使用AnsyncTask异步类从网络上下载图片

• 1,activity.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_alignParentLeft="true"
        android:layout_alignParentTop="true"/>
    <ProgressBar
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/progressBar1"
        android:visibility="gone"
        style="?android:attr/progressBarStyleLarge"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"/>

</RelativeLayout>

• MainActivity.java

package com.example.flanwu.expriment_two;

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.ProgressBar;

import java.net.URL;
import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URLConnection;

public class MainActivity extends AppCompatActivity {
    private ImageView mImageView = null;
    private ProgressBar mProgressBar = null;
    private String URLs = "https://ss3.bdstatic.com/70cFv8Sh_Q1YnxGkpoWK1HF6hhy/it/u=1591194216,3468862819&fm=26&gp=0.jpg";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.layout);
        //实例化控件
        this.mImageView = (ImageView)findViewById(R.id.imageView1);
        this.mProgressBar = (ProgressBar)findViewById(R.id.progressBar1);
        //实例化异步任务
        ImageDownloadTask task = new ImageDownloadTask();
        // 执行异步任务
        task.execute(URLs);

    }
    class ImageDownloadTask extends AsyncTask<String, Void, Bitmap>{
        @Override
        protected Bitmap doInBackground(String... params){
            Bitmap bitmap = null;
            String url = params[0];
            URLConnection connection;
            InputStream is;
            try{
                connection = new URL(url).openConnection();
                is = connection.getInputStream(); //获取输入流
                BufferedInputStream buf = new BufferedInputStream(is);
                bitmap = BitmapFactory.decodeStream(buf);
                is.close();
                buf.close();
            }catch (MalformedURLException e){
                e.printStackTrace();
            }catch (IOException e){
                e.printStackTrace();
            }
            return bitmap;
        }

        @Override
        protected void onPreExecute(){
            // 显示等待圆环
            mProgressBar.setVisibility(View.VISIBLE);
        }

        @Override
        protected void onPostExecute(Bitmap result){
            // 下载完毕，隐藏等待圆环
            mProgressBar.setVisibility(View.GONE);
            mImageView.setImageBitmap(result);
        }
    }
}

结果