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。
[java]
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://up.2cto.com/2012/0515/20120515100913728.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。
摘自 浪人的星空
