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" />

本文地址：https://blog.csdn.net/m0_46267375/article/details/110203009