AsyncTask使用和源码分析

使用

// 定义
private class ExecuteTask extends AsyncTask<String,Integer,Void> {
		// 执行线程任务前的操作
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }
		// 后台线程，执行耗时操作。
        @Override
        protected Void doInBackground(String... strs) {
            Log.e(TAG,"线程名称 doInBackground == " + Thread.currentThread().getName());
            publishProgress(0);
            return null;
        }
		// 响应 publishProgress 方法。
        @Override
        protected void onProgressUpdate(Integer... values) {
            Log.e(TAG,"线程名称 onProgressUpdate == " + Thread.currentThread().getName());
            Log.e(TAG,"onProgressUpdate == " + values);
        }
		// 后台线程任务执行完成之后，在UI线程的回调方法。
        @Override
        protected void onPostExecute(Void aVoid) {
            Log.e(TAG,"线程名称 onPostExecute == " + Thread.currentThread().getName());
        }
		// 异步任务取消之后的回调。
        @Override
        protected void onCancelled() {
            super.onCancelled();
        }
    }

// 开始执行
ExecuteTask executeTask = new ExecuteTask();
executeTask.execute("value1","value2","value3");

源码分析之前你需要知道一些线程的知识点：

带返回值的线程
线程池的一些知识
关于Handler和Looper
关于队列的一些知识

源码分析

①. ExecuteTask executeTask = new ExecuteTask();

		// 创建绑定主线程Looper的Handler
        mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);
		// WorkerRunnable 实现 Callable,为了创建带返回值的线程
        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
                    // 开始执行doInBackground()
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                // 这个值可以在FutureTask.done()方法中通过 get() 方法获取。
                return result;
            }
        };
		// 创建带返回值的任务，该任务可提交到线程池。
        mFuture = new FutureTask<Result>(mWorker) {
        	// 异步任务执行完成之后的回调
            @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);
                }
            }
        };
    }

②. executeTask.execute(“value1”,“value2”,“value3”);

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

    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;
		// 异步任务开始之前的调用
        onPreExecute();

        mWorker.mParams = params;
        // exec == sDefaultExecutor == SerialExecutor
        // mFuture == FutureTask
        exec.execute(mFuture);		

        return this;
    }

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

    private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
        	// 将任务加入到队列中
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                    	// 执行mFuture (FutureTask)
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            
            if (mActive == null) { 
            	// 没有可执行任务时，调用。
                scheduleNext();
            }
        }
		
        protected synchronized void scheduleNext() {
        	// 将队列中的任务取出。
            if ((mActive = mTasks.poll()) != null) {
            	// 执行任务  THREAD_POOL_EXECUTOR == ThreadPoolExecutor
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }
	
	// 创建线程池。
	private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(128);
    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;
    }

// THREAD_POOL_EXECUTOR.execute(mActive); 执行之后 FutureTask.do() 方法被执行。
// 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 = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }
    private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;

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

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

        @Override
        public void handleMessage(Message msg) {
            AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
            switch (msg.what) {
            	// 执行完成之后的消息
                case MESSAGE_POST_RESULT:
                    // There is only one result
                    result.mTask.finish(result.mData[0]);
                    break;
                // publishProgress 执行
                case MESSAGE_POST_PROGRESS:
                    result.mTask.onProgressUpdate(result.mData);
                    break;
            }
        }
    }

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