Android异步加载访问网络图片-解析json

来自：http://www.imooc.com/video/7871 
推荐大家去学习这个视频，讲解的很不错。 
慕课网提供了一个json网址可以用来学习：http://www.imooc.com/api/teacher?type=4&num=30。我们的任务就是建立一个listview，将json提供的一些参数，主要是name，picSmall，description显示出来，效果图如下： 

主要思路如下：listview中图片的加载，程序中使用了两种方式，一种是使用Thread类，一种是使用AsyncTask，为了提高listview滑动的效率，并节省流量，使用了LruCache类，更改加载图片的处理逻辑为：当滑动listview时不加载图片，停止滑动listview时加载界面可视范围内的图片。具体程序如下： 
1 NewsBean.Java listview每一项的封装类

 package com.example.imoocnews;

 public class NewsBean {
     private String newsIconUrl;//图片的网址即picSmall
     private String newsTitle;//图片的标题即json中的name属性
     private String newsContent;//图片的内容即json中的description

     public NewsBean(String newsIconUrl, String newsTitle, String newsContent)
     {
         this.newsIconUrl = newsIconUrl;
         this.newsTitle = newsTitle;
         this.newsContent = newsContent;
     }
     public String getNewsIconUrl() {
         return newsIconUrl;
     }
     public void setNewsIconUrl(String newsIconUrl) {
         this.newsIconUrl = newsIconUrl;
     }
     public String getNewsTitle() {
         return newsTitle;
     }
     public void setNewsTitle(String newsTitle) {
         this.newsTitle = newsTitle;
     }
     public String getNewsContent() {
         return newsContent;
     }
     public void setNewsContent(String newsContent) {
         this.newsContent = newsContent;
     }

 }

2 适配器NewsAdapter.java

 package com.example.imoocnews;

 import java.util.List;

 import android.content.Context;
 import android.view.LayoutInflater;
 import android.view.View;
 import android.view.ViewGroup;
 import android.widget.BaseAdapter;
 import android.widget.ImageView;
 import android.widget.ListView;
 import android.widget.TextView;
 import android.widget.AbsListView;

 /**
  * listview的适配器，包括上下文对象和数据源
  * 提高listview的效率：当listview滚动时不去加载可见项图片，停止滚动后再开始加载
  */
 public class NewsAdapter extends BaseAdapter implements AbsListView.OnScrollListener{

     private LayoutInflater mInflater;
     private List<NewsBean> mList;
     private ImageLoader mImageLoader;
     private int mStart, mEnd;//listview屏幕可视范围内的第一条item和最后一个item
     public static String URLS[];//设置一个数组，用来保存所有图片的URL
     private boolean mFirstIn;//判断是否是第一次启动程序

     public NewsAdapter(Context context, List<NewsBean> data, ListView listView) {
         mInflater = LayoutInflater.from(context);
         this.mList = data;
         mImageLoader = new ImageLoader(listView);//在这里初始化，能够保证只有一个imageloader的实例，即只有一个LruCache的实例
         URLS = new String[data.size()];
         for (int i = 0; i < data.size(); i++) {
             URLS[i] = data.get(i).getNewsIconUrl();//将data中的每一个URL赋值给数组
         }
         listView.setOnScrollListener(this);
         mFirstIn = true;//写在构造函数中，第一次调用newsAdapter时表示第一次启动程序，显示listview
     }

     @Override
     public int getCount() {
         return mList.size();
     }

     @Override
     public Object getItem(int position) {
         // TODO Auto-generated method stub
         return mList.get(position);
     }

     @Override
     public long getItemId(int position) {
         // TODO Auto-generated method stub
         return position;
     }

     @Override
     public View getView(int position, View convertView, ViewGroup parent) {
         ViewHolder holder = null;
         if (convertView == null) {
             convertView = mInflater.inflate(R.layout.item, null);
             holder = new ViewHolder();
             holder.ivIcon = (ImageView) convertView.findViewById(R.id.iv_icon);
             holder.tvTitle = (TextView) convertView.findViewById(R.id.tv_title);
             holder.tvContent = (TextView) convertView.findViewById(R.id.tv_content);
             convertView.setTag(holder);
         }else {
             holder = (ViewHolder) convertView.getTag();
         }
         //holder.ivIcon.setImageResource(R.drawable.ic_launcher);
         String url = mList.get(position).getNewsIconUrl();
         holder.ivIcon.setTag(url);//给ImageView设置标志，即对应的图片网址
         //利用thread类实现异步加载图片，我们这里将其注释，使用AsyncTask的方式
         //new ImageLoader().showImageByThread(holder.ivIcon, mList.get(position).getNewsIconUrl());
         mImageLoader.showImages(holder.ivIcon, url);

         holder.tvTitle.setText(mList.get(position).getNewsTitle());
         holder.tvContent.setText(mList.get(position).getNewsContent());
         return convertView;
     }
     class ViewHolder
     {
         public ImageView ivIcon;
         public TextView tvTitle, tvContent;
     }
     /*
      * 当listview滑动的时候调用
      */
     @Override
     public void onScroll(AbsListView view, int firstVisibleItem,
             int visibleItemCount, int totalItemCount) {
         mStart = firstVisibleItem;
         mEnd = mStart + visibleItemCount;
         //只在第一次加载的时候调用
         if (mFirstIn && visibleItemCount >0) {//表示第一次加载listview并且已经绘制了可见范围内的item
             mImageLoader.loadImages(mStart, mEnd);
             mFirstIn = false;//加载图片后即设为false
         }
     }

     /*
      * 当listview滑动状态变化时调用
      */
     @Override
     public void onScrollStateChanged(AbsListView view, int scrollState) {
         if (scrollState == SCROLL_STATE_IDLE) {//listview停止滚动
             //加载可见项
             mImageLoader.loadImages(mStart, mEnd);

         }else {
             //停止加载任务
             mImageLoader.cancelAllTasks();
         }

     }

 }

3 访问图片的实现ImageLoader.java

 package com.example.imoocnews;

 import java.io.BufferedInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.net.HttpURLConnection;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.util.HashSet;
 import java.util.Set;

 import android.graphics.Bitmap;
 import android.graphics.BitmapFactory;
 import android.os.AsyncTask;
 import android.os.Handler;
 import android.os.Message;
 import android.util.LruCache;
 import android.widget.ImageView;
 import android.widget.ListView;

 public class ImageLoader {
     private ImageView mImageView;
     private String url;
     //当图片加载过后就将图片缓存到本地，下次便不用重新联网获取，直接从本地缓存获取即可,一个图片即string url --> bitmap
     private LruCache<String, Bitmap> mCache;
     private ListView mListView;
     private Set<NewsAsyncTask> mTasks;//从start到end范围每次执行加载图片任务的集合

     public ImageLoader(ListView listView)
     {
         mListView = listView;
         mTasks = new HashSet<ImageLoader.NewsAsyncTask>();
         int maxMemory = (int) Runtime.getRuntime().maxMemory();//获取最大可用内存
         int cacheSize = maxMemory/4;//设置缓存的大小
         mCache = new LruCache<String, Bitmap>(cacheSize){
             @Override
             protected int sizeOf(String key, Bitmap value) {
                 // 每次将图片存入缓存时返回图片的大小
                 return value.getByteCount();
             }
         };
     }
     /**
      * 将已联网获取成功的图片加入到缓存中
      * @param bitmap
      */
     public void addBitmapToCache(String url, Bitmap bitmap)
     {
         //在将图片缓存到本地之前要判断这个图片是否已经缓存过了
         if (getBitmapFromCache(url) == null) {
             mCache.put(url, bitmap);
         }
     }
     /**
      * 通过URL从缓存中取出相应的图片
      */
     public Bitmap getBitmapFromCache(String url)
     {
         return mCache.get(url);
     }

     private Handler mHandler = new Handler(){
         public void handleMessage(Message msg) {
             super.handleMessage(msg);
             //通过tag使得ImageView和它对应的URL绑定，这样在上下滑动listview时ImageView显示的图片就始终是正确的
             //否则，由于listview的缓存机制，ImageView会先显示出上次加载成功时的图片，然后再显示正确的图片
             if (mImageView.getTag().equals(url)) {
                 mImageView.setImageBitmap((Bitmap) msg.obj);//使用handler在主线程中更新UI，并将URL对应的图片设置给控件imageview
             }

         }
     };

     /**
      * 通过使用Thread的方式从网络上获取图片
      */
     public void showImageByThread(ImageView imageView, final String iconUrl)
     {
         mImageView = imageView;
         url = iconUrl;
         new Thread(){
             @Override
             public void run() {
                 // 在新的进程中实现图片的加载
                 super.run();
                 //从url中获得bitmap，将bitmap发送给主线程
                 Bitmap bitmap = getBitmapFromUrl(iconUrl);
                 Message message = Message.obtain();
                 message.obj = bitmap;
                 mHandler.sendMessage(message);
             }
         }.start();
     }
     public Bitmap getBitmapFromUrl(String urlString)
     {
         InputStream is = null;
         Bitmap bitmap;
         try {
             URL url = new URL(urlString);
             HttpURLConnection connection = (HttpURLConnection) url.openConnection();
             is = new BufferedInputStream(connection.getInputStream());
             bitmap = BitmapFactory.decodeStream(is);
             connection.disconnect();
             //Thread.sleep(1000);
             return bitmap;
         } catch (MalformedURLException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         } catch (IOException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         } finally
         {
             try {
                 is.close();
             } catch (IOException e) {
                 // TODO Auto-generated catch block
                 e.printStackTrace();
             }
         }
         return null;
     }

     /**
      * 加载listview可见范围内的所有图片
      *
      */
     public void loadImages(int start, int end)
     {
         for (int i = start; i < end; i++) {
             String url = NewsAdapter.URLS[i];
             //看是否能从缓存中取出对应的图片
             Bitmap bitmap = getBitmapFromCache(url);
             //如果缓存中没有，就要对每个url执行异步加载任务去获取图片
             if (bitmap == null) {
                 NewsAsyncTask task = new NewsAsyncTask(url);
                 task.execute(url);
                 mTasks.add(task);

             }else {
                 //如果缓存中存在此图片，直接将其设置给对应的imageview即可
                 //因为我们之前给imageview设置的tag就是URL,可以利用findViewWithTag直接在这里获取到
                 ImageView imageView = (ImageView) mListView.findViewWithTag(url);
                 imageView.setImageBitmap(bitmap);
             }
         }
     }
     /**
      * 取消所有正在进行的图片加载任务
      */
     public void cancelAllTasks()
     {
         if (mTasks != null) {
             for(NewsAsyncTask task : mTasks)
             {
                 task.cancel(false);
             }
         }
     }

     public void showImages(ImageView imageView, String iconUrl)
     {
         //是否能从缓存中取出对应的图片
         Bitmap bitmap = getBitmapFromCache(iconUrl);
         if (bitmap == null) {
             imageView.setImageResource(R.drawable.ic_launcher);//显示默认图片
         }else {
             //如果缓存中存在此图片，直接将其设置给对应的imageview即可
             imageView.setImageBitmap(bitmap);
         }

     }
     /**
      * 使用AsyncTask实现图片的异步加载
      */
     class NewsAsyncTask extends AsyncTask<String, Void, Bitmap>
     {
         //private ImageView mImageView;
         private String mUrl;
         public NewsAsyncTask(String url) {
         //  mImageView = image;
             mUrl = url;
         }
         @Override
         protected Bitmap doInBackground(String... params) {
             String url = params[0];
             Bitmap bitmap = getBitmapFromUrl(url);//从网络上得到图片
             if (bitmap != null) {
                 addBitmapToCache(url, bitmap);//获取图片成功将图片存入缓存中
             }
             return bitmap;
         }
         @Override
         protected void onPostExecute(Bitmap bitmap) {
             // 后台获取图片的任务完成时调用此方法
             super.onPostExecute(bitmap);
             //给imageview设置图片
             ImageView imageView = (ImageView) mListView.findViewWithTag(mUrl);
             if (imageView != null && bitmap != null) {
                 imageView.setImageBitmap(bitmap);
             }
             mTasks.remove(this);
         }
     }

 }

4. MainActivity.java

 package com.example.imoocnews;

 import java.io.BufferedReader;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.io.UnsupportedEncodingException;
 import java.util.ArrayList;
 import java.util.List;

 import org.json.JSONArray;
 import org.json.JSONException;
 import org.json.JSONObject;

 import android.app.Activity;
 import android.os.AsyncTask;
 import android.os.Bundle;
 import android.util.Log;
 import android.view.Menu;
 import android.view.MenuItem;
 import android.widget.ListView;

 public class MainActivity extends Activity {

     private ListView mListView;
     private static String jsonURL = "http://www.imooc.com/api/teacher?type=4&num=30";//json数据网址

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         mListView = (ListView) findViewById(R.id.lv_main);
         new NewsAsyncTask().execute(jsonURL);
     }
     /**
      * 将URL网址上的json数据转化为我们所需的newsbean对象
      * @return
      */
     private List<NewsBean> getJsonData(String url)
     {
         List<NewsBean> newsBeanList = new ArrayList<NewsBean>();//保存解析出来的所有的数据
         try {
             //获取到json字符串
             String jsonString = readStream(new URL(url).openStream());//和url.openConnection().getInputStream()一样
             //Log.d("MainActivity", jsonString);
             //将获取到的json字符串变为jsonObject对象，打开网址可以看出data节点是一个jsonArray,array里面存放了一个个的jsonObject
             NewsBean newsBean;
             JSONObject jsonObject;
             String newsUrl = null;
             String newsTitle = null;
             String newsContent = null;
             jsonObject = new JSONObject(jsonString);
             JSONArray jsonArray = jsonObject.getJSONArray("data");
             for (int i = 0; i < jsonArray.length(); i++) {
                 jsonObject = jsonArray.getJSONObject(i);
                 newsUrl = jsonObject.getString("picSmall");//图片网址
                 newsTitle = jsonObject.getString("name");//title
                 newsContent = jsonObject.getString("description");//content
                 newsBean = new NewsBean(newsUrl, newsTitle, newsContent);
                 newsBeanList.add(newsBean);
             }
         } catch (MalformedURLException e) {
             e.printStackTrace();
         } catch (IOException e) {
             e.printStackTrace();
         } catch (JSONException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
         return newsBeanList;
     }

     /**
      * 解析网络返回的数据
      */
     private String readStream(InputStream is)
     {
         InputStreamReader isReader;
         String result = "";
         String line = "";
         try {
             isReader = new InputStreamReader(is, "utf-8");//将字节流转化为字符流
             BufferedReader buffReader = new BufferedReader(isReader);//从字符输入流中读取文本，缓冲各个字符，从而实现字符、数组和行的高效读取
             while ((line = buffReader.readLine()) != null) {
                 result += line;
             }
         } catch (UnsupportedEncodingException e) {
             e.printStackTrace();
         } catch (IOException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }

         return result;
     }
     /**
      * 构造一个AsyncTask，传入String类型的URL，返回一个NewsBean对象，每一个对象就是
      * listview中的每一行数据，包括一个icon,title,content
      */
     class NewsAsyncTask extends AsyncTask<String, Void, List<NewsBean>>
     {

         @Override
         protected List<NewsBean> doInBackground(String... params) {
             return getJsonData(params[0]);
         }

         @Override
         protected void onPostExecute(List<NewsBean> result) {
             super.onPostExecute(result);
             // 访问网络并解析json成功后返回结果，即我们设置的List<NewsBean>
             NewsAdapter adapter = new NewsAdapter(MainActivity.this, result, mListView);
             mListView.setAdapter(adapter);
         }

     }

 }

源码在这里：http://download.csdn.net/detail/hnyzwtf/9418993