understanding asynctask
asynctask是android 1.5 cubake加入的用于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平臺自己的異步工具，融入了android平臺的特性，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，加上了平臺相關的特性，所以對于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平臺的特性，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())。關于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

先后順序分別是下載前，下載中和下載后
總結
關于怎么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()后，發生的事情只有:asynctask對象的標識位變了，和doinbackground()執行完成后，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，只是最后圖片不會放到ui上(因為onpostexecute()沒被回調)!

這里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，然后在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，后來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，為什么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，它僅適合于一個非常獨立的異步任務，也即不需要與主線程交互，對于其他情況，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，并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。