public abstract class AsyncTask<Params, Progress, Result> { 

package com.scott.async; 
 
import java.io.ByteArrayOutputStream; 
import java.io.InputStream; 
 
import org.apache.http.HttpEntity; 
import org.apache.http.HttpResponse; 
import org.apache.http.HttpStatus; 
import org.apache.http.client.HttpClient; 
import org.apache.http.client.methods.HttpGet; 
import org.apache.http.impl.client.DefaultHttpClient; 
 
import android.app.Activity; 
import android.os.AsyncTask; 
import android.os.Bundle; 
import android.util.Log; 
import android.view.View; 
import android.widget.Button; 
import android.widget.ProgressBar; 
import android.widget.TextView; 
 
public class MainActivity extends Activity { 
 
  private static final String TAG = "ASYNC_TASK"; 
 
  private Button execute; 
  private Button cancel; 
  private ProgressBar progressBar; 
  private TextView textView; 
 
  private MyTask mTask; 
 
  @Override 
  public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.main); 
 
    execute = (Button) findViewById(R.id.execute); 
    execute.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
        //注意每次需new一个实例,新建的任务只能执行一次,否则会出现异常 
        mTask = new MyTask(); 
        mTask.execute("http://www.baidu.com"); 
 
        execute.setEnabled(false); 
        cancel.setEnabled(true); 
      } 
    }); 
    cancel = (Button) findViewById(R.id.cancel); 
    cancel.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View v) { 
        //取消一个正在执行的任务,onCancelled方法将会被调用 
        mTask.cancel(true); 
      } 
    }); 
    progressBar = (ProgressBar) findViewById(R.id.progress_bar); 
    textView = (TextView) findViewById(R.id.text_view); 
 
  } 
 
  private class MyTask extends AsyncTask<String, Integer, String> { 
    //onPreExecute方法用于在执行后台任务前做一些UI操作 
    @Override 
    protected void onPreExecute() { 
      Log.i(TAG, "onPreExecute() called"); 
      textView.setText("loading..."); 
    } 
 
    //doInBackground方法内部执行后台任务,不可在此方法内修改UI 
    @Override 
    protected String doInBackground(String... params) { 
      Log.i(TAG, "doInBackground(Params... params) called"); 
      try { 
        HttpClient client = new DefaultHttpClient(); 
        HttpGet get = new HttpGet(params[0]); 
        HttpResponse response = client.execute(get); 
        if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) { 
          HttpEntity entity = response.getEntity(); 
          InputStream is = entity.getContent(); 
          long total = entity.getContentLength(); 
          ByteArrayOutputStream baos = new ByteArrayOutputStream(); 
          byte[] buf = new byte[1024]; 
          int count = 0; 
          int length = -1; 
          while ((length = is.read(buf)) != -1) { 
            baos.write(buf, 0, length); 
            count += length; 
            //调用publishProgress公布进度,最后onProgressUpdate方法将被执行 
            publishProgress((int) ((count / (float) total) * 100)); 
            //为了演示进度,休眠500毫秒 
            Thread.sleep(500); 
          } 
          return new String(baos.toByteArray(), "gb2312"); 
        } 
      } catch (Exception e) { 
        Log.e(TAG, e.getMessage()); 
      } 
      return null; 
    } 
 
    //onProgressUpdate方法用于更新进度信息 
    @Override 
    protected void onProgressUpdate(Integer... progresses) { 
      Log.i(TAG, "onProgressUpdate(Progress... progresses) called"); 
      progressBar.setProgress(progresses[0]); 
      textView.setText("loading..." + progresses[0] + "%"); 
    } 
 
    //onPostExecute方法用于在执行完后台任务后更新UI,显示结果 
    @Override 
    protected void onPostExecute(String result) { 
      Log.i(TAG, "onPostExecute(Result result) called"); 
      textView.setText(result); 
 
      execute.setEnabled(true); 
      cancel.setEnabled(false); 
    } 
 
    //onCancelled方法用于在取消执行中的任务时更改UI 
    @Override 
    protected void onCancelled() { 
      Log.i(TAG, "onCancelled() called"); 
      textView.setText("cancelled"); 
      progressBar.setProgress(0); 
 
      execute.setEnabled(true); 
      cancel.setEnabled(false); 
    } 
  } 
} 

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
  android:orientation="vertical" 
  android:layout_width="fill_parent" 
  android:layout_height="fill_parent"> 
  <Button 
    android:id="@+id/execute" 
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:text="execute"/> 
  <Button 
    android:id="@+id/cancel" 
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content" 
    android:enabled="false" 
    android:text="cancel"/> 
  <ProgressBar  
    android:id="@+id/progress_bar"  
    android:layout_width="fill_parent"  
    android:layout_height="wrap_content"  
    android:progress="0" 
    android:max="100" 
    style="?android:attr/progressBarStyleHorizontal"/> 
  <ScrollView 
    android:layout_width="fill_parent"  
    android:layout_height="wrap_content"> 
    <TextView 
      android:id="@+id/text_view" 
      android:layout_width="fill_parent"  
      android:layout_height="wrap_content"/> 
  </ScrollView> 
</LinearLayout> 

<uses-permission android:name="android.permission.INTERNET"/> 

//初始状态 
private volatile Status mStatus = Status.PENDING; 
 
public enum Status { 
  /** 
   * Indicates that the task has not been executed yet. 
   */ 
  PENDING, 
  /** 
   * Indicates that the task is running. 
   */ 
  RUNNING, 
  /** 
   * Indicates that {@link AsyncTask#onPostExecute} has finished. 
   */ 
  FINISHED, 
} 
 
/** 
 * Returns the current status of this task. 
 * 
 * @return The current status. 
 */ 
public final Status getStatus() { 
  return mStatus; 
} 

public final AsyncTask<Params, Progress, Result> execute(Params... params) { 
  if (mStatus != Status.PENDING) { 
    switch (mStatus) { 
      case RUNNING: 
        //如果该任务正在被执行则抛出异常 
        //值得一提的是,在调用cancel取消任务后,状态仍未RUNNING 
        throw new IllegalStateException("Cannot execute task:" 
            + " the task is already running."); 
      case FINISHED: 
        //如果该任务已经执行完成则抛出异常 
        throw new IllegalStateException("Cannot execute task:" 
            + " the task has already been executed " 
            + "(a task can be executed only once)"); 
    } 
  } 
 
  //改变状态为RUNNING 
  mStatus = Status.RUNNING; 
 
  //调用onPreExecute方法 
  onPreExecute(); 
 
  mWorker.mParams = params; 
  sExecutor.execute(mFuture); 
 
  return this; 
} 

private static final int CORE_POOL_SIZE = 5; 
private static final int MAXIMUM_POOL_SIZE = 128; 
private static final int KEEP_ALIVE = 10; 
 
//新建一个队列用来存放线程 
private static final BlockingQueue<Runnable> sWorkQueue = 
    new LinkedBlockingQueue<Runnable>(10); 
//新建一个线程工厂 
private static final ThreadFactory sThreadFactory = new ThreadFactory() { 
  private final AtomicInteger mCount = new AtomicInteger(1); 
  //新建一个线程 
  public Thread newThread(Runnable r) { 
    return new Thread(r, "AsyncTask #" + mCount.getAndIncrement()); 
  } 
}; 
//新建一个线程池执行器,用于管理线程的执行 
private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE, 
    MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory); 

private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { 
  Params[] mParams; 
} 

/** 
 * A cancellable asynchronous computation. 
 * ... 
 */ 
public class FutureTask<V> implements RunnableFuture<V> { 

public interface RunnableFuture<V> extends Runnable, Future<V> { 
  /** 
   * Sets this Future to the result of its computation 
   * unless it has been cancelled. 
   */ 
  void run(); 
} 

private final WorkerRunnable<Params, Result> mWorker; 
private final FutureTask<Result> mFuture; 
 
public AsyncTask() { 
  mWorker = new WorkerRunnable<Params, Result>() { 
    //call方法被调用后,将设置优先级为后台级别,然后调用AsyncTask的doInBackground方法 
    public Result call() throws Exception { 
      Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); 
      return doInBackground(mParams); 
    } 
  }; 
 
  //在mFuture实例中,将会调用mWorker做后台任务,完成后会调用done方法 
  mFuture = new FutureTask<Result>(mWorker) { 
    @Override 
    protected void done() { 
      Message message; 
      Result result = null; 
 
      try { 
        result = get(); 
      } catch (InterruptedException e) { 
        android.util.Log.w(LOG_TAG, e); 
      } catch (ExecutionException e) { 
        throw new RuntimeException("An error occured while executing doInBackground()", 
            e.getCause()); 
      } catch (CancellationException e) { 
        //发送取消任务的消息 
        message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, 
            new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); 
        message.sendToTarget(); 
        return; 
      } catch (Throwable t) { 
        throw new RuntimeException("An error occured while executing " 
            + "doInBackground()", t); 
      } 
 
      //发送显示结果的消息 
      message = sHandler.obtainMessage(MESSAGE_POST_RESULT, 
          new AsyncTaskResult<Result>(AsyncTask.this, result)); 
      message.sendToTarget(); 
    } 
  }; 
} 

private static final int MESSAGE_POST_RESULT = 0x1; //显示结果 
private static final int MESSAGE_POST_PROGRESS = 0x2;  //更新进度 
private static final int MESSAGE_POST_CANCEL = 0x3; //取消任务 
 
private static final InternalHandler sHandler = new InternalHandler(); 
 
private static class InternalHandler extends Handler { 
  @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"}) 
  @Override 
  public void handleMessage(Message msg) { 
    AsyncTaskResult result = (AsyncTaskResult) msg.obj; 
    switch (msg.what) { 
      case MESSAGE_POST_RESULT: 
        // There is only one result 
        //调用AsyncTask.finish方法 
        result.mTask.finish(result.mData[0]); 
        break; 
      case MESSAGE_POST_PROGRESS: 
        //调用AsyncTask.onProgressUpdate方法 
        result.mTask.onProgressUpdate(result.mData); 
        break; 
      case MESSAGE_POST_CANCEL: 
        //调用AsyncTask.onCancelled方法 
        result.mTask.onCancelled(); 
        break; 
    } 
  } 
} 

private void finish(Result result) { 
  if (isCancelled()) result = null; 
  onPostExecute(result); //调用onPostExecute显示结果 
  mStatus = Status.FINISHED; //改变状态为FINISHED 
} 

AsyncTaskResult result = (AsyncTaskResult) msg.obj; 

@SuppressWarnings({"RawUseOfParameterizedType"}) 
private static class AsyncTaskResult<Data> { 
  final AsyncTask mTask; 
  final Data[] mData; 
 
  AsyncTaskResult(AsyncTask task, Data... data) { 
    mTask = task; 
    mData = data; 
  } 
} 

//发送取消任务的消息 
message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, 
    new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); 
message.sendToTarget(); 

//发送显示结果的消息 
message = sHandler.obtainMessage(MESSAGE_POST_RESULT, 
     new AsyncTaskResult<Result>(AsyncTask.this, result)); 
message.sendToTarget(); 

result.mTask.finish(result.mData[0]); 
result.mTask.onProgressUpdate(result.mData);