Android多线程AsyncTask详解

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
 xmlns:tools="http://schemas.android.com/tools" 
 android:layout_width="match_parent" 
 android:layout_height="match_parent" > 
 
 <ImageView 
 android:id="@+id/imageView" 
 android:layout_width="wrap_content" 
 android:layout_height="200dp" 
 android:layout_alignParentRight="true" 
 android:layout_alignParentTop="true" 
 android:scaleType="fitCenter"/> 
 
 <Button 
 android:id="@+id/button" 
 android:layout_width="wrap_content" 
 android:layout_height="wrap_content" 
 android:layout_below="@+id/imageView" 
 android:layout_centerHorizontal="true" 
 android:layout_marginTop="41dp" 
 android:text="从网络上下载一张图片" /> 
 
</RelativeLayout> 

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
 package="com.xiaoluo.android_asynctast" 
 android:versionCode="1" 
 android:versionName="1.0" > 
 
 <uses-sdk 
 android:minSdkVersion="8" 
 android:targetSdkVersion="18" /> 
 
 <!-- 授权手机能够访问网络 --> 
 <uses-permission android:name="android.permission.INTERNET"/> 
 
 <application 
 android:allowBackup="true" 
 android:icon="@drawable/ic_launcher" 
 android:label="@string/app_name" 
 android:theme="@style/AppTheme" > 
 <activity 
 android:name="com.xiaoluo.android_asynctast.MainActivity" 
 android:label="@string/app_name" > 
 <intent-filter> 
 <action android:name="android.intent.action.MAIN" /> 
 
 <category android:name="android.intent.category.LAUNCHER" /> 
 </intent-filter> 
 </activity> 
 </application> 
 
</manifest> 

public class MainActivity extends Activity 
{ 
 private Button button; 
 private ImageView imageView; 
 private ProgressDialog progressDialog; 
 private final String IMAGE_PATH = "http://developer.android.com/images/home/kk-hero.jpg"; 
// private final String IMAGE_PATH2 = "http://ww2.sinaimg.cn/mw690/69c7e018jw1e6hd0vm3pej20fa0a674c.jpg"; 
 @Override 
 protected void onCreate(Bundle savedInstanceState) 
 { 
 super.onCreate(savedInstanceState); 
 setContentView(R.layout.activity_main); 
 
 button = (Button)findViewById(R.id.button); 
 imageView = (ImageView)findViewById(R.id.imageView); 
 // 弹出要给ProgressDialog 
 progressDialog = new ProgressDialog(MainActivity.this); 
 progressDialog.setTitle("提示信息"); 
 progressDialog.setMessage("正在下载中，请稍后......"); 
 // 设置setCancelable(false); 表示我们不能取消这个弹出框，等下载完成之后再让弹出框消失 
 progressDialog.setCancelable(false); 
 // 设置ProgressDialog样式为圆圈的形式 
 progressDialog.setProgressStyle(ProgressDialog.STYLE_SPINNER); 
 
 button.setOnClickListener(new View.OnClickListener() 
 { 
 @Override 
 public void onClick(View v) 
 { 
　　　　　　　　　// 在UI Thread当中实例化AsyncTask对象，并调用execute方法 
 new MyAsyncTask().execute(IMAGE_PATH); 
 } 
 }); 
 } 
 
 /** 
 * 定义一个类，让其继承AsyncTask这个类 
 * Params: String类型，表示传递给异步任务的参数类型是String，通常指定的是URL路径 
 * Progress: Integer类型，进度条的单位通常都是Integer类型 
 * Result：byte[]类型，表示我们下载好的图片以字节数组返回 
 * @author xiaoluo 
 * 
 */ 
 public class MyAsyncTask extends AsyncTask<String, Integer, byte[]> 
 { 
 @Override 
 protected void onPreExecute() 
 { 
 super.onPreExecute(); 
 // 在onPreExecute()中我们让ProgressDialog显示出来 
 progressDialog.show(); 
 } 
 @Override 
 protected byte[] doInBackground(String... params) 
 { 
 // 通过Apache的HttpClient来访问请求网络中的一张图片 
 HttpClient httpClient = new DefaultHttpClient(); 
 HttpGet httpGet = new HttpGet(params[0]); 
 byte[] image = new byte[]{}; 
 try 
 { 
 HttpResponse httpResponse = httpClient.execute(httpGet); 
 HttpEntity httpEntity = httpResponse.getEntity(); 
 if(httpEntity != null && httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK) 
 { 
  image = EntityUtils.toByteArray(httpEntity); 
 } 
 } 
 catch (Exception e) 
 { 
 e.printStackTrace(); 
 } 
 finally 
 { 
 httpClient.getConnectionManager().shutdown(); 
 } 
 return image; 
 } 
 @Override 
 protected void onProgressUpdate(Integer... values) 
 { 
 super.onProgressUpdate(values); 
 } 
 @Override 
 protected void onPostExecute(byte[] result) 
 { 
 super.onPostExecute(result); 
 // 将doInBackground方法返回的byte[]解码成要给Bitmap 
 Bitmap bitmap = BitmapFactory.decodeByteArray(result, 0, result.length); 
 // 更新我们的ImageView控件 
 imageView.setImageBitmap(bitmap); 
 // 使ProgressDialog框消失 
 progressDialog.dismiss(); 
 } 
 } 
 
 @Override 
 public boolean onCreateOptionsMenu(Menu menu) 
 { 
 getMenuInflater().inflate(R.menu.main, menu); 
 return true; 
 } 
 
} 

public class MainActivity extends Activity 
{ 
 private Button button; 
 private ImageView imageView; 
 private ProgressDialog progressDialog; 
 private final String IMAGE_PATH = "http://developer.android.com/images/home/kk-hero.jpg"; 
// private final String IMAGE_PATH2 = "http://ww2.sinaimg.cn/mw690/69c7e018jw1e6hd0vm3pej20fa0a674c.jpg"; 
 @Override 
 protected void onCreate(Bundle savedInstanceState) 
 { 
 super.onCreate(savedInstanceState); 
 setContentView(R.layout.activity_main); 
 
 button = (Button)findViewById(R.id.button); 
 imageView = (ImageView)findViewById(R.id.imageView); 
 // 弹出要给ProgressDialog 
 progressDialog = new ProgressDialog(MainActivity.this); 
 progressDialog.setTitle("提示信息"); 
 progressDialog.setMessage("正在下载中，请稍后......"); 
 // 设置setCancelable(false); 表示我们不能取消这个弹出框，等下载完成之后再让弹出框消失 
 progressDialog.setCancelable(false); 
 // 设置ProgressDialog样式为水平的样式 
 progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL); 
 
 button.setOnClickListener(new View.OnClickListener() 
 { 
 @Override 
 public void onClick(View v) 
 { 
 new MyAsyncTask().execute(IMAGE_PATH); 
 } 
 }); 
 } 
 
 /** 
 * 定义一个类，让其继承AsyncTask这个类 
 * Params: String类型，表示传递给异步任务的参数类型是String，通常指定的是URL路径 
 * Progress: Integer类型，进度条的单位通常都是Integer类型 
 * Result：byte[]类型，表示我们下载好的图片以字节数组返回 
 * @author xiaoluo 
 * 
 */ 
 public class MyAsyncTask extends AsyncTask<String, Integer, byte[]> 
 { 
 @Override 
 protected void onPreExecute() 
 { 
 super.onPreExecute(); 
 // 在onPreExecute()中我们让ProgressDialog显示出来 
 progressDialog.show(); 
 } 
 @Override 
 protected byte[] doInBackground(String... params) 
 { 
 // 通过Apache的HttpClient来访问请求网络中的一张图片 
 HttpClient httpClient = new DefaultHttpClient(); 
 HttpGet httpGet = new HttpGet(params[0]); 
 byte[] image = new byte[]{}; 
 try 
 { 
 HttpResponse httpResponse = httpClient.execute(httpGet); 
 HttpEntity httpEntity = httpResponse.getEntity(); 
 InputStream inputStream = null; 
 ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream(); 
 if(httpEntity != null && httpResponse.getStatusLine().getStatusCode() == HttpStatus.SC_OK) 
 { 
  // 得到文件的总长度 
  long file_length = httpEntity.getContentLength(); 
  // 每次读取后累加的长度 
  long total_length = 0; 
  int length = 0; 
  // 每次读取1024个字节 
  byte[] data = new byte[1024]; 
  inputStream = httpEntity.getContent(); 
  while(-1 != (length = inputStream.read(data))) 
  { 
  // 每读一次，就将total_length累加起来 
  total_length += length; 
  // 边读边写到ByteArrayOutputStream当中 
  byteArrayOutputStream.write(data, 0, length); 
  // 得到当前图片下载的进度 
  int progress = ((int)(total_length/(float)file_length) * 100); 
  // 时刻将当前进度更新给onProgressUpdate方法 
  publishProgress(progress); 
  } 
 } 
 image = byteArrayOutputStream.toByteArray(); 
 inputStream.close(); 
 byteArrayOutputStream.close(); 
 } 
 catch (Exception e) 
 { 
 e.printStackTrace(); 
 } 
 finally 
 { 
 httpClient.getConnectionManager().shutdown(); 
 } 
 return image; 
 } 
 @Override 
 protected void onProgressUpdate(Integer... values) 
 { 
 super.onProgressUpdate(values); 
 // 更新ProgressDialog的进度条 
 progressDialog.setProgress(values[0]); 
 } 
 @Override 
 protected void onPostExecute(byte[] result) 
 { 
 super.onPostExecute(result); 
 // 将doInBackground方法返回的byte[]解码成要给Bitmap 
 Bitmap bitmap = BitmapFactory.decodeByteArray(result, 0, result.length); 
 // 更新我们的ImageView控件 
 imageView.setImageBitmap(bitmap); 
 // 使ProgressDialog框消失 
 progressDialog.dismiss(); 
 } 
 } 
 
 @Override 
 public boolean onCreateOptionsMenu(Menu menu) 
 { 
 getMenuInflater().inflate(R.menu.main, menu); 
 return true; 
 } 
 
}