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

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