android.os.NetworkOnMainThreadException

NetworkOnMainThreadException: The exception that is thrown when an application attempts to perform a networking operation on its main thread.

You should call sendfeedback method on asynctask then only above code will work. As webserver is taking lot of time to response main thread 
becomes unresponsive. To avoid it you should call it on another thread. Hence asynctask is better.

当在主线程中执行网络操作时，NetworkOnMainThreadException会抛出这个异常；

你应该调用“asynctask”的“sendfeedback”方法来执行网络操作；当Web服务器花费很多时间响应主线程而变成迟钝，为了避免这种情况，使用“asynctask”更好

package com.example.lj.imagedemo;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.ImageView;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";
    private ImageView imageView;

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

        try {
            imageView = (ImageView) findViewById(R.id.imageView);
/*
            new Thread(new Runnable() {
                @Override
                public void run() {
                    //图片资源
                    String url = "http://s16.sinaimg.cn/orignal/89429f6dhb99b4903ebcf&690";
                    //得到可用的图片
                    Bitmap bitmap = getHttpBitmap(url);
                    if (bitmap == null) {
                        Log.i(TAG, "bitmap is null");
                    }
                    //显示
                    imageView.setImageBitmap(bitmap);
                }
            }).start();
*/
            new DownloadUrlBitmap().execute("http://192.168.118.120:8888/img/dn1.jpg");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private class DownloadUrlBitmap extends AsyncTask<String, Void, Bitmap> {
        @Override
        protected Bitmap doInBackground(String... params) {
            return loadImageFromNetwork(params[0]);
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            imageView.setImageBitmap(bitmap);
        }
    }


    private Bitmap loadImageFromNetwork(String url) {
        //得到可用的图片
        Bitmap bitmap = simpleNetworkImage(url);
        if (bitmap == null) {
            Log.i(TAG, "bitmap is null");
        }
        return bitmap;
    }


    public Bitmap simpleNetworkImage(String url) {
        Bitmap pngBM = null;
        try {
            URL picUrl = new URL(url);
            pngBM = BitmapFactory.decodeStream(picUrl.openStream());
        } catch (IOException e) {
            e.printStackTrace();
        }
        return pngBM;
    }

    /**
     * 获取网落图片资源
     *
     * @param url
     * @return
     */
    public Bitmap getHttpBitmap(String url) {
        URL myFileURL;
        Bitmap bitmap = null;
        try {
            myFileURL = new URL(url);
            //获得连接
            HttpURLConnection conn = (HttpURLConnection) myFileURL.openConnection();
            //设置超时时间为6000毫秒，conn.setConnectionTiem(0);表示没有时间限制
            conn.setConnectTimeout(6000);
            //连接设置获得数据流
            conn.setDoInput(true);
            //不使用缓存
            conn.setUseCaches(false);
            //这句可有可无，没有影响
            //conn.connect();
            //得到数据流
            InputStream is = conn.getInputStream();
            //解析得到图片
            bitmap = BitmapFactory.decodeStream(is);
            //关闭数据流
            is.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return bitmap;
    }
}