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 Android教程網 >> Android技術 >> Android開發 >> 關於android開發 >> Android中AsyncTask分析--你所不注意的坑,androidasynctask

Android中AsyncTask分析--你所不注意的坑,androidasynctask

編輯:關於android開發

Android中AsyncTask分析--你所不注意的坑,androidasynctask


AsyncTask,是android提供的輕量級的異步類,可以直接繼承AsyncTask,在類中實現異步操作,提供接口反饋當前異步執行的程度(可以通過接口實現UI進度更新),最後反饋執行的結果給UI主線程.

本文不分析AsyncTask的使用,它的使用教程網上一搜一大堆,本文主要分析它的內部邏輯和實現,它是怎麼實現異步的,它是怎麼處理多個任務的,是並發麼??

一、線程任務的調度

在AsyncTask內部會創建一個類相關的線程池來管理要運行的任務,也就就是說當你調用了AsyncTask的execute()後,AsyncTask會把任務交給線程池,由線程池來管理創建Thread和運行Therad。

在Android4.0版本中它內部是有兩個線程池:SerialExecutor和ThreadPoolExecutor,SerialExecutor是串行的,ThreadPoolExecutor是並發的,而默認的就是SerialExecutor的,所以你一個程序中如果用了好幾個AsyncTask你就得注意了:不要忘了換成並發的線程池執行。下面演示一下,穿行的調度

1.一個簡單的例子:可以看出他是一個個執行的   代碼如下:
public class AsyncTaskDemoActivity extends Activity {  
    private static int ID = 0;  
    private static final int TASK_COUNT = 9;  
    private static ExecutorService SINGLE_TASK_EXECUTOR;  
    private static ExecutorService LIMITED_TASK_EXECUTOR;  
    private static ExecutorService FULL_TASK_EXECUTOR;  
      
    static {  
        SINGLE_TASK_EXECUTOR = (ExecutorService) Executors.newSingleThreadExecutor();  
        LIMITED_TASK_EXECUTOR = (ExecutorService) Executors.newFixedThreadPool(7);  
        FULL_TASK_EXECUTOR = (ExecutorService) Executors.newCachedThreadPool();  
    };  
      
    @Override  
    public void onCreate(Bundle icicle) {  
        super.onCreate(icicle);  
        setContentView(R.layout.asynctask_demo_activity);  
        String title = "AsyncTask of API " + VERSION.SDK_INT;  
        setTitle(title);  
        final ListView taskList = (ListView) findViewById(R.id.task_list);  
        taskList.setAdapter(new AsyncTaskAdapter(getApplication(), TASK_COUNT));  
    }  
      
    private class AsyncTaskAdapter extends BaseAdapter {  
        private Context mContext;  
        private LayoutInflater mFactory;  
        private int mTaskCount;  
        List<SimpleAsyncTask> mTaskList;  
          
        public AsyncTaskAdapter(Context context, int taskCount) {  
            mContext = context;  
            mFactory = LayoutInflater.from(mContext);  
            mTaskCount = taskCount;  
            mTaskList = new ArrayList<SimpleAsyncTask>(taskCount);  
        }  
          
        @Override  
        public int getCount() {  
            return mTaskCount;  
        }  
  
        @Override  
        public Object getItem(int position) {  
            return mTaskList.get(position);  
        }  
  
        @Override  
        public long getItemId(int position) {  
            return position;  
        }  
  
        @Override  
        public View getView(int position, View convertView, ViewGroup parent) {  
            if (convertView == null) {  
                convertView = mFactory.inflate(R.layout.asynctask_demo_item, null);  
                SimpleAsyncTask task = new SimpleAsyncTask((TaskItem) convertView);  
                /* 
                 * It only supports five tasks at most. More tasks will be scheduled only after 
                 * first five finish. In all, the pool size of AsyncTask is 5, at any time it only 
                 * has 5 threads running. 
                 */  
                task.execute();
                // use AsyncTask#SERIAL_EXECUTOR is the same to #execute();  
//                task.executeOnExecutor(AsyncTask.SERIAL_EXECUTOR);  
                // use AsyncTask#THREAD_POOL_EXECUTOR is the same to older version #execute() (less than API 11)  
                // but different from newer version of #execute()  
//                task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);  
                // one by one, same to newer version of #execute()  
//                task.executeOnExecutor(SINGLE_TASK_EXECUTOR);  
                // execute tasks at some limit which can be customized  
//                task.executeOnExecutor(LIMITED_TASK_EXECUTOR);  
                // no limit to thread pool size, all tasks run simultaneously  
                //task.executeOnExecutor(FULL_TASK_EXECUTOR);  
                  
                mTaskList.add(task);  
            }  
            return convertView;  
        }  
    }  
      
    private class SimpleAsyncTask extends AsyncTask<Void, Integer, Void> {  
        private TaskItem mTaskItem;  
        private String mName;  
          
        public SimpleAsyncTask(TaskItem item) {  
            mTaskItem = item;  
            mName = "Task #" + String.valueOf(++ID);  
        }  
          
        @Override  
        protected Void doInBackground(Void... params) {  
            int prog = 1;  
            while (prog < 101) {  
                SystemClock.sleep(100);  
                publishProgress(prog);  
                prog++;  
            }  
            return null;  
        }  
          
        @Override  
        protected void onPostExecute(Void result) {  
        }  
          
        @Override  
        protected void onPreExecute() {  
            mTaskItem.setTitle(mName);  
        }  
          
        @Override  
        protected void onProgressUpdate(Integer... values) {  
            mTaskItem.setProgress(values[0]);  
        }  
    }  
}  
  
class TaskItem extends LinearLayout {  
    private TextView mTitle;  
    private ProgressBar mProgress;  
      
    public TaskItem(Context context, AttributeSet attrs) {  
        super(context, attrs);  
    }  
  
    public TaskItem(Context context) {  
        super(context);  
    }  
      
    public void setTitle(String title) {  
        if (mTitle == null) {  
            mTitle = (TextView) findViewById(R.id.task_name);  
        }  
        mTitle.setText(title);  
    }  
      
    public void setProgress(int prog) {  
        if (mProgress == null) {  
            mProgress = (ProgressBar) findViewById(R.id.task_progress);  
        }  
        mProgress.setProgress(prog);  
    }  
}  

  2.你想要的並發執行

上面的情況肯定不是你想要的,你想要的應該是這種情況: 只需要修改一行代碼:
 task.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);  

  當然也可以換成你自己的線程池。

二、源碼分析

  1.成員變量:

定義了需要用到的成員,可以根據名字就能知道干什麼的

//生產線程的工廠
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());
        }
    };
//存放任務的阻塞隊列
    private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(10);

    /**
     * 可以平行的執行任務!就是並發的
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

    /**
     * 線性執行的執行器
     * An {@link Executor} that executes tasks one at a time in serial
     * order.  This serialization is global to a particular process.
     */
    public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
    //內部交互的handler
    private static final InternalHandler sHandler = new InternalHandler();
    //默認的Executor
    private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;

定義了需要用到的成員,可以根據名字就能知道干什麼的,另外注意都是static修飾的:

  第二行的sThreadFactory是創建線程的;

  第十行的sPoolWorkQueue阻塞隊列,存放任務的;

 第十七行是 THREAD_POOL_EXECUTOR是線程池,這個是並發執行的線程池;

 第26行是線性調度的線程池,SERIAL_EXECUTOR,執行完一個才會執行下一個;

 第28行是一個內部封裝的Handler:InternalHandler

 第30行可以看出他默認的是線性調度的線程池, Executor sDefaultExecutor = SERIAL_EXECUTOR,看到這裡你應該注意一個問題,如果程序裡有好多個AsyncTask,它們就是線性調度的,這肯定可你預想的不一樣,所以你別忘了換成並發的執行器。

2.內部Handler的使用:

2.1 自定義的InternalHandler(內部handler)

private static class InternalHandler extends Handler {
        @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
        @Override
        //接受message的處理,可以看到根據狀態選擇是完成了,還是更新著
        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;
            }
        }
    }

  在上邊handleMessage中,根據msg進行判斷,是完成了還是在更新;

     任務完成調用finish方法,在其中執行你定義的onPostExecute方法,

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

  3.構造方法

public AsyncTask() {
        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);

                Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                return postResult(doInBackground(mParams));
            }
        };

        mFuture = new FutureTask<Result>(mWorker) {
            @Override
            protected void done() {
                try {
                    final Result result = get();

                    postResultIfNotInvoked(result);
                } 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);
                } catch (Throwable t) {
                    throw new RuntimeException("An error occured while executing "
                            + "doInBackground()", t);
                }
            }
        };
    }

構造方法中其實隱藏的信息很多,WorkerRunnable和FutureTask;

其中WorkerRunnable繼承了Callable接口,應該是用於在未來某個線程的回調接口,在其中執行了ostResult(doInBackground(mParams));調用doInBackground並用postResult方法,把result發送到主線程。

FutureTask你看類的介紹是說控制任務的,控制任務的開始、取消等等,在這不細究,跟本文關系不大,而且我也沒看明白。

第17行有一個方法:postResultIfNotInvoked(result);根據名字可以看出來是如果沒有調用把把結果post出去,所以他應該是處理取消的任務的。

構造方法就分析到這,下一步就是execute():

看下postResult方法:代碼很少也很簡單,就是把msg發送給handler:

//用shandler把設置message,並發送。
private Result postResult(Result result) {
        Message message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }

3.1 按照執行過程流程,實例化完,就可以調用execute():

//Params... 就相當於一個數組,是傳給doInBackground的參數
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }
//執行邏輯
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)");
            }
        }
        //改變狀態
        mStatus = Status.RUNNING;
        //准備工作
        onPreExecute();
     
        mWorker.mParams = params;
        exec.execute(mFuture);
        return this;
    } 

代碼邏輯很清晰,沒有幾行:

20行:修改了狀態;

21行:准備工作;

24行:設置參數;

25行:線程池調用執行,注意參數是mFuture。 

3.2 execute的執行邏輯

就以它定義SerialExecutor為例:

/*An {@link Executor} that executes tasks one at a time in serial
     * order.  This serialization is global to a particular process.*/
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);
            }
        }
    }

 可以看到它的方法都是用synchronized關鍵字修飾的,而其讓他的介紹裡也說了在同一時間只有一個任務在執行。

   在裡邊調用r.run()方法,執行完了在調用下一個。按先後順序每次只運行一個

 

三、AsyncTask中異步的處理邏輯

沒有忘了前邊構造方法中的postResult(doInBackground(mParams))和postResultIfNotInvoked(result);方法吧,如果忘了翻前邊去看。這兩個方法把執行成功的和失敗的任務都包含了。

所以我們可以設想一下它是怎麼執行的:

1.在executeOnExecutor方法中給變量賦值

2.用執行器Executor另起線程執行任務

3.在Executor中一些復雜的邏輯,用FutureTask進行判斷任務是否被取消,如果沒有就調用回調接口call()方法,

4.在call()方法中調用了postResult(doInBackground(mParams));

5.postResult發送給主線程的Handler進行後續處理。

 

看的時候可以畫下圖,會很清晰,基本邏輯就這樣,好AsyncTask就分析到這,歡迎糾錯。。。

 轉發請注明出處,原文地址:http://www.cnblogs.com/jycboy/p/asynctask_1.html

 

 

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