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