MainActivity如下:

package com.example.asynctasktest;
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.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
public class MainActivity extends Activity {
    private Button satrtButton;
    private Button cancelButton;
    private ProgressBar progressBar;
    private TextView textView;
    private DownLoaderAsyncTask downLoaderAsyncTask;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        initView();
    }
    public void initView() {
        satrtButton=(Button) findViewById(R.id.startButton);
        cancelButton=(Button) findViewById(R.id.cancelButton);
        satrtButton.setOnClickListener(new ButtonOnClickListener());
        cancelButton.setOnClickListener(new ButtonOnClickListener());
        progressBar=(ProgressBar) findViewById(R.id.progressBar);
        textView=(TextView) findViewById(R.id.textView);
    }
   private class ButtonOnClickListener implements OnClickListener{
        public void onClick(View v) {
            switch (v.getId()) {
            case R.id.startButton:
                //注意:
                //1 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
                //2 异步任务的实例必须在UI线程中创建
                //3 execute()方法必须在UI线程中调用。
                downLoaderAsyncTask=new DownLoaderAsyncTask();
                downLoaderAsyncTask.execute("http://www.baidu.com");
                break;
            case R.id.cancelButton:
                //取消一个正在执行的任务,onCancelled()方法将会被调用   
                downLoaderAsyncTask.cancel(true);
                break;
            default:
                break;
            }
        }

   }
   //构造函数AsyncTask<Params, Progress, Result>参数说明: 
   //Params   启动任务执行的输入参数
   //Progress 后台任务执行的进度
   //Result   后台计算结果的类型
   private class DownLoaderAsyncTask extends AsyncTask<String, Integer, String>{
    //onPreExecute()方法用于在执行异步任务前,主线程做一些准备工作   
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        textView.setText("调用onPreExecute()方法--->准备开始执行异步任务");
        System.out.println("调用onPreExecute()方法--->准备开始执行异步任务");
    }

    //doInBackground()方法用于在执行异步任务,不可以更改主线程中UI 
    @Override
    protected String doInBackground(String... params) {
       System.out.println("调用doInBackground()方法--->开始执行异步任务");
        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 bos = new ByteArrayOutputStream();
                byte[] buffer = new byte[1024];
                int count = 0;
                int length = -1;
                while ((length = is.read(buffer)) != -1) {
                    bos.write(buffer, 0, length);
                    count += length;
                    //publishProgress()为AsyncTask类中的方法
                    //常在doInBackground()中调用此方法
                    //用于通知主线程,后台任务的执行情况.
                    //此时会触发AsyncTask中的onProgressUpdate()方法
                    publishProgress((int) ((count / (float) total) * 100));
                    //为了演示进度,休眠1000毫秒
                    Thread.sleep(1000);
                }
                return new String(bos.toByteArray(), "UTF-8");
            }
        } catch (Exception e) {
            return null;
        }
        return null;
    }

    //onPostExecute()方法用于异步任务执行完成后,在主线程中执行的操作
    @Override
    protected void onPostExecute(String result) {
        super.onPostExecute(result);    
        Toast.makeText(getApplicationContext(), "调用onPostExecute()方法--->异步任务执行完毕", 0).show();
        //textView显示网络请求结果
        textView.setText(result);
        System.out.println("调用onPostExecute()方法--->异步任务执行完毕");
    }

    //onProgressUpdate()方法用于更新异步执行中,在主线程中处理异步任务的执行信息   
    @Override
    protected void onProgressUpdate(Integer... values) {
        super.onProgressUpdate(values);
        //更改进度条
        progressBar.setProgress(values[0]);
        //更改TextView
        textView.setText("已经加载"+values[0]+"%");
    }

    //onCancelled()方法用于异步任务被取消时,在主线程中执行相关的操作 
    @Override
    protected void onCancelled() {
        super.onCancelled();
        //更改进度条进度为0
        progressBar.setProgress(0);
        //更改TextView
        textView.setText("调用onCancelled()方法--->异步任务被取消");
        System.out.println("调用onCancelled()方法--->异步任务被取消");
    }
   }
}

<LinearLayout 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"
    android:orientation="vertical" >

    <Button
        android:id="@+id/startButton"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="开始异步任务" />

    <Button
        android:id="@+id/cancelButton"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="取消异步任务" />

    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:max="100"
        android:progress="0" />

    <ScrollView
        android:id="@+id/scrollView"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" >

        <TextView
            android:id="@+id/textView"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text="test test" />
    </ScrollView>

</LinearLayout>