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AsyncTask源码分析

AsyncTask源码分析

作者: 我叫王菜鸟 | 来源:发表于2018-01-03 19:50 被阅读0次
    public AsyncTask() {
        //[1]
        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                return postResult(doInBackground(mParams));//doInBackground(mParams)对外使用
            }
        };
        //[2]
        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);
                }
            }
        };
    }
    
    [1]

    其中WorkerRunnable

    @FunctionalInterface
    public interface Callable<V> {
        V call() throws Exception;
    }
    
    private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
        Params[] mParams;
    }
    

    将doInBackground的结果返回到postResult方法中

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

    其中sHandler是InternalHandler的实例

    private static class InternalHandler extends Handler {
        @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;
            }
        }
    }
    

    生成AsyncTaskResult

    private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;
    
        AsyncTaskResult(AsyncTask task, Data... data) {
            mTask = task;
            mData = data;
        }
    }
    

    这里在Handler中进程处理的是:

    result.mTask.finish(result.mData[0]);
    
    private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);//运行在UI线程
        }
        mStatus = Status.FINISHED;
    }
    

    ==到这里明白了在WorkerRunnable中回调方法执行doInBackground()并且将返回结果交给主线程Handler处理==

    [2]
    mFuture = new FutureTask<Result>(mWorker) {
        @Override
        protected void done() {
            try {
                postResultIfNotInvoked(get());
            } catch (Exception e) {
                postResultIfNotInvoked(null);
            }
        }
    };
    

    对于FutureTask来说,将mWorker设置进去命名callable

    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }
    

    此时构造完成,下面看看当我们使用的时候

    String[] urls = {
                    "http://blog.csdn.net/iispring/article/details/47115879",
                    "http://blog.csdn.net/iispring/article/details/47180325",
                    "http://blog.csdn.net/iispring/article/details/47300819",
                    "http://blog.csdn.net/iispring/article/details/47320407",
                    "http://blog.csdn.net/iispring/article/details/47622705"
            };
    
    DownloadTask downloadTask = new DownloadTask();//自定义
    downloadTask.execute(urls);
    

    我们会执行execute方法

    public static final Executor SERIAL_EXECUTOR = Utils.hasHoneycomb() ? new SerialExecutor() :
            Executors.newSingleThreadExecutor(sThreadFactory);
    
    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
    
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }
    

    如果SDK版本大于11,生成SerialExecutor对象,否则用一个线程池

    对于SerialExecutor是

    @TargetApi(11)
    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);
            }
        }
    }
    

    这里暂时不看对应这里的分析

    我们继续看主逻辑

    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();//刚进来走这里
        mWorker.mParams = params;//将参数给mWorker
        exec.execute(mFuture);//执行mFutrue
    
        return this;
    }
    

    ==执行onPreExecute方法,这个实现是空,所以是对于子类进行复写的。也就是任务执行前的逻辑==

    ==着重看exec.execute(mFuture);==

    所以执行的是SerialExecutor类的execute方法

    public synchronized void execute(final Runnable r) {
        mTasks.offer(new Runnable() {
            public void run() {
                try {
                    r.run();
                } finally {
                    scheduleNext();
                }
            }
        });
        if (mActive == null) {
            scheduleNext();
        }
    }
    

    SerialExecutor将对应的mFuture添加到mTasks中,完了执行scheduleNext()

    
    public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
            TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory,
            new ThreadPoolExecutor.DiscardOldestPolicy());
    
    protected synchronized void scheduleNext() {
        if ((mActive = mTasks.poll()) != null) {
            THREAD_POOL_EXECUTOR.execute(mActive);
        }
    }
    

    用一个线程池去执行刚才的任务,对于任务先执行run方法

    也就是说执行的是FutureTask的run方法,然后执行下一个任务的run方法。

    ==也就是一个线程池执行FutureTask的run方法==

    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();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            runner = null;
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }
    

    run方法在调用c.call(),c是callable,是构造时候传递进来的mWorker,所以调用的是mWorker的call

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

    执行的是回调里面的东西。与此同时在run方法中有set(result);
    也就是说将WorkerRunnable.call()的返回结果返进行set。

    protected void set(V v) {
        if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = v;//核心
            U.putOrderedInt(this, STATE, NORMAL); // final state
            finishCompletion();//[1]这个方法调用down
        }
    }
    

    ==现在知道了outcome是我们在mWorker中call的返回结果==

    [1]
    private void finishCompletion() {
        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();//调用
        callable = null;        // to reduce footprint
    }
    

    也就是执行

    mFuture = new FutureTask<Result>(mWorker) {
        @Override
        protected void done() {
            try {
                postResultIfNotInvoked(get());
            } catch (Exception e) {
                postResultIfNotInvoked(null);
            }
        }
    };
    

    其中get

    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }
    
    private V report(int s) throws ExecutionException {
        Object x = outcome;//这个就是worker产生的结果
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }
    

    ==所以在子线程中执行了FutureTask.run(),得到了传递构造中的worker调用了call()得到的返回值set到了outcome中,随后调用了FutureTask.done(),get得到的返回值传入postResultIfNotInvoked(get())==

    private void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }
    
    private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }
    

    将结果放入了主线程中执行。

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