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();
}
} 关于Android应用中如何异步下载图片并将图片保存到本地DEMO中问题的解答就分享到这里了，希望以上内容可以对大家有一定的帮助，如果你还有很多疑惑没有解开，可以关注亿速云行业资讯频道了解更多相关知识。
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