如果是小的文件，可以直接使用 URLConnection.openStream() 打开网络输入流,然后将流写入到文件中

1. 创建一个 空的 Android 项目 cn.twle.android.NormalDownload

2. 修改 AndroidManifest.xml 添加相关权限

   <!-- 访问 internet 权限 -->
   <uses-permission android:name="android.permission.INTERNET"/>
   <!-- 往 SDCard 写入数据权限 -->
   <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
   

3. 修改 activity_main.xml 创建布局

   <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:orientation="vertical" >
   
       <TextView
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:text="请输入要下载的文件地址" />
   
       <EditText
           android:id="@+id/editpath"
           android:layout_width="match_parent"
           android:layout_height="wrap_content"
           android:text="https://www.twle.cn/static/i/meimei.jpg"
           />
   
       <Button
           android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:id="@+id/btndown"
           android:text="下载" />
   
       <TextView
           android:layout_marginTop="32dp"
           android:id="@+id/ms_log"
           android:layout_width="match_parent"
           android:layout_height="wrap_content"
           android:text="" />
   </LinearLayout>
   

4. 修改 MainActivity.java

   package cn.twle.android.normaldownload;
   
   import java.io.InputStream;
   import java.io.RandomAccessFile;
   import java.net.HttpURLConnection;
   import java.net.URL;
   
   import android.app.Activity;
   import android.os.Bundle;
   import android.os.Environment;
   import android.os.Handler;
   import android.os.Message;
   import android.view.View;
   import android.widget.Button;
   import android.widget.EditText;
   import android.widget.TextView;
   
   public class MainActivity extends Activity {
   
       private TextView ms_log;
       private EditText editpath;
       private Button btndown;
   
       private StringBuilder sb;
   
       private Handler handler = new UIHander();
   
       private final class UIHander extends Handler {
           public void handleMessage(Message msg) {
               ms_log.setText(sb.toString());
           }
       }
   
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);
   
           sb = new StringBuilder();
   
           ms_log = (TextView) findViewById(R.id.ms_log);
           editpath = (EditText) findViewById(R.id.editpath);
           btndown = (Button) findViewById(R.id.btndown);
   
           btndown.setOnClickListener(new View.OnClickListener() {
   
               public void onClick(View v) {
   
                   if (!Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
                       sendMessage("sd卡读取失败\n");
                       return;
                   }
   
                   final String path = editpath.getText().toString();
                   final String filename = "meimei.jpg";
                   final String saveDir = Environment.getExternalStorageDirectory().getAbsolutePath();
                   final String savePath = saveDir + "/" + filename;
                   new Thread(new Runnable() {
                       @Override
                       public void run() {
   
                           try {
                               URL url = new URL(path);
                               HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                               conn.setConnectTimeout(5000);
                               conn.setRequestMethod("GET");
   
                               //获得需要下载的文件的长度(大小)
                               int filelength = conn.getContentLength();
   
                               sendMessage("文件总大小" + filelength + "\n");
                               sendMessage("文件保存地址: " + saveDir + "/" + filename + "\n");
   
                               //生成一个大小相同的本地文件
                               RandomAccessFile file = new RandomAccessFile(savePath, "rwd");
                               file.setLength(filelength);
   
                               InputStream is = url.openStream();
   
                               byte[] buffer = new byte[1024];
                               int len = 0;
   
                               //从输入六中读取数据,读到缓冲区中
                               while((len = is.read(buffer)) > 0)
                               {
   
                                   file.write(buffer,0,len);
                               }
   
                               sendMessage("文件下载完成\n");
                               //关闭输入输出流
                               is.close();
                               file.close();
                               conn.disconnect();
                           } catch (Exception e) {
                               e.printStackTrace();
                           }
                       }
                   }).start();
               }
   
           });
       }
   
       public void sendMessage(String msg) {
   
           sb.append(msg);
   
           handler.sendEmptyMessage(0x001);
       }
   }