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。