深入解析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。

1package daimajiaoliu.concurrent;   2    3import java.io.IOException; 4import java.io.InputStream; 5import java.io.OutputStream; 6import java.net.HttpURLConnection; 7import java.net.MalformedURLException; 8import java.net.URL; 9 10import android.app.Activity; 11import android.content.Context; 12import android.graphics.Bitmap; 13import android.graphics.BitmapFactory; 14import android.os.AsyncTask; 15import android.os.Bundle; 16import android.os.SystemClock; 17import android.view.View; 18import android.widget.Button; 19import android.widget.ImageView; 20import android.widget.ProgressBar; 21 22import daimajiaoliu.R; 23 24/* 25 * AsyncTask cannot be reused, i.e. if you have executed one AsyncTask, you must discard it, you cannot execute it again. 26 * If you try to execute an executed AsyncTask, you will get "java.lang.IllegalStateException: Cannot execute task: the task is already running" 27 * In this demo, if you click "get the image" button twice at any time, you will receive "IllegalStateException". 28 * About cancellation: 29 * You can call AsyncTask\#cancel() at any time during AsyncTask executing, but the result is onPostExecute() is not called after 30 * doInBackground() finishes, which means doInBackground() is not stopped. AsyncTask\#isCancelled() returns true after cancel() getting 31 * called, so if you want to really cancel the task, i.e. stop doInBackground(), you must check the return value of isCancelled() in 32 * doInBackground, when there are loops in doInBackground in particular. 33 * 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 34 * the flag every time in Thread\#run(), if flag is set, run() aborts. 35 */ 36 37public class AsyncTaskDemoActivity extends Activity { 38 39 private static final String ImageUrl = "http://i1.cqnews.net/sports/attachement/jpg/site82/2011-10-01/2960950278670008721.jpg"; 40 41 private ProgressBar mProgressBar; 42 43 private ImageView mImageView; 44 45 private Button mGetImage; 46 47 private Button mAbort; 48 49 50 @Override 51 52 53 public void 54 onCreate(Bundle icicle) { 55 56 super 57 .onCreate(icicle); 58 setContentView(R.layout.async_task_demo_activity); 59 mProgressBar = (ProgressBar) findViewById(R.id.async_task_progress); 60 mImageView = (ImageView) findViewById(R.id.async_task_displayer); 61 62 final ImageLoader loader = 63 new 64 ImageLoader(); 65 mGetImage = (Button) findViewById(R.id.async_task_get_image); 66 mGetImage.setOnClickListener( 67 new 68 View.OnClickListener() { 69 70 public void 71 onClick(View v) { 72 loader.execute(ImageUrl); 73 } 74 }); 75 mAbort = (Button) findViewById(R.id.asyc_task_abort); 76 mAbort.setOnClickListener( 77 new 78 View.OnClickListener() { 79 80 public void 81 onClick(View v) { 82 loader.cancel( 83 true 84 ); 85 } 86 }); 87 mAbort.setEnabled( 88 false 89 ); 90 } 91 92 93 private class ImageLoader extends AsyncTask<String, Integer, Bitmap> { 94 95 private static final String TAG = "ImageLoader"; 96 97 @Override 98 protected void onPreExecute() { 99 100 // Initialize progress and image 101 mGetImage.setEnabled(false); 102 mAbort.setEnabled(true); 103 mProgressBar.setVisibility(View.VISIBLE); 104 mProgressBar.setProgress(0); 105 mImageView.setImageResource(R.drawable.icon); 106 } 107 108 109 @Override 110 protected Bitmap doInBackground(String... url) { 111 112 /* 113 * Fucking ridiculous thing happened here, to use any Internet connections, either via HttpURLConnection 114 * or HttpClient, you must declare INTERNET permission in AndroidManifest.xml. Otherwise you will get 115 * "UnknownHostException" when connecting or other tcp/ip/http exceptions rather than "SecurityException" 116 * which tells you need to declare INTERNET permission. 117 */ 118 119 120 try { 121 URL u; 122 HttpURLConnection conn = null; 123 InputStream in = null; 124 OutputStream out = null; 125 126 final String filename = "local_temp_image"; 127 128 try { 129 u = new URL(url[0]); 130 conn = (HttpURLConnection) u.openConnection(); 131 conn.setDoInput(true); 132 conn.setDoOutput(false); 133 conn.setConnectTimeout(20 * 1000); 134 in = conn.getInputStream(); 135 out = openFileOutput(filename, Context.MODE_PRIVATE); 136 137 byte[] buf = new byte[8196]; 138 139 int seg = 0; 140 141 final long total = conn.getContentLength(); 142 143 long current = 0; 144 145 /* 146 * Without checking isCancelled(), the loop continues until reading whole image done, i.e. the progress 147 * continues go up to 100. But onPostExecute() will not be called. 148 * By checking isCancelled(), we can stop immediately, i.e. progress stops immediately when cancel() is called. 149 */ 150 151 152 while (!isCancelled() && (seg = in.read(buf)) != -1) { 153 out.write(buf, 0, seg); 154 current += seg; 155 int progress = (int) ((float) current / (float) total * 100f); 156 publishProgress(progress); 157 SystemClock.sleep(1000); 158 } 159 } finally { 160 161 if (conn != null) { 162 conn.disconnect(); 163 } 164 165 if (in != null) { 166 in.close(); 167 } 168 169 if (out != null 170 ) { 171 out.close(); 172 } 173 } 174 175 return BitmapFactory.decodeFile(getFileStreamPath(filename).getAbsolutePath()); 176 } catch (MalformedURLException e) { 177 e.printStackTrace(); 178 } catch (IOException e) { 179 e.printStackTrace(); 180 } 181 182 return null; 183 } 184 185 186 @Override 187 protected void onProgressUpdate(Integer... progress) { 188 mProgressBar.setProgress(progress[0]); 189 } 190 191 192 @Override 193 protected void onPostExecute(Bitmap image) { 194 if (image != null) { 195 mImageView.setImageBitmap(image); 196 } 197 mProgressBar.setProgress(100); 198 mProgressBar.setVisibility(View.GONE); 199 mAbort.setEnabled(false); 200 } 201 } 202}   203 204

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。

  1. 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。

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