创建子线程向列队消息发送消息使主线程执行handleMessage()

Question

后台打印“从网络获取图片” 但是在ImageView的UI界面未显示图片
如图

代码：
public class MainActivity extends Activity {
Handler handler = new Handler(){
//只要消息列队里面有消息，此方法就调用
public void handleMessage(android.os.Message msg) {
//8，显示
ImageView iv = (ImageView) findViewById(R.id.iv);
//把图片设置进ListView
iv.setImageBitmap((Bitmap) msg.obj);

}
};

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

public void click(View v){

    final String path = "http://192.168.0.102:8080/mm.jpg";
    //判断缓存是否存在
    final File file = new File(getCacheDir(), getFileName(path));
    if(file.exists()){
        System.out.println("从缓存中读取图片");
        Bitmap bm = BitmapFactory.decodeFile(file.getAbsolutePath());
        ImageView iv = (ImageView) findViewById(R.id.iv);
        iv.setImageBitmap(bm);
    }else{
        System.out.println("从网络获取图片");
        Thread t = new Thread(){
            @Override
            public void run() {
                //请求网址，获取图片
                //1，确认网址
                try {
                    //2,获取URL对象
                    URL url = new URL(path);
                    //3，获取连接对象，此时还未建立连接
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                    //4，初始化连接对象
                    conn.setRequestMethod("GET");   //设置请求方法
                    conn.setReadTimeout(5000);      //设置读取超时
                    conn.setConnectTimeout(5000);   //设置连接超时
                    //5，与服务器建立连接
                    conn.connect();

                    //判断是否请求成功，通过请求码对比
                    if(conn.getResponseCode() == 200){
                        //6，拿到服务器返回的流，客户请求的数据就保存在流中
                        InputStream is = conn.getInputStream();
                        //7，开始文件输出流，把读取的字节写到本地
                        FileOutputStream fos = new FileOutputStream(file);
                        //从流中读数据的同时将数据写到内存
                        int len = 0;
                        byte[]b = new byte[1024];
                        while((len = is.read()) != -1){
                            fos.write(b, 0, len);
                        }
                        fos.close();

                        //将图片显示到屏幕
                        //通过图片的绝对路径构造出一个Bitmap对象
                        Bitmap bm = BitmapFactory.decodeFile(file.getAbsolutePath());

                        //创建消息对象
                        Message msg = new Message();
                        //将bm放入消息中
                        msg.obj = bm;
                        //子线程向列队消息发送消息
                        handler.sendMessage(msg);

                    }else{
                        Looper.prepare();
                        Toast.makeText(MainActivity.this, "请求失败", 0).show();
                        Looper.loop();
                    }
                } catch (Exception e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        };
        t.start();  
    }       
}
public String getFileName(String path){
    int index = path.lastIndexOf("/");
    return path.substring(index + 1);
}

}