Android异步从网络下载图片并且缓存图片到本地的demo

openkk 7年前

     在Android开发中我们经常有这样的需求,从服务器上下载xml或者JSON类型的数据,其中包括一些图片资源,本demo模拟了这个需求,从网络上加载XML资源,其中包括图片,我们要做的解析XML里面的数据,并且把图片缓存到本地一个cache目录里面,并且用一个自定义的Adapter去填充到LIstView,demo运行效果见下图:

Android异步从网络下载图片并且缓存图片到本地的demo

通过这个demo,要学会有一下几点

1.怎么解析一个XML

2.demo中用到的缓存图片到本地一个临时目录的思想是怎样的?

3.AsyncTask类的使用,因为要去异步的加载数据,就必须开启线程,但是在开启线程的时有时候不能很好的控制线程的数量,线程数量太大的时候手机会很快被卡死 这里就采用AsynsTask类的去解决这个问题,这个类里面封装了线程池的技术,从而保证不会因开启过多的线程而消耗太多的资源

4.本demo中的Handler类的使用情况 5.自定义adapter的使用 

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
</div>
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
</div>
<?xml version="1.0" encoding="UTF-8"?>  <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();      }  }

      以上省略了Contact.java这个domain类,通过这个demo,可以看出Android中会经常需要进行异步任务的处理,所以我们会常常用到自己手动开启线程,handler机制,或者AsyncTask类等手段来保证应用的性能。