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.

當在主線程中執行網絡操作時，NetworkOnMainThreadException會拋出這個異常；

你應該調用“asynctask”的“sendfeedback”方法來執行網絡操作；當Web服務器花費很多時間響應主線程而變成遲鈍，為了避免這種情況，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() {
                    //圖片資源
                    String url = "http://s16.sinaimg.cn/orignal/89429f6dhb99b4903ebcf&690";
                    //得到可用的圖片
                    Bitmap bitmap = getHttpBitmap(url);
                    if (bitmap == null) {
                        Log.i(TAG, "bitmap is null");
                    }
                    //顯示
                    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) {
        //得到可用的圖片
        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;
    }

    /**
     * 獲取網落圖片資源
     *
     * @param url
     * @return
     */
    public Bitmap getHttpBitmap(String url) {
        URL myFileURL;
        Bitmap bitmap = null;
        try {
            myFileURL = new URL(url);
            //獲得連接
            HttpURLConnection conn = (HttpURLConnection) myFileURL.openConnection();
            //設置超時時間為6000毫秒，conn.setConnectionTiem(0);表示沒有時間限制
            conn.setConnectTimeout(6000);
            //連接設置獲得數據流
            conn.setDoInput(true);
            //不使用緩存
            conn.setUseCaches(false);
            //這句可有可無，沒有影響
            //conn.connect();
            //得到數據流
            InputStream is = conn.getInputStream();
            //解析得到圖片
            bitmap = BitmapFactory.decodeStream(is);
            //關閉數據流
            is.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return bitmap;
    }
}