一个文件下载的Servlet

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 > download servlet-name>
        <servlet-class>org.mstar.servlet.DownloadServlet servlet-class>
        <init-param>
            <param-name>fileRoot param-name>
            <param-value>d:/temp param-value>
         init-param>
        <init-param>
            <param-name>contentType param-name>
            <param-value>application/x-msdownload param-value>
         init-param>
        <init-param>
            <param-name>enc param-name>
            <param-value>utf-8 param-value>
         init-param>
     servlet>
    <servlet-mapping>
        <servlet-name>download servlet-name>
        <url-pattern>/down url-pattern>
     servlet-mapping>