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
<?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(); } }
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