前臺顯示:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/imageView_img"
        android:src="@mipmap/ic_launcher"
        android:layout_height="200dp"
        android:layout_width="200dp"
        android:layout_gravity="center_horizontal"
        />
     <Button
         android:onClick="click"
         android:layout_height="wrap_content"
         android:layout_width="wrap_content"
         android:text="下載"
         android:layout_gravity="center_horizontal"
         />
</LinearLayout>
 

後臺邏輯：

package com.example.lenovo.async_imagedownload;

import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.ImageView;
import android.widget.Toast;

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

public class MainActivity extends AppCompatActivity {
    private ImageView imageView_img;
    private ProgressDialog progressDialog;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        this.imageView_img = (ImageView) this.findViewById(R.id.imageView_img);

    }
    public void click(View view){
        this.progressDialog = new ProgressDialog(this);
        this.progressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
        String path = "https://gss0.bdstatic.com/94o3dSag_xI4khGkpoWK1HF6hhy/baike/c0%3Dbaike92%2C5%2C5%2C92%2C30/sign=61e019ca7dec54e755e1124cd851f035/7a899e510fb30f24027878f7ce95d143ac4b0367.jpg ";
        new myAsyncTask().execute(path);
    }
    private final class myAsyncTask extends AsyncTask<String,Integer,Bitmap>{
        @Override
        protected  void onPreExecute(){
            progressDialog.setMessage("正在下載網路圖片,請稍等。。。");
            progressDialog.show();
        }

        /**
         * 網路下載操作
         * @param params
         * @return
         */

        @Override
        protected Bitmap doInBackground(String... params) {
            try{
                //1.根據網址得到統一資源定位符物件
                URL url = new URL(params[0]);
                //2.開啟連線
                HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();
                //3.設定相關引數
                httpURLConnection.setConnectTimeout(3000);//三秒後如果連線不上，
                //向伺服器傳送請求並得到伺服器返回的狀態碼:200表示 ok
                   //4.得到輸入流物件
                InputStream inputStream = httpURLConnection.getInputStream();
                //5.得到網路圖片的總大小
                int size = httpURLConnection.getContentLength();
                publishProgress(0,size);//0:代表報告的是檔案的總大小

                ByteArrayOutputStream byteArrayOuputStream = new ByteArrayOutputStream();
                byte[] buffer = new byte[1024];

                int  len = 0;
                while((len = inputStream.read(buffer))!=-1){
                    byteArrayOuputStream.write(buffer,0,len);
                    publishProgress(1,len);//1.代表報告下載的進度
                }
                byte[] data = byteArrayOuputStream.toByteArray();
                //將位元組陣列還原成原始圖片物件
                Bitmap bitmap = BitmapFactory.decodeByteArray(data,0,data.length);
                return bitmap;

        }catch(Exception e){
            e.printStackTrace();
        }
        return null;
    }
    @Override
    protected  void onProgressUpdate(Integer...values){
        switch (values[0]){
            case 0:
                progressDialog.setMax(values[1]);
                break;
            case 1:
                progressDialog.incrementProgressBy(values[1]);
                break;
        }
    }
    @Override
    protected  void onPostExecute(Bitmap bitmap) {
        progressDialog.dismiss();
        if (bitmap == null) {
            Toast.makeText(MainActivity.this, "網路圖片下載失敗",Toast.LENGTH_LONG).show();
            return;
        }
        //將下載好的bitmap影象顯示到imageView控制元件上
        imageView_img.setImageBitmap(bitmap);
         }
     }
}




 

<uses-permission  android:name="android.permission.INTERNET"/>