {
                    System.out.println("done:" + get()); //get()获取异步任务的返回值，这是个阻塞方法
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (ExecutionException e) {
                    e.printStackTrace();
                }
            }
        };
        //线程池（使用了预定义的配置）
        ExecutorService executor = Executors.newCachedThreadPool();
        executor.execute(future);
    }

    //异步任务
    static class Task implements Callable<Integer>{

        @Override
        public Integer call() throws Exception {//返回异步任务的执行结果
            int i = 0;
            for (; i < 10; i++) {
                try {
                    System.out.println(Thread.currentThread().getName() + "_" + i);
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            return i;
        }
    }
}

上面的例子可以看出，在使用了Callable的时候，需要借助FutureTask来包装，然后使用Executor的execute()方法来执行，那么是怎么得到异步任务的返回值呢，在上面的例子中，我们可以看到，其返回值的获取是通过future.get()得到的，然而这个get()方法确是被阻塞的，只有在异步任务完成的时候才能获取到其结果，那我们怎么才能知道异步任务时候执行完毕呢，这里就可以实现FutureTask的done()方法，当异步任务执行完毕以后会回调这个方法，上述例子其实解释了AsyncTask的实现逻辑，call()方法是在子线程中完成，这也就是doInBackground()的实现，在主线程中获得结果，这是在onPostExecute()使用了get()方法，那也就是说AsyncTask就是通过这一套方法去实现的

从这里我们可以总结出FutureTask为异步任务提供了诸多便利性，包括

1. 获取异步任务的返回值
2. 监听异步任务的执行情况
3. 取消异步任务

那么在AsyncTask中，WorkerRunnable又是啥呢，其实就是一个内部类，对Callable进行了封装

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

源代码分析

有了以上知识储备，我们就可以动手分析AsyncTask源代码了
拿到源代码，不同的人有不同的分析习惯，这里我按照我的习惯对源代码进行一次分析

构造方法分析

首先，因为我们分析源代码是为了更好的去使用，而使用的话，第一个关注的就应该是构造方法，回到之前的的构造方法，这里要开始对构造方法开始入手分析了

private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
private final AtomicBoolean mCancelled = new AtomicBoolean();
private final AtomicBoolean mTaskInvoked = new AtomicBoolean();
···
public AsyncTask() {
    mWorker = new WorkerRunnable<Params, Result>() {
        public Result call() throws Exception {
            //设置线程调用
            mTaskInvoked.set(true);
            Result result = null;
            try {
                //设置线程优先级，其给定值为10
                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                //调用doInBackground()方法得到返回值
                result = doInBackground(mParams);
                //将当前线程中的Binder命令发送至kernel
                Binder.flushPendingCommands();
            } catch (Throwable tr) {
                //发生异常则取消线程调用设置
                mCancelled.set(true);
                throw tr;
            } finally {
                //执行postResult()方法
                postResult(result);
            }
            return result;
        }
    };

    mFuture = new FutureTask<Result>(mWorker) {
        @Override
        protected void done() {
            try {
                //执行postResultIfNotInvoked()方法
                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);
            }
        }
    };
}

以上就是AsyncTask的构造方法了，在构造方法上有一句说明，这个构造方法必须在UI线程中创建，这一点很好理解，因为其有需要再主线程中执行的地方，后面会说到，那么这个构造方法干了什么事情呢，很简单，这里新建了两个对象，首先是WorkerRunnable，而这个WorkerRunnable则是实现自Callable接口，主要是要使用其call()方法，为了返回参数，并没有什么特别之处，再其内部则实现了call()方法，而其自生是无法执行的，需要找一个包装类，而这个包装类就是FutureTask，通过之前的分析，这里就不再多赘述关于FutureTask的东西了，这里实现了done()方法，也就是线程执行完毕调用的方法，简单点来说就是在call()方法中执行，在done()中获得执行的返回结果，上述涉及到一个内部类和三个自定义的方法，那么接下来我们看一看这个内部类和三个方法都干了啥

构造方法中出现的内部类

这里的WorkerRunnable，正如前面所说，这里除了实现Callable就啥也没干，还是个抽象方法，这里将实现放在了构造方法中

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

构造方法中调用的方法

首先是doInBackground()方法，前面讲到，这个方法在子线程中完成，那么这里的子线程是哪个呢，其实就是WorkerThread，这个方法是一个抽象方法，放在子线程中执行，其具体实现由调用者完成

@WorkerThread protected abstract Result doInBackground(Params... params);

再看postResult()方法，这里获取了一个Handler，然后发送了一个消息，这里就是子线程能够通信主线程的地方了

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

那么到这里，我们关注的重点就来了，子线程是怎么告诉主线程的呢，要知道其中的原因，我们就需要去看看代码里面是怎么实现的
我们先看发送了什么消息，也就是AsyncTaskResult里面干了啥，查看代码发现，其就是做了参数传递的任务

@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
    final AsyncTask mTask;
    final Data[] mData;

    AsyncTaskResult(AsyncTask task, Data... data) {
        mTask = task;
        mData = data;
    }
}

那么接下来的重点就是getHandler()方法了，这里拿到AsyncTask.class就上了锁了，这也很好理解，不上锁其他线程走到这里会产生安全隐患，然后返回sHandler，那再继续看看InternalHandler又是个什么吧

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

这里就是了，在构造方法里面super(Looper.getMainLooper())，也就说明了这个方法是在主线程中执行的，在主线程中对Message进行处理，这里又涉及到两个方法，一个是finish()，还有一个是onProgressUpdate()，那么好吧，再去看看这两个方法在干啥

private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
···
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;
        }
    }
}

首先是又调了isCancelled()，判断是否取消，前面构造函数的时候见过这个，这是在异常发生的时候才设置为true的，那么如果不发生异常，这里应该就是为false的，但在找源代码时发现，另一个方法也对这个参数进行了设置，那就是cancel()，所以在不发生异常和取消的时候应该是为true的，接下来是onCancelled()方法，这里是不做任何操作的，这也是主线程中的方法，还有就是onPostExecute()方法，然后会设置状态，其默认状态是PENDING

private volatile Status mStatus = Status.PENDING;
···
public enum Status {
    PENDING,
    RUNNING,
    FINISHED,
}
···
private void finish(Result result) {
    if (isCancelled()) {
        onCancelled(result);
    } else {
        onPostExecute(result);
    }
    mStatus = Status.FINISHED;
}
@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onPostExecute(Result result) {
}

@SuppressWarnings({"UnusedParameters"})
@MainThread
protected void onCancelled(Result result) {
    onCancelled();
}    

@MainThread
protected void onCancelled() {
}

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

然后是onProgressUpdate()方法，那么这里做了啥呢，嗯~啥也没有，交给调用者在继承时可以使用

@SuppressWarnings({"UnusedDeclaration"})
@MainThread
protected void onProgressUpdate(Progress... values) {
}

来看构造方法中涉及到的最后一个方法postResultIfNotInvoked()，这个方法又干了啥了，首先获得了mTaskInvoked的状态，整个AsyncTask只有构造方法处设置了这个值，然后判断是否执行postResult()方法

private void postResultIfNotInvoked(Result result) {
    final boolean wasTaskInvoked = mTaskInvoked.get();
    if (!wasTaskInvoked) {
        postResult(result);
    }
}

至此构造方法分析完成，可以看到在构造方法中，其主要做的工作最主要的就是搭建好了子线程和主线程沟通的桥梁

执行入口分析

在新建AsyncTask对象以后，要执行的话，需要使用execute()去开始执行
那么我们就从这里入手，看看其具体是怎么工作的，可以看到无论是构造方法还是启动方法，都是需要在主线程中完成的，在execute()中，做了些什么呢，在这之前我们先看看传递的sDefaultExecutor是啥

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

这其实是一个线程池，任务调度的线程池，可以看到SerialExecutor实际上是实现了Executor接口，其作用就是将任务添加到双向队列，然后不断地取出执行取出执行，那么THREAD_POOL_EXECUTOR也应该是一个线程池，那这又是啥呢，去看一看这个玩意儿就是到了

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
···
private static class SerialExecutor implements Executor {
    //定义了一个双向队列，用来存储线程
    final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
    Runnable mActive;

    public synchronized void execute(final Runnable r) {
        //向队列中添加线程
        mTasks.offer(new Runnable() {
            public void run() {
                try {
                    //线程运行
                    r.run();
                } finally {
                    //执行scheduleNext()方法
                    scheduleNext();
                }
            }
        });
        if (mActive == null) {
            scheduleNext();
        }
    }

    //从队列中取出线程并执行
    protected synchronized void scheduleNext() {
        if ((mActive = mTasks.poll()) != null) {
            THREAD_POOL_EXECUTOR.execute(mActive);
        }
    }
}

下列代码就是初始化了线程池的参数，指定了线程数量

//获得可用CPU数量
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
//设置核心线程池数量其范围[2,4]，无论是否使用都存在
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
//设置最大线程数量
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
//设置闲置回收时间，也就是说线程在这个时间内没有活动的话，会被回收
private static final int KEEP_ALIVE_SECONDS = 30;
//设置线程工厂，通过这个创建线程
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
    //创建线程安全的线程个数计数器
    private final AtomicInteger mCount = new AtomicInteger(1);

    public Thread newThread(Runnable r) {
        return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
    }
};
//设置任务队列大小
private static final BlockingQueue<Runnable> sPoolWorkQueue =
        new LinkedBlockingQueue<Runnable>(128);
//设置线程池
public static final Executor THREAD_POOL_EXECUTOR;

//初始化线程池
static {
    ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
            CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
            sPoolWorkQueue, sThreadFactory);
    //打开核心线程池的超时时间
    threadPoolExecutor.allowCoreThreadTimeOut(true);
    THREAD_POOL_EXECUTOR = threadPoolExecutor;
}

所以在执行scheduleNext()的时候，会将THREAD_POOL_EXECUTOR中设置好的线程全部取出来，用来执行后面的任务，其执行的任务就是execute()方法所指定的任务，在executeOnExecutor()方法中，由于前面初始化完成，这里的状态应该是PENDING，之后还设置了mWorker的参数，然后会执行线程池的方法，然后据开始执行任务了，前面没有涉及到的方法还有一个，那我们接下来看看

@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;
}

onPreExecute()，这个方法由调用者在继承时候能够使用

@MainThread
protected void onPreExecute() {
}

至此，AsyncTask的执行方法也分析完了，那么我们接下来看看还有什么方法没有涉及到，没有涉及到的方法都是public属性和方法

AsyncTask的公共方法

//这个方法设置为public，那么就意味着我们可以自定义线程池
public static void setDefaultExecutor(Executor exec) {
    sDefaultExecutor = exec;
}

//这个方法意味着我们可以获得其状态，配合枚举值使用
public enum Status {
    PENDING,
    RUNNING,
    FINISHED,
}
···
public final Status getStatus() {
    return mStatus;
}

//查看是非被取消
public final boolean isCancelled() {
    return mCancelled.get();
}

//取消异步任务
public final boolean cancel(boolean mayInterruptIfRunning) {
    mCancelled.set(true);
    return mFuture.cancel(mayInterruptIfRunning);
}

//获取返回结果，注意：这个方法是阻塞式的
public final Result get() throws InterruptedException, ExecutionException {
    return mFuture.get();
}

//同上
public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
            ExecutionException, TimeoutException {
    return mFuture.get(timeout, unit);
}

//自定义的线程池可以从这个方法启动
@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;
}

//使用默认线程池启动异步任务
@MainThread
public static void execute(Runnable runnable) {
    sDefaultExecutor.execute(runnable);
}

总结

AsyncTask的实例化过程，其本质上就是实例化了一个FutureTask

其执行过程Executor.execute(mFuture) -> SerialExecutor.mTasks(队列) -> (线程池)THREAD_POOL_EXECUTOR.execute

线程池中的所有线程，为了执行异步任务

如果当前线程池中的数量小于corePoolSize，创建并添加的任务
如果当前线程池中的数量等于corePoolSize，缓冲队列workQueue未满，那么任务被放入缓冲队列、等待任务调度执行
如果当前线程池中的数量大于corePoolSize，缓冲队列workQueue已满，并且线程池中的数量小于maximumPoolSize，新提交任务会创建新线程执行任务
如果当前线程池中的数量大于corePoolSize，缓冲队列workQueue已满，并且线程池中的数量等于maximumPoolSize，新提交任务由Handler处理
当线程池中的线程大于corePoolSize时，多余线程空闲时间超过keepAliveTime时，会关闭这部分线程

线程池在添加时候是串行的，在执行任务的时候是并行的

附录(源代码)

package android.os;

import android.annotation.MainThread;
import android.annotation.WorkerThread;

import java.util.ArrayDeque;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * <p>AsyncTask enables proper and easy use of the UI thread. This class allows you
 * to perform background operations and publish results on the UI thread without
 * having to manipulate threads and/or handlers.</p>
 *
 * <p>AsyncTask is designed to be a helper class around {@link Thread} and {@link Handler}
 * and does not constitute a generic threading framework. AsyncTasks should ideally be
 * used for short operations (a few seconds at the most.) If you need to keep threads
 * running for long periods of time, it is highly recommended you use the various APIs
 * provided by the <code>java.util.concurrent</code> package such as {@link Executor},
 * {@link ThreadPoolExecutor} and {@link FutureTask}.</p>
 *
 * <p>An asynchronous task is defined by a computation that runs on a background thread and
 * whose result is published on the UI thread. An asynchronous task is defined by 3 generic
 * types, called <code>Params</code>, <code>Progress</code> and <code>Result</code>,
 * and 4 steps, called <code>onPreExecute</code>, <code>doInBackground</code>,
 * <code>onProgressUpdate</code> and <code>onPostExecute</code>.</p>
 *
 * <div class="special reference">
 * <h3>Developer Guides</h3>
 * <p>For more information about using tasks and threads, read the
 * <a href="{@docRoot}guide/components/processes-and-threads.html">Processes and
 * Threads</a> developer guide.</p>
 * </div>
 *
 * <h2>Usage</h2>
 * <p>AsyncTask must be subclassed to be used. The subclass will override at least
 * one method ({@link #doInBackground}), and most often will override a
 * second one ({@link #onPostExecute}.)</p>
 *
 * <p>Here is an example of subclassing:</p>
 * <pre class="prettyprint">
 * private class DownloadFilesTask extends AsyncTask&lt;URL, Integer, Long&gt; {
 *     protected Long doInBackground(URL... urls) {
 *         int count = urls.length;
 *         long totalSize = 0;
 *         for (int i = 0; i < count; i++) {
 *             totalSize += Downloader.downloadFile(urls[i]);
 *             publishProgress((int) ((i / (float) count) * 100));
 *             // Escape early if cancel() is called
 *             if (isCancelled()) break;
 *         }
 *         return totalSize;
 *     }
 *
 *     protected void onProgressUpdate(Integer... progress) {
 *         setProgressPercent(progress[0]);
 *     }
 *
 *     protected void onPostExecute(Long result) {
 *         showDialog("Downloaded " + result + " bytes");
 *     }
 * }
 * </pre>
 *
 * <p>Once created, a task is executed very simply:</p>
 * <pre class="prettyprint">
 * new DownloadFilesTask().execute(url1, url2, url3);
 * </pre>
 *
 * <h2>AsyncTask's generic types</h2>
 * <p>The three types used by an asynchronous task are the following:</p>
 * <ol>
 *     <li><code>Params</code>, the type of the parameters sent to the task upon
 *     execution.</li>
 *     <li><code>Progress</code>, the type of the progress units published during
 *     the background computation.</li>
 *     <li><code>Result</code>, the type of the result of the background
 *     computation.</li>
 * </ol>
 * <p>Not all types are always used by an asynchronous task. To mark a type as unused,
 * simply use the type {@link Void}:</p>
 * <pre>
 * private class MyTask extends AsyncTask&lt;Void, Void, Void&gt; { ... }
 * </pre>
 *
 * <h2>The 4 steps</h2>
 * <p>When an asynchronous task is executed, the task goes through 4 steps:</p>
 * <ol>
 *     <li>{@link #onPreExecute()}, invoked on the UI thread before the task
 *     is executed. This step is normally used to setup the task, for instance by
 *     showing a progress bar in the user interface.</li>
 *     <li>{@link #doInBackground}, invoked on the background thread
 *     immediately after {@link #onPreExecute()} finishes executing. This step is used
 *     to perform background computation that can take a long time. The parameters
 *     of the asynchronous task are passed to this step. The result of the computation must
 *     be returned by this step and will be passed back to the last step. This step
 *     can also use {@link #publishProgress} to publish one or more units
 *     of progress. These values are published on the UI thread, in the
 *     {@link #onProgressUpdate} step.</li>
 *     <li>{@link #onProgressUpdate}, invoked on the UI thread after a
 *     call to {@link #publishProgress}. The timing of the execution is
 *     undefined. This method is used to display any form of progress in the user
 *     interface while the background computation is still executing. For instance,
 *     it can be used to animate a progress bar or show logs in a text field.</li>
 *     <li>{@link #onPostExecute}, invoked on the UI thread after the background
 *     computation finishes. The result of the background computation is passed to
 *     this step as a parameter.</li>
 * </ol>
 *
 * <h2>Cancelling a task</h2>
 * <p>A task can be cancelled at any time by invoking {@link #cancel(boolean)}. Invoking
 * this method will cause subsequent calls to {@link #isCancelled()} to return true.
 * After invoking this method, {@link #onCancelled(Object)}, instead of
 * {@link #onPostExecute(Object)} will be invoked after {@link #doInBackground(Object[])}
 * returns. To ensure that a task is cancelled as quickly as possible, you should always
 * check the return value of {@link #isCancelled()} periodically from
 * {@link #doInBackground(Object[])}, if possible (inside a loop for instance.)</p>
 *
 * <h2>Threading rules</h2>
 * <p>There are a few threading rules that must be followed for this class to
 * work properly:</p>
 * <ul>
 *     <li>The AsyncTask class must be loaded on the UI thread. This is done
 *     automatically as of {@link android.os.Build.VERSION_CODES#JELLY_BEAN}.</li>
 *     <li>The task instance must be created on the UI thread.</li>
 *     <li>{@link #execute} must be invoked on the UI thread.</li>
 *     <li>Do not call {@link #onPreExecute()}, {@link #onPostExecute},
 *     {@link #doInBackground}, {@link #onProgressUpdate} manually.</li>
 *     <li>The task can be executed only once (an exception will be thrown if
 *     a second execution is attempted.)</li>
 * </ul>
 *
 * <h2>Memory observability</h2>
 * <p>AsyncTask guarantees that all callback calls are synchronized in such a way that the following
 * operations are safe without explicit synchronizations.</p>
 * <ul>
 *     <li>Set member fields in the constructor or {@link #onPreExecute}, and refer to them
 *     in {@link #doInBackground}.
 *     <li>Set member fields in {@link #doInBackground}, and refer to them in
 *     {@link #onProgressUpdate} and {@link #onPostExecute}.
 * </ul>
 *
 * <h2>Order of execution</h2>
 * <p>When first introduced, AsyncTasks were executed serially on a single background
 * thread. Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
 * to a pool of threads allowing multiple tasks to operate in parallel. Starting with
 * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are executed on a single
 * thread to avoid common application errors caused by parallel execution.</p>
 * <p>If you truly want parallel execution, you can invoke
 * {@link #executeOnExecutor(java.util.concurrent.Executor, Object[])} with
 * {@link #THREAD_POOL_EXECUTOR}.</p>
 */
public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    // We want at least 2 threads and at most 4 threads in the core pool,
    // preferring to have 1 less than the CPU count to avoid saturating
    // the CPU with background work
    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);

    /**
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    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;
    }

    /**
     * An {@link Executor} that executes tasks one at a time in serial
     * order.  This serialization is global to a particular process.
     */
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;

    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
    private static InternalHandler sHandler;

    private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture;

    private volatile Status mStatus = Status.PENDING;

    private final AtomicBoolean mCancelled = new AtomicBoolean();
    private final AtomicBoolean mTaskInvoked = new AtomicBoolean();

    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);
            }
        }
    }

    /**
     * Indicates the current status of the task. Each status will be set only once
     * during the lifetime of a task.
     */
    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,
    }

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

    /** @hide */
    public static void setDefaultExecutor(Executor exec) {
        sDefaultExecutor = exec;
    }

    /**
     * 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);
                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);
                }
            }
        };
    }

    private void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }

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

    /**
     * Returns the current status of this task.
     *
     * @return The current status.
     */
    public final Status getStatus() {
        return mStatus;
    }

    /**
     * Override this method to perform a computation on a background thread. The
     * specified parameters are the parameters passed to {@link #execute}
     * by the caller of this task.
     *
     * This method can call {@link #publishProgress} to publish updates
     * on the UI thread.
     *
     * @param params The parameters of the task.
     *
     * @return A result, defined by the subclass of this task.
     *
     * @see #onPreExecute()
     * @see #onPostExecute
     * @see #publishProgress
     */
    @WorkerThread
    protected abstract Result doInBackground(Params... params);

    /**
     * Runs on the UI thread before {@link #doInBackground}.
     *
     * @see #onPostExecute
     * @see #doInBackground
     */
    @MainThread
    protected void onPreExecute() {
    }

    /**
     * <p>Runs on the UI thread after {@link #doInBackground}. The
     * specified result is the value returned by {@link #doInBackground}.</p>
     *
     * <p>This method won't be invoked if the task was cancelled.</p>
     *
     * @param result The result of the operation computed by {@link #doInBackground}.
     *
     * @see #onPreExecute
     * @see #doInBackground
     * @see #onCancelled(Object)
     */
    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onPostExecute(Result result) {
    }

    /**
     * Runs on the UI thread after {@link #publishProgress} is invoked.
     * The specified values are the values passed to {@link #publishProgress}.
     *
     * @param values The values indicating progress.
     *
     * @see #publishProgress
     * @see #doInBackground
     */
    @SuppressWarnings({"UnusedDeclaration"})
    @MainThread
    protected void onProgressUpdate(Progress... values) {
    }

    /**
     * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground(Object[])} has finished.</p>
     *
     * <p>The default implementation simply invokes {@link #onCancelled()} and
     * ignores the result. If you write your own implementation, do not call
     * <code>super.onCancelled(result)</code>.</p>
     *
     * @param result The result, if any, computed in
     *               {@link #doInBackground(Object[])}, can be null
     *
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    @SuppressWarnings({"UnusedParameters"})
    @MainThread
    protected void onCancelled(Result result) {
        onCancelled();
    }    

    /**
     * <p>Applications should preferably override {@link #onCancelled(Object)}.
     * This method is invoked by the default implementation of
     * {@link #onCancelled(Object)}.</p>
     *
     * <p>Runs on the UI thread after {@link #cancel(boolean)} is invoked and
     * {@link #doInBackground(Object[])} has finished.</p>
     *
     * @see #onCancelled(Object)
     * @see #cancel(boolean)
     * @see #isCancelled()
     */
    @MainThread
    protected void onCancelled() {
    }

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally. If you are calling {@link #cancel(boolean)} on the task,
     * the value returned by this method should be checked periodically from
     * {@link #doInBackground(Object[])} to end the task as soon as possible.
     *
     * @return <tt>true</tt> if task was cancelled before it completed
     *
     * @see #cancel(boolean)
     */
    public final boolean isCancelled() {
        return mCancelled.get();
    }

    /**
     * <p>Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when <tt>cancel</tt> is called,
     * this task should never run. If the task has already started,
     * then the <tt>mayInterruptIfRunning</tt> parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.</p>
     *
     * <p>Calling this method will result in {@link #onCancelled(Object)} being
     * invoked on the UI thread after {@link #doInBackground(Object[])}
     * returns. Calling this method guarantees that {@link #onPostExecute(Object)}
     * is never invoked. After invoking this method, you should check the
     * value returned by {@link #isCancelled()} periodically from
     * {@link #doInBackground(Object[])} to finish the task as early as
     * possible.</p>
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
     *        task should be interrupted; otherwise, in-progress tasks are allowed
     *        to complete.
     *
     * @return <tt>false</tt> if the task could not be cancelled,
     *         typically because it has already completed normally;
     *         <tt>true</tt> otherwise
     *
     * @see #isCancelled()
     * @see #onCancelled(Object)
     */
    public final boolean cancel(boolean mayInterruptIfRunning) {
        mCancelled.set(true);
        return mFuture.cancel(mayInterruptIfRunning);
    }

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return The computed result.
     *
     * @throws CancellationException If the computation was cancelled.
     * @throws ExecutionException If the computation threw an exception.
     * @throws InterruptedException If the current thread was interrupted
     *         while waiting.
     */
    public final Result get() throws InterruptedException, ExecutionException {
        return mFuture.get();
    }

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result.
     *
     * @param timeout Time to wait before cancelling the operation.
     * @param unit The time unit for the timeout.
     *
     * @return The computed result.
     *
     * @throws CancellationException If the computation was cancelled.
     * @throws ExecutionException If the computation threw an exception.
     * @throws InterruptedException If the current thread was interrupted
     *         while waiting.
     * @throws TimeoutException If the wait timed out.
     */
    public final Result get(long timeout, TimeUnit unit) throws InterruptedException,
            ExecutionException, TimeoutException {
        return mFuture.get(timeout, unit);
    }

    /**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     *
     * <p>Note: this function schedules the task on a queue for a single background
     * thread or pool of threads depending on the platform version.  When first
     * introduced, AsyncTasks were executed serially on a single background thread.
     * Starting with {@link android.os.Build.VERSION_CODES#DONUT}, this was changed
     * to a pool of threads allowing multiple tasks to operate in parallel. Starting
     * {@link android.os.Build.VERSION_CODES#HONEYCOMB}, tasks are back to being
     * executed on a single thread to avoid common application errors caused
     * by parallel execution.  If you truly want parallel execution, you can use
     * the {@link #executeOnExecutor} version of this method
     * with {@link #THREAD_POOL_EXECUTOR}; however, see commentary there for warnings
     * on its use.
     *
     * <p>This method must be invoked on the UI thread.
     *
     * @param params The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException If {@link #getStatus()} returns either
     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
     *
     * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
     * @see #execute(Runnable)
     */
    @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }

    /**
     * Executes the task with the specified parameters. The task returns
     * itself (this) so that the caller can keep a reference to it.
     *
     * <p>This method is typically used with {@link #THREAD_POOL_EXECUTOR} to
     * allow multiple tasks to run in parallel on a pool of threads managed by
     * AsyncTask, however you can also use your own {@link Executor} for custom
     * behavior.
     *
     * <p><em>Warning:</em> Allowing multiple tasks to run in parallel from
     * a thread pool is generally <em>not</em> what one wants, because the order
     * of their operation is not defined.  For example, if these tasks are used
     * to modify any state in common (such as writing a file due to a button click),
     * there are no guarantees on the order of the modifications.
     * Without careful work it is possible in rare cases for the newer version
     * of the data to be over-written by an older one, leading to obscure data
     * loss and stability issues.  Such changes are best
     * executed in serial; to guarantee such work is serialized regardless of
     * platform version you can use this function with {@link #SERIAL_EXECUTOR}.
     *
     * <p>This method must be invoked on the UI thread.
     *
     * @param exec The executor to use.  {@link #THREAD_POOL_EXECUTOR} is available as a
     *              convenient process-wide thread pool for tasks that are loosely coupled.
     * @param params The parameters of the task.
     *
     * @return This instance of AsyncTask.
     *
     * @throws IllegalStateException If {@link #getStatus()} returns either
     *         {@link AsyncTask.Status#RUNNING} or {@link AsyncTask.Status#FINISHED}.
     *
     * @see #execute(Object[])
     */
    @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;
    }

    /**
     * Convenience version of {@link #execute(Object...)} for use with
     * a simple Runnable object. See {@link #execute(Object[])} for more
     * information on the order of execution.
     *
     * @see #execute(Object[])
     * @see #executeOnExecutor(java.util.concurrent.Executor, Object[])
     */
    @MainThread
    public static void execute(Runnable runnable) {
        sDefaultExecutor.execute(runnable);
    }

    /**
     * This method can be invoked from {@link #doInBackground} to
     * publish updates on the UI thread while the background computation is
     * still running. Each call to this method will trigger the execution of
     * {@link #onProgressUpdate} on the UI thread.
     *
     * {@link #onProgressUpdate} will not be called if the task has been
     * canceled.
     *
     * @param values The progress values to update the UI with.
     *
     * @see #onProgressUpdate
     * @see #doInBackground
     */
    @WorkerThread
    protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

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

    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;
            }
        }
    }

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

    @SuppressWarnings({"RawUseOfParameterizedType"})
    private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;

        AsyncTaskResult(AsyncTask task, Data... data) {
            mTask = task;
            mData = data;
        }
    }
}