把文件目录直接暴露给用户是很不安全的。所以要用Servlet来做,而且这样做,文件的存储方式就更丰富了,可以是从文件系统上取来的,也可以是数据库中经过计算生成的,或者从其它什么稀奇古怪的地方取来的。

public class DownloadServlet extends HttpServlet {
    
private String contentType = "application/x-msdownload";
    
private String enc = "utf-8";
    
private String fileRoot = "";


    
/**
     * 初始化contentType,enc,fileRoot
     
*/
    
public void init(ServletConfig config) throws ServletException {
        String tempStr 
= config.getInitParameter("contentType");
        
if (tempStr != null && !tempStr.equals("")) {
            contentType 
= tempStr;
        }
        tempStr 
= config.getInitParameter("enc");
        
if (tempStr != null && !tempStr.equals("")) {
            enc 
= tempStr;
        }
        tempStr 
= config.getInitParameter("fileRoot");
        
if (tempStr != null && !tempStr.equals("")) {
            fileRoot 
= tempStr;
        }
    }

    
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String filepath 
= request.getParameter("filepath");
        String fullFilePath 
= fileRoot + filepath;
        
/*读取文件*/
        File file 
= new File(fullFilePath);
        
/*如果文件存在*/
        
if (file.exists()) {
            String filename 
= URLEncoder.encode(file.getName(), enc);
            response.reset();
            response.setContentType(contentType);
            response.addHeader(
"Content-Disposition""attachment; filename=\"" + filename + "\"");
            
int fileLength = (int) file.length();
            response.setContentLength(fileLength);
            
/*如果文件长度大于0*/
            
if (fileLength != 0) {
                
/*创建输入流*/
                InputStream inStream 
= new FileInputStream(file);
                
byte[] buf = new byte[4096];
                
/*创建输出流*/
                ServletOutputStream servletOS 
= response.getOutputStream();
                
int readLength;
                
while (((readLength = inStream.read(buf)) != -1)) {
                    servletOS.write(buf, 
0, readLength);
                }
                inStream.close();
                servletOS.flush();
                servletOS.close();
            }
        }
    }

web.xml
    <servlet>
        
<servlet-name>downloadservlet-name>
        
<servlet-class>org.mstar.servlet.DownloadServletservlet-class>
        
<init-param>
            
<param-name>fileRootparam-name>
            
<param-value>d:/tempparam-value>
        
init-param>
        
<init-param>
            
<param-name>contentTypeparam-name>
            
<param-value>application/x-msdownloadparam-value>
        
init-param>
        
<init-param>
            
<param-name>encparam-name>
            
<param-value>utf-8param-value>
        
init-param>
    
servlet>
    
<servlet-mapping>
        
<servlet-name>downloadservlet-name>
        
<url-pattern>/downurl-pattern>
    
servlet-mapping>