首先先声明访问网络权限

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.asktaskdownload"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="14"
        android:targetSdkVersion="17" />
    <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=".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>

<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="com.example.asktaskdownload.MainActivity" >

     <Button
        android:id="@+id/button1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="29dp"
        android:text="下载网络图片" 
        />

     <ImageView
         android:id="@+id/imageView1"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_centerInParent="true"
         android:src=http://www.mamicode.com/"@drawable/ic_launcher" />>
package com.example.asktaskdownload;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
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.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {

	private ImageView imageView;
	private Button button;
	//图片的网络地址
	private String image_path = "http://pic4.nipic.com/20090828/2366808_092035042961_2.jpg";
	private ProgressDialog dialog;
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		imageView = (ImageView) findViewById(R.id.imageView1);
		button = (Button) findViewById(R.id.button1);
		dialog = new ProgressDialog(this);
		dialog.setTitle("提示");
		dialog.setMessage("正在下载，请稍后...");
		button.setOnClickListener(new OnClickListener() {
			//Android3以后不允许在UI主线程访问网络
			@Override
			public void onClick(View v) {
				// TODO Auto-generated method stub
				new MyTask().execute(image_path);
			}
		});
	}
	/*
	 * 第一个参数:表示要执行的任务的参数,一般传递一个路径
	 * 第二个参数:进度的刻度
	 * 第三个参数:任务执行的返回结果
	 */
	public class MyTask extends AsyncTask<String, Void, Bitmap>{

		//任务执行之前的操作
		@Override
		protected void onPreExecute() {
			// TODO Auto-generated method stub
			super.onPreExecute();
			dialog.show();
		}
		//完成下载或耗时任务操作
		@Override
		protected Bitmap doInBackground(String... params) {
            HttpClient httpClient = new DefaultHttpClient();
            HttpGet httpGet = new HttpGet(params[0]);
            Bitmap bitmap = null;
            try {
				HttpResponse httpResponse = httpClient.execute(httpGet);
				//返回的状态码为200表示操作成功
				if(httpResponse.getStatusLine().getStatusCode() == 200){
					//获取实体类
					HttpEntity httpEntity = httpResponse.getEntity();
					//将实体类转换成字节数组
				    byte[] data = http://www.mamicode.com/EntityUtils.toByteArray(httpEntity);>

主页面，点击下载按钮

最后下载完成的结果

异步操作AsyncTask（一）从网络下载图片