android异步向服务器请求数据

    下面就android向服务器请求数据的问题分析如下：

1、在android4.0以后的版本，主线程（UI线程）不在支持网络请求，原因大概是影响主线程，速度太慢，容易卡机，所以需要开启新的线程请求数据；

  

<span style="font-family:System;font-size:12px;">thread1 = new Thread(){
            @Override
            public void run() {
                try {
                    URL url = new URL(WebUrlManager.CARSEVER_GetCarsServlet);
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();

                    BufferedInputStream bis = new BufferedInputStream(conn.getInputStream());
                    //缓冲读取
                    byte[] data = new byte[1024];
                    int len = 0;
                    String bufferString = "";
                    while((len = bis.read(data)) != -1){
                        bufferString+=new String(data, 0, len);
                    }
                    carList = new JSONArray(bufferString.trim());
                    //System.out.println(carList);
                    /*
                    for(int i=0;i<carList.length();i++){
                        System.out.println("加载图片");
                        JSONObject json = (JSONObject) carList.get(i);
                        String imageName = json.getString("image");
                        bm = BitmapFactory.decodeStream(new URL(WebUrlManager.CARSERVER_CAR_IMAGE+imageName).openStream());
                        carImageArray.add(bm);
                    }
                    */
                } catch (MalformedURLException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                } catch (JSONException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                flag = true;
            }
        };
        thread1.start();</span>

 

2、新线程完成后一启动，发现报错，空指针nullpointerexception，要等待线程完毕后才能得到数据，下面是两种解决方法：

1）要么判断线程是否还活着；

2）要么在线程中设置一flag，结束后，更改其状态 

<span style="font-family:System;font-size:12px;">/*
        //等待线程thread1执行完毕
        while(true){
                    if(thread1.isAlive()){
                        try {
                            Thread.sleep(500);
                        } catch (InterruptedException e) {
                            // TODO Auto-generated catch block
                            e.printStackTrace();
                        }
                    }else{
                        break;
                    }
            }
        */
        //当线程还没结束，就睡500毫秒ms
        while(!flag){
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
            }
        }

    }</span>

3、处理返回的json数据

1）向服务器请求Json数据，保存在carList 

<span style="font-family:System;font-size:12px;">URL url = new URL(WebUrlManager.CARSEVER_GetCarsServlet);
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();

                    BufferedInputStream bis = new BufferedInputStream(conn.getInputStream());
                    //缓冲读取
                    byte[] data = new byte[1024];
                    int len = 0;
                    String bufferString = "";
                    while((len = bis.read(data)) != -1){
                        bufferString+=new String(data, 0, len);
                    }
                    carList = new JSONArray(bufferString.trim());</span>

2)解析Json数据

  

<span style="font-family:System;font-size:12px;">JSONObject car = (JSONObject) getItem(position);
        try {
            //this.pic.setImageBitmap(carImageArray.get(position));
            this.title.setText(car.getString("title"));
            this.describe.setText(car.getString("describe"));
            this.updateTime.setText(car.getString("updateTime"));
            this.price.setText(String.format("%.1f", car.getDouble("price"))+"万");
            this.pic.setTag(WebUrlManager.CARSERVER_CAR_IMAGE+car.getString("image"));
            new AsyncViewTask().execute(this.pic);
        } catch (JSONException e1) {
            e1.printStackTrace();
        }</span>

4、图片加载通常很慢，最好异步请求

1）先贴出异步请求的类源代码 

<span style="font-family:System;font-size:12px;">import java.io.InputStream;
import java.lang.ref.SoftReference;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.HashMap;
import android.graphics.drawable.Drawable;
import android.os.AsyncTask;
import android.util.Log;
import android.view.View;
import android.webkit.URLUtil;
import android.widget.ImageView;

/**
 * @author wzy qq:290581825  http://blog.csdn.net/wzygis
 */
public class AsyncViewTask extends AsyncTask<View, Void, Drawable> {
    private View mView;
    private HashMap<String, SoftReference<Drawable>> imageCache;

    public AsyncViewTask() {
        imageCache = new HashMap<String, SoftReference<Drawable>>();
    }

    protected Drawable doInBackground(View... views) {
        Drawable drawable = null;
        View view = views[0];
        if (view.getTag() != null) {
            if (imageCache.containsKey(view.getTag())) {
                SoftReference<Drawable> cache = imageCache.get(view.getTag().toString());
                drawable = cache.get();
                if (drawable != null) {
                    return drawable;
                }
            }
            try {
                if (URLUtil.isHttpUrl(view.getTag().toString())) {// 如果为网络地址。则连接url下载图片
                    URL url = new URL(view.getTag().toString());
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                    conn.setDoInput(true);
                    conn.connect();
                    InputStream stream = conn.getInputStream();
                    drawable = Drawable.createFromStream(stream, "src");
                    stream.close();
                } else {// 如果为本地数据，直接解析
                    drawable = Drawable.createFromPath(view.getTag().toString());
                }
            } catch (Exception e) {
                Log.v("img", e.getMessage());
                return null;
            }
        }
        this.mView = view;
        return drawable;
    }

    protected void onPostExecute(Drawable drawable) {
        if (drawable != null) {
            ImageView view = (ImageView) this.mView;
            view.setImageDrawable(drawable);
            this.mView = null;
        }
    }

}</span>

结果如下：