AsyncTask的代碼

AsyncTask的學習

When first introduced, AsyncTasks were executed serially on a single background thread. Starting with DONUT(API level1.6), this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with HONEYCOMB(API level3.0), tasks are executed on a single thread to avoid common application errors caused by parallel execution. If you truly want parallel execution, you can invoke executeOnExecutor(java.util.concurrent.Executor, Object[]) with THREAD_POOL_EXECUTOR.

1.6以前的AsyncTask在一個後臺線程中串行serially,

後來1.6之後又在一個線程池中並行.

再後來的3.0以後又只在一個線程中執行了.因此全部的AsyncTask都會排隊.可是你也能夠調用executeOnExecutor(Executor)傳入你的線程池而後去並行這些任務.

AsyncTask爲何後來要改爲串行? 同一個進程中的AsyncTask使用的都是一個線程池.那麼若是是並行的. 當A,B兩個AsyncTask的doInbackGround方法都訪問操做了同一個資源的話就煞筆了. 通常不會有人記得去synchronized一把doInbackground方法吧.

android2.0_r1中部分AsyncTask源碼

看這個東西是想知道爲何剛開始AsyncTask的任務都是並行的?

public final AsyncTask<Params, Progress, Result> execute(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;
        sExecutor.execute(mFuture);

        return this;
    }

關於這個sExecutor的聲明以下:因此很容易看出來這些任務都是在這個線程池中並行的.

private static final int CORE_POOL_SIZE = 5;
    private static final int MAXIMUM_POOL_SIZE = 128;
    private static final int KEEP_ALIVE = 10;

    private static final BlockingQueue<Runnable> sWorkQueue =
            new LinkedBlockingQueue<Runnable>(10);

    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 ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
            MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);

後來的代碼變成了默認串行,是由於這段代碼嗎?
synchronized致使這個方法只會同時有一個線程進入.
必須等這個線程執行完了纔會釋放鎖讓其餘線程有機會進入execute方法.
那爲何真正執行線程代碼的地方線程池的核心線程要超過一個?
不是串行麼?一個不就夠了嗎?

```
 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);
        }
    }
}
```

android4.1.1的代碼.在ActivityThread這個類的main方法中有以下:

public static void  main(String[] args) {
        ...
    AsyncTask.init();
}

這一樣的4.1.1的AsyncTask的代碼中

首先這個sHandler的聲明以下

private static final InternalHandler sHandler = new InternalHandler();

    //Used to force static handler to be created.
    public static void init() {
    sHandler.getLooper();
    }

這個InternalHandler是AsyncTask的一個內部類的代碼以下:

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

可是在5.1(API level22)之後這些有一點點變化 首先AsyncTask的init靜態方法被去除了. 而後就是sHandler在聲明的時候是空,後續操做該handler都經過getHander()方法.

private static InternalHandler sHandler;
    
    private static Handler getHandler() {
        //這裏爲啥不像double check似的寫啊.    
         synchronized (AsyncTask.class) {
            if (sHandler == null) {
                sHandler = new InternalHandler();
            }
            return sHandler;
         }
    }

而後這個InternalHandler的內容和以前有了一個點小小的改變

private static class InternalHandler extends Handler {
        
        //這裏是重點確保該handler的looper爲主線程的looper
        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;
            }
        }
    }

注意這個構造方法super(Looper.getMainLooper())這個就確保了這個sHandler的Looper必定是主線程的Looper. 那麼經過這個sHandler發送的消息的handleMessage方法必定在主線程執行. 因此在這個時候after API level22繼承AsyncTask的實例就無需在主線程實例化了.

AsyncTask的構造方法代碼

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

這裏的WorkRunnable代碼以下:

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

可見這就是一個Callable. 下面開始跟蹤代碼.從

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

這個sDefaultExecutor的聲明是這個樣子:

private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();

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;
        //這裏的exec就是上面的sDefaultExecutor
        //mFuture就是構造方法建立的那個FutureTask
        exec.execute(mFuture);

        return this;
    }

而後再到SerialExecutor

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();
                    }
                }
            });
            
            //A start
            if (mActive == null) {
                scheduleNext();
            }
            //A end
        }

        protected synchronized void scheduleNext() {
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

SerialExecutor原本就是串行的線程池的意思. 它只負責把runnable放到mTasks中. execute方法中把new的runnable加入mTasks. 至於這個runnnable執行的內容先無論. 而後就到了代碼段A.剛開始的時候mActive變量必須是null因此會執行scheduleNext()方法.而後這個方法裏會從mTasks中取出東西並賦值給mActive變量. 並把這個mActive放到THREAD_POOL_EXECUTOR中執行.

到這裏會先直接調用傳入Runnable的run方法. 就是執行mFuture的run方法.

大概的瞭解:FutureTask實現了Runnnable接口.調用run方法會致使 構造該FutureTask的callable的call方法被調用.

因此到了這裏構造方法中的WorkerRunnable的call方法就會被調用.該call方法以下:

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);
    }

到了這裏AsyncTask的doInBackground(mParams)方法就被調用了.並且是在THREAD_POOL_EXECUTOR這個線程池中執行.

public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
    private static final int KEEP_ALIVE = 1;
    
    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);

而後接着的代碼

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

把doInbackground方法的結果經過handler進行發送.而後看handler的handleMessage方法.

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;
            }
        }

這裏的result.mTask就是AsyncTask.見代碼

private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;

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

因此代碼就到了AsyncTask的finish方法.

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

到這裏就看到onPostExecute方法裏,就差很少了.