Android开发笔记之:深入理解多线程AsyncTask

Understanding AsyncTask
AsyncTask是Android 1.5 Cubake加入的用于实现异步操作的一个类，在此之前只能用Java SE库中的Thread来实现多线程异步，AsyncTask是Android平台自己的异步工具，融入了Android平台的特性，让异步操作更加的安全，方便和实用。实质上它也是对Java SE库中Thread的一个封装，加上了平台相关的特性，所以对于所有的多线程异步都强烈推荐使用AsyncTask，因为它考虑，也融入了Android平台的特性，更加的安全和高效。
AsyncTask可以方便的执行异步操作（doInBackground)，又能方便的与主线程进行通信，它本身又有良好的封装性，可以进行取消操作（cancel())。关于AsyncTask的使用，文档说的很明白，下面直接上实例。
实例
这个实例用AsyncTask到网络上下载图片，同时显示进度，下载完图片更新UI。

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);
 }
    }
}

运行结果

先后顺序分别是下载前，下载中和下载后
总结
关于怎么使用看文档和这个例子就够了，下面说下，使用时的注意事项：
1. AsyncTask对象不可重复使用，也就是说一个AsyncTask对象只能execute()一次，否则会有异常抛出"java.lang.IllegalStateException: Cannot execute task: the task is already running"

2. 在doInBackground()中要检查isCancelled()的返回值，如果你的异步任务是可以取消的话。
cancel()仅仅是给AsyncTask对象设置了一个标识位，当调用了cancel()后，发生的事情只有:AsyncTask对象的标识位变了，和doInBackground()执行完成后，onPostExecute()不会被回调了，而doInBackground()和onProgressUpdate()还是会继续执行直到doInBackground()结束。所以要在doInBackground()中不断的检查isCancellled()的返回值，当其返回true时就停止执行，特别是有循环的时候。如上面的例子，如果把读取数据的isCancelled()检查去掉，图片还是会下载，进度也一直会走，只是最后图片不会放到UI上(因为onPostExecute()没被回调)!

这里的原因其实很好理解，想想Java SE的Thread吧，是没有方法将其直接Cacncel掉的，那些线程取消也无非就是给线程设置标识位，然后在run()方法中不断的检查标识而已。

3. 如果要在应用程序中使用网络，一定不要忘记在AndroidManifest中声明INTERNET权限，否则会报出很诡异的异常信息，比如上面的例子，如果把INTERNET权限拿掉会抛出"UnknownHostException"。刚开始很疑惑，因为模拟器是可以正常上网的，后来Google了下才发现原来是没权限，但是疑问还是没有消除，既然没有声明网络权限，为什么不直接提示无网络权限呢?

对比Java SE的Thread
Thread是非常原始的类，它只有一个run()方法，一旦开始，无法停止，它仅适合于一个非常独立的异步任务，也即不需要与主线程交互，对于其他情况，比如需要取消或与主线程交互，都需添加额外的代码来实现，并且还要注意同步的问题。
而AsyncTask是封装好了的，可以直接拿来用，如果你仅执行独立的异步任务，可以仅实现doInBackground()。
所以，当有一个非常独立的任务时，可以考虑使用Thread，其他时候，尽可能的用AsyncTask。