实现一个输入url可以查看图片及文件下载的小功能

布局文件

<?xml version="1.0" encoding="utf-8"?> <RelativeLayout         xmlns:android="http://schemas.android.com/apk/res/android"         xmlns:tools="http://schemas.android.com/tools"         xmlns:app="http://schemas.android.com/apk/res-auto"         android:layout_width="match_parent"         android:layout_height="match_parent"         tools:context="com.br.NetActivity">     <!--   centerCrop属性 拉伸图片填满imageView     如果原图大于imageView的大小，则按比例缩小并居中        -->     <ImageView             android:layout_width="match_parent"             android:layout_height="match_parent"             android:layout_below="@+id/ll"             android:id="@+id/iv"             android:scaleType="centerCrop" />      <LinearLayout             android:id="@+id/ll"             android:orientation="vertical"             android:layout_marginBottom="5dp"             android:layout_marginTop="10dp"             android:layout_width="match_parent"             android:layout_height="wrap_content">         <EditText                 android:layout_width="match_parent"                 android:layout_height="match_parent"                 android:inputType="textUri"                 android:layout_margin="5dp"                 android:layout_weight="1"                 android:hint="请输入图片的url"                 android:ems="10"                 android:id="@+id/et"/>         <Button                 android:text="浏览"                 android:layout_weight="4"                 android:onClick="click"                 android:layout_width="wrap_content"                 android:layout_height="wrap_content" tools:ignore="OnClick"/>         <Button                 android:hint="下载"                 android:onClick="down"                 android:layout_width="wrap_content"                 android:layout_height="wrap_content" android:layout_weight="1"/>         <Button                 android:text="多线程下载"                 android:layout_width="wrap_content"                 android:layout_height="wrap_content" android:id="@+id/mb" android:layout_weight="1"/>     </LinearLayout>    </RelativeLayout>  

NetActivity类

package com.br;  import android.content.Context; import android.graphics.Bitmap; import android.graphics.BitmapFactory; import android.os.Environment; import android.os.Handler; import android.os.Message; import android.support.v7.app.AppCompatActivity; import android.os.Bundle; import android.text.TextUtils; import android.util.Log; import android.view.View; import android.widget.Button; import android.widget.EditText; import android.widget.ImageView; import android.widget.Toast;  import java.io.*; import java.net.HttpURLConnection; import java.net.MalformedURLException; import java.net.ProtocolException; import java.net.URL;  public class NetActivity extends AppCompatActivity {      protected static final int CHANGE_UI = 1;     protected static final int ERROR = 2;     private EditText editText;     private ImageView imageView;     private Button down;      //主线程创建消息处理器     private Handler handler = new Handler(){          public void handleMessage(android.os.Message message){              Log.i("handleMessage","handleMessage方法里");              if(message.what == CHANGE_UI){                  Bitmap bitmap = (Bitmap) message.obj;                  //显示图片                  imageView.setImageBitmap(bitmap);              }else if(message.what ==ERROR){                  Toast.makeText(NetActivity.this,"图片显示错误",Toast.LENGTH_SHORT).show();              }         }     };     @Override     protected void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_net);           imageView = findViewById(R.id.iv);         editText = findViewById(R.id.et);     }     public void click(View view) {         final String path = editText.getText().toString().trim();         if(TextUtils.isEmpty(path)){             Toast.makeText(NetActivity.this,"图片路径不能为空",Toast.LENGTH_SHORT).show();         }         else{             new Thread(){                 private HttpURLConnection conn;                 private Bitmap bitmap;             //子线程请求网络，Android4.0后访问网络不能放在主线程中                 @Override                 public void run() {                     try {                         //创建url对象                         URL url = new URL(path);                         //根据url发送http请求                         conn = (HttpURLConnection)url.openConnection();                         conn.setRequestMethod("GET");  //请求方式为大写                         conn.setConnectTimeout(5000);  //设置超时时间                         int code = conn.getResponseCode();  //获取请求状态码                         //请求成功                         if(code==200){                             //获取输入流                             InputStream is = conn.getInputStream();                             //将流 转换成bitmap对象                             bitmap = BitmapFactory.decodeStream(is);                             Message msg = new Message();                             msg.what = CHANGE_UI;                             msg.obj = bitmap;                             handler.sendMessage(msg);                             is.close();                         }                         else {  //请求失败                             Message msg = new Message();                             msg.what = ERROR;                             handler.sendMessage(msg);                         }                     }catch (Exception e){                         e.printStackTrace();                         Message msg = new Message();                         msg.what = ERROR;                         handler.sendMessage(msg);                     }                     conn.disconnect();  //关闭连接                 }             }.start();         }     }      public void down(View view) {         final String path = editText.getText().toString().trim();         if(TextUtils.isEmpty(path)){             Toast.makeText(NetActivity.this,"图片路径不能为空",Toast.LENGTH_SHORT).show();             return;         }         //异步操作一定要开辟线程         new Thread(){             @Override             public void run() {                 InputStream is = null;                 try {                     URL url = new URL(path);                     //根据url发送http请求                     HttpURLConnection conn = (HttpURLConnection) url.openConnection();                     conn.setRequestMethod("GET");  //请求方式为大写                     conn.setConnectTimeout(5000);  //设置超时时间                     int contentLength = conn.getContentLength();                     Log.i("文件的大小",contentLength+"");                     //获取输入流                     is = conn.getInputStream();                 }catch (Exception e){                  }                 //  在存储卡下创建文件(sdcard文件夹中)                 String files = Environment.getExternalStorageDirectory()+"/";                 //path.substring(path.lastIndexOf("."));截取文件后缀名                 String fileName = files + "4"+ path.substring(path.lastIndexOf("."));                 File file =new File(fileName);                 byte[] b = new byte[1024];                 try {                     OutputStream os = new FileOutputStream(file);                     int len=0;                     //写入数据                     while ((len=is.read(b)) != -1){                         os.write(b,0,len);                     }                     Log.i("msg","文件下载成功");                     os.close();                     is.close();                 }catch (IOException e){                     Log.i("msg","文件下载错误");                     e.printStackTrace();                 }             }         }.start();         }      public void mulDown(View view) {     } }  

最后在AndroidManifest.xml加入这三个权限，若Android6.0以上需动态申请权限

  <!-- 网络请求权限   -->     <uses-permission android:name="android.permission.INTERNET"/>      <!--  写入文件权限  -->     <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>     <!--  读取文件权限  -->     <uses-permission android:name="android.permission.READ_PHONE_STATE" />