Android開發筆記之:深入理解多線程AsyncTask

Understanding AsyncTask
AsyncTask是Android 1.5 Cubake加入的用於實現異步操作的一個類，在此之前只能用Java SE庫中的Thread來實現多線程異步，AsyncTask是Android平台自己的異步工具，融入了Android平台的特性，讓異步操作更加的安全，方便和實用。實質上它也是對Java SE庫中Thread的一個封裝，加上了平台相關的特性，所以對於所有的多線程異步都強烈推薦使用AsyncTask，因為它考慮，也融入了Android平台的特性，更加的安全和高效。
AsyncTask可以方便的執行異步操作（doInBackground)，又能方便的與主線程進行通信，它本身又有良好的封裝性，可以進行取消操作（cancel())。關於AsyncTask的使用，文檔說的很明白，下面直接上實例。
實例
這個實例用AsyncTask到網絡上下載圖片，同時顯示進度，下載完圖片更新UI。
復制代碼 代碼如下:
package com.hilton.effectiveandroid.concurrent;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.SystemClock;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import com.hilton.effectiveandroid.R;
/*
 * AsyncTask cannot be reused, i.e. if you have executed one AsyncTask, you must discard it, you cannot execute it again.
 * If you try to execute an executed AsyncTask, you will get "java.lang.IllegalStateException: Cannot execute task: the task is already running"
 * In this demo, if you click "get the image" button twice at any time, you will receive "IllegalStateException".
 * About cancellation:
 * You can call AsyncTask#cancel() at any time during AsyncTask executing, but the result is onPostExecute() is not called after
 * doInBackground() finishes, which means doInBackground() is not stopped. AsyncTask#isCancelled() returns true after cancel() getting
 * called, so if you want to really cancel the task, i.e. stop doInBackground(), you must check the return value of isCancelled() in
 * doInBackground, when there are loops in doInBackground in particular.
 * This is the same to Java threading, in which is no effective way to stop a running thread, only way to do is set a flag to thread, and check
 * the flag every time in Thread#run(), if flag is set, run() aborts.
 */
public class AsyncTaskDemoActivity extends Activity {
    private static final String ImageUrl = "http://i1.cqnews.net/sports/attachement/jpg/site82/2011-10-01/2960950278670008721.jpg";
    private ProgressBar mProgressBar;
    private ImageView mImageView;
    private Button mGetImage;
    private Button mAbort;

    @Override
    public void onCreate(Bundle icicle) {
 super.onCreate(icicle);
 setContentView(R.layout.async_task_demo_activity);
 mProgressBar = (ProgressBar) findViewById(R.id.async_task_progress);
 mImageView = (ImageView) findViewById(R.id.async_task_displayer);
 final ImageLoader loader = new ImageLoader();
 mGetImage = (Button) findViewById(R.id.async_task_get_image);
 mGetImage.setOnClickListener(new View.OnClickListener() {
     public void onClick(View v) {
  loader.execute(ImageUrl);
     }
 });
 mAbort = (Button) findViewById(R.id.asyc_task_abort);
 mAbort.setOnClickListener(new View.OnClickListener() {
     public void onClick(View v) {
  loader.cancel(true);
     }
 });
 mAbort.setEnabled(false);
    }

    private class ImageLoader extends AsyncTask<String, Integer, Bitmap> {
 private static final String TAG = "ImageLoader";
 @Override
 protected void onPreExecute() {
     // Initialize progress and image
     mGetImage.setEnabled(false);
     mAbort.setEnabled(true);
     mProgressBar.setVisibility(View.VISIBLE);
     mProgressBar.setProgress(0);
     mImageView.setImageResource(R.drawable.icon);
 }

 @Override
 protected Bitmap doInBackground(String... url) {
     /*
      * Fucking ridiculous thing happened here, to use any Internet connections, either via HttpURLConnection
      * or HttpClient, you must declare INTERNET permission in AndroidManifest.xml. Otherwise you will get
      * "UnknownHostException" when connecting or other tcp/ip/http exceptions rather than "SecurityException"
      * which tells you need to declare INTERNET permission.
      */
     try {
  URL u;
  HttpURLConnection conn = null;
  InputStream in = null;
  OutputStream out = null;
  final String filename = "local_temp_image";
  try {
      u = new URL(url[0]);
      conn = (HttpURLConnection) u.openConnection();
      conn.setDoInput(true);
      conn.setDoOutput(false);
      conn.setConnectTimeout(20 * 1000);
      in = conn.getInputStream();
      out = openFileOutput(filename, Context.MODE_PRIVATE);
      byte[] buf = new byte[8196];
      int seg = 0;
      final long total = conn.getContentLength();
      long current = 0;
      /*
       * Without checking isCancelled(), the loop continues until reading whole image done, i.e. the progress
       * continues go up to 100. But onPostExecute() will not be called.
       * By checking isCancelled(), we can stop immediately, i.e. progress stops immediately when cancel() is called.
       */
      while (!isCancelled() && (seg = in.read(buf)) != -1) {
   out.write(buf, 0, seg);
   current += seg;
   int progress = (int) ((float) current / (float) total * 100f);
   publishProgress(progress);
   SystemClock.sleep(1000);
      }
  } finally {
      if (conn != null) {
   conn.disconnect();
      }
      if (in != null) {
   in.close();
      }
      if (out != null) {
   out.close();
      }
  }
  return BitmapFactory.decodeFile(getFileStreamPath(filename).getAbsolutePath());
     } catch (MalformedURLException e) {
  e.printStackTrace();
     } catch (IOException e) {
  e.printStackTrace();
     }
     return null;
 }

 @Override
 protected void onProgressUpdate(Integer... progress) {
     mProgressBar.setProgress(progress[0]);
 }

 @Override
 protected void onPostExecute(Bitmap image) {
     if (image != null) {
  mImageView.setImageBitmap(image);
     }
     mProgressBar.setProgress(100);
     mProgressBar.setVisibility(View.GONE);
     mAbort.setEnabled(false);
 }
    }
}

運行結果

先後順序分別是下載前，下載中和下載後
總結
關於怎麼使用看文檔和這個例子就夠了，下面說下，使用時的注意事項：
1. AsyncTask對象不可重復使用，也就是說一個AsyncTask對象只能execute()一次，否則會有異常拋出"java.lang.IllegalStateException: Cannot execute task: the task is already running"

2. 在doInBackground()中要檢查isCancelled()的返回值，如果你的異步任務是可以取消的話。
cancel()僅僅是給AsyncTask對象設置了一個標識位，當調用了cancel()後，發生的事情只有:AsyncTask對象的標識位變了，和doInBackground()執行完成後，onPostExecute()不會被回調了，而doInBackground()和onProgressUpdate()還是會繼續執行直到doInBackground()結束。所以要在doInBackground()中不斷的檢查isCancellled()的返回值，當其返回true時就停止執行，特別是有循環的時候。如上面的例子，如果把讀取數據的isCancelled()檢查去掉，圖片還是會下載，進度也一直會走，只是最後圖片不會放到UI上(因為onPostExecute()沒被回調)!

這裡的原因其實很好理解，想想Java SE的Thread吧，是沒有方法將其直接Cacncel掉的，那些線程取消也無非就是給線程設置標識位，然後在run()方法中不斷的檢查標識而已。

3. 如果要在應用程序中使用網絡，一定不要忘記在AndroidManifest中聲明INTERNET權限，否則會報出很詭異的異常信息，比如上面的例子，如果把INTERNET權限拿掉會拋出"UnknownHostException"。剛開始很疑惑，因為模擬器是可以正常上網的，後來Google了下才發現原來是沒權限，但是疑問還是沒有消除，既然沒有聲明網絡權限，為什麼不直接提示無網絡權限呢?

對比Java SE的Thread
Thread是非常原始的類，它只有一個run()方法，一旦開始，無法停止，它僅適合於一個非常獨立的異步任務，也即不需要與主線程交互，對於其他情況，比如需要取消或與主線程交互，都需添加額外的代碼來實現，並且還要注意同步的問題。
而AsyncTask是封裝好了的，可以直接拿來用，如果你僅執行獨立的異步任務，可以僅實現doInBackground()。
所以，當有一個非常獨立的任務時，可以考慮使用Thread，其他時候，盡可能的用AsyncTask。