androidj基础：从网上下载图片

<ImageView
android:id="@+id/ImageView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:background="#ffffff" />
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
>
<Button
android:id="@+id/bt1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="下载图片"
/> <Button
android:id="@+id/bt2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:layout_toRightOf="@+id/bt1" android:text="读取图片"
/>

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.administrator.httpexe">
<uses-permission android:name="android.permission.INTERNET"/></manifest>

import...//import语句自动生成，略
public class MainActivity extends AppCompatActivity { ImageView imageView;
Button button;
Button button1; @Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
imageView = (ImageView) findViewById(R.id.ImageView);
button = (Button) findViewById(R.id.bt1);
button1=(Button)findViewById(R.id.bt2); button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
MyTask task = new MyTask();//创建异步任务对象
task.execute("https://www.baidu.com/img/bd_logo1.png");//创建下载任务
}
});
button1.setOnClickListener(new View.OnClickListener() {//读取图片 @Override
public void onClick(View v) {
Bitmap bmp=null;
try{
FileInputStream fis= openFileInput("a.png");
bmp=BitmapFactory.decodeStream(fis);
if(bmp!=null) {
imageView.setImageBitmap(bmp);
}
}catch (Exception ex) {
}
}
});
}
public class MyTask extends AsyncTask<String, Void, Bitmap> { //bmp位图,泛型
@Override
protected Bitmap doInBackground(String... params) { //真正的下载
String urlStr = params[0]; //取第一个变参
Bitmap bmp = null;
try {
URL url = new URL(urlStr);
bmp=BitmapFactory.decodeStream(url.openStream()); //位图工厂,返回bitmap
InputStream is= url.openStream();
byte[] buffer=new byte[4096];//byte[]缓冲区
FileOutputStream fos= MainActivity.this.openFileOutput("a.png",MainActivity.MODE_PRIVATE);
int hasRead=is.read(buffer);
while (hasRead>0){
fos.write(buffer,0,hasRead);//文件大小，起始位置，读取的量
hasRead=is.read(buffer);
}
fos.close();//关闭
is.close();//关闭
}
catch (Exception ex) {
}finally {
}
return bmp;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
if(bitmap!=null){
imageView.setImageBitmap(bitmap);
}
}
}