Android应用中如何异步下载图片并将图片保存到本地DEMO中

发布时间：2020-12-03 16:40:58 作者：Leah
来源：亿速云阅读：476

Android应用中如何异步下载图片并将图片保存到本地DEMO中？针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。

通下面是demo中的Activity。

public class MainActivity extends Activity { 
 protected static final int SUCCESS_GET_CONTACT = 0; 
 private ListView mListView; 
 private MyContactAdapter mAdapter; 
 private File cache; 
 private Handler mHandler = new Handler(){ 
  public void handleMessage(android.os.Message msg) { 
   if(msg.what == SUCCESS_GET_CONTACT){ 
    List<Contact> contacts = (List<Contact>) msg.obj; 
    mAdapter = new MyContactAdapter(getApplicationContext(),contacts,cache); 
    mListView.setAdapter(mAdapter); 
   } 
  }; 
 }; 
 @Override 
 public void onCreate(Bundle savedInstanceState) { 
  super.onCreate(savedInstanceState); 
  setContentView(R.layout.main); 
  mListView = (ListView) findViewById(R.id.listview); 
  //创建缓存目录，系统一运行就得创建缓存目录的， 
  cache = new File(Environment.getExternalStorageDirectory(), "cache"); 
  if(!cache.exists()){ 
   cache.mkdirs(); 
  } 
  //获取数据，主UI线程是不能做耗时操作的，所以启动子线程来做 
  new Thread(){ 
   public void run() { 
    ContactService service = new ContactService(); 
    List<Contact> contacts = null; 
    try { 
     contacts = service.getContactAll(); 
    } catch (Exception e) { 
     e.printStackTrace(); 
    } 
    //子线程通过Message对象封装信息，并且用初始化好的， 
    //Handler对象的sendMessage()方法把数据发送到主线程中，从而达到更新UI主线程的目的 
    Message msg = new Message(); 
    msg.what = SUCCESS_GET_CONTACT; 
    msg.obj = contacts; 
    mHandler.sendMessage(msg); 
   }; 
  }.start(); 
 } 
 @Override 
 protected void onDestroy() { 
  super.onDestroy(); 
  //清空缓存 
  File[] files = cache.listFiles(); 
  for(File file :files){ 
   file.delete(); 
  } 
  cache.delete(); 
 } 
}

Activity中，注意以下几点，

1.初始化了一个缓存目录，这个目录最好是应用开启就去创建好，为手续缓存图片做准备，在这里把数据存放在SDCard上

2.要去服务器加载数据，这个耗时操作最好是去开启线程加载数据，加载完毕后去异步的更新UI线程，利用Handler机制能很好的解决这个问题，

3.最后退出应用的时候，要删掉缓存目录和目录里面的数据，避免给手机制造很多的垃圾文件

下面就是一个Service类了，

public class ContactService { 
 /* 
  * 从服务器上获取数据 
  */ 
 public List<Contact> getContactAll() throws Exception { 
  List<Contact> contacts = null; 
  String Parth = "http://192.168.1.103:8080/myweb/list.xml"; 
  URL url = new URL(Parth); 
  HttpURLConnection conn = (HttpURLConnection) url.openConnection(); 
  conn.setConnectTimeout(3000); 
  conn.setRequestMethod("GET"); 
  if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) { 
   InputStream is = conn.getInputStream(); 
   // 这里获取数据直接放在XmlPullParser里面解析 
   contacts = xmlParser(is); 
   return contacts; 
  } else { 
   return null; 
  } 
 } 
 // 这里并没有下载图片下来，而是把图片的地址保存下来了 
 private List<Contact> xmlParser(InputStream is) throws Exception { 
  List<Contact> contacts = null; 
  Contact contact = null; 
  XmlPullParser parser = Xml.newPullParser(); 
  parser.setInput(is, "UTF-8"); 
  int eventType = parser.getEventType(); 
  while ((eventType = parser.next()) != XmlPullParser.END_DOCUMENT) { 
   switch (eventType) { 
   case XmlPullParser.START_TAG: 
    if (parser.getName().equals("contacts")) { 
     contacts = new ArrayList<Contact>(); 
    } else if (parser.getName().equals("contact")) { 
     contact = new Contact(); 
     contact.setId(Integer.valueOf(parser.getAttributeValue(0))); 
    } else if (parser.getName().equals("name")) { 
     contact.setName(parser.nextText()); 
    } else if (parser.getName().equals("image")) { 
     contact.setImage(parser.getAttributeValue(0)); 
    } 
    break; 
   case XmlPullParser.END_TAG: 
    if (parser.getName().equals("contact")) { 
     contacts.add(contact); 
    } 
    break; 
   } 
  } 
  return contacts; 
 } 
 /* 
  * 从网络上获取图片，如果图片在本地存在的话就直接拿，如果不存在再去服务器上下载图片 
  * 这里的path是图片的地址 
  */ 
 public Uri getImageURI(String path, File cache) throws Exception { 
  String name = MD5.getMD5(path) + path.substring(path.lastIndexOf(".")); 
  File file = new File(cache, name); 
  // 如果图片存在本地缓存目录，则不去服务器下载  
  if (file.exists()) { 
   return Uri.fromFile(file);//Uri.fromFile(path)这个方法能得到文件的URI 
  } else { 
   // 从网络上获取图片 
   URL url = new URL(path); 
   HttpURLConnection conn = (HttpURLConnection) url.openConnection(); 
   conn.setConnectTimeout(5000); 
   conn.setRequestMethod("GET"); 
   conn.setDoInput(true); 
   if (conn.getResponseCode() == 200) { 
    InputStream is = conn.getInputStream(); 
    FileOutputStream fos = new FileOutputStream(file); 
    byte[] buffer = new byte[1024]; 
    int len = 0; 
    while ((len = is.read(buffer)) != -1) { 
     fos.write(buffer, 0, len); 
    } 
    is.close(); 
    fos.close(); 
    // 返回一个URI对象 
    return Uri.fromFile(file); 
   } 
  } 
  return null; 
 } 
}

Serivce类中，注意以下几点

1.HttpURLConnection conn = (HttpURLConnection) url.openConnection();获取一个链接，从而进行通讯

2.怎么利用XxmlPullPaser类去解析XML，从而把数据封装成对象

3.getImageURI(String path, File cache) 这个方法具体实现

4.Uri.fromFile(file);这个方法能够直接返回一个Uri来

下面是自定义的Adapter类，

public class MyContactAdapter extends BaseAdapter { 
 protected static final int SUCCESS_GET_IMAGE = 0; 
 private Context context; 
 private List<Contact> contacts; 
 private File cache; 
 private LayoutInflater mInflater; 
 // 自己定义的构造函数 
 public MyContactAdapter(Context context, List<Contact> contacts, File cache) { 
  this.context = context; 
  this.contacts = contacts; 
  this.cache = cache; 
  mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE); 
 } 
 @Override 
 public int getCount() { 
  return contacts.size(); 
 } 
 @Override 
 public Object getItem(int position) { 
  return contacts.get(position); 
 } 
 @Override 
 public long getItemId(int position) { 
  return position; 
 } 
 @Override 
 public View getView(int position, View convertView, ViewGroup parent) { 
  // 1获取item,再得到控件 
  // 2 获取数据 
  // 3绑定数据到item 
  View view = null; 
  if (convertView != null) { 
   view = convertView; 
  } else { 
   view = mInflater.inflate(R.layout.item, null); 
  } 
  ImageView iv_header = (ImageView) view.findViewById(R.id.iv_header); 
  TextView tv_name = (TextView) view.findViewById(R.id.tv_name); 
  Contact contact = contacts.get(position); 
  // 异步的加载图片 (线程池 + Handler ) ---> AsyncTask 
  asyncloadImage(iv_header, contact.image); 
  tv_name.setText(contact.name); 
  return view; 
 } 
 private void asyncloadImage(ImageView iv_header, String path) { 
  ContactService service = new ContactService(); 
  AsyncImageTask task = new AsyncImageTask(service, iv_header); 
  task.execute(path); 
 } 
 private final class AsyncImageTask extends AsyncTask<String, Integer, Uri> { 
  private ContactService service; 
  private ImageView iv_header; 
  public AsyncImageTask(ContactService service, ImageView iv_header) { 
   this.service = service; 
   this.iv_header = iv_header; 
  } 
  // 后台运行的子线程子线程 
  @Override 
  protected Uri doInBackground(String... params) { 
   try { 
    return service.getImageURI(params[0], cache); 
   } catch (Exception e) { 
    e.printStackTrace(); 
   } 
   return null; 
  } 
  // 这个放在在ui线程中执行 
  @Override 
  protected void onPostExecute(Uri result) { 
   super.onPostExecute(result);  
   // 完成图片的绑定 
   if (iv_header != null && result != null) { 
    iv_header.setImageURI(result); 
   } 
  } 
 } 
 /** 
  * 采用普通方式异步的加载图片 
  */ 
 /*private void asyncloadImage(final ImageView iv_header, final String path) { 
  final Handler mHandler = new Handler() { 
   @Override 
   public void handleMessage(Message msg) { 
    super.handleMessage(msg); 
    if (msg.what == SUCCESS_GET_IMAGE) { 
     Uri uri = (Uri) msg.obj; 
     if (iv_header != null && uri != null) { 
      iv_header.setImageURI(uri); 
     } 
    } 
   } 
  }; 
  // 子线程，开启子线程去下载或者去缓存目录找图片，并且返回图片在缓存目录的地址 
  Runnable runnable = new Runnable() { 
   @Override 
   public void run() { 
    ContactService service = new ContactService(); 
    try { 
     //这个URI是图片下载到本地后的缓存目录中的URI 
     Uri uri = service.getImageURI(path, cache); 
     Message msg = new Message(); 
     msg.what = SUCCESS_GET_IMAGE; 
     msg.obj = uri; 
     mHandler.sendMessage(msg); 
    } catch (Exception e) { 
     e.printStackTrace(); 
    } 
   } 
  }; 
  new Thread(runnable).start(); 
 }*/ 
}

自定义Adapter中，我们要注意 AsyncImageTask这个类继承了AsyncTask类，AsyncTask是Android中常用来做异步任务的类，对线程池进行了封装，详细分析稍后再贴出一篇Blog。

下面是我们从服务器上获取并且解析的Xml文件

<&#63;xml version="1.0" encoding="UTF-8"&#63;> 
<contacts> 
 <contact id="1"> 
  <name>张飞</name> 
  <image src="http://192.168.1.103:8080/mymyweb/images/1.gif"/> 
 </contact> 
 <contact id="2"> 
  <name>博文</name> 
  <image src="http://192.168.1.103:8080/myweb/images/2.gif"/> 
 </contact>  
 <contact id="3"> 
  <name>张天佑</name> 
  <image src="http://192.168.1.103:8080/myweb/images/3.gif"/> 
 </contact>   
 <contact id="4"> 
  <name>松德</name> 
  <image src="http://192.168.1.103:8080/myweb/images/4.gif"/> 
 </contact>   
 <contact id="5"> 
  <name>赵薇</name> 
  <image src="http://192.168.1.103:8080/myweb/images/5.gif"/> 
 </contact> 
 <contact id="6"> 
  <name>李静</name> 
  <image src="http://192.168.1.103:8080/myweb/images/6.gif"/> 
 </contact>  
 <contact id="7"> 
  <name>李明</name> 
  <image src="http://192.168.1.103:8080/myweb/images/7.gif"/> 
 </contact>   
 <contact id="8"> 
  <name>黎明</name> 
  <image src="http://192.168.1.103:8080/myweb/images/8.gif"/> 
 </contact>   
 <contact id="9"> 
  <name>秦桧</name> 
  <image src="http://192.168.1.103:8080/myweb/images/9.gif"/> 
 </contact> 
 <contact id="10"> 
  <name>朱德</name> 
  <image src="http://192.168.1.103:8080/myweb/images/10.gif"/> 
 </contact>  
 <contact id="11"> 
  <name>冯巩</name> 
  <image src="http://192.168.1.103:8080/myweb/images/11.gif"/> 
 </contact>   
 <contact id="12"> 
  <name>dylan</name> 
  <image src="http://192.168.1.103:8080/myweb/images/12.gif"/> 
 </contact>   
 <contact id="13"> 
  <name>黄单</name> 
  <image src="http://192.168.1.103:8080/myweb/images/13.gif"/> 
 </contact> 
 <contact id="14"> 
  <name>含蕊</name> 
  <image src="http://192.168.1.103:8080/myweb/images/14.gif"/> 
 </contact>  
 <contact id="15"> 
  <name>欣琪</name> 
  <image src="http://192.168.1.103:8080/myweb/images/15.jpg"/> 
 </contact>   
 <contact id="16"> 
  <name>李忠华</name> 
  <image src="http://192.168.1.103:8080/myweb/images/16.jpg"/> 
 </contact>  
 <contact id="17"> 
  <name>方产员</name> 
  <image src="http://192.168.1.103:8080/myweb/images/17.jpg"/> 
 </contact>   
 <contact id="18"> 
  <name>张光</name> 
  <image src="http://192.168.1.103:8080/myweb/images/18.jpg"/> 
 </contact>  
</contacts>

本demo中为了安全起见，还对下载下来的图片的文件名进行了MD5加密，下面是MD5加密的代码，

public class MD5 { 
 public static String getMD5(String content) { 
  try { 
   MessageDigest digest = MessageDigest.getInstance("MD5"); 
   digest.update(content.getBytes()); 
   return getHashString(digest); 
  } catch (NoSuchAlgorithmException e) { 
   e.printStackTrace(); 
  } 
  return null; 
 } 
 private static String getHashString(MessageDigest digest) { 
  StringBuilder builder = new StringBuilder(); 
  for (byte b : digest.digest()) { 
   builder.append(Integer.toHexString((b >> 4) & 0xf)); 
   builder.append(Integer.toHexString(b & 0xf)); 
  } 
  return builder.toString(); 
 } 
}

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Android应用中如何异步下载图片并将图片保存到本地DEMO中

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