（1）该文件下载主要演示异步任务下载图片

（2）布局代码如下：一个ImageView用于存放下载的图片，Button用于开始异步任务下载

<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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"
        android:layout_marginLeft="141dp"
        android:layout_marginTop="135dp" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_alignRight="@+id/imageView1"
        android:layout_marginBottom="80dp"
        android:text="网页下载图片" />

</RelativeLayout>

package com.example.asynctask_download;

import java.io.IOException;

import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;

import android.app.Activity;
import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {

	private Button button1;
	private ImageView imageView1;
	private final String IMAGE_PATH = "http://www.baidu.com/img/bd_logo1.png";// 设置网页图片的地址
	private ProgressDialog progressDialog;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		button1 = (Button) this.findViewById(R.id.button1);
		imageView1 = (ImageView) this.findViewById(R.id.imageView1);
		/*
		 * 下载进度条的设置
		 */
		progressDialog = new ProgressDialog(this);
		progressDialog.setTitle("提示");
		progressDialog.setCancelable(false);
		progressDialog.setMessage("正在下载图片，请耐心等候！");

		button1.setOnClickListener(new OnClickListener() {

			@Override
			public void onClick(View v) {
				/*
				 * 异步任务、执行下载图片
				 */
				new MyTask().execute(IMAGE_PATH);
			}
		});
	}

	/*
	 * 异步任务：三个参数、四个步骤onPreExecute()、
	 * doInBackground()、onProgressUpdate（）、onPostExecute（）、没有参数传传进去的时候可以使用void
	 */
	public class MyTask extends AsyncTask<String, Integer, byte[]> {

		/*
		 * onPreExecute()方法在后台操作开始前运行在UI线程上
		 */
		@Override
		protected void onPreExecute() {
			super.onPreExecute();
			progressDialog.show();
		}

		/*
		 * doInBackground方法运行在后台并处理后台操作
		 */
		@Override
		protected byte[] doInBackground(String... params) {
			HttpClient httpClient = new DefaultHttpClient();
			HttpGet httpGet = new HttpGet(params[0]);
			byte[] result = null;

			try {
				HttpResponse httpResponse = httpClient.execute(httpGet);
				if (httpResponse.getStatusLine().getStatusCode() == 200) {
					result = EntityUtils.toByteArray(httpResponse.getEntity());
				}
			} catch (ClientProtocolException e) {
				e.printStackTrace();
			} catch (IOException e) {
				e.printStackTrace();
			} finally {
				httpClient.getConnectionManager().shutdown();
			}

			return result;
		}

		@Override
		protected void onProgressUpdate(Integer... values) {
			super.onProgressUpdate(values);
		}

		/*
		 * 一旦后台操作完毕，onPostExecute()方法就会在UI线程中运行
		 */
		@Override
		protected void onPostExecute(byte[] result) {
			super.onPostExecute(result);
			/*
			 * 将下载的bitmap放置在imagView中去
			 * result就是下载的图片
			 */
			Bitmap bitmap = BitmapFactory.decodeByteArray(result, 0,
					result.length);
			imageView1.setImageBitmap(bitmap);
			progressDialog.dismiss();
		}

	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

}

Android学习笔记之AsyncTask实现文件下载任务