AsyncTask深入解读(下)

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前言

在上一节AsyncTask深入解读(上)中已经介绍了AsyncTask的基本知识,本节将从源码出发,深层次分析AsyncTask的工作原理。


1 AsyncTask对象创建

使用AsyncTask时,一般根据具体的任务继承AsyncTask类,例如:

private class MyAsyncTask extends AsyncTask<String, Integer, String> {

        protected String doInBackground(String... args1) {
		
            Log.i(TAG, "doInBackground in:" + args1[0]);
            int times = 10;
            for (int i = 0; i < times; i++) {
                publishProgress(i);//提交之后,会执行onProcessUpdate方法
            }
            Log.i(TAG, "doInBackground out");
            return "over";
        }

        /**
         * 在调用cancel方法后会执行到这里
         */
        protected void onCancelled() {
            Log.i(TAG, "onCancelled");
        }

        /**
         * 在doInbackground之后执行
         */
        protected void onPostExecute(String args3) {
            Log.i(TAG, "onPostExecute:" + args3);
        }

        /**
         * 在doInBackground之前执行
         */
        @Override
        protected void onPreExecute() {
            Log.i(TAG, "onPreExecute");
        }

        /**       
         * @param args2
         */
        @Override
        protected void onProgressUpdate(Integer... args2) {
            Log.i(TAG, "onProgressUpdate:" + args2[0]);
        }
    }

MyAsyncTask对象的创建必须在主线程中,使用方式为:

new MyAsyncTask().execute("AsyncTask Test");

2 execute方法

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

execute方法调用了executeOnExecutor方法并传递参数sDefaultExecutor和params。 再看executeOnExecutor方法:

public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
            Params... params) {
			
		//判断当前状态
        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)");
            }
        }

		//将状态置为运行态
        mStatus = Status.RUNNING;

		//主线程中最先调用onPreExecute方法,进行准备工作
        onPreExecute();

		//将参数传给mWorker
        mWorker.mParams = params;
		
		//调用线程池,执行任务
        exec.execute(mFuture);

        return this;
}

executeOnExecutor方法首先判断状态,若处于可执行态,则将状态置为RUNNING。然后调用了onPreExecute方法,交给用户进行执行任务前的准备工作。核心部分在于 exec.execute(mFuture)。exec即sDefaultExecutor。


3 sDefaultExecutor串行线程池

查看sDefaultExecutor定义:

public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

sDefaultExecutor是一个串行线程池,作用在于任务的排队执行。其源码如下:

	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);
            }
        }
    }

从SerialExecutor的源码可以看出,mFuture是插入到mTasks任务队列的对象。当mTasks中没有正在活动的AsyncTask任务,则调用scheduleNext方法执行下一个任务。若一个AsyncTask任务执行完毕,则继续执行下一个AsyncTask任务,直至所有任务执行完毕。通过分析可以发现真正去执行后台任务的是线程池THREAD_POOL_EXECUTOR。


4 线程池THREAD_POOL_EXECUTOR

THREAD_POOL_EXECUTOR定义如下:

	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;
    }

5 mFuture并发执行对象

线程池THREAD_POOL_EXECUTOR中执行的Runnable对象为mFuture。

mFuture的定义:

private final FutureTask<Result> mFuture;

mFuture为线程池执行的真正任务,mFuture的执行过程过程是怎样的呢?

再看AsyncTask的构造:

	public AsyncTask(@Nullable Looper callbackLooper) {
	
        mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);
        //创建mworker对象
        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;
            }
        };
        //创建mFuture对象
        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);
                }
            }
        };
    }

通过AsyncTask的构造中可以看到mWorker为FutureTask的构造函数参数,则FutureTask对象中持有mWorker的引用。

FutureTask的构造函数:

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

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 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);
        }
    }

FutureTask的run方法中调用了 result = c.call(); 即调用了mWorker的call方法。


6 mWorker对象

mWorker的call方法:

		mWorker = new WorkerRunnable<Params, Result>() {
		
			//call方法
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //noinspection unchecked
					//执行任务
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

通过代码可以发现,最终在线程池中执行的是mWorker的call方法,call方法中调用了doInBackground方法,因此可以看出doInBackground方法是在线程池中调用的。 当任务执行完毕后则调用postResult方法:

	//任务执行完后调用方法
	private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }
	

postResult方法发送MESSAGE_POST_RESULT消息和result。


7 InternalHandler接收处理消息

InternalHandler代码:

	private static class InternalHandler extends Handler {
        public InternalHandler(Looper looper) {
            super(looper);
        }

        @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;
            }
        }
    }

InternalHandler接收到MESSAGE_POST_RESULT时调用result.mTask.finish(result.mData[0]);


8 finish方法:

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

finish方法中若任务没有取消则调用onPostExecute方法发送结果,若任务取消则调用onCancelled方法。finish方法是在主线程中执行的。


9 onProgressUpdate

通过上述流程已经顺序找到了onPreExecute、doInBackground、onPostExecute方法,那么onProgressUpdate是如何执行的呢? 首先查看 publishProgress方法:

	protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

MyAsyncTask示例中在doInBackground中调用publishProgress方法,publishProgress方法发送MESSAGE_POST_PROGRESS消息和进度values,InternalHandler在接收到MESSAGE_POST_PROGRESS消息中调用onProgressUpdate方法。因此onProgressUpdate也是在主线程中调用。


10 总结

通过上述一步步的源码分析过程,已经掌握了AsyncTask任务的执行过程。AsyncTask中有两个线程池串行线程池sDefaultExecutor和线程池THREAD_POOL_EXECUTOR。sDefaultExecutor用于任务的排队,THREAD_POOL_EXECUTOR真正的执行任务。线程的切换使用Handler(InternalHandler)实现。


11 问题回答

1)为什么AsyncTask在主线程创建执行? 因为AsyncTask需要在主线程创建InternalHandler,以便onProgressUpdate, onPostExecute , onCancelled 可以正常更新UI。

2)为什么AsyncTask不适合特别耗时任务? AsyncTask实际上是一个线程池。如果有线程长时间占用,且没有空闲,则其他线程只能处于等待状态,会造成阻塞。

3)AsyncTask内存泄漏问题 如果AsyncTask被声明为Activity的非静态的内部类,那么AsyncTask会保留一个对创建了AsyncTask的Activity的引用。如果Activity已经被销毁,AsyncTask的后台线程还在执行,它将继续在内存里保留这个引用,导致Activity无法被回收,引起内存泄露。