Java Servlet：选择要下载的文件

Question

我是Java Servlets / Apache Tomcat / etc的新手，但我能找到一些方便的资源来说明如何实现下载和上传功能。

当用户想要上传文件时，他们会单击该按钮，系统会提示他们选择文件。但是，当他们想要下载文件时，他们只能选择一个文件（在servlet中指定）。

所以，我的问题是：有没有办法以某种方式列出目录，让用户选择一个特定的文件，然后下载该文件？

如果你需要一个参考点，下面是我的代码：

注意：此代码有效，我只是想知道如何以我想要的效果扩展它。

package testPackage;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/DownloadServlet")
public class DownloadServlet extends HttpServlet 
{
    private static final String absFilePath = "/test/files/download/test.txt";
    private static final String relFilePath = "/files/download/test.txt";

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {   
        //Quick switch to determine what type of path to user
        boolean useAbsolute = false;    //Defaults to relative

        //Initialize a new File
        File downloadFile = new File("");

        //Check if we want to use the absolute file path
        if(useAbsolute)
        {
            //Read the input file from the absolute path
            downloadFile = new File(absFilePath);
        }

        //Otherwise, we are using relative
        else
        {
            //Read the input file from the relative path
            String relativePath = getServletContext().getRealPath(relFilePath);
            downloadFile = new File(relativePath);
            //System.out.println("Relative Path: " + relativePath);         
        }

        //Open up the FileInputStream
        FileInputStream inputStream = new FileInputStream(downloadFile);

        //Obtain the Servlet Context
        ServletContext context = getServletContext();

        //Get the MIME type of the file
        String mimeType = context.getMimeType(absFilePath);

        //If we don't get a type, default to binary
        if(mimeType == null)
        {
            //Set to binary type if the MIME mapping is not found
            mimeType = "application/octet-stream";
        }

        //Just some logging
        System.out.println("MIME Type: " + mimeType);

        //Modify Servlet Response
        response.setContentType(mimeType);
        response.setContentLength((int) downloadFile.length());

        //Set the header key/value
        String headerKey = "Content-Disposition";
        String headerValue = String.format("attachment; filename=\"%s\"", downloadFile.getName());
        response.setHeader(headerKey, headerValue);

        //Obtain the response output stream
        OutputStream outputStream = response.getOutputStream();

        byte[] buffer = new byte[4096];
        int bytesRead = -1;

        //While there are bytes to write
        while((bytesRead = inputStream.read(buffer)) != -1)
        {
            outputStream.write(buffer, 0, bytesRead);
        }

        //Close the streams
        inputStream.close();
        outputStream.close();
    }
}