AsyncTask實現原理和內部代碼

Android從1.5開始引入了AsyncTask這個類，可以幫助我們解決線程和界面刷新問題，主要是對Thread+Handler這樣的封裝，但在設計模式和代碼維護方面都有不錯的表現。對於AsyncTask的實現原理和內部的代碼如何實現android123一起和大家分享，早在2008年時Google推出了一個示例應用叫PhotoStream來演示UI在多線程網絡慢速I/O下的刷新問題，裡面的線程構造使用的正是AsyncTask的雛形，由於內部使用Java 1.5的並發庫比普通初級Android開發者編寫的Thread+Handler穩定很多，下面我們就android.os.AsyncTask的實現

  public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";

    private static final int CORE_POOL_SIZE = 5; //線程池數量
    private static final int MAXIMUM_POOL_SIZE = 128; //線程池中最大線程數
    private static final int KEEP_ALIVE = 1;

    private static final BlockingQueue<Runnable> sWorkQueue =
            new LinkedBlockingQueue<Runnable>(10); //使用並發庫的阻塞隊列初始時保存10個Runnable對象

    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); //創建線程池

    private static final int MESSAGE_POST_RESULT = 0x1;
    private static final int MESSAGE_POST_PROGRESS = 0x2;
    private static final int MESSAGE_POST_CANCEL = 0x3;

    private static final InternalHandler sHandler = new InternalHandler(); //這個是對Handler的封裝，內部處理Thread的狀態。

    private final WorkerRunnable<Params, Result> mWorker;  //該類對Runnable做簡單封裝
    private final FutureTask<Result> mFuture; //對於並發庫而言FutureTask是最重要的，有興趣的網友可以看下JDK源碼

    private volatile Status mStatus = Status.PENDING; //保存當前線程狀態

    public enum Status { //枚舉類記錄當前線程狀態
        PENDING,
        RUNNING,
        FINISHED,
    }

    public AsyncTask() {
        mWorker = new WorkerRunnable<Params, Result>() { //構造Runable對象
            public Result call() throws Exception {
                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //設置線程優先級為後台，這裡android開發網提示大家低於標准線程優先級
                return doInBackground(mParams);
            }
        };

        mFuture = new FutureTask<Result>(mWorker) {
            @Override
            protected void done() {
                Message message;
                Result result = null;

                try {
                    result = get();
                } catch (InterruptedException e) { //處理Thread中斷異常
                    android.util.Log.w(LOG_TAG, e);
                } catch (ExecutionException e) {
                    throw new RuntimeException("An error occured while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
                            new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); 

                    message.sendToTarget();
                    return;
                } catch (Throwable t) {
                    throw new RuntimeException("An error occured while executing "
                            + "doInBackground()", t);
                }

                message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
                        new AsyncTaskResult<Result>(AsyncTask.this, result)); //執行完後通過Handler通知結果
                message.sendToTarget();
            }
        };
    }

    public final Status getStatus() {
        return mStatus;
    }

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

    protected void onPreExecute() {
    }

    protected void onPostExecute(Result result) {
    }

    protected void onProgressUpdate(Progress... values) {
    }

    protected void onCancelled() {
    }

    public final boolean isCancelled() {
        return mFuture.isCancelled();
    }

    public final boolean cancel(boolean mayInterruptIfRunning) {
        return mFuture.cancel(mayInterruptIfRunning);
    }

    public final Result get() throws InterruptedException, ExecutionException {
        return mFuture.get();
    }

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

    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;
    }

    protected final void publishProgress(Progress... values) { //通過Handler通知UI刷新
        sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
                new AsyncTaskResult<Progress>(this, values)).sendToTarget();
    }

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

    private static class InternalHandler extends Handler { //和我們的Handler沒有什麼不同
        @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;
                case MESSAGE_POST_CANCEL:
                    result.mTask.onCancelled();
                    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;
        }
    }
}

 經過上面的簡單分析相信大家對android AsyncTask會有更加深刻的理解，整個AsyncTask實現基於Thread+Handler，但對於Thread使用的是Java的並發包的FutureTask具體的可以參考JDK5以後的源碼。